Counting on cowpeas and other grain legumes

Credit: B.M.D.Tran, NRI

Grain legumes, such as cowpeas, are sold soon after harvest in many semi-arid areas of Africa, either because producers need cash to meet debts or because they cannot prevent losses due to storage insect pest damage. Selling early in the storage season results in a loss of income because prices rise as grain legumes become increasingly scarce. However, deterioration in grain quality is not just a problem faced by farmers. Traders at all levels within the system also suffer storage losses as a result of insect pest damage and it is also a major problem for food aid agencies.

Bruchid beetles (Callosobruchus maculatus and C.subinnotatuts), major insect pests of stored legumes also attack mature pods in the field before harvest (with the exception of subterranean bambara groundnut pods). Damage and weight loss in stored seeds is caused by larvae which develop within the grain, consuming the seed. Although surveys conducted during 1996-9 in northern Ghana, unexpectedly revealed that average weight losses (<5% each month) in cowpea and bambara during on-farm storage are of limited importance, the damage incurred is highly significant as damaged beans command a much reduced market price.

On-farm trials in farmers' stores, to test the most promising treatments for protecting grain pulses identified in station trials, revealed that hermetic storage in plastic buckets is very effective. Unfortunately it was also the most expensive form of protection tested and is therefore unlikely to be adopted by farmers. Thermal disinfestation (seeds laid out in the midday sun for 3hrs) proved to be very valuable followed by treatment with kim-kim (Synedrella nodiflora) solution or admixture with shea nut butter. However, although kim-kim is a treatment traditionally used on bambara participating farmers have more recently commented that it discolours cowpea grain, which deters consumers and reduces the market value.

The surveys have also revealed desirable grain characteristics (good appearance, taste and cooking qualities) which, together with the resistance of local varieties to insect attack, is providing plant breeders with important features to incorporate into improved varieties.

Treatment for traders

Damage incurred during storage at trader level is highly significant as traders purchase considerable quantities of cowpeas, bambara groundnut, local maize and many other commodities, which may be stored for six to seven months. Other than bruchids, a major pest is the Khapra beetle, Trogoderma granarium. Studies at Tamale market in the Northern Region of Ghana have shown that pest control is usually ineffective, if not hazardous, particularly in the case of indiscriminate use of aluminium phosphide fumigation tablets and the incorrect admixture of synthetic insecticides. The conditions in stores have been observed to lack the most basic storage hygiene, which makes cross-infestation (from neighbouring stores) and re-infestation from one season to the next, unavoidable.

Credit: Peter Golob, NRI

Whilst renovating and upgrading market stores for safe storage is a priority, a communal Fumigation Centre has been provided in Tamale where trained personnel carry out management of the site and fumigation work. Traders pay a small fee per bag for treatment of stored grain. However, although fumigation allows the disinfestation of large quantities of stored grain, it does not provide long-term protection. To provide long lasting protection, alternative procedures have been tested such as cotton or plastic sheeting placed over stacks of fumigated bags (to act as a physical barrier to insects) and dusting stack surfaces with the inert dust, Dryacide® (see also Inert Dust: using the past to protect the future). With good hygiene in the store, re-infestation did not occur until after five months of storage. Traders were impressed with the effectiveness of these cheap and simple techniques as they had been provided with the means to store and sell good quality produce and obtain premium prices. They are now anxious to have storage facilities improved and discussions are being held with the Tamale Municipal Council which is keen to offer support.

The various technologies tested in Ghana have demonstrated that solutions are available. In addition, it has been found that similar pulse grain storage problems have been identified in the farming systems of north-east Uganda. Although much of this region receives more rainfall than northern Ghana, safe crop storage could be provided by similar techniques. Participatory research methods are currently being conducted in Ghana to identify the constraints and opportunities of the proposed storage solutions and these are to be extended to Uganda. Extension material to disseminate information to farmers and traders is also to be produced.

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