Cassava is one of the most drought tolerant crops and can besuccessfully grown on marginal soils, giving reasonable yields where many othercrops 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 thousandmeters above sea level, in temperatures ranging from 18-25°C to rainfall offifty to five thousand millimetres annually and to poor soils with a pH from 4to 9. It is a perennial plant growing to a height ranging from 1 to 5 m withthree-core single or multitude branching stems. The leaves are deeply, palmatelylobed and the roots are enlarged by deposition of starch cells which constitutethe principal source of nutrients. Roots' bulking occurs usually between the45th and 60th day after planting and storage rootsbuilding is a continuous process. An average storage root yield of 5-12tonnes/ha has been reported by traditional methods of cultivation; but bycultivating high yielding varieties and following better production practices,yield can increase to 40-60 tonnes/ha. Cassava's productivity in terms ofcalories per unit land area per unit of time is significantly higher than otherstaple crops as cassava can produce 250 x 103 cal/ha/day comparedwith 176 x 103 for rice, 110 x 103 for wheat, 200 x103 for maize and 114 x 103 for sorghum (Balagopalanetal.,1988).
The principal parts of the mature cassava plant expressed as apercentage of the whole plant are leaves 6 percent; stem 44 percent and storageroots 50 percent. The roots and leaves of the cassava plant are the twonutritionally valuable parts, which offer potential as a feed source. Thecassava storage root is essentially a carbohydrate source. Its composition shows60-65 percent moisture, 20-31 percent carbohydrate, 0.2-0.6 percent etherextracts, 1-2 percent crude protein and a comparatively low content of vitaminsand minerals. However, the roots are rich in calcium and vitamin C and contain anutritionally significant quantity of thiamine, riboflavin and nicotinic acid.Of its carbohydrate, 64-72 percent is made up of starch. The starch contentincreases with the growth of the storage roots and reaches a maximum between the8th and 12th month after planting. Thereafter, the starchdecreases and the fibre content increases. Cassava starch contains 20 percentamylose and 70 percent amylopectin. Cassava roots also contain sucrose, maltose,glucose and fructose in limited levels. The raw starch of the cassava root has adigestibility of 48.3 percent while cooked starch has a digestibility of 77.9percent.
Cassava root is a poor source of protein. The quality ofcassava root protein is however, fairly good as far as the proportion ofessential amino acid as a percentage of total nitrogen is concerned. Methionine,cysteine and cystine are however limiting amino acids in the root. Only about 60percent of the total nitrogen is derived from amino acids and about one percentof it is in the form of nitrates, nitrites and hydrocyanic acid. The remaining38-40 percent of the total nitrogen remains unidentified. Peeling results in theloss of part of the valuable protein content of the root because the peelcontains more protein than is found in the root flesh. The amino acid level ofcassava roots show higher levels of lysine and trypophan in its true proteinfraction.
The lipid fraction of cassava flour is 2.5 percent and is 50percent extractable with conventional solvents (Hudson and Ogunsua, 1974). Theextractable lipids are mainly polar in nature, the principal group beinggalactosy/diglycerides. The cassava root is a relatively poor source of mineralsand vitamins, however, there is a high content of calcium and phosphorus in thestorage roots. The mineral content of the dry bark is higher than that of thecortex. Calcium values in the whole root range from 15-129 mg/100 g, whilephosphorous values are approximately 100 mg/100 g. The content of iron in thecentral cylinder is 32 mg/100 g, while in the bark, it is 77 mg/100 g. Vitamin Ccontent of raw roots range from 38.5-64.6 mg. Drying reduces the vitamin Ccontent apparently, with values going down to 2-13 mg/100 g.
The annual yield of cassava foliage has been reported to be ashigh as 90 tonnes fresh leaves/ha/per annum if harvested three times a year(Sicco, 2002 personal communication). This however, has a depressing effect onstorage root yield. Lower values up to 12 tonnes/ha/annum have been obtainedwithout reduction in root yield. Cassava foliage is therefore a highly nutritiveand economically feasible high protein ingredient of animal feed rations. Driedcassava leaves have vast scope as a protein ingredient in compound feeds forlivestock and poultry. Cassava leaf blades are especially rich in protein(average 30.5 percent) and the protein content reduces to 13 percent for wholefoliage (Gramacho, 1973). Essential and non-essential amino acids can be foundin good levels in cassava leaves. Cassava leaves and roots are low in methioninewith values of 1.7 and 1.2 g/100 g of crude protein compared with 2.2 g, for theFAO reference pattern. Lysine content is high in the leaves (7.2 g) and low inthe roots (3.9 g) compared with the FAO pattern (4.2 g). The biological value ofcassava is inferior to that of animal protein. A major proportion of the leafcarbohydrate is starch. The amylose content of the leaf starch has been reportedto range from 19-24 percent.
The crude fibre content of cassava leaves is low which makesit palatable as poultry feed. However, when harvested with the tender stem thefibre becomes as high as 13.9-16.9 percent. The leaves are rich in calcium butlow in phosphorus compared with maize and sorghum (Teweet al.,1976).
Cassava foliage meal contains as high as 56 000 IU of vitaminA as compared with 14 200 IU in alfalfa meal, 66 IU in ground yellow maize and264 IU in wheat flour. This high content of vitamin A is significant in thepigmentation of egg yolk and skin of poultry.
The cyanogenic glucosides of cassava (Linamarin andLotaustralin) on hydrolysis releases hydrocyanic acid (HCN). The presence ofcyanide in cassava has caused a global scare as to the safety of cassava and itsproducts for human and animal consumption. The concentration of the glycosidesvaries considerably between varieties and also with climatic and culturalconditions. The normal range of cyanoglucosides content in fresh roots is from15-400 ppm calculated as mg HCN/kg fresh weight but occasionally varieties withvery low HCN content of 10 mg/kg or very high HCN content of 2 000 mg/kg havebeen reported. Cassava is often classified as "bitter or sweet" according to theamount of cyanide present. However, several studies have shown that bitternessor sweetness could not be exactly correlated with the level of cyanogenicglucosides. Earlier classifications of cassava safety limits provided byBolhuis, 1954 indicate:
A comprehensive study on the cyanogenic character of cassavaand its implication in productivity of livestock has been reported (Tewe, 1997).Safe levels of cyanide in cassava-based rations have been deduced from variousstudies 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 livestockprovided the feed is adequately supplemented with protein (or specificallymethionine) and iodine.
In long-term trials, the carry over effect of cyanide,particularly for gestating animals, can be deleterious as placental thiocyanatetransfer occurs in gestating pigs consuming cassava-based feeds with a HCN levelof 500 ppm. Through proper processing however, cyanide levels of less than 50ppm can be obtained particularly in sundries samples.
Presently, safety limits for cyanide in cassava food (CodexAlimentarius 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 pelletsimported into the European Union (EU) from Indonesia and Thailand for feeding ofdifferent classes of livestock. For pregnant stock only a consumption of up to500 ppm cyanide breaks the placental barrier against thiocyanate transfer (Tewe,1978). This hydrocyanic acid level is rarely encountered in fresh or driedcassava samples. Moreover, since balanced livestock rations only contain aproportion of energy, cassava is rarely fed at levels of more than 50 percent ofthe rations. Hydrocyanic acid levels above 250 ppm will rarely be encountered inpractical cassava-based rations. It is important to note that levels ofhydrocyanic acid in cassava leaves can be as high as 2 000 mg/kg of fresh leaveswhile chopping and drying reduces the level by at least 90 percent within 24hours of exposure.
According to Nwekeet al. (2002), the collaborativestudy on cassava in Africa (COSCA) revealed that between 1961 and 1999, totalcassava production in Africa nearly tripled from 33 million tonnes per year from1961 to 1965 to 87 million tonnes per year from 1995 to 1999, in contrast to themore moderate increases in Asia and Latin America. Most of the dramatic increasein cassava production in Africa was achieved in Ghana and Nigeria. In each ofthese countries, the production growth rate was greater than the rate ofpopulation growth. In other countries, D.R. of the Congo, Côte d'Ivoire,United Republic of Tanzania and Uganda, the increase in cassava production keptpace with population growth.
From 1961 to 1965, Nigeria produced only 7.8 million tonnes ofcassava 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, Nigeriaproduced 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 anannual production of only 1.2 million tonnes. From 1995 to 1999, however, Ghanaproduced 7.2 million tonnes annually and advanced to the position of the thirdlargest producer in Africa after Nigeria and the D.R. of the Congo.
The dramatic increase in cassava production in Ghana andNigeria was achieved through an increase in both area and yield. In 1951, theaverage yield in Africa was between 5 and 10 tonnes/ha (Jones, 1959). The COSCAstudy showed that the average yield was between 10 and 15 tonnes/ha inCôte d'Ivoire, Democratic Republic of the Congo (D.R. of the Congo),Ghana, Nigeria, United Republic of Tanzania and Uganda. Cassava yield thereforeincreased in Africa in the early 1960s due mainly to the planting of highyielding varieties and the adoption of better agronomic practices. The averagefarm yield was highest in Nigeria with 14.7 tonnes/ha, followed by Ghana with13.1 tonnes, Côte d'Ivoire, 10.8 tonnes, Uganda, 10.6 tonnes, UnitedRepublic of Tanzania, 10.5 tonnes and the D.R. of the Congo, 9.7tonnes/ha.
Cassava production in South Africa is a fairly recentdevelopment coming with the advent of production of high quality starch fromcassava on an industrial scale (Caysey, 2002 personal communication). Theaverage yield on a 5 000 ha cassava farm in South Africa is presently 50tonnes/ha at a production cost of US$20/tonne. Modern agronomic practicescoupled with use of improved varieties and other inputs have made this model areference 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 traditionalsystems. By contrast, in Latin America, 32.4 percent of its cassava is used forlivestock feeding while in Asia, over 40 percent of its products is exported inthe form of chips and pellets for the European Union livestock industry withanother 2.9 percent used for domestic livestock production (International Fundfor Agricultural Development - IFAD and Food and Agriculture Organization of theUnited Nations - FAO, 2000).
The share of African cassava production used as livestock feedis probably underestimated because cassava roots and leaves are fed to sheep,goat and pigs on small-scale farms in the cassava producing areas, either infresh or cut-and-dried form (Nwekeet al.2002). Cassava production,sheep and rearing are highly complementary because cassava processing is carriedout around homes, and sheep, goats and chicken are fed by-products of cassavaprocessing.
African cassava pellets are presently not competitive inEuropean livestock feed markets because of the high cost of production andtransportation within Africa and Europe and because Africa has been anunreliable supplier of pellets (Philips, 1973). The rising cost of grains on thecontinent due to weather induced fluctuations, high foreign debts and currencydevaluation has forced a number of countries in Africa to look inwards foralternatives to maize particularly for its livestock industry. In 1985 theGovernment of Nigeria banned the importation of maize and compelled livestockfeed mills to look for local crop sources such as cassava. As a result, theproportion of total cassava production used as livestock feed increased from3-10 percent from 1985 to 1990 (FAOSTAT). The Nigeria feed milling industry hastherefore adjusted their facilities to utilize cassava chips as long as theprice of cassava is competitive.
Presently, price considerations keep the usage of cassava lowin the African livestock industry. However, with higher productivity expectedfrom improved varieties and cost saving production and processing technologies,a surplus is anticipated that could lower the farm price of cassava. Thisscenario has led to growing interest among government authorities, the privatesector and researchers in Africa on the improvement of processing andutilization of cassava for its livestock industry currently faced with a limitedsupply of raw materials for the feed industry. This has resulted in a continuousincrease in the cost of production, causing a phenomenal rise in the unit costof livestock products, which has become too expensive and unaffordable for themajority of the population of the continent. Cassava can play a significant rolein stemming this tide of animal protein shortage.
