Production of single cell protein for livestock feed

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As an additional feed for livestock these root crops could serve as suitable substrates for a range of micro-organisms. Under optimal conditions from 100 kg of sweet potato containing 6.9 kg protein, a range of Fungi imperfect) species could produce 8.12 kg of dried mycelia, and a residue of unutilized sweet potato tissue for livestock feed, contain 31.6 kg protein i.e. the protein concentration can be increased fourfold. Analysis showed that the mycelia contain more Iysine, histidine, tryptophan, methionine and tyrosine than casein (Gray and Abou-el-Seoud, 1966). Dawson et al. (1951) found that waste water from the starch industry could be used as a medium for the growth of the yeast Torulopsis utilis. With addition of ammonium hydroxide as a source of nitrogen, as much as one tonne of dried yeast containing 50 percent protein could be obtained from 100 tonnes fresh weight of sweet potato processed for starch. Similar results have been reported for cocoyam and cassava. Production of single cell protein for animal feed has reached a level of about one million tonnes per annum in the USSR, with several plants capable of producing 0.1 million tonnes annually under construction elsewhere. A pilot plant in the International Centre for Tropical Agriculture (CIAT), Colombia, using cassava as a substrate, produces a dried product with a crude protein content of 28 percent. This final dried product was incorporated into the feed of growing rats to determine the nutritive value of the unsupplemented protein. Total weight gains over a 28-day experimental period were very poor for diets based on this material. When supplemented with methionine it gave body weight gains equivalent to casein. It is not yet certain whether small-scale plants could be technically and economically feasible as at least 60 percent of the production cost is made up of the costs of the raw material.

An important factor requiring further study is the possible health risks to individuals from continued exposure to spores of the micro-organism employed in the fermentation process. Equally important is the need to examine the effects on animals to which the single cell protein is fed. Research results to date on these aspects have not revealed any disadvantageous effects (Goering, 1979).

9. Food security in developing countries

Food security has been defined by the FAO Committee on World Food Security as the "economic and physical access to food, of all people, at all times". This implies that food should be available throughout the year to sustain household energy and health, and to meet nutritional requirements. The availability of food must be coupled with the ability of every household to acquire it: it must be affordable, especially by the poor. A food security system should act as a food bank during periods of crop failure, natural disasters and external or internal hostilities.

During periods of seasonal or national food shortages the groups nutritionally at risk include poor rural and urban families, with little or no land and limited resources to meet the nutritional needs of vulnerable infants and pregnant women. To ensure access to food supplies for such groups will involve increasing their income-earning opportunities and providing adequate supplies of basic foodstuffs at prices within their reach.

Rural food security can best be met by local measures to raise farm output of locally consumed basic foodstuffs. Production of a food surplus, in response to guaranteed markets, will provide additional income for the producers, and increased food supplies which can be processed to supply convenience food products to urban areas.

The most needy families will want to increase production if promotion is focused on foods which make up a large proportion of their diet. Ideally such foods should be adapted to existing farming systems, and be capable of producing high returns to land and to labour within the constraints of unpredictable rainfall and limited inputs of capital.

Considering the situation in a number of countries, roots and tubers have many advantages as food crops for household food security, with cassava as possibly the most significant.

Cassava is already a staple food in the tropics, where it is processed in numerous ways. It has further potential for yield improvement and for conversion into a greater range of convenience products, but this will require research in food technology to design appropriate small-scale equipment for their manufacture.

Processing root crops into convenience foods will improve their being accepted by the urban population. This will lead to expanded markets and thus encourage the increased production of root crops. Use of processed foods based on local products to replace imported foodstuffs will also conserve foreign exchange.

Malnutrition has important seasonal dimensions in many countries. The hungry season is shortened when crop and variety selection extends the harvesting period, and crops are mixed to lessen the risk. Food security is improved by cultivation of drought-resistant crops, which are grown as food reserves.

During prolonged drought periods, cassava is often the only crop to survive. When properly processed as gari, it is safe and convenient to eat and may be stored for up to 12 months. In the South Pacific, especially in Fiji, fermented cassava products are buried in pits in the ground for months or even years and used when required. Mature cassava plants can also be left in the ground for up to three years. However, this reduces the effective land area available for subsequent crops, and also reduces the processing quality of most varieties of cassava. Root crops can provide food for consumption during preharvest periods. In Nigeria, cassava is usually the last crop in the rotation system, as it will produce reasonable yields on depleted land.

Cassava grown as a food reserve need not be harvested if domestic food supplies are plentiful. Such a crop produces a considerable biomass as roots, stems and leaves which could be incorporated into balanced feed formulas for small livestock, such as pigs. Possession of livestock provides some buffer in times of adversity, as cash raised by their sale can be used to purchase additional food.

Rosling (1987) referred to cassava as the " Cinderella of the poor" because its rapid spread in Africa led originally to an improvement in agricultural productivity and evened potential famine in some areas. This important role will decline if current agricultural productivity in Africa continues to fall. Population pressure on land means a reduced fallow period. Lack of crop rotation brings an increase in diseases and pests resulting in lower yields. It is essential to develop and improve farming systems so as to increase productivity as well as to secure and maintain soil fertility. This will enable farmers to obtain high yields from their root crops and with an appropriate government support policy will lead to the availability of root crops all year round at affordable prices. Since food energy is still a limiting nutrient in many tropical countries the effective adoption of such a policy will provide additional energy resources to improve the health of disadvantaged groups.

In many tropical countries most of the population lives in the rural areas and practices subsistence farming. Among the main crops grown for home consumption arc root crops. Policy-makers often consider root crops to be cheap food meant for the poor, and direct agricultural attention toward the major cereal crops. These are relied on to increase local food production, as they did during the Green Revolution in India. This is not always feasible because the inputs required and the marketing infrastructure are not always present. Regular rainfall is a prerequisite for the successful establishment of rice, wheat and maize as well. If the rains fail, local food security may depend on drought-resistant traditional staples such as sorghum, millet and cassava.

In some parts of Asia irrigation agriculture is already established, fertilizers and pesticides are available, and conditions are appropriate for the introduction of high-yielding cereal varieties. On the other hand these countries also have their own local root crops. The yields of these crops could be considerably improved by selective breeding and increased production inputs. A production or consumption system based on only two or three food crops is extremely vulnerable, and is likely to be nutritionally unbalanced. Cultivation of roots and tubers in addition to cereals provides greater food security and a more varied and interesting diet.

Villareal (1982) has given sweet potato as an example of root crop yield potential. Experimental yields are up to 600 percent higher than farmers' yields (Table 9.1). This gap is typical for other root crops as well.

TABLE 9.1 - Sweet potato yields obtained In experimental stations compared with the national average


Potential yieldı

Farmer's yield² (t/ha)

Yield gap

Possible improvement (%)































ıBased on yields chained horn experimental stations.
²1979 national average.
Source: FAO 1980.

Unfortunately these root crops only attain special status in times of war, calamity and famine. Yet these are staple crops that farmers are already very familiar with, offering several ideal qualities as crops for food security in the tropics. They have high tolerance for the poorer soils resulting from reduced fallowing and population pressure on the land and, in the case of cassava, tolerance for periods of drought, encountered in arid areas. Processing of root crops could also provide cottage industry employment for rural women. Some harvest of certain root crops can, if necessary, be made during the growth cycle within 70 to 90 days, though harvest should preferably not begin before a minimum period of 120 days.

In many lowland areas of Papua New Guinea, rainfall and subsistence agriculture show marked seasonal patterns. Seasonal variation in food supply is considerably reduced by the cultivation of taro, or cocoyam. As shown in Fig. 9.1 the cultivation of two species of cocoyam, Colocasia and Xanthosoma, complement one another in providing a supply of calories throughout the growing season. Banana also, as a non-seasonal producer of calories, helps to even out the food supply and ensures household food security throughout the year.

Chandra (1979) calculated the energy efficiency with which a crop uses resources to produce returns under a given cropping system in Fiji. He obtained relative values of 66 for yam, sweet potato 60, cassava 52, cocoyam 21, while other crops on the same farm including maize, rice, pulses and vegetables gave significantly lower figures, indicating that the return to energy expenditure was higher in root crops, under his experimental conditions.

As discussed earlier sweet potato can produce more calories/unit area than cereals and most other crops except sugar cane. In terms of gross monetary return/ha potato is most profitable as shown in Table 9.2 (Horton et al., 1984) with US$1500/ha, followed closely by yam (US$1 469), then sweet potato, cassava and cocoyam with lower figures, owing to their relatively low prices and yields. Cereals show lower monetary returns ranging from US$366/ha for rice to US$117 for sorghum, confirming the superiority of root crops in gross return/unit of land. Cassava, yam, potato and sweet potato rank highest in the list of major food crops in terms of dry matter production per hectare (Table 9.3), potato and yam rank first and second in terms of edible energy production per hectare per day, with sweet potato ranking sixth and cassava ninth. Potato is third in the list of the most productive crops for developing market economies, in terms of edible protein per hectare per day (Horton et al., 1984). Idusogie (1971) pointed out that under West African conditions, yams can provide more protein per hectare per year than maize, rice, sorghum or even soybean. Doku (1984) estimated that the use of improved varieties of root crops under conditions of good husbandry could result in an annual production of about 140 t/ha for cassava and yam, and up to 200 t/ha for sweet potato and cocoyam.

Figure 9-1 - Seasonal pattern to the harvesting of selected food crops in Papua New Guinea

TABLE 9.2 - Average root crop and cereal grain yields, prices and gross returns/ha in developing market economies


Yieldı (t/ha)

Price² (US$/t)

Gross return³ (US$/ha)







1 63


Sweet potato




























ıYield: average estimate for 1979/81 from FAO (1982).
²Price: weighted average farm-gate price corresponding to 1977, provided by FAO Basic Data Unit (unpublished).
³Gross return: yield multiplied by price.
Source: Horton e, al., 1984.

With the establishment of international institutes like the International Centre for Tropical Agriculture (CIAT), the International Institute of Tropical Agriculture (IITA), and the International Potato Center (CIP), which are mandated to carry out research into these root crops, it is expected that governments will appreciate their economic and nutritional importance and formulate appropriate policies to encourage their production and consumption.

TABLE 9.3 - Top food crops in terms of dry matter production/ha and edible energy and protein production/ha/day In developing market economies

Dry matter production Energy production Protein production
crap t/ha crop mj/ha/day crop kg/ha/day
Cassava 3.0 Potatoes 216 Cabbages 2.0
Yams 2.4 Yams 182 Dry broad beans 1.6
Potatoes 2.2 Carrots 162 Potatoes 1.4
Sweet potatoes 2.1 Maize 159 Dry peas 1.4
Rice 1.9 Cabbages 156 Eggplants 1.4
Carrots 1.7 Sweet potatoes 152 Wheat 1.3
Cabbages 1.6 Rice 151 Lentils 1.3
Bananas 1.5 Wheat 135 Tomatoes 1.2
Wheat 1.3 Cassava 121 Chickpeas 1.1
Maize 1.3 Eggplants 120 Carrots 1.0

Yield FAO(1982) and FAO (unpublished). Vegetative period: FAO (1981) and Goering (1979). Edible portion, dry matter, and food composition USDA (1975) and INCAP (1961)
Source: Horton et al., 1984.

Constraints to production

Root crops contain about two-thirds of their weight as water. This poses two marketing problems, transportation and perishability. Cassava starts to develop a blue or brown discolouration after 24 hours, referred to as "vascular streaking". Within two days it starts to rot through the action of fungi and bacteria (Booth, 1974). If there is no central provision for processing or storage of yams and cassava these tubers have to be consumed soon after harvest within the producer's own locality. Distances to urban markets and the poor condition of rural roads often mean that the farmer has to accept poor prices offered by intermediaries. The alternative would involve construction of storage facilities in the localities of production.

TABLE 9.4 - Comparative retail prices of some crops per 100 calories In Oceania, 1982-84





1982-84 average

(Australian cents per 100 calories)
Sweet potatoes










Rice (milled)





Wheat flour





Source: FAO, 1987b.

Production of some root crops is labour intensive. Clearing the land, weeding, planting and staking in the case of yam, and harvesting, which can be single or double harvesting, all require labour. Women, who are already fully occupied with domestic duties, are also heavily involved with many of these farming activities. Yams and potatoes are regrown from previously harvested tubers. The use of small yam sets may result in reduced yields and so a proportion of the harvest, about one-fifth, is usually reserved for replanting. Assuming a yield of 12.5 t/ha and a multiplication ratio of five this can amount to as much as 2.5 t/ha reserved for replanting (Onwueme, 1978). The multiplication factor for most root crops is low when compared to cereals such as maize or guinea corn which show a multiplication ratio of 70 to 80. Some root crops such as cassava require more than a year to reach full maturity, and market handling costs are high. When root crop production is not mechanized production costs are also high. In some regions certain preferred root crops are more expensive than cereals, particularly if the latter are imported at low prices (Table 9.4).

There are a number of other production constraints involving biological problems of diseases and pests. These include the cassava mealy bug and cassava viral diseases, sweet potato viruses, root-knot nematodes and weevils. Root crops are propagated vegetatively from local planting material, which increases the risk of disease transmission.

Land holdings are small and inputs are limited in subsistence farming. In some countries the individual's land tenure is insecure as ownership of the land is vested in chiefs, landlords or the state. Pressure on agricultural land owing to population increase has resulted in much fallow periods and hence less fertile soils. Traditional farming systems involve intercropping. This ensures the most effective use of family labour throughout the cropping period and provides some insurance against failure of one or more crops, but it is not conducive to the highest yields for each component.

Extension services usually limit their attention to cash crops and cereals. Subsistence farmers do not benefit from the limited research and development on root crops. The marketing and distributing infrastructure is not well developed so the farmer is reluctant to increase his acreage of root crops, and without credit facilities for fertilizers, insecticides and pesticides his crop yields are poor. The average yield of yam is only about 14 percent of the potential yield possible with optimal inputs and conditions.

The most profitable avenue for stimulating higher production of roots and tubers will be the establishment of a guaranteed market, through encouraging such industrial processing enterprises as are based on sound economic principles and are consistent with overall national development.


Root crops are essential components of the diet in many countries. In tropical Africa it has been estimated that 37 percent of the dietary energy comes from cassava. Root crops have the potential to provide more dietary energy per hectare than cereals; and some root crops, such as taro and cassava, can be grown in tropical climates all the year round, to provide increased food security. This is of particular importance during the preharvest period of cereal crops, when other foods are expensive or unavailable.

Attainment of food security requires that a nation should produce those products from which it will enjoy some natural and economic advantages. For many developing countries root crops of per considerable benefits and potential.

Many food-deficit countries are forced to import large quantities of grain to meet local production shortfalls. Nationally, payments for food imports are a heavy drain on foreign exchange. Increased production and consumption of domestically produced food staples such as roots, tubers and plantains will increase food supplies and broaden the food base at household and national level.

Development for the urban market of low-cost convenience foods will increase household income and stimulate increased consumption of, and demand for, these valuable foods.

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