Cassava (Manihot esculenta)
This is grown in large quantities in Africa, Asia and Latin America, both for human consumption and as a livestock feed. Cassava and its by-products (in the form of leaves, small tubers, pulp, peels, chaff, gari [fermented grated tubers], gari sievings, whole fermented roots and ensiled cassava meal) are used. The dried chips are high in energy and fibre but low in protein. In regions where cassava is used for human food, the peels are the most useful part of the cassava plant for feeding livestock. Amounts of 20 to 45 percent cassava peel meal (CPM) have been fed to chickens, but its use is limited because of the high content of the poison hydrogen cyanide (HCN), as well as high Crude Fibre, low protein content and dust. There is a considerable range of HCN levels in cassava, according to variety. When cassava completely replaces grains in a ration, there is a consequent reduction in egg weight and a change in egg yolk colour. Whether or not there are negative effects on egg fertility and hatchability is not known. Cassava meal gives good growth in meat chickens, although protein and other nutrients must be carefully balanced. Molasses or sugar may be added to sweeten the bitterness of the cyanide and thus improve palatability. Oilseeds such as full fat soybean can compensate for the high fibre and low protein content and for the dustiness. To remove the cyanide, detoxification methods include ensiling, sun-drying, air-drying, roasting, boiling and soaking. For smallholders, the most practical method is sun-drying (Sonaiya and Omole, 1977). Palm oil can also moderate the effects of cyanide on poultry. Some “sweet” varieties of cassava (which do not contain cyanide) are used in human food preparation, and these are often fed to poultry, particularly ducks.
Sweet potato (Ipomoea batatas)
Dried sweet potato forming up to 35 percent of the ration has been fed successfully to broilers and layers. The tubers are boiled before use, which overcomes any problems with dust or fungal growth from storage.