Organisation:
Food and Agriculture Organization of the United Nations (FAO),
Rome, Italy
Author: Carlos Gómez
Edited by AGST/FAO: Danilo Mejía, PhD, FAO (Technical), Emanuela
Parrucci (HTML transfer)
CHAPTER XXXII - COWPEA: Post-Harvest Operations
Cowpea suffers terribly from its natural enemies. Insects are the worst of these enemies, but nematodes, bacterial diseases and viruses also cause losses. There are published data providing evidence that insects cause devastating losses in cowpea yields. Weevils - post-harvest pest - can destroy a granary full of cowpeas within two or three months. But people need to have the grain to eat for 12 months a year. (A BIOTECH, 2002).
If any broad spectrum insecticide is used on the growing crop, grain yields range from 1 400 to 1 700 kg per hectares. The insect pressure on cowpeas is an important weight that reduces yields to almost nothing. Yield losses in the field are only half the problem. Even when the crop has been harvested, the grain has still not escaped its insect enemies. (Murdock, L., 2002).
Murdock, L. states that traditional method and chemical insecticides have failed to control the insects. Despite millions of dollars spent and despite a great deal of research, average cowpea yields in Africa are still far below the yield potential. Insecticides are not the answer. They are widely available, they require expensive equipment and training for their use, and they are themselves expensive, polluting and potentially dangerous to users.
Murdock, L, continues to affirm "If we decided to solve the problem of insect control in cowpea solely by using insecticides, we would have to spray insecticides at probably a minimum of 250g of insecticide per hectare on the 8.8 million hectares of cowpea grown in Africa, that is, spreading 2.2 million kg of insecticide into the african environment every year, not only onto the plants of course, but also on the soil, the air and the water".
Field losses in West Africa are very high, because insecticide use is not frequent. Even when insecticides are available, farmers rarely have the money to buy them. In contrast, field losses in the United States (and probably in Australia) are quite low because of heavy use of insecticides.
Annual losses caused by Bacterial blight (Xanthomonas vignicola) and Aphid borne mosaic virus (CabMV) in Senegal have been estimated at 40 percent and 20 percent respectively (Gaikwad cited by Cissé, N. et al., 2000).
The parasitic weed Striga gesnerioides attack cowpeas particularly in the semiarid regions of West and Central Africa a mean yield loss of 30 percent (Aggarwal et al., 1989 cited by Cissé, N. et al., 2000). In Nigeria, the largest producer and consumer country in the world, the low yield is attributed to farmers' use of local land races that have low yield potential and high susceptibility to diseases, insect pests and parasitic flowering plants like Striga and Alectra. (NAQAS, 2001)
On the other hand, storage losses in West Africa are substantial in spite of the use of storage insecticides by merchants. Except in Senegal most West African farmers sell cowpea shortly after harvest, in part because they do not want to deal with the storage problems. A related problem is the lack of capital to invest in storing cowpea. In Senegal, farmers have slightly more resources than elsewhere in West Africa region and there is widespread use of hermetic storage methods developed by the Bean/Cowpea CRSP and the Senegalese Institute for Agricultural Research (ISRA).
The Fig. 25 represents the average damage recorded on stored cowpea in the Ghana northern region during the storage season 96-97. The number of farmers decreased over the storage season as they sold or consumed their cowpea. Few farmers keep their cowpea in store over the entire storage season.
These levels of damage were recorded on farmers' cowpea, under normal storage conditions and management. The weight loss remains lower than expected but observed levels of damage cause significant losses, hence price reduction.
Cowpea that are not stored with either chemical or the CRSP non-chemical methods are often completely consumed by bruchids in the first 10 to 12 months of storage. Even if the cowpeas are not completely consumed, West African consumers demand a substantial price discount before they will buy bruchid damaged cowpea. (J. Lowenberg-DeBoer, personal communication, 2003)
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. (New Agriculturist
on line.)