Cassava is one of the most important staple crops in sub-Saharan Africa, yet it stands out from other crops in many ways. In fact, cassava has been even imposing challenges for us to analyze its production data and modeling growth and productivity. This post briefly explains why.
Cassava (also known as manioc and by many other local names) is grown in South America – especially Brazil – in south-east Asia and in Africa, where it is one of the most important starchy staple crops. Cassava (Manihot esculenta) is a native of Brazil, where there are many wild relatives, but was spread into Africa and Asia from about 1520 AD by Portuguese and Spanish colonisers.It probably first arrived in Africa in Angola, and spread throughout the more humid areas of the continent in the nearly 500 years since its introduction, mainly through farmer-to-farmer contact, but also by some deliberate introductions during colonial times.
It is now grown from Senegal to Kenya, and from Sudan to Mozambique, at elevations from sea level to 2,200 metres, and in environments ranging from moist savanna to humid forest.
What's So Unusual?
Cassava is a very unusual staple crops in many ways.
Firstly, it is propagated only from cuttings, not from seeds or tubers. This is a serious constraint in trying to introduce new varieties, since cuttings are relatively bulky and have a severely limited storage life (3-5 days in most cases), and cannot therefore be sold through seed merchants or similar commercial channels. This propagation mechanism is also problematic in breeding new varieties, and in reducing the spread of disease. New laboratory techniques allow in vitro germination and the use of micro-cuttings, but these techniques are currently out of the reach of almost all peasant farmers.
A second unusual feature is the post-harvest processing and storage. Cassava tubers, although relatively unaffected by ageing or decay while still in the ground, decay rapidly after harvest, and must be eaten or treated rapidly. Cassava tubers have a fairly thick skin which is high in cyanide-type compounds which protect them from insect attack but are poisonous to humans. The skin must be removed, usually manually after softening in water, and the tuber must then be treated by chipping and drying, by grinding or in some countries by fermentation to ensure preservation. The cyanide toxicity can be a problem for new farmers inexperienced in the post-harvest requirements, and tubers of “bitter” cassavas still have trace amounts of cyanide after removal of the skin, but this is tolerated and the flavour is even preferred by many consumers.
The third characteristic which distinguishes cassava from all other starchy staples, and which leads to much confusion and inaccuracy in production statistics is that cassava has no clear harvest season. Tubers can be edible, although small, after as little as six months after planting, but very little is harvested until nine months or one year after planting. Even then, the whole crop is not harvested at a single time. Rather, individual plants are harvested as required for consumption in the household or for sale, and the field may not be completely harvested until as much as two years after planting. Cassava is often treated as a reserve crop. If the main starchy staple, often maize, fails due to poor rainfall, then cassava in the ground provides interim nutrition and income, and can also keep the household fed between harvests of other more seasonal crops. In some parts of Africa, notably in northern Zambia, cassava was introduced by the colonial government early in the 20th century to provide improved food security during a time of regular locust plagues. The African Red Locust, breeding in swamps in northern Zambia and south-western Tanzania, attacked all above-ground vegetation, devastating the maize crop, but underground cassava tubers were unaffected and the tough stems were not eaten and soon regenerated.
Although yields of cassava are improved by good husbandry, particularly regular weeding, it is still possible to simply plant a crop and then to return after a year to reap some kind of harvest. In areas where land is scarce, households can plant cassava in remote fields far from the village, visiting the fields only for planting and harvest. In most African farming cultures, land preparation (felling forest, clearing fallow) is done by men, who may also do the actual planting of cassava cuttings. All subsequent work, especially weeding and harvesting, is done by women.
Subsistence Crop with Commercial Potential
Much cassava in Africa is grown as a subsistence crop, although it is rarely grown only for household consumption. Tubers are sold in nearby markets to raise essential cash in times of need. The increasing urbanisation of some major cassava producing countries, for example Nigeria, has resulted in an important market for cassava tubers and cassava-derived products in the growing towns. This in turn has led to the growth of a significant cassava processing industry, using simple locally manufactured machinery for chipping, grinding and drying cassava, and also to commercial-scale farming, with growers planting tens or even hundreds of hectares of cassava instead of the more traditional fractional hectare plots.
Cassava has wider commercial potential. It is an important source of starch for industrial use and for stock-feed, and industrial processing is starting in Nigeria and other important cassava-growing countries in Africa, following the example of Thailand which is currently the largest producer of industrial cassava. The Nigerian government has recently introduced a law to ensure that at least ten percent of the flour used in making bread and pizza in Nigeria is derived from cassava, in order to reduce wheat imports and to provide support for local farmers. Other countries, including Mozambique, may follow this example.
Breeding of improved cassava varieties has been directed by two main pressures.
One has been to increase yield. Traditional cassava varieties, grown by peasant smallholders, provide yields of 2-5 tonnes per hectare, depending on climate and agronomic techniques. New varieties have been developed which can yield more than 40 tonnes per hectare, although very high levels of fertiliser input are necessary in most African soils to achieve this level. Without inputs, which are beyond the means and experience of most smallholders, these high-yielding varieties will rapidly impoverish the soil.
The second main pressure has been from a range of diseases, many originating in the original home of cassava, in Brazil, but mutating in new African environments. Cassava varieties tolerant to new virus diseases may not provide higher yields in disease-free situations than the traditional varieties, but at least yield is maintained under disease pressure.
Much smallholder cassava in Africa is grown inter-cropped with other food crops. In rainforest areas of Cameroon, for example, cassava is inter-cropped with maize and groundnuts, and sometimes also with plantains and beans. Groundnuts are harvested first, then maize, and finally the cassava. These traditional inter-cropping patterns have probably evolved to provide a more or less continuous supply of food, but also to maintain soil fertility and even to mitigate the effects of disease.
Cassava tubers provide substantial starch for the farming household, but the tubers are notably deficient in protein, minerals and vitamins. The cassava must therefore be eaten in combination with other more nutritious foods. Fortunately, cassava leaves are high in iron and some vitamins, and are eaten as a vegetable in many African cultures. Attempts are being made, for example in the HarvestPlus Project, to develop cassava varieties naturally more nutritious than traditional varieties, in much the same way that orange-fleshed sweet potato is more nutritious than white-fleshed. The new biofortifying cassava with provitamin A are to be released in D.R. Congo and Nigeria in 2011.