Crop and Industrial By-Products:
Roots and Tubers

Many tubers and other vegetable crops are grown for human consumption, but their foliage, or tops, can also be used as animal feed. The commonest by-products from root crops in many areas of the tropics are the tops from cassava and sweet potato

Cassava Sweet Potato References and Further Reading

Cassava or tapioca (Manihot esculenta) is grown primarily for its starchy root, but can yield up to 6 t/ha of foliage. This foliage has a high protein content (between 170 and 400 g/kg dry matter), but has a low carbohydrate content. It is a succulent feed, and so either needs to be fed straight away or preserved in some way (by either drying or ensiling). If it is ensiled, its nutritive value and ensiling properties are improved if molasses are added since its low sugar content and high protein content mean that it is deficient in fermentable energy for the microbes involved in ensiling. (For more information on ensiling, see Silage).

Cassava leaves have relatively high concentrations of hydrocyanic acid (HCN), which is toxic and could limit the amount of cassava leaves that could be fed to pigs and poultry. Ruminant animals (sheep and goats) are able to neutralize the effects of HCN by the action of microbes in the rumen. They can therefore eat diets with a much higher content of cassava tops (up to 35% in the case of dairy cows). Wilting the leaves substantially reduces their HCN content making them safe to feed to pigs and poultry. Work in Vietnam showed that there was no difference in pig performance if the leaves were fed fresh or wilted, which might suggest that the HCN in cassava was not toxic to pigs, and cassava can constitute up to 25% of the diet. However, cassava is very low in some essential amino acids (most notably methionine). Poultry production was greatly improved when a diet containing cassava (constituting 20% of the diet) was supplemented with methionine. Rich sources of methionine are animal proteins and there are also reasonable concentrations of methionine in mustard or soya bean meal if these crops are processed on the farm.

Sweet potato (Ipomoea batatas) is a creeping plant with a perennial vine that grows in rich, well-drained soils. The fresh vines have a protein content of between 150 and 200 g/kg dry matter. Small amounts can be fed to pigs and poultry, and they are a good forage for sheep and goats.

 

Cramb, R. and Purcell, T. (2001). Developing Forage Technologies with Smallholder Farmers: How to Monitor and Evaluate Impacts. ACIAR Working Paper Series, No. 41

Chin, F.Y. (2002). Ensilaging of tropical forages with particular reference to South East Asian systems. Paper presented at the XIIIth International Silage Conference, 11-13th September, 2002. [this paper includes Internet links to additional references]