Bioscience in brief
Crop Improvement & Management
Did you know?
African cereal farmers discovered the wild ancestors of the cowpea (Vigna unguiculata) and developed the plant as a crop some 5,000 years ago. Other farmers adopted it around the world – calling it various names, including southern pea, blackeyed pea, crowder pea, lubia, niebe, coupe or frijole.
- Cowpea is grown in millions of smallholder farms in the drier zones of Africa, forming a great arc stretching from Senegal to Sudan and Somalia and southward to Zimbabwe, Botswana and Mozambique. 200 million people in Africa consume cowpea. Cowpea is often referred to as “the women’s crop” since they mostly grow it for their families’ the nutrition and to earn a little cash from trading it. Cowpeas are the most important legume in Africa.
- Africa leads the world in cowpea output, and Nigeria accounts for 58% of worldwide production. In addition of being the biggest producer, Nigeria is also the world’s largest importer since it takes in a large proportion of neighbouring countries’ production.
- Cowpea is one of the most drought-tolerant crops: some cowpea cultivated varieties can produce a good harvest with as little as 300 mm of rainfall a year. It can also thrive in poor soils. Like other legumes, cowpea fixes atmospheric nitrogen and therefore is an ideal plant for intercropping with sorghum, millet or maize and providing them with nitrogen in the soil.
- Cowpea seed is high in proteins rich in the amino acids lysine and tryptophan, although low in the sulphur-containing amino acids. Since cereals and cowpeas complement each other in terms of amino acids, a diet combining both provides a balanced protein intake.
- The tender green leaves are an important food source in Africa and are prepared as a pot herb like spinach. Leaves contain 25% protein of their dry weight and the protein quality is high. Immature snapped pods are used in the same way as snap beans, often mixed with other foods. Leaves and fresh pods are often the first food available at the end of the dry season (the “hungry time”): leaves can be harvested as soon as 21 days after planting, and some cowpea varieties crop after only 60 days.
THREATS AND CHALLENGES
The cowpea plant is attacked by pests during every stage of its life cycle. Insects are the biggest constraint to cowpea production, both in the field as well as when the grain is stored after harvest. Insects can reduce yields by 95% unless the crop is treated with insecticides and an effective insecticide treatment can increase yields by 20-fold. Insecticides are however expensive (sometimes prohibitively so for small holder farmers), are not always available while farmers often lack the necessary equipment and training for their safe use.
COMBATING THE FOESCowpea improvement breeding programmes
IITA scientists have developed high-yielding varieties that mature early or in a medium length of time and have characteristics that consumers prefer such as large seeds, seed coat texture and colour. A number of the varieties have resistance to some of the major diseases, pests, nematodes, and parasitic weeds. They are also well-adapted to sole or intercropping. The varieties include improved dual-purpose cowpea varieties that produce both grain for human consumption and fodder for livestock in the dry season. IITA has released improved varieties to 68 countries in all of the world’s regions.
Bt cowpea safe from pod borers
One of the most destructive cowpea pests is the legume pod borer Maruca vitrata which can be responsible for up to losses of 80%. The pod borer can be controlled with insecticides but these are expensive and toxic, and their continuous use results in the development of resistance in pests.
An international team of scientists have developed cowpea varieties with “built-in” insect protection – the Bt gene. The Bt gene works by interfering with the digestive system of pod borers, but it is very specific to these pests and so safe for all other organisms, including beneficial insects and humans. Bt cowpeas have been tested in confined field trials in Nigeria and Burkina Faso.
The Bt gene conferring Maruca resistance to the cowpea is the same Bt gene introduced into maize, soybean and cotton crops which planted on several millions of hectares in the US, Brazil, Argentina, South Africa, Canada, Spain and the Philippines for more than a decade now.
Bt cowpea is expected to become the next GM crop to be adopted in West Africa, and may be the first GM crop to be released commercially by the public sector.
Attacked by Witchweed: cowpea gene to the rescue
Witchweed, from the genus Striga, is a plant that parasitises many crops including maize, sorghum, millet, sugarcane, rice, and legumes. It attaches itself to the roots of host plants stealing their water and nutrients and stunting their growth. Witchweed can cause yield losses of between 30% and 100%, and in Sub-Sahara Africa alone the disease costs are estimated at US$ 1 billion, affecting the livelihood of 100 million people. Because the damage occurs underground, by the time the parasite is visible to farmers it is too late.
Most varieties of cowpea are also susceptible to Witchweed, but scientists have identified one variety able to fend off the parasite. And in this variety a single cowpea gene confers resistance by activating a common plant defense mechanism which works by killing the root cells that have been attacked by Witchweed. This prevents the parasite from siphoning off the plant’s nutrients and water. This finding is crucial for transferring resistance to other preferred cowpea cultivars but, more importantly, it also opens a way for developing a line of defense against this parasite in other crops. Currently no natural sources of Witchweed resistance have been identified in maize or sorghum, two major cereal crops in Africa.
Sacking the cowpea beetle
After cowpeas are harvested, seeds are plagued by seed-feeding beetles (bruchids) which can destroy the whole crop in a few months. Because farmers cannot store the crop they have to sell at harvest times when the price is lowest. Scientists from Purdue University in the United States and Niger’s Institute of National Agriculture Research developed and disseminated inexpensive hermetic grain storage bags that cut off oxygen to destructive weevils and stop them from feeding. An estimated 3.4 million households throughout West and Central Africa are expected to adopt this system by 2012 saving about a half billion US$ dollars each year overall.
We thank T. J. Higgins for information, comments and photographs.