Cassava is one of the most drought tolerant crops and can be successfully grown on marginal soils, giving reasonable yields where many other crops do not grow well. Cassava is adapted to the zone within latitudes 30° north and south of the equator, at elevations of not more than two thousand meters above sea level, in temperatures ranging from 18-25°C to rainfall of fifty to five thousand millimetres annually and to poor soils with a pH from 4 to 9. It is a perennial plant growing to a height ranging from 1 to 5 m with three-core single or multitude branching stems. The leaves are deeply, palmately lobed and the roots are enlarged by deposition of starch cells which constitute the principal source of nutrients. Roots' bulking occurs usually between the 45th and 60th day after planting and storage roots building is a continuous process. An average storage root yield of 5-12 tonnes/ha has been reported by traditional methods of cultivation; but by cultivating high yielding varieties and following better production practices, yield can increase to 40-60 tonnes/ha. Cassava's productivity in terms of calories per unit land area per unit of time is significantly higher than other staple crops as cassava can produce 250 x 103 cal/ha/day compared with 176 x 103 for rice, 110 x 103 for wheat, 200 x 103 for maize and 114 x 103 for sorghum (Balagopalan et al., 1988).
The principal parts of the mature cassava plant expressed as a percentage of the whole plant are leaves 6 percent; stem 44 percent and storage roots 50 percent. The roots and leaves of the cassava plant are the two nutritionally valuable parts, which offer potential as a feed source. The cassava storage root is essentially a carbohydrate source. Its composition shows 60-65 percent moisture, 20-31 percent carbohydrate, 0.2-0.6 percent ether extracts, 1-2 percent crude protein and a comparatively low content of vitamins and minerals. However, the roots are rich in calcium and vitamin C and contain a nutritionally significant quantity of thiamine, riboflavin and nicotinic acid. Of its carbohydrate, 64-72 percent is made up of starch. The starch content increases with the growth of the storage roots and reaches a maximum between the 8th and 12th month after planting. Thereafter, the starch decreases and the fibre content increases. Cassava starch contains 20 percent amylose and 70 percent amylopectin. Cassava roots also contain sucrose, maltose, glucose and fructose in limited levels. The raw starch of the cassava root has a digestibility of 48.3 percent while cooked starch has a digestibility of 77.9 percent.
Cassava root is a poor source of protein. The quality of cassava root protein is however, fairly good as far as the proportion of essential amino acid as a percentage of total nitrogen is concerned. Methionine, cysteine and cystine are however limiting amino acids in the root. Only about 60 percent of the total nitrogen is derived from amino acids and about one percent of it is in the form of nitrates, nitrites and hydrocyanic acid. The remaining 38-40 percent of the total nitrogen remains unidentified. Peeling results in the loss of part of the valuable protein content of the root because the peel contains more protein than is found in the root flesh. The amino acid level of cassava roots show higher levels of lysine and trypophan in its true protein fraction.
The lipid fraction of cassava flour is 2.5 percent and is 50 percent extractable with conventional solvents (Hudson and Ogunsua, 1974). The extractable lipids are mainly polar in nature, the principal group being galactosy/diglycerides. The cassava root is a relatively poor source of minerals and vitamins, however, there is a high content of calcium and phosphorus in the storage roots. The mineral content of the dry bark is higher than that of the cortex. Calcium values in the whole root range from 15-129 mg/100 g, while phosphorous values are approximately 100 mg/100 g. The content of iron in the central cylinder is 32 mg/100 g, while in the bark, it is 77 mg/100 g. Vitamin C content of raw roots range from 38.5-64.6 mg. Drying reduces the vitamin C content apparently, with values going down to 2-13 mg/100 g.
The annual yield of cassava foliage has been reported to be as high as 90 tonnes fresh leaves/ha/per annum if harvested three times a year (Sicco, 2002 personal communication). This however, has a depressing effect on storage root yield. Lower values up to 12 tonnes/ha/annum have been obtained without reduction in root yield. Cassava foliage is therefore a highly nutritive and economically feasible high protein ingredient of animal feed rations. Dried cassava leaves have vast scope as a protein ingredient in compound feeds for livestock and poultry. Cassava leaf blades are especially rich in protein (average 30.5 percent) and the protein content reduces to 13 percent for whole foliage (Gramacho, 1973). Essential and non-essential amino acids can be found in good levels in cassava leaves. Cassava leaves and roots are low in methionine with values of 1.7 and 1.2 g/100 g of crude protein compared with 2.2 g, for the FAO reference pattern. Lysine content is high in the leaves (7.2 g) and low in the roots (3.9 g) compared with the FAO pattern (4.2 g). The biological value of cassava is inferior to that of animal protein. A major proportion of the leaf carbohydrate is starch. The amylose content of the leaf starch has been reported to range from 19-24 percent.
The crude fibre content of cassava leaves is low which makes it palatable as poultry feed. However, when harvested with the tender stem the fibre becomes as high as 13.9-16.9 percent. The leaves are rich in calcium but low in phosphorus compared with maize and sorghum (Tewe et al., 1976).
Cassava foliage meal contains as high as 56 000 IU of vitamin A as compared with 14 200 IU in alfalfa meal, 66 IU in ground yellow maize and 264 IU in wheat flour. This high content of vitamin A is significant in the pigmentation of egg yolk and skin of poultry.
The cyanogenic glucosides of cassava (Linamarin and Lotaustralin) on hydrolysis releases hydrocyanic acid (HCN). The presence of cyanide in cassava has caused a global scare as to the safety of cassava and its products for human and animal consumption. The concentration of the glycosides varies considerably between varieties and also with climatic and cultural conditions. The normal range of cyanoglucosides content in fresh roots is from 15-400 ppm calculated as mg HCN/kg fresh weight but occasionally varieties with very low HCN content of 10 mg/kg or very high HCN content of 2 000 mg/kg have been reported. Cassava is often classified as "bitter or sweet" according to the amount of cyanide present. However, several studies have shown that bitterness or sweetness could not be exactly correlated with the level of cyanogenic glucosides. Earlier classifications of cassava safety limits provided by Bolhuis, 1954 indicate:
A comprehensive study on the cyanogenic character of cassava and its implication in productivity of livestock has been reported (Tewe, 1997). Safe levels of cyanide in cassava-based rations have been deduced from various studies for different classes of livestock and poultry. At a level of 100 ppm (100 mg HCN/kg, dried chips) satisfactory growth can be obtained in livestock provided the feed is adequately supplemented with protein (or specifically methionine) and iodine.
In long-term trials, the carry over effect of cyanide, particularly for gestating animals, can be deleterious as placental thiocyanate transfer occurs in gestating pigs consuming cassava-based feeds with a HCN level of 500 ppm. Through proper processing however, cyanide levels of less than 50 ppm can be obtained particularly in sundries samples.
Presently, safety limits for cyanide in cassava food (Codex Alimentarius Commission of FAO/WHO, 1988) is 10 ppm (or 10 mg/kg dry weight). However, levels below 100 ppm are considered safe in cassava chips and pellets imported into the European Union (EU) from Indonesia and Thailand for feeding of different classes of livestock. For pregnant stock only a consumption of up to 500 ppm cyanide breaks the placental barrier against thiocyanate transfer (Tewe, 1978). This hydrocyanic acid level is rarely encountered in fresh or dried cassava samples. Moreover, since balanced livestock rations only contain a proportion of energy, cassava is rarely fed at levels of more than 50 percent of the rations. Hydrocyanic acid levels above 250 ppm will rarely be encountered in practical cassava-based rations. It is important to note that levels of hydrocyanic acid in cassava leaves can be as high as 2 000 mg/kg of fresh leaves while chopping and drying reduces the level by at least 90 percent within 24 hours of exposure.
According to Nweke et al. (2002), the collaborative study on cassava in Africa (COSCA) revealed that between 1961 and 1999, total cassava production in Africa nearly tripled from 33 million tonnes per year from 1961 to 1965 to 87 million tonnes per year from 1995 to 1999, in contrast to the more moderate increases in Asia and Latin America. Most of the dramatic increase in cassava production in Africa was achieved in Ghana and Nigeria. In each of these countries, the production growth rate was greater than the rate of population growth. In other countries, D.R. of the Congo, Côte d'Ivoire, United Republic of Tanzania and Uganda, the increase in cassava production kept pace with population growth.
From 1961 to 1965, Nigeria produced only 7.8 million tonnes of cassava per year and was the fourth-largest producer in the world after Brazil, Indonesia and the D.R. of the Congo (FAOSTAT). From 1995 to 1999, Nigeria produced 30 million tonnes per year and became the largest producer worldwide; Ghana was only the seventh largest producer in Africa from 1961 to 1965 with an annual production of only 1.2 million tonnes. From 1995 to 1999, however, Ghana produced 7.2 million tonnes annually and advanced to the position of the third largest producer in Africa after Nigeria and the D.R. of the Congo.
The dramatic increase in cassava production in Ghana and Nigeria was achieved through an increase in both area and yield. In 1951, the average yield in Africa was between 5 and 10 tonnes/ha (Jones, 1959). The COSCA study showed that the average yield was between 10 and 15 tonnes/ha in Côte d'Ivoire, Democratic Republic of the Congo (D.R. of the Congo), Ghana, Nigeria, United Republic of Tanzania and Uganda. Cassava yield therefore increased in Africa in the early 1960s due mainly to the planting of high yielding varieties and the adoption of better agronomic practices. The average farm yield was highest in Nigeria with 14.7 tonnes/ha, followed by Ghana with 13.1 tonnes, Côte d'Ivoire, 10.8 tonnes, Uganda, 10.6 tonnes, United Republic of Tanzania, 10.5 tonnes and the D.R. of the Congo, 9.7 tonnes/ha.
Cassava production in South Africa is a fairly recent development coming with the advent of production of high quality starch from cassava on an industrial scale (Caysey, 2002 personal communication). The average yield on a 5 000 ha cassava farm in South Africa is presently 50 tonnes/ha at a production cost of US$20/tonne. Modern agronomic practices coupled with use of improved varieties and other inputs have made this model a reference point for potential of cassava on the continent.
Of a total production of 87 million tonnes annually in Africa, only 6 percent of this is used in livestock production mainly in traditional systems. By contrast, in Latin America, 32.4 percent of its cassava is used for livestock feeding while in Asia, over 40 percent of its products is exported in the form of chips and pellets for the European Union livestock industry with another 2.9 percent used for domestic livestock production (International Fund for Agricultural Development - IFAD and Food and Agriculture Organization of the United Nations - FAO, 2000).
The share of African cassava production used as livestock feed is probably underestimated because cassava roots and leaves are fed to sheep, goat and pigs on small-scale farms in the cassava producing areas, either in fresh or cut-and-dried form (Nweke et al. 2002). Cassava production, sheep and rearing are highly complementary because cassava processing is carried out around homes, and sheep, goats and chicken are fed by-products of cassava processing.
African cassava pellets are presently not competitive in European livestock feed markets because of the high cost of production and transportation within Africa and Europe and because Africa has been an unreliable supplier of pellets (Philips, 1973). The rising cost of grains on the continent due to weather induced fluctuations, high foreign debts and currency devaluation has forced a number of countries in Africa to look inwards for alternatives to maize particularly for its livestock industry. In 1985 the Government of Nigeria banned the importation of maize and compelled livestock feed mills to look for local crop sources such as cassava. As a result, the proportion of total cassava production used as livestock feed increased from 3-10 percent from 1985 to 1990 (FAOSTAT). The Nigeria feed milling industry has therefore adjusted their facilities to utilize cassava chips as long as the price of cassava is competitive.
Presently, price considerations keep the usage of cassava low in the African livestock industry. However, with higher productivity expected from improved varieties and cost saving production and processing technologies, a surplus is anticipated that could lower the farm price of cassava. This scenario has led to growing interest among government authorities, the private sector and researchers in Africa on the improvement of processing and utilization of cassava for its livestock industry currently faced with a limited supply of raw materials for the feed industry. This has resulted in a continuous increase in the cost of production, causing a phenomenal rise in the unit cost of livestock products, which has become too expensive and unaffordable for the majority of the population of the continent. Cassava can play a significant role in stemming this tide of animal protein shortage.