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Make way for super cassava

A quiet revolution is taking place in the development of cassava, Africa's most widely grown staple food.

In the mid-1980s, the root crop was yielding an average of six tons per hectare on African farms. Then, the International Institute of Tropical Agriculture (IITA) in Nigeria developed the high-yielding variety TMS 30572. It was widely adopted, and on-farm yields doubled to 12 tons per ha. Now, cassava yields are set to double again, giving farmers four times more than they enjoyed less than a decade ago and offering important possibilities for improved food self-sufficiency.

Crop breeders at IITA in Ibadan have been working on the improved cassava varieties since the institute was set up in 1967. Part of the Consultative Group on International Agricultural Research (CGIAR), IITA's mandate is to increase the productivity of key African food crops and develop sustainable systems that replace slash-and-burn techniques.

Cassava, also known as manioc and yuca, grows in parts of Latin America and Asia as well as in Africa. The largest producers, along with Nigeria, are Brazil and Thailand. But it is of special importance to Africa where mostly small-scale farmers cultivate it under rain-fed conditions and some 200 million Africans in 30 countries-nearly half the continent's population-eat it daily.

What helps make cassava so important for Africa is that it grows in poor soils, does not require chemical fertilizer and can survive with little water. It needs only 60 millimetres of rain a month during the growing period, and if rainfall is less the plant's deep root system can tap underground soil moisture. In times of drought, when soil moisture is exhausted, cassava has a defensive mechanism-its leaves bear the lack of moisture. The leaves droop, but the tubers survive. Thus the claim that “where there is cassava, there is no hunger.”

About half of Africa's cassava is used for subsistence needs and half for commercial purposes. Because the tubers can be stored in the ground for more than a year, giving farmers the freedom to eat or sell when the need arises, it is a security crop. Tubers are processed into many different products, depending on local custom, including gari and fufu. In Latin America, the processing of cassava tubers into flour is thought to date back to 2000 BC.

New processing techniques

While it requires the least labor of all root crops, cassava processing by traditional methods needs a great deal of labor and is often done by families working together. But this too is changing. A post-harvest technology unit at IITA recently developed low-cost technology that substantially reduces processing time, saving about four hours a day of labor.

While cassava contains more calories per kilogram than any other root crop, it has the disadvantage of being low in protein. But nutritional value can be increased by fortifying the processed tubers with protein-rich crops like soybean. In areas where cassava is increasing, agricultural extension workers also stress the importance of growing vegetables.

To develop high-yielding varieties of cassava, IITA breeders crossed TMS 30572 with a wild species of the plant. This produced spontaneous polyploids, meaning the crossing of plant varieties had multiplied the number of chromosomes. The polyploids have up to four times the normal number of chromosomes and so offer the possibility of much higher yields.

The polyploids are undergoing extensive evaluations in four different agro-ecological zones in Nigeria-humid forest, moist savanna, semi-arid and mid-altitude areas. They are being grown and tested as far as possible under the conditions that exist on African farms, and several of the new varieties have reached the stage of testing on selected Nigerian farms. The results are highly encouraging. More than 200 triploids and tetraploids have been generated through open and controlled pollination, and two spontaneous somatic tetraploids have been adopted. Whereas TMS 30572 yields about 23 tons per ha on-station, some of the triploids are yielding 50 to 70 tons per ha.

Sang-Ki Hahn, a Korean crop breeder who heads IITA's cassava program, hopes the polyploids will yield up to 40 tons per ha, giving farmers two to three times more than present harvests. In the view of Lukas grader, IITA director-general, “The polyploids offer the potential for a significant breakthrough.”

A hedge against drought

The advent of this super cassava offers significant prospects not just for cassava-growing countries, but also for those where cassava is still little known. “In time of drought, cassava will survive when other crops wither,” says IITA crop breeder Alfred Dixon. Although it grows extensively across West and Central Africa, the crop only grows in small pockets in Southern Africa, which was hard-hit by drought in 1992. Maize, the staple food across most of Southern Africa, is vulnerable to drought, and scientists have been unable to develop maize varieties that can cope. “In Southern Africa, there is now a tremendous interest in increasing cassava production,” Brader says.

An Eastern and Southern Africa Root Crops Network, set up in 1987, helps countries of the regions develop root crops. IITA officials say that Zimbabwe is now trying to start a large-scale cassava program and that Zambia is expressing considerable interest in the crop. In Uganda, a World Bank project is helping expand the area under cassava.

Like maize, a small portion of cassava output is used for animal feed and for making beer, cosmetics and starches. “Cassava could be used instead of maize for such purposes, leaving more maize for food,” Dixon says.

IITA officials believe the new varieties will make cassava attractive to African farmers, both for their own food security and as a profitable commercial crop. The continent's growing population is raising demand and prices.

An IITA study of cassava in Africa found that in half of 275 villages in six countries that had not experienced famine, cassava was grown as the main crop. The study found that in villages where cassava was increasing, 30 per cent of growers said they were planting more of the crop as a hedge against hunger and famine, 25 per cent said it was because of growing population, and 20 per cent said it was because there was a good local market.

Cassava is planted out in stems, and in the next few years the all important link between research and higher farm yields will be the multiplying of stem material in order to bring super cassava to farmers' fields. Multiplication rates tend to be slow, and this could mean a delay in farmers reaping the benefits. But crop breeders are cautiously optimistic the new varieties will give Africa a long-overdue breakthrough to higher food output. “Africa will survive,” Hahn believes, “because of the advent of polyploidy in cassava.”

John Madeley