|DUCKWEED: A tiny aquatic plant with enormous potential for agriculture and environment|
A considerable proportion of the world's surface is covered by saline waters, and the land areas from which the salts of the sea mostly originated are continuously leached of minerals by the run-off of rain water. Aquatic habitats abound; these may be temporary following rains or permanent largely through impediments to drainage . From the beginning of time these aquatic habitats have been harvested for biomass in many forms (food, fuel and building materials) by animals and man. From the time of the Industrial Revolution and with the onset of intensive land use enormous changes occurred. Agriculturists harvested both water and dry lands for biomass and minerals were applied to stimulate biomass yields, the aquatic habitats often became enriched (or contaminated) and water bodies were more temporary because of water use in agriculture or were lost through drainage or the establishment of major dams for irrigation, human water supplies and/or hydro-electric power generation. On the other hand other human activities, created aquatic areas for such purposes as the control of soil erosion, for irrigation, storage of water, sewage disposal and industrial waste storage or treatment and for recreational use.
Aquatic habitats have, in general, degenerated throughout the world because of pollution by both industry and other activities. Human activities have, in general, resulted in much higher flows of minerals and organic materials through aquatic systems, often leading to eutrophication and a huge drop in the biomass produced in such systems. The lack of dissolved oxygen in water bodies, through its uptake by microbes for decomposition of organic compounds, produces degrees of anaerobiosis that results in major growth of anaerobic bacteria and the evolution of methane gases.
Despite this in the areas of high rainfall, particularly in the wet-tropics, there remain major aquaculture industries, which vary from small farmers with 'manure fed' ponds producing fish through to large and extensive cultivation of fish and shellfish that are replacing the biomass harvested from the seas. The distribution of global aquaculture is shown in Figure 1. Fish production from ocean catches appear to be reduced, but production from farming practices are increasing which clearly demonstrates how important aquaculture is (and will become) in protein food production. This trend is illustrated by the trend in world prawn (shrimp) production shown in Figure 2.
Although traditional or staple crops can be produced from water bodies and in many situations traditional people often harnessed these resources, the aquatic habitat has been considered too costly and to difficult to farm other than for extremely high value crops such as algae harvested for high value materials such as b -carotene or essential long-chain fatty acids. Intensive aquaculture (hydroponics) of crops in highly mechanised farms have been developed but require highly sophisticated management systems and are expensive.
Throughout the world, and particularly in Asia, farmers have harvested naturally produced aquatic plants for a number of purposes including animal feed, green manure and for their family feed resources. The best known of these include the free floating plants; water lettuce (Pistia), water hyacinth (Eichhcornia), duckweed (Lemna) and Azolla and some bottom growing plants.
Azolla, which is a member of the fern family grows extensively in association with nitrogen fixing bacteria, which allows it to produce on waters low in N but containing phosphorus. Azolla has been comprehensively discussed by van Hove (1989).
In recent years a commonly occurring aquatic plant, "duckweed", has become prominent, because of its ability to concentrate minerals on heavily polluted water such as that arising from sewage treatment facilities. However, it has also attracted the attention of scientists because of its apparent high potential as a feed resource for livestock (Skillicorn et al., 1993; Leng, et al., 1994). Duckweed grows on water with relatively high levels of N, P and K and concentrates the minerals and synthesises protein. These are the nutrients which are often critically deficient in traditional fodders and feeds given to ruminants and to pigs and poultry particularly where the former depend on agro-industrial byproducts and crop residues.
The growing awareness of water pollution and its threat to the ecology of a region and agriculture per se has also focussed attention on potential biological mechanisms for cleansing water of these impurities making it potable and available for reuse. In general, water availability is becoming a primary limitation to expanding human activities and also the capacity of agricultural land to feed the ever increasing population of the world.
Another pressure that has stimulated interest in aquatic plants has been the over-use of fertilisers, particularly in Europe that has led to contamination of ground water supplies that can no longer be tolerated.
In the early 1960's a number of scientists warned of the pending shortage of fossil fuels, the expanding population and the potential for mass-starvation from an inability of agriculture to produce sufficient food.
The prophesies have proved wrong in the short term, largely because of the extent of the then undiscovered fossil fuel, but also because of the impact of the development of high yielding crop varieties, particularly of cereal grain. The "Green Revolution" whilst increasing cereal crop yields faster than human population increase has had serious side effects such as increased erosion and greater water pollution in some places and a huge increase in demand for water and fertiliser. Fertiliser availability and water use are often highly dependent on fossil fuel costs. Water resources in many of the world's aquifers are being used at rates far beyond their renewal from rainfall (see World Watch 1997).
At the present time it appears that potentially the application of scientific research could maintain the momentum for increased food production to support an increasing world population, but it is rather obvious that if this is to occur it must be without increased pollution, and with limited increases in the need for water and fertiliser and therefore also fossil fuel.
Global warming has now been accepted as inevitable. It is now a major political issue in most countries. Governments are now considering the need to reduce the combustion of fuels which contribute most to the build up of greenhouse gases and thus the increase in the thermal load that is presently occurring. A second problem for fossil fuel devouring industries is the potential for scarcer, and therefore, more costly oil resources in the near future. As Fleay (1996) in his book "The decline in the age of oil" has pointed out there have been no major discoveries of oil in the last ten years. This suggests that we have already discovered the major resources. Many of the oil wells are approaching or have passed the point at which half the reserves have been extracted. At this stage the cost in fuel to extract the remaining fuel increases markedly. The need to reduce fuel combustion and the potential for large increases in costs of extraction of oil from the major deposits all indicate major increases in fuel costs and the need to stimulate alternative energy strategies for industry and agriculture alike.
Fuel is a major economic component of all industries, and in particular, industrialised agriculture. Therefore food prices are highly influenced by fuel prices. The energy balance for grain production has consistently decreased with mechanisation as is illustrated by the fuel costs for grain production which is approaching 1MJ in as fossil fuel used in all activities associated with growing that crop to 1.5MJ out in the grain. A major component of the costs are in traction, fertiliser, herbicides and water use, particularly the energy costs of irrigation.
In recent times, a movement has begun to examine a more sustainable future for agriculture, particularly in the developing countries. The need in developing countries of Asia and Africa where most of the world's population lives and where population growth is the highest is to:
This suggests that small farmers need to be targeted and that farming should be integrated so that fertiliser and other chemical use is minimised together with lowered gaseous pollution. At the same time a country must ensure its security of food supplies. In the 1998 financial crisis in Asia, the small farmer was seriously effected because of the relatively high cost of fuel. This is bound to have serious effects on food production in the next few years if fertiliser applications are restricted. This will show up as a decline in crop yields over the next few years.
The problem of decreasing world supplies of fuels, increased legislation to decrease use of fossil fuel to reduce pollution, and the economic disincentive to use fertilisers in developing countries indicates to this writer that there is a massive need to consider a more integrated farming systems approach, rather than the monocultures that have developed to the present time.
Integrated farming systems use require three major components to minimise fertiliser use:
It also requires incorporation of animals into the system to utilise the major byproducts from human food production.
Duckweed aquaculture is an activity that fits readily into many crop/animal systems managed by small farmers and can be a major mechanism for scavenging nutrient loss. It appears to have great potential in securing continuous food production, particularly by small farmers, as it can provide fertiliser, food for humans and feed for livestock and in addition decrease water pollution and increase the potential for water re-use.
The production and use of duckweed is not restricted to this area and there is immense scope to produce duckweeds on industrial waste waters, providing a feed stock particularly for the animal production industries, at the same time purifying water.
In this presentation, duckweed production and use, particularly in small farmer systems, is discussed to highlight its potential in food security, particularly in countries where water resources abound and have been misused. On the other hand, duckweed aquaculture through its water cleansing abilities can make a greater amount of potable water available to a population living under arid conditions, providing certain safeguards are applied.
CHAPTER 2: The plant and its habitat
Duckweed is the common name given to the simplest and smallest flowering plant that grows ubiquitously on fresh or polluted water throughout the world. They have been, botanical curiosities with an inordinate amount of research aimed largely at understanding the plant or biochemical mechanisms. Duckweeds have great application in genetic or biochemical research. This has been more or less in the same way that drosophila (fruit flies) and breadmoulds have been used as inexpensive medium for genetic, morphological, physiological and biochemical research.
Duckweeds are small, fragile, free floating aquatic plants. However, at times they grow on mud or water that is only millimetres deep to water depths of 3 metres. Their vegetative reproduction can be rapid when nutrient densities are optimum. They grow slowly where nutrient deficiencies occur or major imbalances in nutrients are apparent. They are opportunistic in using flushes of nutrients and can put on growth spurts during such periods.
Duckweeds belong to four genea; Lemna, Spirodela, Wolfia and Wolffiella. About 40 species are known world wide. All of the species have flattened minute, leaflike oval to round "fronds" from about 1mm to less than 1cm across. Some species develop root-like structures in open water which either stabilise the plant or assist it to obtain nutrients where these are in dilute concentrations.
When conditions are ideal, in terms of water temperature, pH, incident light and nutrient concentrations they compete in terms of biomass production with the most vigorous photosynthetic terrestrial plants doubling their biomass in between 16 hours and 2 days, depending on conditions. An idea of their rapid growth is illustrated by the calculation that shows that if duckweed growth is unrestricted and therefore exponential that a biomass of duckweed covering 10cm2 may increase to cover 1 hectare (100 million cm2) in under 50 days or a 10 million fold increase in biomass in that time.
Obviously when biomass doubles every 1-2 days, by 60 days this could extend to a coverage of 32ha. In natural or farming conditions, however, the growth rate is altered by crowding, nutrient supply, light incidence and both air and water temperature in addition to harvesting by natural predators (fish, ducks, crustaceans and humans).
In addition to the above limiting factors there also appears to be a senescence and rejuvenation cycle which is also apparent in Azolla.
Vegetative growth in Lemna minor exhibits cycles of senescence and rejuvenation under constant nutrient availability and consistent climatic conditions (Ashbey & Wangermann, 1949). Fronds of Lemna have a definite life span, during which, a set number of daughter fronds are produced; each of these daughter fronds is of smaller mass than the one preceding it and its life span is reduced. The size reduction is due to a change in cell numbers. Late daughter fronds also produce fewer daughters than early daughters.
At the same time as a senescence cycle is occurring an apparent rejuvenation cycle, in which the short lived daughter fronds (with half the life span of the early daughters) produce first daughter fronds that are larger than themselves and their daughter fronds are also larger, and this continues until the largest size is produced and senescence starts again. This has repercussions as there will be cyclical growth pattern if the plants are sourced from a single colony and are all the same age. Under natural conditions it is possible to see a mat of duckweeds, apparently wane and explode in growth patterns.
The cyclic nature of a synchronised duckweed mat (i.e. all the same age) could be over at least 1 month as the life span of fronds from early to late daughters can be 33 or 19d respectively with a 3 fold difference in frond rate production (See Wangermann & Ashby, 1950).
The phenomena of cyclical senescence and rejuvenation may cause considerable errors of interpretation in studies that examine, for example, the response of a few plants to differing nutrient sources over short time periods.
In practice this cycle may be responsible for the need to restock many production units after a few weeks of harvesting. In Vietnam, with small growth chambers the duckweed required reseeding every 4-6 weeks (T.R. Preston personal communication) to be able to produce a constant harvestable biomass growing on diluted biogas digestor fluid. There is also the possibility in such systems of a build up in the plant of compounds that eventually become toxic or at least diminish their growth rate.
Root length appears to be a convenient relative measure of frond-age.
The senescence-rejuvenation cycle is increased by high temperatures through a decrease in individual frond life span but there is a concomitant increase in daughter frond production so that the biomass of fronds produced in a shorter life span is the same.
The rejuvenation cycle appears to be unaffected by either light density or temperature.
The cyclical changes appear to be mediated by chemicals secreted by the mother frond and growth patterns may be modified greatly by harvesting methods which mix water, wind effects and shelter as well as light intensity and temperature.
The increased death rate of duckweed mats exposed to direct sunlight has been recognised in work in Bangladesh where workers are set to cool duckweed mats by splashing them with water from below the surface and in Vietnam, Preston (personal communication) observed that the incidence of showers stimulated very rapid growth of duckweed in small ponds.
Duckweeds appear to have evolved, so as, to make good use of the periodic flushes of nutrients that arise from natural sources. However, in recent times they are more likely to be found growing in water associated with cropping and fertiliser washout, or down stream from human activities, particularly from sewage works, housed animal production systems and to some extent industrial plants.
For the many purposes related in this publication, the selection of duckweed to farm will depend on what grows on a particular water body and the farmer has little control over the species present. The various duckweeds have different characteristics. The fronds of Spirodela and Lemna are flat, oval and leaf like. Spirodela has two or more thread-like roots on each frond, Lemna has only one. Wolffiella and Wolfia are thalloid and have no roots; they are much smaller than Spirodela or Lemna. Wolfia fronds are usually sickle shaped whereas Wolffiella is boat shaped and neither has roots. Differentiation and identification is difficult and is perhaps irrelevant to the discussion. This is mainly because the species that grows on any water is the one with the characteristic requirement of that particular water and the dominant species will change with the variations in water quality, topography, management and climate, most of which are not easily or economically manipulated
The structure of the fronds of duckweed is simple. New or daughter fronds are produced alternatively and in a pattern from two pockets on each side of the mature frond in Spirodela and Lemma. In Wolffiella and Wolfia only one pocket exists. These pockets are situated in Spirodela or Lemna close to where the roots arise. Each frond, as they mature, may remain attached to the mother frond and each in turn, under goes this process of reproduction.
In all four genea each mother frond produces a considerable number of daughter fronds during its lifetime. However, after six deliveries of daughter fronds, the mother frond tends to die. Colonies produced in laboratory or naturally are always spotted with brown dead mother fronds.
The bulk of the frond is composed of chlorenchymatous cells separated by large intracellular spaces that are filled with air (or other gases) and provide buoyancy. Some cells of Lemna and Spirodela have needle like raphides which are presumably composed of calcium oxalate.
The upper epidermis in the Lemna is highly cutinized and is unwettable. Stomata are on the upper side in all four genea. Anthocyanin pigments similar to that in Azolla also form in a number species of Lemnacae. Both Spirodela and Lemna have greatly reduced vascular bundles.
Roots in both Spirodela and Lemna are adventitious. The roots are usually short but this depends on species and environmental conditions and vary from a few millimetres up to 14cm. They often contain chloroplasts which are active photosynthetically. However, there are no root-hairs.
The plant reproduces both vegetatively and sexually, flowering occurs sporadically and unpredictably. The fruit contains several ribbed seeds which are resistant to prolonged desiccation and quickly germinate in favourable conditions.
The Lemnacae family is world wide, but most diverse species appear in the subtropical or tropical areas. These readily grow in the summer months in temperate and cold regions; they occur in still or slowly moving water and will persist on mud. Luxurious growth often occurs in sheltered small ponds, ditches or swamps where there are rich sources of nutrients. Duckweed mats often abound in slow moving backwaters down-stream from sewage works.
In the aquatic habitat of crocodiles and alligators, duckweeds often have luxurious growth on the nutrients from the excrement of these reptiles and the local zoo can often provide a convenient source of duckweed for experimental purposes (see Photo 2).
Some species appear to tolerate saline waters but they do not concentrate sodium ions in their growth. The apparent limit for growth appears to be between 0.5 and 2.5% sodium chloride for Lemna minor
When the aquatic ecosystem dries out or declining temperatures occur, duckweeds have mechanisms to persist until conditions return that can support growth. This occurs through late summer flowering, or the production of starch filled structures or turin which are more dense than the fronds so the plants sink to the bottom of the water body and become embedded in dried mud.
The four species of Lemnacae are found in all possible combinations with each other and other floating plants. They are supported by plants that are rooted in the pond. They effect the light penetration of water resources and depending on their coverage of the area they can prevent the growth of algae or plants that grow emersed in water. They provide habitat and protection for a number of insects that associate with the plant but they appear to have few insects that feed on them. The main predators appears to be herbivorous fish, (particularly carp), snails, flatworms and ducks, other birds may also feed on duckweeds but reports are few in the literature. The musk rat appears to enjoy duckweeds and the author suggests that many other animals may occasionally take duckweeds such as pigs and ruminants.
The appearances of duckweed species not previously seen in areas of Europe have been attributed to global warming and/or a strong indication of rising water temperature throughout the world from global warming (Wolff & Landolt 1994).
This is a most difficult area to review since much of the information is by way of the popular press or is only mentioned in scientific papers. However, after a lecture given at the University of Agriculture and Forestry in Ho Chi Minh City in which the potential of duckweed biomass for animal production was discussed, as a novel concept, the writer was most chastened to find that duckweed was used extensively by local farmers as feed for ducks and fish and there was a flourishing market for duckweed.
The duckweed based farming system in Vietnam depended largely on manure and excrement being collected in a small pond where some eutrophication takes place; the water from this pond runs into a larger pond about 0.5m deep on which duckweed grows in a thick mat. This was harvested on a daily basis and immediately mixed with cassava waste (largely peelings) and fed to ducks which were constrained in pens on the side of the pond or lagoon (see Photo 3). The ducks were produced for the local restaurant trade.
In Taiwan, it was traditional to produce duckweeds for sale to pig and poultry producers.
There are reports that Wolfia arrhiza, which is about 1mm across has been used for many generations as a vegetable by Burmese, Laotions and Northern Thailand people. Thai's refer to this duckweed as "Khai-nam" or "eggs of the water" and it was apparently regarded as a highly nutritious food stuff. It could have been a valuable source of vitamins particular of vitamin A to these people. This would have been particularly important source during the long dry season of Northern Thailand when green vegetables may have been scarce. It is also a good source of minerals, again its phosphorous content could have been vital in areas where there are major deficiencies, such as occurs in Northern Thailand.
There are references in the literature to duckweed as both a human food resource and as a component of animal and bird diets in traditional/small farmer systems in most of South Asia.
Like all photosynthetic organisms, duckweeds grow with only requirements for minerals, utilising solar energy to synthesise biomass. They have, however, the capacity to utilise preformed organic materials particularly sugars and can grow without sunlight when provided with such energy substrates. In practice the ability to use sugars in the medium as energy sources is irrelevant, as in most aquatic systems they do not exist. However, they could be of some importance where industrial effluent's need to be purified and duckweed is considered for this process (e.g. waste water from the sugar industry or waste water from starch processing).
Most research on nutrient requirements have centred on the need for nitrogen, phosphorus and potassium (NPK). However, like all plants, duckweeds need an array of trace elements and have well developed mechanisms for concentrating these from dilute sources. From the experience of the Non-Government Organisation PRISM (based in Colombia, Maryland, USA, see chapter 6) in Bangladesh, it appears that providing trace minerals through the application of crude sea salt was sufficient to ensure good growth rates of duckweeds in ponded systems. However, considerable interest has been shown by scientists in the capacities of duckweed to concentrate, in particular, copper, cobalt and cadmium from water resources where these have economic significance.
Mineral nutrients appear to be absorbed through all surfaces of the duckweed frond, however, absorption of trace elements is often centred on specific sites in the frond.
The requirements to fertilise duckweeds depends on the source of the water. Rainwater collected in ponds may need a balanced NPK application which can be given as inorganic fertiliser or as rotting biomass, manure or polluted water from agriculture or industry. Effluent's from housed animals are often adequate or are too highly concentrated sources of minerals and particularly because of high ammonia concentration may need to be diluted to favour duckweed growth. Run-off water from agriculture is often high in P and N but the concentration may need to be more appropriately balanced. Sewage waste water can be high or low in N depending on pretreatments but is almost always adequate in K and P. Industrial waste water from sugar and alcohol industries for example are always low in N.
Little work has been done to find the best balance of nutrients to provide maximum growth of duckweed. The duckweed has been provided with mechanisms that allow it to preferentially uptake minerals and can grow on very dilute medium. The main variables that effect its growth under these circumstances are light incidence and water and air temperatures.
The growth rate and chemical composition of duckweed depends heavily on the concentration of minerals in water and also on their rate of replenishment, their balance, water pH, water temperature, incidence of sunlight and perhaps day length. Its production per unit of pond surface also depends on biomass present at any one time.
Duckweeds grow at water temperatures between 6 and 33° C. Growth rate increase with water temperature, but there is an upper limit of water temperature around 30° C when growth slows and at higher temperature ceases. In open lagoons in direct sunlight duckweed is stressed by high temperature created by irradication and in practice yields are increased by mixing the cooler layers of water low in the pond and splashing to reduce surface temperature of the duckweed matt.
Duckweed survives at pH's between 5 and 9 but grows best over the range of 6.5-7.5. Efficient management would tend to maintain pH between 6.5 and 7. In this pH range ammonia is present largely as the ammonium ion which is the most readily absorbed N form. On the other hand a high pH results in ammonia in solution which can be toxic and can also be lost by volatilisation.
Duckweeds appear to be able to concentrate many macro and micro minerals several hundred fold from water, on the other hand high mineral levels can depress growth or eliminate duckweeds which grow best on fairly dilute mineral media. There is a mass of data on the uptake by duckweed of micro-elements which can be accumulated to toxic levels (for animal feed). However, their ability to concentrate trace elements from very dilute medium can be a major asset where duckweed is to be used as an animal feed supplement. Trace elements are often deficient in the major feed available to the livestock of small resource poor farmers. For example, in cattle fed mainly straw based diets both macro and micro mineral deficiencies are present.
Duckweeds need many nutrients and minerals to support growth. Generally slowly decaying plant materials release sufficient trace minerals to provide for growth which is often more effected by the concentrations of ammonia, phosphorous, potassium and sodium levels. There is by far the greatest literature on the requirements of duckweed for NPK and the ability of the plant to concentrate the requirements of micro nutrients from the aquatic medium is usually considered not to be a limitation. In the work in Bangladesh by PRISM, crude sea salt was considered to be sufficient to provide all trace mineral requirements when added to water at 9kg/ha water surface area when duckweed growth rates were high at around 1,000kg of fresh plant material/day.
Depth of water required to grow duckweed under warm conditions is minimal but there is a major problem with shallow ponds in both cold and hot climates where the temperature can quickly move below or above optimum growth needs. However, to obtain a sufficiently high concentration of nutrients and to maintain low temperatures for prolonged optimal growth rate a balance must be established between volume and surface area. Depth of water is also critical for management, anything greater than about 0.5 metres poses problems for harvesting duckweeds, particularly by resource poor farmers. Whereas, where water purification is a major objective in the production of duckweed, it is impractical to construct ponds shallower than about 2m deep.
Incident sunlight and environmental temperatures are significant in determining the depth of water as undoubtedly duckweed is stressed by temperatures in excess of 30° C and below about 20° C growth rate is reduced.
In practice, depth of water is probably set by the management needs rather than the pool of available nutrients and harvesting is adjusted according to changes of growth rate, climate changes and the nutrient flows into the system.
Duckweeds evolved to take advantage of the minerals released by decaying organic materials in water, and also to use flushes of minerals in water as they occurred when wet lands flooded. Duckweeds now appear to have the potential to be harnessed as a commercial crop for a number of purposes.
Water availability is likely to limit terrestrial crop production particularly of cereals in the coming years (see World Watch 1997). Water purification and re-use particularly that water arising from sewage works, industrial processing and run-off from irrigation appears to be mandatory in the future, both to reduce pollution of existing water bodies and to provide reusable water for many purposes including that required by humans in some places as drinking water.
Duckweeds appear to be able to use a number of nitrogenous compounds either on their own or through the activities of associated plant and animal species. The ammonium ion (NH4+) appears to be the most useful N source and depending on temperatures duckweeds continue to grow down to extremely low levels of N in the water. However, the level of ammonia N in the water effects the accretion of crude protein in the plant (see Table 1).
The value of duckweed as a feed resource for domestic animals increases with increasing crude protein content. In studies at the University of New England, Armidale, Australia, the crude protein content of duckweed growing on diluted effluent from housed pigs increased with increased water levels of N from about 15% crude protein with trace levels of N (1-4mg N/l) to 37% at between 10-15mg N/l. Above 60mg N/l a toxic effect was noticed perhaps due to high levels of free ammonia in the water. Whilst few experiments have been undertaken on the optimum level of ammonia required, these results give a guide-line for the levels of N to be established and maintained in duckweed aquaculture to obtain a consistently high crude protein level in the dry matter.
In most practical situations the approach to growing duckweed is to find the dilution of water where N is not limiting growth and supports high levels of crude protein in the plant. This is usually done by an arbitrary test. Serial dilutions of the water source with relatively pure water (rain water) is carried out and duckweed seeded into each dilution and weight change recorded after, say, 4 weeks. In this way the appropriate N concentration is established.
A useful indicator of whether conditions in the pond are appropriate for growth of duckweed (Lemna spp) of high protein content in the length of the roots. Many experimental observations (Rodriguez and Preston 1996a; Nguyen Duc Anh et al 1997; Le Ha Chau 1998) have shown that over short growth periods there is a close negative relationship between root lenght and protein content of the duckweed and with the N content of the water. Data taken from the experiment of le Ha Chau (1998) are illustrated in Figure 4. In most small-scale farmsituations it is not feasible to determine the protein content of the duckweed that is being used; nor can the nutrient content (especially nitrogen) of the water be estimated easily. To determine the root length of duckweed is a simple operation and and requires neither equipment nor chemicals. By monitoring this characteristic, the user can have an indication of the nutritive corrective measures when the lenght of the roots exceeds about 10mm.
In duckweed aquaculture a source of N essential and in many start-up systems, based on water effluent from sewage or housed animals, the project has been considered by pre-treatments that denitrify the water and reduce ammonia concentrations. Most forms of aeration in sewage works are highly efficient in de-nitrification of waste waters, but this process compounds pollution peoblems. for instance where the effluent is high in P this promotes the growth of algae that fix N. In Australia the contamination of river systems with phosphorus often led to massive blooms of blue-green algae that rae toxic to humans and animals.
Although there is an association of N fixing cyanobacteria with duckweeds, these are certainly not important from a standpoint of farming duckweeds. Duong and Tiedje (1985) were able to demonstrate that duckweeds from many sources had heterocystous cyanobacteria firmly attached to the lower epidermis of older leaves, inside the reproductive pockets and occasionally attached to the roots. They calculated that N fixation via these colonies could amount to 3.7-7.5kg N per hectare of water surface in typical Lemna blooms, but the association of cyanobacteria with Lemna trisulca was 10 times more effective.
Probably, under most practical situations ammonia is the primary limiting nutrient for duckweed growth and the establishment of the optimum level for maximum growth of duckweeds needs research, particularly in the variety of systems the plant may be expected to grow. The effects of time and lowering of N content of sewage water on yield and crude protein content of duckweed is shown in Figure 5 and Figure 6.
From recent research it appears that duckweed require about 20-60mg N/l to grow actively and from two studies [(those of Sutton & Ornes, (1975) compared with those of Leng et al., (1994)] it is apparent that there is a complex relationship between, the initial composition of the duckweed used in research and the level of nutrients required.
Stambolie and Leng (1993) showed with duckweeds harvested from a backwater of a river and with an initial low crude protein content, it was only when the duckweed protein increased to the highest level that rapid growth of biomass commenced (i.e. at 3 weeks after introduction to the water) (Figure 6) even though by that time the N content of the water had declined to levels that were below the optimum that appears to be necessary for maximum protein levels (Figure 3).
In the work of Sutton and Ornes (1975), however, duckweed of a higher protein content was initially used and growth rate again peaked at about the third week (Fig. 6) but by this time the crude protein content had declined to below 15%. This apparent opposite result can be rationalised if there is a stress factor involved which requires 3 weeks to overcome, and under these circumstances its growth may be considered
to be optimised at about 20mg ammonia N/l but to obtain maximum crude protein content it requires ammonia levels to be about 60 mg N/l (see Figure 5). A further implication is that where a high protein content is present in duckweed at the commencement of a growth study, the duckweed can grow through mainly synthesis of only carbohydrate. However, the variable results using duckweeds harvested from the wild and the slow "adaptation" to new conditions is obviously a confusing factor in interpreting any data of the requirements for duckweed for nutrients in such short term studies.
The most important issue is that duckweed increases its protein content according to the ammonia level in an otherwise adequate medium up to levels of 60mg N/l (see Figures 3, 4, & 5). For food or feed purposes there is a vast difference in the value of duckweed biomass depending on its protein content (see later). Rapid growth of duckweed is also associated with high protein accretion and low fibre content and fibre content increases where root growth occurs.
Duckweeds appear to concentrate P up to about 1.5% of their dry weight and as such are able to grow on high P waters provided the N concentrations are maintained. The plant also appears to be able to draw on the pool of P in its biomass for its biochemical activities and once P had been accumulated it will continue to grow on waters devoid of P. On the other hand the P in duckweed appears to be highly soluble and is released rapidly to the medium on death of the plant (Stambolie & Leng 1994).
The relationship between P content of sewage water and P content of duckweeds growing on such water are shown in Figure 6. Leng et al (1994) found a higher concentration of P in duckweed at high water P levels than that found by Sutton and Ornes (1975). The time course of uptake of P by duckweed in static sewage water is shown in Figure 6. The differences in accumulation levels in the two studies cited possibly resides in the rates of growth when the samples were taken. The capacity of duckweed to concentrate P is clear and maximum P levels in tissues (10-14mg P/kg dry weight) are achieved with water P levels as low as 1.0 mg P/litre.
The important issue here is that duckweeds concentrate P when water levels are enriched with P and it appears to be readily available once the plant is disrupted or dies. The P level in duckweed is sufficiently high to be a valuable source of this nutrient for both plants and animals.
Vigorously growing duckweed is a highly efficient K sink, but only low concentration of K in water are needed to support good growth when other mineral requirements are satisfied. Most decaying plant materials would easily produce the K requirements of duckweed.
Little work has been done to examine the S requirements of duckweeds. The mechanisms for sulphate uptake have been studied since uptake of sulphate is the first step in the biosynthesis of S-amino acids. Such biosynthesis needs the integration of pathways providing carbon building blocks and reduced sulphur (Datko & Mudd, 1984). It is possible that S levels are at times limiting to growth or protein accretion because of the high level of S-amino acids in the plant when growth rate is high and ammonia in the medium is non-limiting. Salts of sulphate appears to meet the requirements. As S is so readily leached from soils it is an unlikely candidate for deficiencies in systems that may be established to farm duckweed except where huge dilutions of the water are needed to obtain a suitable N level.
The uptake of NPSK by duckweed from sewage water is shown in Figure 7 and the experimental design for such studies are shown in Photo 4.
Sea salt (9kg/ha/d) has been applied as part of a fertiliser program in pilot studies of duckweed farming in Bangladesh (see the discussion of PRISM's work in Chapter 6). This work suggested a good ability of duckweed to accumulate sodium as there was no apparent problems with salination. It appeared possible that duckweed removed up to 9kg salt/ha/d when grown under fairly optimal conditions, suggesting a potential for duckweed to rehabilitate saline land and water.
In studies undertaken at the University of New England it soon became apparent that the requirement for salt and the capacity of the plant to concentrate sodium was not significant in relation to salt levels that accumulated in lagoons fed by open cut coal mines. However, the exercise pointed a way for the potential use of such waters for duckweed production as duckweeds tolerated the salt levels and grew substantially when additional nutrients were provided. Using small galvanised iron tanks (see Photo 4) the effect of growing duckweed on saline mine waters with or without extra added nutrients was studied. Growth rate and protein content of duckweed is shown in Figure 8 together with the effects on mineral levels in the water. Duckweed grew on the water with or without added fertilisers but the uptake of sodium was low. The quality of the duckweed (indicated by its crude protein content) was maintained for some time by fertiliser application. The phosphorous requirement for growth was apparently low.
This data is introduced here to show that even saline waters can be used to grow duckweed, although research is needed to investigate the needs for additional nutrients on saline waters.
Figure 8a. The effects of growing duckweed on saline mine waters with (D ) or without (o) added NPK fertiliser to optimum levels on crude protein and salt content. (Sell et al., 1993; Sell, 1993).
Figure 8b. The effects of growing duckweed on saline mine waters with (D )or without (o) added NPK fertiliser to optimum levels on dry matter harvest and P content. (Sell, et al 1993, Sell, 1993).
8(c) Duck weed dry matter harvested: cumulative and residual pool8 (d) Phosphorus content of mine waste water growing duckweed
Conclusions on mineral requirements
The well developed system of concentrating minerals in duckweed allows them to grow under a wide range of conditions. The concentrating ability of duckweed for trace elements has been estimated to be many hundreds of thousands times. A simple comparison indicates some of the potential for duckweed to accumulate nutrients by comparing water levels and tissue dry matter levels of a number of minerals.
Table 2: Some mineral compositions of duckweed and their potential to remove minerals from water bodies (calculated from the literature).
Along with this advantage of mineral removal is obviously the potential detrimental effects of accumulation of heavy metals.
All members of the duckweed family concentrate heavy metals in particular cadmium, chromium and lead which may at times reach levels in the plant which are detrimental to both the health and growth of the plant in addition to creating problems where the plant is used in any part of a food chain eventually leading to human consumption.
The accumulation of heavy metals by duckweed is not normally a problem for those wishing to use duckweeds from natural water resources or effluent from human or intensive animal housing as these metals are normally at extremely low concentrations.
Duckweeds, however, are contaminated by such heavy metals from industries such as tanning (chromium) leachates from mining (e.g. cadmium) and great care is needed where water is contaminated to be sure that heavy metals do not get into the human food chain.
On the other hand, duckweeds may find use in stripping heavy metals from industrial water. Also their content of heavy metals can be used to indicate potential pollution levels of waters.
Cadmium appears to be absorbed by both living and dead duckweed plants and the cadmium is actively taken up by the plant (Noraho & Gour 1996). Cadmium at high concentrations, that is the concentration that prevents vegetative reproduction (EC-50) was found to be 800ppb but duckweeds grown in medium of 2.2ppm still accumulated most of the cadmium over 7d and when fed to crayfish increased cadmium in the hepatopncreata 26 fold and in muscles almost 7 fold (Devi et al., 1996). It is therefore, extremely important to be sure of low cadmium levels in water prior to any large scale use of duckweeds as feed for domestic animals or humans.
Many reports are available on the uptake of metal ions by duckweeds and the numerous interactions that occur. Duckweeds will uptake and concentrate Cd, N, Cr, Zn, Sr, Co, Fe, Mn, Cu, Pb, Al and even Au. To attempt to define the rates of accumulation is not important here, except to point out that as the levels of these minerals rise to higher than normal in general they may directly inhibit growth of the plant and any animal that consume significant quantities. At low level accumulation the plants become a very useful source of trace minerals particularly for livestock and fish.
Problems of heavy metal contamination obviously arise where duckweeds grow on industrial and mining waste where the contaminating elements are known and therefore the problem should be apparent from the beginning of any study.
In conclusion, it is only where heavy metals are washed out in effluents from industry and mining that there is potential for duckweeds to become toxic to livestock, and in these situations duckweeds harvested from such sources should be disposed of differently. A most useful disposal method being as a mulch for non-food crops such as trees.
A commercial balanced NPSK with sea salt to provide trace minerals can undoubtedly be used with relative unpolluted waters to meet the growth requirements of duckweeds. In the Mirazapur project, (see chapter 6) muriate of potash (KCl), urea and superphosphate (supplying P+ S) were successfully used to produce duckweeds on inundated lands that also collected the effluent waters from the local hospital. Fertilisers were applied on a daily basis, which together with the need for regular harvesting had a high labour cost. Sea salt was added as a source of trace minerals.
Slow decomposition of manure and other organic materials are good ways to continuously supply a water body with nutrients required for duckweeds to grow. The skill here resides in somehow controlling the nutrient inflow into the growth ponds. In many instances this can be established by trial and error. This has been apparently highly successfully in Vietnam where, commercial producers of duckweed used ruminant and pig excrement. This was collected in a small settling pond, the water from which then passes through a series of duckweed ponds before either entering the river or being used for irrigation. These systems appear to work because of long established experience with growing duckweed. The series of ponds all apparently produced a good harvest of duckweed.
A further extension of this method was seen in Bangladesh. The system was based on a simple toilet block, which may be just a hole in a concrete slab from which human excrement could be directed by gravity through a plastic pipe into a basket (usually split bamboo) situated at the centre of the pond and from which nutrients slowly diffused into the duckweed pond (see Photo 6).
The effluents from biogas digestors, suitably diluted are very effective media for growing duckweed. These can be extremely simple systems, easily incorporated into a small farming areas based on home-biodigestors, constructed from plastic (see Photo 6) through to industrial size biodigestors made of metal. In all cases the excrement plus washings from animals held under penned conditions are collected, held in some form of settling pond to remove solids and then into an enclosed container which allows anaerobic microbes to grow and convert the residual carbohydrates to carbon dioxide and methane. The gaseous effluent containing methane and carbon dioxide is collected and combusted for various purposes including household cooking. The water leaving the biodigestor retains the minerals and with suitable dilution is a good media for the duckweed farm.
Photo 5: A young boy harvests duckweed as a protein source for ducks in Vietna
Figure 9: Schematic representation of present duckweed framing in Vietnam.
Photo 6. Duckweed mats fed from faecal materials through a small basket which collects the solids in the middle of the pond
Biodigestor effluent from animal production is usually pH neutral and has a relatively high ammonia content. The mineral component of the diet effects the levels of nutrients in the water and therefore the need to dilute the effluent depends on the animal's diet. Ammonia treated straw results in an effluent from cows fed this that is high in ammonia. A simple system is shown in Figure 10.
The system advocated by Dr. Preston, (Photo 7) is relatively simply to apply on small farms. A cow and calf, mainly fed crop residues provide both urine and faeces to a biodigestor suitably diluted with wash water. The biodigestor in this case is a simple polyethylene tube placed in a ground pit. The washings from the stall enter the digestor and have a half time of 10-15 days during which time most of the organic matter is converted to carbon dioxide and methane. The effluent is diluted and run into narrow plastic lined channels or concrete cannels in which the duckweeds are seeded and grow for several weeks, harvesting the duckweed occurs every few days. The duckweed is then fed either fresh as a supplement to pigs and poultry or is sun dried for the same use.
Figure 10: Diagrammatic representation of flow of nutrients through simple biogas digestor to feed duckweed.
Photo 7: Duckweed growing on small plastic lined containers fed by biogas digestor fluid
Another system that has been proposed uses the effluent (washings) from large numbers of housed animals under intensive management or from abattoirs. The effluent is channelled through a lagoon covered with a 5-10cm thick plastic film and methane is collected from a convenient site beneath the plastic. The effluent being run into ponds, diluted and duckweed produced on the effluent (Figure 11)
Wherever there is an effluent of polluted water associated with industry or agriculture there is potential to purify the effluent waters with duckweeds. Each process, however, requires particular attention and it is beyond the scope of this presentation to make recommendation here for such systems. Some examples of where duckweed cleansing systems might have application are listed Table 3.
Figure 11: Schematic Diagram showing abattoir or intensive animal production waste processing and biogas flow
Table 3: Some examples of where duckweed might be used to cleanse wastewater of mineral pollution and produce a feedstock of duckweed biomass
Possibly the best case can be put for the use of duckweeds to remove P from human sewage which is mostly collected strategically at a point site in a township and although treated to varying degrees is often finally exported via rivers to the sea. There are now a number of commercially viable duckweed based sewage systems that have been developed. These systems are expensive because of the obvious need for high technology to ensure success in treatment. It appears, however, that both
chemical and microbiological treatment plants are much more costly. On the other hand the use of aquaculture does not necessarily replace such systems and they can often be incorporated into or added on to a number of sewage purification plants. However, in small communities in the tropics the cultivation of duckweed on lagoons may be the only treatment necessary for simple sewage treatment.
The literature contains a great deal of information on the potential growth rates of duckweeds. In many early studies the growth rates were measured under controlled conditions for short periods of time. In the absence of large scale field data obtained over 12 month periods these data have been used to estimate potential production rates. The data needs to be treated with reservations as the data in Figures 4 and 5 point to serious problems in doing growth trials with duckweed under laboratory conditions.
The results in Table 4 are from research with near optimum conditions for duckweed growth. Landolt and Kandeler (1987) concluded that under such conditions a 73ton of dry matter are possibly produced per hectare per year or 20g DM/m2/d. Results up to 180ton DM/ha/y have been recorded. Under less than optimum conditions it is more realistic to target between 5 and 20ton DM/ha/y (Table 5).
In practice the yields of duckweed often depend on the skill of the farmer in solving the problem of how to balance the mineral requirements of duckweeds and to identify with time the need for continuing and varying mineral supplementation. Waters that are high in P and K and trace element need minimal but repeated inputs of an ammonia source, keeping ammonia at around the 60mg N/l when growth and protein accretion is greatest.
Table 4: Field results of duckweed growth in near-optimal conditions
Table 5: Field results of duckweed growth in sub-optimal conditions.
Yield of duckweed will depend on how the farmer monitors the duckweed system and whether a high protein meal is the objective. Industrial waters can only be discharged when the P levels have been depleted by consistent cropping and fertilisation with urea as ammonia levels decrease and become limiting to growth. How often to feed urea into the system and how long can duckweed growth be maintained can only be understood from research in the locality.
Where the production of clean water is a major objective then it becomes necessary often to balance other nutrients as well as ammonia in order to end-up with water that has had its total mineral composition decreased to levels that will allow its re-use.
In Table 6 the residual P in wash water from piggeries shows that with skill, a number of fertilisations with urea followed by harvesting of duckweed could result in very low P levels occurring. Recharging ammonia levels could be expected to stimulate growth so that P levels can drop below 1mg P/litre.
The growth rate of duckweed under ideal light, temperature and pH would be exponential if there were no limitation in terms of mineral (including ammonia) deficiencies or excesses. However, in practice many issues reduce the biomass yield. One of the most important is obviously plant density. The rate of harvesting duckweed is important since there is a minimum biomass at which yields will decrease and an upper biomass where yield will be limited by crowding, all other variables being equal. In a study where most of the conditions for growth were unlimited the effect of harvesting indicated that, above about 1.2kg/m2 duckweed (fresh) growth decreased and below 0.6 kg/m2
Table 6: Removal of nutrients by Lemna from a flow through pond fed by aerated piggery waste water (Instituto de Investigaciones Porcinas, Havana, Cuba (unpublished observations)
duckweed (fresh) biomass limited growth potential. It appeared that if 1.0kg fresh duckweed/m2 could be maintained by frequent harvesting then an extrapolated yield of 32 tonnes DM/ha/yr could be produced under other non-limiting conditions. The data is shown in Figure 12.
Figure 12: The range of densities of duckweed biomass on the water surface after harvesting at which duckweed grows optimally (Stambolie and Leng, 1994). In this case the average yield was 32 tonnes DM/ha/year. The upper density (filled squares) appears to be that at which crowding limits growth and the lower density (unfilled squares) is the density at which growth is insufficient to prevent algal blooms (Stambolie 1994 reported by Leng et al 1994)
Integrated farming systems, so long as they improve soil fertility (or at least maintain the same soil fertility) in the long term have major advantages which can improve both the overall production of land without losing sustainability.
The World Commission on Environment and Development defines sustainability as:
"ensuring that development meets the needs of the present without compromising the ability of future generations to meet their own needs".
However, development opportunities and aspirations change with changing economic considerations. Major increases in the cost of food production is likely to arise where cost of fuel (fossil) increases relative to income. Fuel prices must surely increase in the future in response to:
The cost of food production in a country is highly dependant on fuel prices, and food prices in the 1998 Asian financial crisis rose and must continue to rise.
The two important agricultural cost factors that will be effected most are, mechanisation, where fuel is directly consumed in crop farming, and fertiliser availability and application since the cost of NPK is highly related to gasoline prices. The fuel crisis in Cuba brought about by removal of economic support from Russia and the embargo by the United States has seen a return to animal traction in the past 6 years and a massive decline in crop yields through decreased use of inorganic fertilisers.
Farms export considerable mineral nutrients in products and also effluent from many sources. In the future and for continuing sustainable food production, these minerals must be replaced. In industrialised farming systems this is done largely by inorganic fertilisers produced and delivered at an increasing cost of fossil fuel combustion.
High level use of NPK have resulted in the sustained yields of feed crops in industrialised countries and in the last 20 years greatly improved yields in developing countries where these inputs have been used together with improved crop varieties.
The other major issues in terms of crop production has been the increased use of water resources, some of which are irreplaceable. Levels of fertiliser and water application are almost always in excess of plant needs and water run-off contaminated with minerals has created great problems with salination and eutrophication in river and pond systems throughout the world, changing the aquatic ecology of whole regions in places.
Integrated systems are aimed at minimising (or preventing) loss of nutrients from a farming system and in many situations conserving water for reuse (Preston & Murgueitio, 1992).
Integrated farming systems to be employed by small farmers in developing countries, require considerable skills in operation in order for them to be economic and/or sustainable. Integration may be developed on a single land holding or may be more easily applied where a number of farms combine their requirements to develop an integrated system where the minerals leached from the land by farming are returned to the land via good conservation practices involving a number of farms. In Australia the Land-Care Movement involves usually a number of land-holders to combine their efforts to conserve a whole catchment area. Similarly in India a catchment area approach to sustainability has been implemented through ICRASAT.
Integrated and sustainable systems must be developed in order to prevent land degradation, minimise external (costly) inputs, conserve resources otherwise lost through effluents and potentially increase the income and standard of living of the farmer and at the same time maintain the fertility of the land. Integrated farming systems, that are also sustainable require that:
Integrated systems were traditional in most developing countries prior to the "green revolution" and many ancient societies recognised and put into practice sustainable cropping systems often through application of taboos against practices that caused degeneration of food supplies. In more recent times (say the 1920's) this took the form of integration of crop and animal production. The animals being an intermediate in conversion of crop residues and other wastes to dung which was then returned to the land. In some countries composting and biogas digestors were instrumental in recycling nutrients within the farm(s). The net result was that the minerals in biomass produced on the farm were recycled by re-incorporating the nutrients back to the land via human or animal excrement. Often the animal was a draught animal which is now being replaced by tractors. However, even in these systems effluent loss was considerable in water run off and in product where crops and animal products were sold from the farm.
Within the integrated farming systems, strategies to encourage N fixation and for increasing P availability are primary targets.
N accumulation in land or maintenance of levels through N fixing plants (e.g. legumes) or the extraction from effluent waters by aquatic plants are strategies that were used by small farmers only a few decades ago. Similarly P fertilisers have often been developed from aquatic plants. For example, in Kashmir, aquatic bottom growing plants are harvested from the lakes for use as fertilisers, and seaweeds have been used where ever they are washed ashore for application to soils.
Ruminants in general, if they graze non cultivatable land harvest considerable N and P and this then can find its way into crops via manure and therefore potentially avoid downstream problems.
The major constraint to the establishment of integrated farming systems is the level of management that must be exerted by small farmer. This can often be beyond his presently developed skills. An exception to this statement however, was seen in Vietnam and Bangladesh where the collection of all animal and human wastes into ponds and the subsequent growth of duckweeds has proven to be relatively free of problems and is very skilfully managed by a number of cooperating farmers. However, if these tropical systems had to be managed for production of quality water as well as feed for ducks or fish a greater degree of control of the duckweed growth would need to be exerted.
One of the major reasons for the development of livestock production in cropping systems in developing countries was to utilise crop residues efficiently, thus, eliminating waste and optimising the use of the total biomass produced within the small farm system. The small farmer has often a requirement for draught power with animal products (milk and meat) as secondary considerations.
Integrated crop and livestock production systems can be highly efficient; potentially crop residues are used as livestock feed; the waste products (e.g. faeces and urine) are fed into a biogas digestor and the effluent used to fertilise ponds for aquatic plant/algae production, with fish farming as the terminal activity. These systems are worthwhile pursuing as a means of providing nutrients/fuel for the family, minimising fuel combustion and reducing environmental pollution (Preston, 1990).
The array of integrated strategies that could be developed is large. They all have as a central core a basic flow of nutrients through a number of systems. At each of these steps research can be brought to bear to optimise the partitioning of the available biomass into food, fuel and residues (see Figure 13). The environmental attributes of such systems are the methane emissions into the atmosphere and fuel (fossil fuel and fire wood) use are minimised. In addition the efficient and also
Figure 13: Flow diagram showing the potential recycling of feed and faeces biomass from crop residues in an integrated farm.
total harnessing of the energy from high producing crops reduce the land areas required per unit of product (see Preston, 1990).
A complete discussion of these systems is beyond the scope of this document but two examples are:-
Figure 14: An example of an integrated farming system based on sugar cane and forage trees fractionated to provide feed for pigs and poultry (the juice and tree leaves), sheep (the cane tops and tree leaves), fuel for the family (bagasse and firewood) and litter for sheep and earthworms (bagasse), with recycling of excreta through biodigestors to provide fuel (biogas) and fertiliser (the effluent) for water plants in ponds and for the crops (Preston, 1990)
The publication of a booklet by Skillicorn et al. (1993) on duckweed aquaculture based on the experiences of a project in Bangladesh operated by PRISM stimulated considerable interest the use of duckweed.. In Cuba, major research has been developed as a result of the lead-work of the PRISM group. It is of significance that both these countries have relatively high priced fuel and in Bangladesh draught animal power is still the main farm power, whereas in Cuba, the farmers have reverted to draught animal power mainly because of the increased price of gasoline. Neither country, however, has been able to efficiently establish integrated farms. In both countries the objective behind the research on duckweed has been to provide food for carp and/or tilapia production with some spin off for pig/poultry production.
A system, that appears ready to be put straight into farm practice arises from the work of Dr. Preston and his colleagues in Vietnam. It incorporates a duckweed production system into a rice farm or market garden. It depends on a typical 1 to 0.5ha farm with one milking animal, a calf and a bullock for work and with additional small ruminants (goats or sheep) or rabbits and where possible a pig.
The pig is a crucial animal in much of the cropping system of small farmers in Vietnam particularly to maintain rice yields. They are produced on the byproducts of the household and from small inputs of other feeds. The faeces of these animals has been the mechanism by which soil fertility has been maintained over the centuries in much of the Mekong Delta. However, the systems are decreasing because of the importation of high technology pig production based on European technology and this could be highly detrimental to small farm rice production in the future.
The nutrient requirements for high rates of duckweed growth has been discussed. In practice, however, standards and requirements only provide a basis for the "adviser" to give recommendation to the farmer. Most scientists are distracted by the establishment of "nutrient requirements". The application of precise levels of nutrients into any system is problematic particularly where these are to be met from the farm resources. However, nutrient requirements estimated in research laboratories may possibly emphasise a critical limitation in duckweed production through mineral analysis of the water.
Essentially, duckweeds must grow on the effluent from plant and animal production. Often the effluent from plant production (drainage) is too low in nutrients for high growth rates of duckweed. An exception to this occurs in some areas where high levels of fertiliser are applied to crops under irrigation. In Pakistan close to Faisalabad duckweed growth in drainage ditches can sometimes be so great as to create major problems in water pump blockage. Similarly the run-off from cotton production often provides a good medium for duckweed growth. On the other hand effluent from intensive animal production almost always is a too concentrated source of minerals particularly ammonia and needs dilution with other waste water sources.
Scientists can measure ammonia-N and phosphorous in effluent waters and then make recommendations for the appropriate dilution to provide pond water for duckweed farming. On the other hand chemical analysis of water is not feasible or affordable for large numbers of resource poor farmers that may be involved. However, some practical recommendations based on simple research at the farm level can be given to assist farmers to establish a duckweed farm.
OTHER BENEFITS FROM INTEGRATING DUCKWEED INTO CROP FARMING
Aquaculture of duckweed has been largely promoted as an opportunity crop for use as animal or fish feed. This is particularly appropriate where ponds have become unusable for other purposes because of pollution. This emanates largely from fertiliser run off or from wash out of animal/human manure. Such ponds are abundant in countries such as Bangladesh, where there are an estimated 1.3 million ponds (average size 0.11ha) covering 147,000ha. Only 46% of the ponds contain fish. The ponds have multiple use, bathing (washing) irrigation and livestock watering which interferes with fish culture but which could make them useful for duckweed production. A recent World Bank review suggested that about 40,000 ha of ponds could be brought into duckweed production and that if yields varied from, 4-20ton DM/ha/y then 160,000 to 800,000ton of duckweed could be available to poultry farmers. The potential value of this can be seen from the fact that the higher quantity of duckweed represents twice the availability of 'home grown' feed concentrates in Bangladesh.
The major problems of developing such a system of concentrate production are associated with multiple ownership of ponds, lack of credit and the unavailability of extension services. A major constraint is the lack of a marketing mechanisms including quality control, that can effectively allow a duckweed meal to compete with imported protein meals particularly those from Europe and derived from animal offal.
A major benefit of using duckweeds is emerging. There is accumulating evidence that duckweeds release compounds that have insecticidal properties particular to the larval stages of mosquitoes. Thus the development of duckweed aquaculture in the wet tropics may have implications for mosquito control in rural areas where malaria is again becoming a serious problem.
Eid et al. (1992a) published evidence that an extract of Lemna minor had insecticidal action against the mosquito Culex pipens pipens. The same extract contained synomones which also repelled oviposition by the female mosquito. Where sublethal doses of synomones were added to water it was found that all larval stages of the mosquitoes were malformed. Duckweed synomones added to water also repelled the ovipositing of Piophila casei, and effected larval development and reduced survival. Similarly Spodephera littarolis larvae were malformed when synomones from Lemna minor were added to their culture medium.
If the insecticidal properties of Lemna minor and other duckweeds are sufficient to truly control mosquito populations it will have an immense effect on health of people in areas where mosquito borne diseases are endemic and resistance of the parasitic stage to drugs has increased. It is also a further inducement to cultivate duckweeds widely for water treatment (purification) and animal feed. A further potential is the commercial cultivation of duckweed as a source of insecticides in water where it is difficult to spray for control of mosquito larvae or where the use of other control measure are impractical.
Other research workers have also associated duckweed presence with reduced (or elimination) of mosquito development. For example Marten et al. (1996) showed that Anopheles albimanus populations were negatively correlated with the amount of cover of the water by Lemna. The relative cover of water surface with duckweed was also negatively correlated with populations of fish and other insects indicating how intricate the associations are in natural ecosystems.
Eid et al. (1992b) have made the observation that the mosquito Culex pipens pipiens never colonised sewage water covered with duckweed and Bellini et al. (1994) observed that Lemna covering the surface of rice paddy-fields strongly effected mosquito populations. However, again other organisms might also have been involved in the control of mosquitos.
Lemna trisulca appears to produce allelo chemicals that are active against algae (Crombie & Heavers 1994) and Mesmar and Abussaud (1991) suggested that extracts of Lemna minor were active in inhibiting the growth of Staphylococcus aureus.
The role of duckweeds in preventing algal growth can be by shading, by perhaps the production of algacides if this can be satisfactorily proven and in addition because they lower the nutrient supply, particularly P concentrations either in effluent waters from sewage plants or in water bodies.
Cholera has long been associated with seasonal coastal algal blooms off Bangladesh. Fluorescent antibody techniques have shown that a viable, non cultivatable form of Vibro cholerae in a wide range of marine life, including algae. In unfavourable conditions V. cholerae assumes spore-like forms which as conditions improve reverts to a readily transmittible and infectious state. Algal blooms which are associated with eutrophication have been related to the spread and persistence of cholera. Prevention of algal blooms may therefore be of considerable benefit (see Epstein, 1993).
Although the literature is sparse and not totally convincing on the potential of duckweed to control of mosquito populations, the author has heard farmers in many countries express opinions that growing duckweed on ponds control mosquito populations. Farmers in a group of villages in Vietnam that traditionally produced duckweeds were adamant that mosquitoes were not a problem so long as their lagoons were covered with duckweed.
Research into the insecticidal properties of duckweed is worthy of follow-up. If the production of natural insecticides can be promoted at the same time as improving health conditions, purifying water and providing a natural food resource for animal production it may have far-reaching implications. A new naturally occurring insecticide produced from duckweeds may be as revolutionary as the discovery of pyrethrins and perhaps this potential alone may give the necessary government resolve to support integrated farming with a duckweed crop as a major component of such farms in countries where this is appropriate.
A number of aquatic plants including duckweeds have great potential for development for various purposes.
Essentially aquatic plants may or will be grown in developing countries where:
Candidates for use in any of these applications include duckweeds, Azolla, Pistia, Ecihhornia and a few lessor known aquatic plants.
There are major advantages for floating aquatic plants as the water depth is not critical and harvesting does not necessarily disturb the underlying ecosystem in the mud. Ease of harvest is important and Azolla and duckweed are readily harvested, but have the disadvantage of having to be protected from wind and water currents to encourage total coverage of lagoons and hence maximum yields. There is some evidence for a symbiotic association of Lemna and N fixing bacteria, but on low N waters Lemna growth is slow and the product is low in protein, on high phosphorus water low in N, Azolla with its association with N fixing bacteria is more appropriately grown. However, Azolla has some greater problems associated with its continuous growth as compared to Lemna particularly from insect damage. Addition of N fertiliser in aquatic media mostly removes the major advantage of Azolla, that is, its ability to grow on low N water.
A simple approach to establishing a duckweed pond system is often the only way to start.
If water emanating from an animal production unit is taken as an example of how to approach its use for duckweed production. First the water has to be collected in some suitable settling pond. To obtain information on the water's potential to grow duckweed, the water is serially diluted in small containers with water relatively free of minerals. Duckweed is seeded into each water container and its relative growth monitored by eye. It quickly becomes apparent what is the appropriate dilution and this can be further refined by successive harvesting from the containers to determine the dilution at which duckweed grows at the greatest rate. This system can be recommended where the objective is to produce duckweed and there is no constraint to disposing of the effluent or it is not required to recycle the water.
In general, it appears that in most systems, N quickly becomes the limiting nutrient as duckweed mats grow. The second potentially limiting nutrient is P. Thus there is sometimes great benefits in providing extra N (as urea) at the end of the growth period following harvest of half the duckweed mat. This allows further growth of duckweed and further reduction of P content in the effluent water. Water can be cleaned effectively by growing and harvesting duckweed only when this is a well designed succession of fertiliser applications that rebalance NPK for growth after each harvest. Eventually the minerals may be reduced to acceptable levels.
Although farmers, particularly in South East Asia and probably elsewhere had developed the use of duckweeds as a source of nutrients for livestock, the actual controlled experimentation that has been typically used to develop such commercial crops as soyabeans or maize for livestock feed has not been undertaken. There are, however, a number of reports in the literature on the use of duckweeds as feed supplements for fish and livestock. These report research with domestic animals in which normal feed protein sources have been replaced by duckweed meal on an isonitrogenous bases in complete diets based on compounded concentrate diets.
Duckweeds are highly variable in their composition. They grow slowly on low nutrient waters and are high in fibre, ash and carbohydrates but contain low crude protein. In contrast when grown on waters high in ammonia and minerals they grow rapidly and have a high protein content associated with a high ash and are often lower in fibre. Because duckweeds respond quickly to the availability of nutrients they often have highly variable levels of some nutrients which makes it difficult to prescribe the amounts needed for livestock and fish over a period of, say, one year. Careful interpretation of some studies reported is required when the quality of duckweed given in diets to livestock is not consistent throughout the study. In terms of domestic animal/fish nutrition, duckweeds may be used in many ways. These include:
Duckweeds have been largely researched as a total feed for fish, including carp and tilapia production, as a protein supplement for pigs and poultry (including ducks) and as fermentable N and mineral supplement for ruminants.
The research on duckweeds as a feed are summarised below. The uncertainty of the conclusions and the difficulty in making clear recommendations largely pertains to the fact the quality of the duckweed used by various researchers (i.e. its nutrient densities) were variable. However, as a resource that can be harvested for labour costs alone in natural conditions, it obviously represents a valuable asset to the resource poor farmer. In many countries it could have a low cost where it is grown on sewage and it has to be disposed of from the works at a subsidised price.
It appears to be a resource that is most conveniently used by the small holder farmer, particularly in an integrated farming system. Unfortunately much of the research has attempted to demonstrate the value of duckweed as a protein source in diets that are most commonly used in industrial production systems. This is particularly true of the research with poultry and yet its major application probably lies in the more difficult situation of increasing animal production on small farms.
The crude protein content of duckweeds depend mostly on the N content of the water upon which they grow. Some publications have indicated that there are some variations in amino acid content of duckweed proteins. High levels of lysine have been reported in studies coming from the duckweed research programmes in Bangladesh. However, it appears that the protein component of most aquatic plants including duckweeds have similar amino composition to terrestrial plant proteins in general (Table 7). In this respect the amino acid composition is influenced by the major enzyme protein in plant that is ribose bisphosphate carboxylase. Protein extracted from Lemna minor when fed to rats compared equally with the nutritive value of a wheat flour diet. This indicates that Lemna meal has a relatively high biological value for rat growth (Dewanji & Matai, 1996). Duckweeds therefore are good
Table 7: Amino acid composition in aquatic plants (g/100g protein) grown on wash water from a pig farm in Cuba (unpublished Instituto de Investigaciones Porcinas, Havana) The wash water was collected and aerated to reduce total N (Figueroa, V. personal communication)
Note: that Lemna protein has a lower lysine content considerably below that of soybean protein, which is contrary to the data produced by Skillicorn et al. (1993). It is possible but highly unlikely that the essential amino acid content in Lemna is dependent on the protein content. It is important to emphasise that it is not true that there is a better content of lysine in Lemna as compared to soyabean although extracted protein from Lemna minor has sufficient lysine to meet the FAO/WHO Reference Standards and NRC requirements for chicks (Dewanji, 1993) laying hens or pigs (Hanczakowski et al., 1995) sources of essential amino acids but are not enriched in any particular amino acid in comparison with the usual protein sources used in animal production.
The value of duckweed as a protein supplement to poultry was recognised some time ago (Lautner & Muller, 1954; Muzaffarov, 1968; Abdulayef, 1969).
Truax et al. (1972) showed that dried duckweed was superior in protein quality for poultry as compared to alfalfa meal and could fully substitute for alfalfa meal at 5% of the total diet.
Interpretation and extrapolation from much of the earlier studies are confusing because the duckweed was harvested from natural sources and often the material could have been "old" and low in protein but high in fibre. This situation led Haustein et al. (1990) to reassess the value of duckweed as a protein supplement for pigs and poultry. They established studies to examine the potential to substitute not only alfalfa meal, but also fish meal and/or soyabean meal with Lemna meal in a compounded feed.
The diets used by these researchers at the University of La Molina, Lima, Peru were based on those used in the intensive egg production industry (Table 8). Duckweed was harvested from a tertiary sewage effluent and lagoon run-off and in general had a medium level of protein (33%CP). Both Wolfia and Lemna species were harvested and their estimated metabolisable energy level was 1,200kcal/kg (in young broilers) and 2,000kcal/kg (in mature cockerels) indicating a poor overall digestibility of duckweed for monogastric animals (see also Hanczakowski et al. 1995). It might be inferred here that there was considerable intestinally indigestible carbohydrate (fibre?) present.
Table 8: Composition of diets fed to Topaz layers (Haustein et al. 1990)
* Approximate as the level of wheat middlings was not stated by the authors and small amounts of other carbohydrates were included in the diet to balance the energy.
The diets used are important, because these studies only compared duckweed protein with other sources of protein in an otherwise commercial diet used for intensive production Table 8. The results of replacing soyabean with a meal made from Lemna or Wolfia clearly indicated that the latter two are at least as good as soyabean as a source of essential amino acids as there is virtually no differences between egg production of birds on all diets (Table 9).
Table 9: Performance of Topaz layers fed three isonotrogenous diets based on protein either from soyabean or duckweed after 2 weeks (Wolfia diet) or 10 weeks (Control and Lemna diets (Haustein et al. 1990)
* As a percentage of the egg production when all birds were fed the control diet in the pre-experimental 2 week period
** total egg produced (g) divided by total feed consumed.
Table 10: Performance of poultry kept for egg production when dried Lemna powder replaced soyabean and some of the fishmeal in the diet shown in Table 8. The experiment lasted 18 weeks.
* represents total egg weight produced divided by total feed consumed
**Lemna completely replaced both fish meal and soyabean meal in the diet.
Even when Lemna powder was increased to 40% of the diet, egg laying was sustained for 18 weeks but the birds were losing weight and therefore it could be anticipated that eventually egg production would have decreased (Table 10).
These studies clearly demonstrated the value of a duckweed powder, that was of only medium quality, as a source of essential amino acids for egg production. The excellent outcome from these experiments also led to a number of other issues being researched and the following observations were made:
This demonstrated quite effectively that in areas where protein resources are scarce, duckweed represents potentially a high quality protein source that can be safely exploited for poultry production.
Recent research in Australia investigated egg production and egg characteristics in two strains of layers Tegel Hi-Sex and Tegal Super Brown birds (Nolan et al., 1997). The birds were changed from conventional layer diet to diets in which duckweed (Spirodela) represented 10, 30, 50, 80, 120 and 200g/kg of feed replacing both grain and soyabean to retain a diet with 160g crude protein, 40g Ca, 10g P and 11.3 MJ of metabolisable energy per kg feed. Egg mass production was slightly reduced in the Hi-Sex hens given diets containing higher duckweed content (a 59.7g egg/d was reduced in size by 0.046g per g duckweed included in the diet), but it was more markedly reduced in Super Brown hens (44g egg/d was reduced by 0.088g per g duckweed in diet). Duckweed in a diet increased the pigmentation colour substantially see Figure 15. The more recent results confirm the value of duckweed as a protein source and a source of minerals and pigment for poultry.
The researchers in these particular studies were motivated by the potential for large scale production of duckweeds on for instance sewage lagoons. Large scale production which involves mechanical modification of sewage systems, implementation of mechanised harvesting, drying and processing is an unknown cost and does not have much application to small resource poor farmers who produce eggs and meat from birds often scavenging for a proportion of their feed.
Figure 15: Variation in yolk colour of eggs from hens fed four conventional diets without artificial pigments with duckweed replacing soyabean meal.
Recent studies have demonstrated that on conventional diets for young broiler chickens, replacing a protein source with Lemna meal retarded growth as levels increased (Haustein et al., 1992b, 1994) whereas layers produced efficiently (Haustein et al., 1990) and older broiler birds had excellent growth characteristics when fed relatively high levels of Lemna meal. This is of significance to the factory production systems for poultry where margins per bird are often small and small decreases in profitability per bird are important. The reduction in growth of young birds has little significance where Lemna meal would be used in a small farmer systems, particularly where birds balance their own diet by scavenging from cropping areas.
A major question arising from this work is what would have been the value of the duckweed as a protein supplement had the duckweed been fertilised with a little extra nitrogen (urea) so that protein level had approached the upper limit (about 40% CP)?
Emphasis now needs to go to the other end of the spectrum of production systems for poultry. Major research efforts are needed to find ways by which duckweed can increase egg and meat production from non-conventional diets as used by small farmers at the village level. It appears that dried duckweed could be used very effectively with scavenging poultry, particularly where grain is fed as a separate meal.
Poultry have a well developed ability to select a balanced diet from individual resources made available to them, or by scavenging. The use of both energy supplements (e.g. spoilt grains) and duckweed made available as separate components on poultry production needs considerable research (see Mastika & Cumming 1985). In such practices there is also the additional need for a distinct source of calcium that can be ingested as the bird needs it. This is particularly so in laying birds as the intake and absorption of calcium must occur at the same time as the shell is being synthesised.
Research in Vietnam is attempting to define where duckweed can be fitted into small farmer operations. The major costs of drying and transporting for the feed industry is a real drawback, but it can cost little on the small farm where sun-drying on black plastic sheets is feasible.
If duckweed can be harvested at frequent intervals and fed fresh or partially dried this would be a major advantage at the village level However, the small farmer who is more and more advised to use supplements often feed brans or pollards. These have considerable fibre and therefore the duckweed used should not increase the fibre load. There would be great benefit therefore in growing duckweed as a crop, managed so as to minimise fibre and ensure it blends with primitive diets and equally with the more nutrient rich compounded feeds.
Duckweed is perhaps named because ducks were observed to use it in the wild. The more omnivorous duck appears to utilise it highly effectively under field conditions. On sewage farms in the New England territory of Australia wild ducks so vigorously consumed duckweeds that they initially prevented the high growth rates needed to lower water nutrients to desired levels. In Vietnam duckweed produced on nutrients from animal and human waste is given fresh with cassava waste (Photos 8-12) to ducks. Duckweed provides both energy and protein and also a complement of essential minerals needed to grow ducks to a body composition and a weight for age that was needed by the excellent restaurant trade.
Photo 8: Duckweed harvest is a daily routine for the small boys in a village in Vietnam growing duckweed.
Photo 9: Cassava waste and duckweed being mixed for duck feed
Photo 10:Ducks being fed the harvested duckweed mixed with cassava meal
Photo 11: Ducks grown on duckweed and cassava head for the excellent restaurant trade in Ho Chi Minh City, Vietnam
Photo 12: Ducks fed sugar cane juice and duckweed at The University of Agriculture and Forestry, Ho Chi Minh City, Vietnam.
Duckweed (Lemna trisulca) has been shown to be able to replace 50% of the fish meal in a conventional diet for ducklings (Hamid et al., 1993) but its use as a major feed has not been considered.
In Vietnam there are 30 million ducks raised annually These ducks traditionally scavenge their food supplies from the rice fields. They obtain a considerable amount of spilt grain, especially just after harvest and also consume insects, crustaceans and slugs and snails. Similar systems are well developed in Indonesia where ducks are induced to "graze" various land areas being led in groups after being fixated at birth to some common object.
Ducks are preferred over poultry in these countries because they are highly resistant to poultry diseases. Changing conditions, particularly introduction of short rotation rice varieties and government regulation is resulting in more and more confinement of ducks which are fed on any locally available feed stuffs. Typically such ducks, whilst allowed limited scavenging are fed broken rice of low market value and soyabean meal or roasted soyabean.
Duckweed is proving a valuable replacement for soyabeans in these "more intensive" production practices. The intake by ducks of broken rice and either a combination of roasted soyabean or fresh duckweed has been compared. The intake and dietary ingredients used in these experiments are shown in Table 11.
Table 11: Mean intake of fresh foods by ducks (Men et al., 1995).
It is clear that for fattening ducks, in these "intensive" farm practices duckweed fed fresh, can totally replace soyabean as a protein source without effecting production other than for a small decrease in feed conversion efficiency.
The truly important issue here is that the research confirms the potential of duckweed as a high quality mineral source with protein equivalent to soyabean protein but which is readily produced locally on farm.
Expensive mineral premixes were unnecessary when duckweed was added to a diet indicating a major practical and economic role of duckweed to provide the array of minerals for this level of production. An analysis of duckweed used in all experiments (i.e. in lab and on farm) indicates the ability to supply major mineral elements and trace minerals. They have a well balanced source of calcium and phosphorous. The array of analysed nutrients are shown in Table 12.
In an attempt to find alternative energy sources low in fibre for production, Becerra (1995) studied the use of duckweed as a protein/mineral supplement to ducks given sugar cane juice. Supplements were either boiled soyabeans or freely available fresh Lemna. These studies, however, clearly showed a negative effect on production as Lemna increasingly replaced soyabeans in the diet. The
Table 12: Values for the composition of duckweed, broken rice and soyabeans used in the studies of Men et al (1995,1996)
experiment was perhaps a little over optimistic considering the large intake of water needed to obtain energy (as sugar) from the juice (in actual fact it was sugar in solution mimicking sugar cane juice) and the large intake of water from Lemna may have been a primary limitation. The Lemna in this study contained only 26% CP and would have had 5-10% dry matter.
Table 13: Effects of replacing boiled soyabeans with Lemna in a diet of reconstituted sugar cane juice on intake and production of ducks (initial weight was 920g) (Becerra et al., 1995)
The amount of water ingested by these small ducks (average liveweight 2kg) appeared to plateau at about 800g H2O per day suggesting that the high water content limited the intake of Lemna and sugar. This emphasises again the necessity to use wherever possible the highest quality of duckweed that can be produced (i.e. high in protein) and at least partially dry prior to feeding.
Sugar cane juice, if it can be used in animal production has enormous implication because of the potentially high yields of carbohydrate that can be achieved per unit of land area from this crop (Preston, 1995) and the fact that grain has to be imported for poultry production in many developing countries in the tropics. Further work in this area should be encouraged to find ways of combining energy from sugar cane with duckweed to produce a feed efficiently used by ducks.
Undoubtedly farmers have used aquatic plants to feed pigs in many countries but only a small amount of controlled research has been reported. Haustein et al. (1992a) fed pigs on a conventional grain based diet and replaced part of the protein requirements with a low quality duckweed (23% CP with 7.5% fibre) harvested from a natural lake. In this instance the pigs were relatively young and as the Lemna meal increased in the diet, liveweight gain was significantly reduced. The results of this study are shown in Table 14.
Table 14: The effect of replacing "conventional" protein sources in a concentrate based diet for pigs with Lemna meal (23% CP)
A number of issues may be raised with this study. The liveweight gains of these pigs was low on the control diet and the Lemna meal may have been relatively old since it was low in protein. Other problems may also have occurred, for instance, a high level of oxalate in the duckweed or the accumulation of some toxin or heavy metal is always a possibility and the disappointing result reported here should not detract from further studies.
Pigs are omnivorous and the systems under which they are produced vary from scavenging their food, through to intensive production on high cereal based diets. The results with poultry that were fed duckweed were indicative of a negative influence of low protein Lemna (fed fresh) on production, whereas high protein (33% CP) duckweed supported similar growth rates to soyabean meal. It appears in this case that a Lemna meal, higher in protein may have been much more beneficial than the one used in the reported studies.
In unpublished work from Instituto de Investigaciones Porcinas (Havana, Cuba) the partial replacement of soyabean meal with Lemna-meal produced on the effluent from pig houses demonstrated that Lemna meal appeared to provide a high quality protein to support growth on molasses (molasses here is produced from sugar cane juice after only two extractions of sugar so it is higher in energy than most molasses). The results are shown in Table 15. (Figueroa, V. unpublished).
Table 15: Effects of replacing soyabean meal (SBM) with Lemna on the growth of pigs over 3 months on a basal diet of molasses.
* Lemna replaced 10% of the protein from soybean
More recently Rodriguez and Preston (1996b) have fed three groups of genetically different mature pigs, unconventional diets containing duckweed. The experimental groups were local Monc Cai (MC) pigs, Large Whites (LW) or Mong Cai crossed with Large Whites (MCxLW). These were given sugar cane juice as the major energy source together with duckweed as a protein/mineral supplement. The pigs that had been highly selected for their abilities to utilise high grain/high quality protein diets (the 'so called' high-genetic potential Large White) appeared to be unable to utilise the duckweed, whereas the native pigs and their crosses utilised duckweed efficiently, consuming significant quantities in addition to the free choice sugar cane juice mixture.
No growth data is yet published from these studies but as the intake of drymatter from duckweed was increased in the total dry matter consumed (from sugar cane juice and duckweed) there was a linear increase in N retained. As the proportion of the N intake from duckweed increased, 50% of the N in duckweed consumed was stored in tissues when duckweed was about 50% of the total feed dry matter intake. Dry matter digestibility decreased as the proportion of duckweed in the diet dry matter increased (Figure 16). Rodriguez and Preston (1996b) suggest that the protein of duckweeds are readily utilised and have a higher biological value than meals such as those prepared from cassava leaves which are often fed to pigs in these countries.
The results of this preliminary experiment clearly indicate an interaction of genotype and non-conventional feeds. Protein from duckweed can be well used by mature, native pigs and presumably therefore it is a well balanced source of essential amino acids and minerals.
Figure 16: Relationship between percent of diet DM consumed as duckweed and apparent DM digestibility of Mong Cai (MC) or Mong Cai/Large White crossed (MCxLW) pigs.
The data discussed above for pigs and poultry also indicates that there may be some large differences in the composition or availability of nutrients from low protein duckweeds (i.e. slow growing) in particular there are indications that the protein is lowered in value (availability or essential amino acids or composition) as the protein content of duckweeds are reduced as they age on low-nutrient water.
Unlike monogastric nutrition where feed analysis are indicative of nutrient availability to the animal, ruminants through their fermentative digestive system modify virtually all the protein and carbohydrate in the feed they consume. The nutrients become available as volatile fatty acids (which are the major energy source), and amino acids (produced by enzymatic digestion of microbial cells that have grown and been washed from the rumen in liquor). Forage proteins are in general, degraded to ammonia in the rumen and the animal depends on microbial protein for its essential amino acid supply. The efficiency of production is primarily dependent on the establishment of an efficient microbial ecosystem in the rumen. The potential use of duckweed in ruminants diets is two fold:
These two roles are largely confined to the rumen since an efficient microbial digestive system is dependent on a full complement of essential minerals and a high level of ammonia in the fluid. Deficiency of minerals and/or ammonia (which may be produced from supplemental non-protein-nitrogen sources or by the degradation of dietary protein) results in a lowered microbial growth in the rumen with inefficient growth of the microbial milieu. The consequences of low microbial growth is a reduced protein relative to energy in the nutrients absorbed (see Preston & Leng 1986 for review) and often lowered digestibility of forage and reduced feed intake.
Ruminants under feeding systems found in most areas of the world are often deficient in an array of nutrients required by the microbial fermentative digestive system. This is the case, particularly when consuming mature dry forages or crop residues (straws/stubbles) and at times agro-industrial byproducts (e.g. sugar cane tops, molasses and fruit residues). Duckweed with its high mineral and protein content can provide an array of nutrients for the rumen microbes to function efficiently on such diets. These food resources are the basis of diets for ruminants in large areas of the world, particularly in countries that are considered to be developing. In this way a quantity of duckweed could replace the use of multi-nutritional supplements based on such things as molasses urea block licks (see Leng 1984).
Duckweed also has some potential as a dietary protein source that may be modified or may actually provide bypass protein that is required by productive animals to meet their extra requirements for essential amino acids (see Preston & Leng, 1986).
There are some preliminary research results where duckweed has been fed as a supplement to ruminants, but clearly there is a need for major research effort in this area to develop a clear definition of the strategic importance of duckweed as a N source, as a bypass protein source, as a source of essential minerals including S, P, Na, K, Mg and trace minerals.
A duckweed, corn silage diet (1:2) produced higher growth rates in Holstein heifers than a diet based on corn silage, concentrate and grass (Rusoff et al., 1978, 1980). These studies and preliminary studies at the University of New England indicate the high potential of duckweed as a supplement.
More recently Huque et al. (1996) have commenced work to examine duckweed as a source of N and minerals for ruminant animals in Bangladesh. In a study of the kinetics of the utilisation of duckweed dry matter and protein they used nylon bag incubation techniques to study the breakdown of duckweeds in the rumen of cattle. The duckweeds used in these studies were around 30% crude protein. Overall the studies illustrated that, in cattle fed forage and concentrate, the potential degradation of duckweed dry matter in the rumen was 85% (Spirodela), 72% (Lemna) and 93% (Wolfia). The protein of duckweed were highly soluble in the rumen at 24% (Spirodela), 42% (Lemna) and 18% (Wolfia) and overall 80, 87 and 94% respectively of the protein was apparently degraded in the rumen. At high feed intakes there was apparently some potential for a small amount of the protein from duckweeds to escape degradation in the rumen and provide essential amino acids directly to the animal.
It seems probable that dried duckweed will provide a readily fermentable protein source together with a rich mineral level needed for creating an efficient rumen for animals fed low protein forages such as straw. The extent that duckweed can correct mineral deficiencies in diets for ruminants will depend on the composition of the duckweed which in turn depends on the level of minerals in the water body growing the duckweed.
The major role of duckweed in ruminant diets is likely to be as a major source of minerals and ammonia-N for the rumen and future research should examine its strategic use to stimulate ruminant production on high mature forage diets.
In developing countries, ruminants often subsist on byproducts of agro-industries and crop residues that are often (mostly) low in minerals and a source of ammonia in the rumen. In many situations duckweeds would be a valuable resource to ensure that ruminants utilise these feeds effectively by providing a soluble N source (e.g. ammonia) needed by the cellulolytic organisms for protein synthesis and also a source of particularly P and S which are essential for microbial growth and therefore the animal (Preston & Leng, 1986).
From the research of Huque et al. (1996) it appears that duckweed would require treatment to protect its protein to produce a meal that will deliver protein to the intestines and produce a high protein to energy ratio in the nutrients absorbed that will further advance ruminant productivity from crop residues. The potential responses of cattle to both fermentable N (i.e. N sources that give rise of ammonia in the rumen such as urea or leaf proteins) and to bypass protein have been discussed in many publications. Duckweed might be able in the future through research to replace multi-nutrient blocks used in these feeding trials (Sansoucy, 1995)
Treatment method for duckweed, which provide say half the protein as protected and half soluble to give bypass protein to the animal and ammonia for the rumen microbial digestive system together with the minerals in duckweed could remarkably enhance meat and milk production by ruminants given crop residues that contain major nutrient deficiencies.
Smith and Leng (1993) incubated duckweed meal in rumen fluid from sheep where it was rapidly fermented with the production of ammonia. Unfortunately treatment by heat, formaldehyde or xylose - three methods that have been successful in turning soyabean meal into bypass protein had no effect on the rate of release of ammonia. However, these chemicals were only sprayed on the meal and it is probable that some heat is necessary to effect protection of the protein. Duckweed protein, like terrestrial plant leaf protein is not easily protected from rumen degradation by any presently known methodology.
It is possible that when the protein is at high concentrations in duckweed that some of it is as peptide, amino acid or non protein-nitrogen. On the other hand it could be that leaf proteins have failed to come into contact with the dilute solutions of protecting agents that are used to protect the finely prepared extracted seed meals such as soyabean (formaldehyde or xylose) and future research should examine the application of heat after spraying to complete the reactions.
Most intensive fish farms culture fish that have very high value on national or world markets. In these intensive systems the fish require feed with extremely high protein levels and a well balanced array of essential amino acids. This is quite often provided by high cost fish meals. These farming systems have achieved a high level of production but with high cost inputs (including feed, fresh water, and the prevention of pollution) which makes them expensive. In the context of this book, duckweeds are not easily accommodated into such high technology systems, even though with great care a dry meal with excess of 45% crude protein may be produced and it may be possible to blend this with fish meal as a major protein source for intensive fish farming.
Herbivorous fish are cultured in many parts of the world. They include many varieties of carp and tilapia which provide a protein source of high biological value for humans. These fish are often regarded as inferior in taste but they are of great benefit in the diets of poor people who are often financially confined to largely vegetarian diets. These diets may at times be deficient in essential amino acids, depending on the source and variety of vegetables available. In the same way as milk is a source of potentially deficient essential amino acids in resource poor people, particularly those who are vegetarians, fish can play the same role.
Fresh duckweed (and also the dried meal) is suited to intensive production of herbivorous fish (Gaiger et al., 1984) and duckweed is converted efficiently to liveweight gain by carp and tilapia (Hepher & Pruginin, 1979; Robinette et al., 1980; van Dyke & Sutton, 1977, Hassan & Edwards 1992, Skillicorn et al., 1993).
A major investment in duckweed aquaculture research in Bangladesh (see later) has potentially important repercussions particularly for the host countries where the research has been carried out. This research has been the single most important research in this area and has focussed world attention on duckweed both as a feed for freshwater fish and as a water cleanser (Skillicorn et al., 1993) and the book by these authors is mandatory reading for anyone becoming interested in this area.
The PRISM group initiated the pilot project in Bangladesh to develop farming systems for duckweed and to test its values as a fish feed in polycarp production.
The outcomes of this project points the way for the efficient use of duckweed in many situations. It has important lessons for the development of small farmer systems where integration of crop and animal production benefits from the use of duckweed's ability to scavenge and retain major mineral nutrients within the system.
Carp species are by far the most commonly cultivated freshwater fish in Asia. They grow under diverse conditions, tolerating reduced water quality in even stagnant water ways. Their greatest attribute is the ease with which they can be managed in ponds and the huge production potential under good environmental and nutritional conditions.
Different carp species tend to occupy different ecological niches and therefore a number of species that are highly selective in their dietary preferences can be places in the same pond and will occupy different feeding zones. The development of polycarp culture depends on maximally using the feed biomass in a pond system by having top feeders, middle feeders and bottom feeding fish. These include an herbivorous species capable of feeding on surface plants or plants accessible on the sides of the pond or fed to them freshly harvested from another site as would be the case with duckweed; Two middle feeders which feed on either zooplankton and/or phytoplankton that grow on the detritus produced by the top feeders and a bottom feeding species which use the faecal materials produced by the middle and upper feeding fish.
Carp polyculture depends on most of the fish being produced on the zooplankton and phytoplankton (up to 85%). Providing extra feed for the surface feeders may increase carp production and it was a major reason for establishing the PRISM projects at Mirazapur as the surface feeders were restricted to very low stocking rates because of the small availability of plant biomass from the pond edges.
The polycarp systems depend on balancing biochemical oxygen demand (BOD) and maintaining oxygen levels in the water. Aeration and fertiliser use rates are critical in this way. BOD is created by:
In a traditional manure fed water body for polyculture of carp the main consideration is at what rate should the manures be fed to the water body? Stocking rate then is determined by the resultant demand for oxygen.
The special aspect of carp production from duckweed is that it can be set up using a single species of surface feeding carp or it can be used in polyculture to increase the total stocking rates. By providing fresh duckweed daily which does not decompose and can be fed ad lib, the top feeding carp densities might be increased together with an increase in bottom feeders increasing stocking density overall.
As the biochemical oxygen demand is turned down by reduced aerobic degradation of plant materials (that is replacing dead plant materials with live duckweed), the fish levels can increase to an extent that their respiration approaches the oxygen needs for the degradation of the organic matter entering the pond. The incremental production of the top feeders (Grass, Catla and Mirror carps) and bottom feeders (Mrigal carp) represents the potential extra production from a duckweed system (Skillicorn et al., 1993).
Duckweed-fed carp polyculture
The reader should refer to Skillicorn et al. (1993) for the most authoritative discussion of this subject.
In this document I will take the main issues from these authors. The major arguable criticism of Skillicorn et al. work put forward here is that it is too sophisticated for simple application by a small farmer. Resource-poor farmers need considerable economic support to set up fish farming. It becomes more complex where there is an attempt to integrate the farm in order for it to be sustainable. Under these conditions the use of artificial fertiliser as used in the Mirazapur project limits the application by small farmers. Fertilisers are often too expensive to use even for rice crop production, for example, much of the rice grown on small farms in Vietnam depend on recycling of nutrients via pig manure and fertilisers are either not used or are used sparingly. At the present time the financial crisis in Asia is likely to make fertilisers more expensive and it can be anticipated that there will be decreasing grain production in Asia over the next few years. Despite these reservations about their approach Skillicorn et al. (1993) have provided immensely valuable data which is essential for future small-farm systems to develop. It will be particularly important for the establishment of manure fed duckweed aquaculture and systems based on manure/biogas.
The Mirazapur carp stocking strategy and carp growth rates
Grass carp (Ctenopharyngodon idella) are the major users of duckweed but Catla (Catla catla) and Common carp (Cyprinus carpio) compete aggressively for duckweed. About 50% of the potentially digested nutrients in duckweed are used by the fish and so the faeces from duckweed fed carp are of relatively high in organic materials useable directly or indirectly through microbial action by the bottom feeders and these can be increased to 30% of the total population (see Skillicorn et al. 1993).
In general over 1989-90 the distribution of carp in the Mirazapur venture was:
The initial population was about 23,000 carp fingerlings per hectare. Yields have been difficult to estimate but around 10tons per hectare were claimed to be produced. The weight of fish captured by month is shown in Figure 17 and of fish weights in Figure 18. The feed conversion efficiency was estimated to be between 10-12kg fresh duckweed to 1kg of gain. This suggests that considerable other sources of food were available to the fish. A major cost is the fertiliser application to the duckweed ponds which was put into the water on a daily routine.
The growth rates of fish were largely in middle feeders with Silver and Rohu Carp showing fastest growth rates. Grass carp grew disappointingly slowly. The weight of fish after 13 months feeding of duckweed to the polyculture is shown in Figure 18.
Figure 17: Average weight of fish catch by month in Mirazapur duckweed-fed carp production trials
Table 16: Weight of different carp species in polyculture fed with duckweed
Figure 18: Average weight of fish catch after 13 months in Mirazapur
If the fingerlings weighed only a few grams when placed in the unit, and this is considered negligible to the final weight, then the average growth rate of the carp in polyculture is shown in Figures 17, 18 and Table 16.
In the Mirazapur venture, fresh duckweed is the only food provided to the fish in the water body by the farmer. The rate of feeding is as high as is needed to produce a slight excess after feeding activity and at 30,000 fish per hectare there were no problems of low oxygen levels in the water. The duckweed is fed either at enclosed (surface) feeding stations or simply tipped into the pond.
The data on fish growth as reported by Skillicorn et al. (1993) is unfortunately compromised by the "normal" problems encountered by research workers that to set up demonstrations under practical conditions and then attempt to analyse collected data from rather uncontrolled "research". There were serious logistic problems, at least initially, particularly in providing the feed consistently. The recommended harvesting techniques were based on frequent harvesting with removal of the largest and smallest fish at each harvest. The large ones were presumed to be reaching the weight at which their growth rate slows whereas the small ones were deemed to be "poor doers".
Obviously from their studies Silver carp, Catla and Rohu grew significantly more rapidly than the others, but feed availability problems make the growth rates in Figures 17 and 18 merely guides, as the authors claim that whereas in the first 13 months grass carp grew to less than 1kg, in other studies when they were fed ad libitum duckweed they grew to 4kg in six months (say 20g/day) suggesting that the production of grass carp may be more appropriately promoted as a monoculture.
Some ideas on duckweed use for carp production at small farm level
Carp polyculture in manure fertilised lagoons is an efficient and established method of producing carp. However, it is somewhat specialised and not managed easily by small farmers with limited resources, including availability of land, water and a continuous source of nutrient enriched water. A further constraint is also the capital outlay for the relatively deep lagoons that are required.
Duckweed production can be intermittent, it can be produced in relatively shallow ponds, simply lined with plastic or other inexpensive materials. Duckweed can be fed fresh or after drying and storage or both dried and fresh together. Simple systems that can be fitted into the daily routine of a family farm and according to the availability of water need to be researched. Undoubtedly there are many ways to approach a monoculture strategy for carp. The variations in practice are multiple. One potential system is described below as an indication of the approach that could be taken to such a development.
If grass carp are capable of the high growth rates indicated by the PRISM work it suggests that it could be advantageous to move away from polyculture to the culture of single species perhaps produced in a succession from a pond; Grass carp would be fed duckweed then following their removal from the pond, Rohu and Silver carp would be introduced to utilise the phytoplankton/zooplankton that build up and then the pond could return to duckweed to extract the nutrients in the water liberated from the detritus. The detritus feeders not being very productive would not be included in such an enterprise.
Thus, a potential strategy would be to periodically reverse the roles of the duckweed pond and the fish pond. This would be especially effective where effluent low in organic matter is used as the medium for duckweed culture, particularly the liquid from a biodigestor and where water is scarce and therefore must be used efficiently. The system that could be exploited is shown below (Figure 19).
Trial and error would be needed to work out a routine but a safe bet might be to use 0.2ha of duckweed to produce the necessary food for 0.2ha of fish ponds. At any one time therefore a pond could be producing duckweed or Grass carp or Silver carp. Three ponds could be used. One of these would receive the effluent from a biogas digestor and the other two would be producing fish, one pond fed with duckweed and the other used for phyloplankton/zooplankton feeders. These could be used with a rotation of the duckweed between ponds so that each pond produces fish and then duckweed reuses the minerals released by the faeces of the fish. The fish are then maintained until the oxygen concentration in the water declines to levels where Grass carp cease to produce. Then plankton consuming carp could be introduced. Duckweed is grown to lower the mineral content of the pond water after which the duckweed pond then reverts to a fish pond for Grass carp. Such a system would allow the use of static or slow moving water with a relatively slow rate of turnover. Rotation among ponds might be:
1. duckweed production
2. fish pond (Grass carp)
3. fish pond (Silver carp)
4. duckweed production.
The only major water loss from the system being by evaporation if seepage and use of water from other purposes is minimised. The three pond system over 18 months is indicated in the table below:
Table 17: Potential monoculture of carp using duckweeds in an integrated system
Figure 19: A schematic outline of a potential system for production of mono cultures of carp in a three pond system. Duckweed in pond 1 feeds the top feeding carp in pond 2 and plankton feeding carp are placed in pond 3 after the Grass carp have been harvested. The ponds can then be rotated in their use.
The success of such a project would depend on:
Where water is scarce, either year round or seasonally, this system could have a great advantages. The numbers of animals (large or small ruminants) needed to effectively feed the static ponds would need some considerable research but initially the finding of NPK fertiliser needs in the Mirazapur venture would give some guidelines.
In the studies in Bangladesh or a lagoon producing 100kg fresh duckweed per ha per day about 10kg of urea, 4kg of trisodium phosphate, 4kg of muriate of potash and 9kg of sea salt were needed per hectare of the surface area. Of these nutrients probably urea or ammonia is the nutrient that needs to be most carefully controlled as it can quickly limit duckweed production above 60mg N/l and below about 10mg N/l.
If a cow is consuming ammoniated rice straw supplemented with minerals and some protein meal, the faeces and urine probably contains only slightly less than the concentrations of N present in the ration. A cow would consume 2.5% of its body weight as dry matter of ammoniated straw. Thus a 400kg cow would consume 10kg of feed dry matter and produce 5kg of faeces dry matter plus urine containing most of the N fed as ammonia in the straw.
If 4% urea had been used to treat the straw then in 10kg of straw there would be the equivalent 400g of urea in the wash water from the cows stall. Washwater from the animals stall may be 80 litres/day so we have about 50mgN/l. This is fairly close to the level recorded in laboratory work for the optimal growth of duckweed. With a number of cows producing faecal and urinary N at this rate it would mean that 5 cows would produce 2kg urea and therefore 50 cows could fertilise 1ha or 5 cows to 0.1 ha. of duckweed lagoon.
Undoubtedly tilapia can use duckweeds efficiently when fed at an appropriate rate. In recent studies in Thailand, Hassan and Edwards (1992) have grown tilapia in static water in concrete tanks and fed them two species of duckweed Lemna perpusilla and Spirodela polyrhiza at various levels of duckweed dry matter per kg wet weight of fish per day.
The duckweeds were relatively low in protein (approx. 24% CP). The Spirodela was poorly consumed whereas Lemna was rapidly ingested by fish. The growth rate and feed conversion rates for Lemna-fed tilapia are shown in Table 18.
Table 18: The effects of feeding tilapia increasing levels of Lemna. Tilapia were held in static water in concrete tanks (Hassan & Edwards, 1992). The fish initially weighed approximately 41g
The tilapia in this case were fed duckweed in static water in small concrete tanks and the high death rates above a feeding of 30g dry lemna per kg fish was presumably due to the eventual eutrophication of the water. Over consumption of duckweed, however, cannot be ruled out as a cause of the high death rate.
Where duckweed is fed to tilapia in open waters or lagoons it is fed fresh and does not appear to effect the biochemical oxygen demand directly as the rate of feeding is controlled by the farmer so as to maintain only small amounts of excess duckweed on a daily basis (personal observations).
The work of Hassan and Edwards (1992) indicates the voluntary consumption rates of duckweed and its potential role as a feed that can be added to relatively unpolluted lagoon waters.
The growing importance of farmed tilapia suggests that a greater research effort is needed to develop inexpensive system where tilapia densities can be increased by addition of extra feed without reduction in size of fish. Tilapia could replace the top feeding carp in the systems proposed for research in Figure 19, retaining the rotation of phytoplankton consuming carp and the production of duckweed.
Ogburn and Ogburn (1994) developed an oxidation treatment of sugar-mill waste water using duckweed that appeared to be highly successful. The work was carried out in Negros Oriental, in The Philippines. The mean ammonia concentration in the influent relative to effluent water from the treatment plant over a six month period was reduced before release to the ocean from 0.87 to 0.31mg ammonia/l, orthophosphate from 0.93 to 0.5 mg P205/l and biochemical oxygen demand from 611 to 143 mg BOD/l.
The treatment water over the three years since the inception of the treatment plant apparently prevented the mass death rates of fish that occurred annually in the bay that received the water. Duckweed production represented 8.8g/m2/day. Whilst milk-fish did not consume duckweed, the duckweed was used to generate acceptable food. Duckweed was harvested and applied to the base of the pond prior to introduction of water. The dead duckweed fertilised the production of lablab which the fish consumed. Lablab is Filipino for the biological complex of blue green algae, diatoms, bacteria and animals that form on the bottom or float to the surface of ponds. Fertilising fish ponds in this way, as compared to inorganic fertiliser or cow manure enhanced fish production as shown in Table 19 (Ogburn & Ogburn 1994).
Table 19: The effects of different fertiliser application on the production of fish (Chanos chanos).
DUCKWEED AS A SOURCE OF NUTRIENTS FOR HUMANS
Duckweed has been used as a food by poor people in the past. The major benefit from such an addition to a diet is likely to have been as a supplement rich in phosphorous and/or vitamin A. However, undoubtedly there is a role for Lemna as a source of essential amino acids. Duckweed makes a fine addition to a salad and is quite tasty.
Where vegetable proteins are scarce in some regions of the world and particularly during a prolonged dry season or in normally arid areas, there is considerable scope to improve the nutritional status of the mal-nourished child through the use of duckweed directly or after extraction of a protein from the plant. Many aquatic plants may be used for such purposes with some additional purification to remove any toxic materials and also reduce the level of polyphenols. Dewanji (1993) demonstrated this effectively with several aquatic weeds in India. The aquatic weeds were subjected to pulping and filtration to extract mainly protein which was precipitated by steam injection. The chemical composition of such protein extract is shown in Table 20 and the amino acid composition in Table 21.
Table 20: Composition of protein extracts from three common aquatic weeds (modified from Dewanji & Matai, 1991)
As a source of essential amino acids then the protein of water plants have comparable amino acid compositions to that of most leaf proteins. The protein extract would provide quite considerable benefits to communities constrained to vegetarian diets through their economic situation. This would particularly apply to those without a source of milk and where there is a long period of dependency on dried foodstuffs deficient in vitamin A or in phosphorous as occurs in many of the arid regions of the world. On the other hand with the increasing demand for vegetable proteins in the industrialised world duckweeds could make a fine addition to most mixed salads and could be regarded as a commercial crop, provided quality water was used to grow the plants.
Whilst there appears to be considerable scope to use duckweeds as components of diets for animals and to a small extent humans, it is necessary to be cautious in recommending wide scale application. The nature of duckweed as a scavenger of minerals from water bodies poses
Table 21: Some amino acids in a leaf protein extracted from three aquatic weeds compared with leaf protein form alfalfa (see Dewanji 1993 for details)
potential danger where heavy metal contamination of water occur. This is of increased concern where radionuclides from nuclear reactors have leaked into the environment and these may also be concentrated in duckweeds. Heavy metals can enter the food chain at a number of points and it needs to be stressed strongly that monitoring of heavy metals in any large scale development of duckweed for any food/feed purposes.
A major problem of the 21st Century will be the control of environmental pollution. The major forms are pollution through emissions of gases into the atmosphere which is now an international issue with recognition of the potential (perhaps now unavoidable) global warming. There are both benefits and disadvantages to regions from global warming, but it now appears the disadvantages will certainly vastly outweigh any benefits, with tragic effects if global weather patterns are changed and sea levels rise inundating vast areas of agricultural (the deltas) and other land.
Whilst gaseous pollution is now an international issue the pollution of water (and indirectly land) remain national problems. Some governments view water pollution as a very serious problem, and legislate to prevent it occurring whereas, it is hardly considered by others.
Water pollution may be with industrial wastes which are outside the scope of this book or it maybe caused by residues of plant nutrients, naturally occurring or through fertiliser application. These run-off nutrients can potentially be harvested and provide valuable food and fertiliser sources through the development of aquatic plant crops.
Salination and acidification of previously undisturbed soils because of cultivations is also occurring and are global problems which will also need attention in the future.
Human settlements, provide great problems in the disposal of household wastes. In most industrialised countries the intensification of animal production has been promulgated in order to produce a consistent quality of animal products. This has also created major waste disposal problems. A major issue facing both human settlements and intensive animal housing systems is the concentration of nutrients locally and the difficulty and costs of disposal of these nutrients.
In the United States for instance there were in 1992 about 5,500 waste water treatment lagoons. Sewage systems world wide were developed mainly for the removal of organic materials and lowering of N levels but often major minerals (P,K etc) remain in the water when it is discharged from the works.
Intensive animal industries have now multiplied world wide, largely stimulated by world surpluses of inexpensive cereal grains and the increasing demand for animal protein as the standard of living of people improves.
The Environmental Protection Agency of the United States estimates that from all animals other than humans, 13,250 million tonnes of waste are produced per year (Hogan, 1993). A considerable proportion of which is concentrated in small areas. A figure for human waste may be around 5,700 million tonnes. The loss of P in sewage water discharged into oceans where it is non-recoverable is particularly concerning as P is rapidly becoming the most deficient nutrient for plant growth world wide. Disposal, and/or redistribution of nutrients and prevention of water contamination from the excrements of humans and animals will surely be one of the great problems of the 21st Century. Feedlots may concentrate 100,000 cattle or more at one site, poultry likewise may concentrate up to the same numbers with pigs and other animals usually in smaller concentrations. On the other hand humans are concentrated in cities with more than 10 million inhabitants.
The accumulation of animal wastes at one site, whilst having a number of overriding negative aspects, should potentially aid the economic extraction of the available minerals with the harvest of the "energy" through the controlled production and collection of methane in a biogestor. Growth of aquatic plants combined with other treatments may well serve the triple functions of extracting nutrients from waste water effluents for use as fertilisers or feed and at the same time allowing re-use of the water.
Some use has already been made of Lemna to treat sewage lagoons in USA, Europe and Australia. Intensive animal production industries have, however, taken little notice of such developments, perhaps because they have not been forced to pay the cost of an appropriate dispersal of nutrients they concentrate in one place. The problems in disposing of the nutrients are vast and mostly economic.
Skillicorn et al. (1993) devote a chapter in their book to duckweed based wastewater treatment systems. Urban wastewater treatment systems occur throughout the tropical developing countries but service only a small percentage of the total population. Skillicorn et al. (1993) argue that duckweed-based wastewater treatment systems provide genuine solutions to the problems of urban and rural human waste management with simple infrastructure at low cost. Their arguments should be read by anyone interested in this particular aspect of the use of duckweeds.
Social structures in densely populated developing countries and the problems and costs of installation and management of sewage works are problematic, particularly in a country where these costs of installation must be met by poor people with an income of only between $100-$500 US per annum. It is difficult, under these conditions to visualise large scale sewage works being implemented in rural areas or shanty towns except where people can pay for the service. In many countries nightsoil is regarded as a valuable asset particularly for recycling of nutrients back to the farm and sewage farm in rural areas may not be regarded as an asset, particularly if it had to be funded from local resources.
The use of duckweed as envisaged by Skillicorn et al. (1993) appears to have only limited application in the rural areas of developing countries because it largely exports the nutrients to a central site where sewage works are installed and the cost of transporting nutrients back to the farm where they can be an asset would be extremely high.
However, the valuable contribution of the work of Skillicorn et al. should not be neglected as there is some scope to develop such systems in crowded cities and urbanised areas populated by the relatively rich and where sewage disposal is often via open drains to the river, lake or sea. The resultant duckweed availability may assist urban-animal production or it is likely to be used as fertiliser on vegetable gardens. Many cities in developing countries have huge animal populations; intensive poultry production is often close to sea ports that import grains and in India milch animals abound in cities such as Bombay.
Duckweed waste water treatment systems are based on stand alone lagoons, and a single or a series of lagoons may be used depending on the size of the treatment plant. The settling tanks need to be dug out once in a while and two tanks are often needed so that whilst one is cleaned the other is in use. Following the sedimentation tank are a series of duckweed ponds and depending on the ultimate water use a 'polishing pond'. In the latter pond sunlight largely removes any pathogens that remain in the water. Generally these systems require about a 30 day turnover rate of water to be sure of minimum mineral contamination and low bacterial counts in water leaving the works. The potential efficiency of a duckweed treatment plant can be gauged by the fact that in the Mirazapur pilot scale plant the effluent water from such an operation was lower in ammonia, phosphorous and had a lower biochemical oxygen demand and turbidity than required by US Standards for the Washington DC area.
In most modern sewage works the ammonia levels have been reduced by a combination of microbial treatment methods. Usually for these systems to effectively grow duckweed either the denitrification step in the treatment works needs to be bypassed or urea must be applied to provide ammonia so that duckweed growth is vigorous enough to remove the residual phosphorus and other minerals. Even where no denitrification is brought about, fertilisation with urea in some of the 'downstream' ponds may be necessary to capture as much P as possible.
There are probably as many as 100 duckweed sewage treatment plants throughout the world, some of enormous complexity. Management has to be well informed and skillful for optimum performance. When this level of sophistication is achieved then management can be aided by dynamic modelling, which is able to simulate the behaviour of waste water treatment plants based on using Lemna gibba. A model developed by Vatta et al. (1995) clearly accounts for the main biochemical and chemical changes owing to such variables as water temperature, light incidence, nutrient levels and harvesting and replenishment rates etc. This opens up the way for more informed application of duckweed in the management of wastewaters.
The sophisticated lagoon systems, together with the need to provide floating chambers or grids, mechanised harvesting and good management do not preclude their use in developing countries but emphasise their unlikely rapid acceptance and development. However, with more simplified systems there are clear opportunities to develop small units easy to manage, that may be regarded as assets to the household.
In this context the potential of duckweed is to:
Most people in developed countries have access to relatively pure water free of pathogens and low in mineral components,. However, in the OECD countries more than half the population drink water that has passed through waste water treatment works.
In many developing countries, safe water is a luxury, enjoyed by a small proportion of the population. Particularly in arid areas water is a precious resource. It was estimated in 1988 that the health of between 9-22 million children of less than five years old are compromised each year in developing countries because of lack of water, inadequate sanitary facilities and water born diseases and around fifty per cent of people in third world countries have inadequate supplies (Maywald et al. (1988).
The recycling of water through waste water treatment works or purification of water for human use from presently unused surface water, whilst aesthetically not very acceptable, may be an only recourse in many developing countries. However, care must be taken to effectively treat such water to ensure that health standards are achieved and duckweed growth is only one potential step in such treatment processes.
CHAPTER 7: Overview
The future for duckweed farming may reside in establishment of duckweed cooperatives alone the line of the milk cooperatives in India. It may surprise the reader that there are many things in common between milk and duckweed production including:
In Bangladesh the Grameen bank has sponsored a highly successful small-scale loan system which encourages the development of small scale farming practices. This particularly applies to the raising of poultry. Much of this program is precarious because it is supported largely by imported protein meals (offal, meat, soyabean etc) from Europe and inexpensive grain often from India. Bangladesh could be genuinely termed the home of duckweed.
Taking Bangladesh as an example, the organised production of duckweed could provide in sufficient quantities to replace at a minimum 50% of the protein meals required by the small poultry producer if a simple and economic system of collection/sun drying and marketing could be put in place.
There are thousands of hectares of derelict ponds polluted to eutrophication levels in Bangladesh alone, that could potentially be cleansed of much of their pollutants and resurrected for duckweed aqua-culture and fish farming at the family farm level. In these systems the objective would be largely to provide protein of high biological value for the family of small farmers, who often have no animal protein in their diets.
To resurrect derelict ponds, the approach might be to first establish duckweed aqua-culture as a source of nutrients for terrestrial crop production (e.g. mulches and organic fertiliser) and as the ponds oxygen levels rise with harvesting of the crop to introduce fish farming either in part of the pond or in adjacent (clean water) ponds.
A further interesting approach would be to create a market for duckweed locally, as is the case presently in Vietnam, in order to encourage duckweed aquaculture as a cash crop. Undoubtedly a cash flow from such a market would then stimulate village people to clean-up the huge number of polluted ponds. In this case duckweed collection centres may be established to either sell duckweed directly or after drying for pig, duck, poultry or even ruminant production through local outlets or to blend duckweeds for use in compounded feed. The latter in Bangladesh is largely imported at great cost.
Creation of markets is essential if duckweed is to realise its potential in all the countries in the subtropics and tropics that have large areas of ponds or swamps that are presently eutrophic because of man's activities.
Many countries in the wet tropics could maximally use duckweeds where guaranteed acceptance of the product with payment on quality terms is introduced. Processing and product development could quickly develop with great potential for chemical extraction of the protein (and perhaps other chemicals including insecticides), production of feed and food production could encourage the very poor to participate. The bottom line would be healthier conditions for particularly the poor in a less polluted society.
Duckweed will remain an underutilised resource unless governments accepts that polluted water cannot be released into water bodies without removal of minerals and that there is some form of organised trade in duckweed introduced to ensure small farmers receive an adequate compensation for it's production. There is a vast need for research support for this little plant with such a great potential.
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