Manure and nutrient cycling
After the harvest a part of the crop residues tends to be freely accessible to all livestock including those from outside the village. The residues consist of straw that is sometimes left in the field or carried home and they contain stubble and ratoon (green regrowth of the crop) which can be grazed. Another part of the residue is fallen grain - in Asia this is used by ducks. Weeds can also be considered as crop residues and in wetter areas or seasons they can form an important feed resource. Sometimes they are cut and carried home during the cultivation period when access to grazing grounds is difficult for animals.
The nutrient content of manure and other organic fertilizers varies according to the quality of feed and the way it is stored and handled. Dry matter content of manure also varies widely; in cows on lush pasture it can be less than 15 percent but in sheep on dry forage it can be higher than 50 percent. The amount and quality of urine and dung produced depends on the type of animal, its size and the type of feed as well as on the management of the farmer. One way to calculate the amount of manure produced is:
· One animal of 250 kg live weight has a feed intake that averages 2.5 percent dry matter of its live weight. It therefore consumes 250*365*0.025 = 2 280 kg of dry matter. With an average digestibility of 55 percent, the animal will produce 0.45*2 280 = 1 026 kg of dung every year.
· Small ruminants weighing 25 kg and feeding 3.2 per cent daily on average of their live weight, consume 25*365*0.032 = 292 kg dry matter. If their average digestibility of the feed is estimated at 60 percent it can be calculated that one small ruminant produces some 117 kg of dry matter faeces per year (Defoer et al., 2000).
The amount and proportion of nitrogen excreted depend on animal diet. The urine and solid dung of animals fed highly digestible diets with a lot of protein contain much more nitrogen and, therefore, are more susceptible to nitrogen losses than excreta from diets containing greater amounts of roughage. Much of the urine nitrogen is lost via ammonia volatilization. Where animal management tends towards increased stall-feeding, the composting of fresh manure will have to play a greater role in minimizing nutrient losses. Pits or heaps that capture feed refusals, manure and urine and household waste need to be designed to minimize nutrient losses. Low-cost appropriate implements to spread the compost at the appropriate time over large cultivated areas such as the Sahel are also needed (based on Powell and Williams, 1993).
In various systems draught oxen, donkeys, horses and small ruminants are kept overnight in pens in the compound throughout the year, and the manure they produce is transported to the fields during the dry season. Droppings of donkeys, sheep and goats are often added to the heap of household waste. However, droppings from small ruminants are sometimes used separately, for example on infields to manure certain spots in the millet crop. Manure from small ruminants takes longer to have an effect on crop yields than cow dung, but once it has started the effect lasts for several years. As this dung is kept the nitrogen losses can be reduced by mixing with low quality biomass, i.e. with straws, leftover feeds, dry leaves, etc. The greener leaves still contain much nitrogen themselves and they are less capable of capturing surplus nitrogen from urine and dung.
Nutrient losses and their prevention
Different kinds of nutrient losses can occur during crop cultivation and post harvest:
· Stray animals can damage the standing crop during crop cultivation and post-harvest period, in terms of losses of nitrogen, phosphorus and potassium and in terms of feeding value.
· Straw stubble that is left on the field can be trampled or soiled and incompletely eaten: careful harvest could save valuable nutrients.
· Leaving the crop residues in the field causes a loss of nutritive quality and sometimes leaching of nutrients through rains and degradation processes that involve fungi.
· Lush regrowth, in some cases after sudden rain, can be poisonous, such as the formation of prussic acid in sorghum ratoon.
· When the soil is bare for long periods nutrients can be lost due to erosion (wind and water).
Animals leave their droppings while they graze the stubble, so even farmers without livestock will receive some on their fields. However, the quality of the dung is likely to deteriorate if left lying on the field unprotected for up to six months, even when it is stacked and stored in the open. Activities to prevent or reduce nutrient losses are:
· Penning of animals with or without bedding.
· Storage of crop residues.
· Mulching of the soil with residues unfit for animal feed.
· Breeding or managing for crop varieties with better or more straws or for cultivars that have more ratoon.
· Covering of soil with so-called catch crops that retain leached nutrients to build their own organic matter that can be used for animal feed or mulch.
· Harvesting of crop residues before they overmature.
Cropping patterns, livestock and nutrients
The survival of farmers in mixed systems depends to a large extent on a proper adjustment of soils, plants and animals. Techniques suggested in this report include:
· better use of fallow;
· use of leys and catch crops.
These techniques can help in several ways either to reduce erosion, to add nitrogen, to enhance phosphorus flow rates, to pick up leached nutrients or to provide feed.
Farmers in most EXPAGR regions tend to depend on fallow as the technique for restoring soil fertility. This is possible because there is plenty of agricultural land available and entire fields can be left fallow for long periods of ten years or more. The system resembles shifting cultivation and the location where crops are cultivated changes every couple of years. However, over the years, with increasing population and changing technology, the fallow period has become shorter and, in areas with a dense population, it has disappeared almost entirely.
Leys and catch crops
A ley is a fallow that is planted with crops such as grass or legumes to regenerate the soil more rapidly. The grass is used as fodder if animals are present on the farm, but similar systems are used when only mixing between crops takes place. In Mali, for example, farmers grow millet and cowpea on the same infield year after year. When they find that the fertility of part of the plot has fallen significantly, they change the association to pure stands of groundnuts or Bambara groundnuts. The following year they apply organic fertilizer and then start cultivating millet/cowpea again. Such a strategy is widely used in the Sahel. In other words, a ley is a crop rotation that includes animals. To some extent it is also the system used in the Gangetic plains and in the Nile delta, where berseem and/or mustard are rotation crops used exclusively for livestock feeding, either on-farm or off-farm. The system is being tried in northern Thailand (Gibson, 1987), and is also practised in the Krishna Valley of eastern India where a legume (pilipesare) is the main rotation crop. However, use of legumes is also common in the United States, in Western Europe and in the so-called Mediterranean systems that even occur in Australia.
Towards sustainable land use
Integration of crops and livestock is often considered as a step towards sustainable agricultural production because of the associated intensified organic matter and nutrient cycling. Residues of the different crops represent the main on-farm source of organic matter and nutrients. This combines well with the presence of livestock since animals play a vital role as capital assets for security and as a means of saving, for cash income and in nutrient flows. Management of crop residues in such regions is closely related to their utilization in animal feeding.
An advantage of the integration of livestock and crops is the added value derived from crop residues (especially those of legumes) in terms of animal products and income. However, to maintain the system in the long term it is necessary that nutrients from external sources are added. In the past, this was covered to some extent through fallowing and manuring contracts with pastoralists, but the growing demand for feed by the increasing herd (from both arable farmers and pastoralists), the shrinking area of cropland per capita and declining crop yields dictate the scope for improvement through integration of crops and livestock. The extra input must come from inorganic fertilizer as well as from concentrates or both. Often, the price ratios of fertilizer and grains are not conducive to the utilization of fertilizers, and development of institutional and physical infrastructure for cost-effective use of fertilizers and concentrate is required to trigger sustainable land use in the region. Use of concentrate can be remunerative for farmers but credit facilities may stimulate intensification of livestock production and thereby increase availability of nutrients for crop land (based on Savadogo, 2000).
Energy, biogas and nutrients
Animals can also play a role in the provision of energy. Sometimes this role is very negative where livestock keeping contributes to deforestation in large parts of South America, Asia and Africa (Photo 91), but it can also be positive, such as by transforming plant energy into useful work or by providing dung used for fuel through dungcakes or biogas to replace charcoal (see Photos 92, 93 and 94). Wood and charcoal will con-tinue for a long time to provide energy for household cooking and for rural industries but increasing energy demands and increasing pressure on natural resources call for measures such as a more efficient use of biomass, improved natural resource management, fuel switching from, for example, charcoal to kerosene or liquefied petroleum gas, and for the development of alternatives such as biogas. The principal uses of biogas are for household energy, such as cooking and lighting, although larger installations can produce sufficient gas to fuel engines, for example, for powering mills and water pumps.
Despite the merits of biogas technology, bio digesters have only been widespread in India (over 5 million installations) and in China (nearly 3 million installations). In these countries it is common to find biodigesters even in remote villages provided there is enough water. For example, an ambitious programme in Nepal is accelerating market development of small biodigesters. In many countries, some practical experience has now been gained with the dissemination of biodigesters but larger-scale dissemination has not yet become popular, despite the assumed economic, environmental and social benefits of the technology. The main reasons seem to be that the multiple benefits of biodigesters must be acknowledged by the end-users to convince them to invest in the technology, and that a market-driven institutional infrastructure should be in place to facilitate large-scale dissemination of the technology. Much work has been done on mixed crop-livestock systems including the use of biogas, for example in the well-known CIPAV project in Colombia where pigs, sheep, sugar cane and biogas are just some of the components studied (FAO, 1992c).
Three designs can be distinguished among the small-scale and low-cost biodigesters:
· the floating-drum, also known as the "Indian design";
· the fixed-dome, known as the "Chinese design"; and
· the flexible-bag digester.
Roughly speaking, a well-constructed fixed-dome biodigester has the longest lifetime: 20 years and more. The floating-drum digester can have a comparable lifetime, but the recurrent costs are higher as the steel drums have to be replaced every five to ten years due to corrosion. The lifetime of the flexible-bag digester is hard to indicate, as some do not last more than a couple of months, while others function for several years.
Complex microbiological processes take place within a biodigester. In a common farm-type digester, the temperature ideally varies between 30 and 40 degrees Celsius. The process can be divided into three steps. During the hydrolysis phase, the complex molecules in the feedstock are broken down into smaller molecules. During the second phase, volatile fatty acids are formed. Finally, during the methanogenic phase, other bacteria use the acids to produce methane. The methanogenic bacteria can only do their work under anaerobic conditions (without oxygen), i.e. it is important for a biogas plant to be built of leak-tight materials. In practice, the three phases of the process take place simultaneously in a digester where the different bacteria work together in a symbiotic relationship.
Many different types of organic matter can be used to "feed" a biodigester, but to allow the bacteria to do their work the organic matter should be accessible to them. This means that pretreatment is sometimes necessary, e.g. through chopping and/or composting of crop wastes. The easiest feedstock to use is cattle dung, as it already contains the right bacteria and the vegetable matter has been broken down by passing through the guts (and teeth) of the cow. Human excrement and manure from chickens and pigs are also useful, but they do not contain the right bacteria and they need a starter in the form of, for example, some slurry from a working biodigester. Neither the feedstock nor the water should contain toxins such as antibiotics, detergents and disinfectants. Toxins cause the digester to "go sour", causing it to give off an unpleasant smell of acids and, therefore, the dung of cows that receive antibiotics should not be fed into the digester.
Initially, biodigesters were promoted only for their energy production. Once again, the thinking about a technology in a mixed system focused on only one aspect. Unfortunately little attention has been paid to the excellent properties of the slurry as an organic fertilizer of crops and trees. In other words, slurry has the potential of supporting crop production and, in addition, the process kills or substantially reduces the amount of pathogenic germs and seeds of weeds present in the feedstock. Another advantage is that nutrients that are already present in the feedstock, such as nitrogen, potassium and phosphorus, are made available to crops more quickly and efficiently, i.e. the biodigester affects the nutrient dynamics of the system. Further, it reduces the odours given off by manure and compost. Emissions of greenhouse gases are also reduced, as the biodigester produces renewable energy (biogas) and because fossil fuel based chemical fertilizer is replaced by an organic fertilizer (the slurry). A major disadvantage of the slurry is that it is liquid, with a dry matter content of less than 10 percent. Transport to the fields is therefore often complicated and the use of the slurry is usually limited to fields near the digester. Larger installations in industrialized countries separate the slurry into a liquid and a solid fraction; the solid fraction can be applied on fields at a distance from the digester.