Can cassava replace maize? An increasing number of African governments are looking to cassava, rich in starch, as an alternative to maize that has been the traditional staple food in large parts of Africa. But cassava can be lethal. Kennie Ntonga examines the arguments.

Unpredictable weather conditions and high cost of farm inputs have forced many African governments to earmark cassava as the most suitable alternative food crop, but scientists say continuous consumption of the crop may result in serious health implications on both people and livestock.

Many Africans depend on maize as their staple food, but adverse weather conditions, environmental degradation and the ever increasing cost of chemical fertilisers have resulted in low production of the crop over recent years. Many countries, especially in the southern African region have been forced to imported maize to feed their ever increasing populations.

Under its food diversification programme, the Malawi government through its Ministry of Agriculture and Food Security has been encouraging the local population to diversify its eating habits and switch to cassava which doesn't require fertiliser and is resistant to drought and various plant diseases.

But the head of the bio-chemistry department at the College of Medicine of the University of Malawi, Professor Emmanuel Maduagwu says cassava contains a toxic element known as hydrocyanic acid (HCN), popularly known as cyanide which is one of the most lethal poisons known to man.

Maduagwu says there are many different varieties of cassava but the plant can mainly be differentiated in terms 'sweetness' and 'bitterness' of taste; and it has been suggested that generally, the 'sweet' variety usually has a low cyanide content while the 'bitter' is high in cyanide concentration. Ironically, most people prefer to grow the bitter variety as it is high yielding and has a higher amount of carbohydrates (starch).

According to Maduagwu, who is a postdoctoral fellow at the International Institute of Tropical Agriculture (IITA) in Nigeria, the toxicity of cassava attributable to its cyanide content has been recognised as a serious health hazard in the tropics for many years. He says: "In 1966, two lethal cases of cyanide poisoning following ingestion of sweet cassava roots were reported on the East African island of Pemba and three years later, a similar case was reported of a three-and-half year old Indian child who died after eating tapioca (another name for cassava)."

Maduagwu says that in 1992 some doctors working in a hospital in Lagos, Nigeria, have reported the case of patients who died from acute poisoning after consuming 'gari', a roasted cassava grit derivative.

"Animals have also been shown to be highly susceptible to toxic feed stuffs formulated with cassava derivatives," he says. Apart from Tanzania and Nigeria, cassava toxicity cases including neurological disorders and goitre have also been reported in Mozambique, the Democratic Republic of Congo (DRC) and West Africa.

The World Health Organisation (WHO) is currently investigating 60 cases of upper motor neuron disease with paralysis of the limbs known as "Konzo" in Pemba, Tanzania, which occurred in September and have been attributed to cassava.

POISONS CAN BE NEUTRALISED

Studies have shown that the monotonous ingestion of some species of cassava may cause chronic health problems which include "nutritional neuropathies, endemic goiter and upper motor neuron disease" that causes paralysis. The studies further indicate that raw or insufficiently processed cassava may cause headaches, nausea, loss of appetite, vomiting and collapse, among other symptoms.

But the Cassava Newsletter from the College of Medicine says cyanide can be removed or reduced substantially by simple traditional processing techniques which include drying, boiling, roasting, shredding or grating, and the de-watering of pulp and fermentation of tissue.

"Cassava roots are a particularly perishable staple food and therefore, well managed post harvest handling is of immense economic importance. In general, this involves fresh cassava storage, cassava drying and development of new by-products in order to reduce post-harvest deterioration and wastage to the barest minimum," says the newsletter.

Nthutang Seleka is the South African High Commissioner to Malawi. He says: "The issue of cassava cyanide poses a challenge to all countries in the Southern African Development Community (SADC) region as they are all considering diversification from seasonal crops like maize to the more resilient cassava which survives even drought."

Seleka says SADC countries may not have reached the stage where cassava is a main food, but the fact initiatives are being proposed for it to be an alternative food substance means "we too may be affected by the challenge".

But an official from Malawi's Ministry of Agriculture, Jonathan Mkumbira says Malawians, especially those living along lakeshore areas already have cassava as their staple food but no one has ever complained of such effects.

"Farmers in Malawi prefer to grow the bitter cassava for its high yielding capacity but it has had no effects on them because they process it well," says Mkumbira.

Maduagwu however insists; "To-date, no cassava plant anywhere has been found to be free of cyanide. This means that eating unprocessed raw cassava roots, peeled or unpeeled, as a normal dietary relish in any community would en tail ingestion of huge amounts of cyanide." The controversy continues.

RELATED ARTICLE: Science finds cure for maize disease.

One of the worst enemies that maize farmers in Africa face is the stem borer, an insect that can devastate entire crops. But now science has come up with new weapons to deal with it. Stephen Williams reports.

Small scale farmers in East Africa who cannot afford expensive chemical inputs, now have a new means to ward off one of the most common and devastating pests that attack maize crops--the stem borer moth. It's taken almost 10 years of research--conducted by the International Centre of Insect Physiology and Ecology (ICIPE) who lead and co-ordinate the project under Dr. Zeyaur Khan; the Kenya Agricultural Research Institute (KARI); and Lester Wadhams and John Pickett from the UK's Rothamsted Agricultural Laboratory. Early results from the research programme are exceeding everyone's expectations.

The principal is clear and easily understood. It has long been known that various plants send out signals, known as volatile semio-chemicals that can either attract or repel certain insects. That's why, throughout the ages, farmers have known that intermixing two different crops can make good sense.

But could the scientists come up with a plant that would repel the stem borer moth? This insect drills into the stems of immature maize plants to lay their eggs, and the resulting caterpillars hatch to feast on the host plant until it collapses, usually long before the edible ears have ripened.

Research that began in 1994 identified Napier grass that could be planted around the edge of the maize field, and Molasses grass between the rows. Starting with just two experimental 50sq metre plots, one planted just with maize, the other with maize and the two grasses, early trials proved that the maize planted with the two grasses had far fewer incidences of stem borer moths. In addition, it was discovered that the molasses grass attracted a parasitic wasp that preyed on the stem borers.

Completing the picture, it was also found that the Napier grass around the maize field was attracting the stem borer to lay its larvae in its stem. When the larvae hatched and began to bore into the grass' stem, the Napier grass produced a super sticky type of sap that killed the majority of the invaders--leaving just enough to tempt the parasitic wasps to remain in the area to pick off the survivors.

So, in effect, the grasses were providing protection in three different ways. First, they were repelling the stem borer moth away from the crop. Secondly, the stem borer was being attracted to a trap crop that killed most of them, and thirdly the parasitic wasp was being encouraged to feed on the remaining stem borers.

A win-win-win situation? Not quite, because there was a fourth bonus. Both Napier and Molasses grasses provide nutritious fodder for cattle. When the experimental crops were first planted, in an arid region on the shores of Lake Victoria, there were just four cows producing milk--today more than 400 farmers taking part in the experiments are producing milk. And maize yields are up by a minimum of 30% for the 1,500 farmers who have started to interplant the grasses with their maize.

HITTING ANOTHER AFRICAN PEST

The research team has also been conducting experiments over the last six years to tackle another problem for African can increase farmers, the parasitical plant known as Striga or Witchweed. This weed attacks crops, living off the host plant's roots, eventually killing it, flowering and producing tens of thousands of seeds that can lie dormant in the soil for up to 20 years. What the scientists have come up with here are a pair of south American legumes known as Silverleaf and Greenleaf which, when intercropped with maize, virtually eliminates Striga. And, like Napier and Molasses grasses, they provide quality fodder for cattle.

When Silverleaf or Greenleaf and the grasses are used side-by-side with maize, farmers that would once have expected to harvest one or two tons of maize per hectare are now reaping some five tons. It is all looking extremely positive and the experimental plantings are being extended to sites in Ethiopia, Malawi, South Africa, Tanzania and Uganda--but the present cost of the imported Silverleaf and Greenleaf seed is a drawback.