The potential bene®ts of weeds with reference to small holder agriculture in Africa

Weed control is one of the most important crop protection activities undertaken in both intensive and low-input farming systems. However, even under intensive systems, crop protection which is less dependent on pesticides may require that weeds be managed to obtain a balance between crop and non-crop vegetation to encourage an increase in natural enemies of crop pests. In the low-input farming systems which sustain much of the rural population of Africa, weed control is usually done by hand and `clean weeding ' is often beyond the labour resources of the farming family. The vegetational diversity of peasant agriculture in Africa to which weeds make their contribution, helps to decrease the risk of disease and pest epidemics. In addition to the pest control bene®ts of a diverse agroecosystem, weeds contribute to the resource base of the rural community, providing a source of secondary foods, medicines and insecticides. Weed control within an integrated crop protection system appropriate to the needs of the resource-poor farmer, requires that weeds are managed in such a way that their biodiversity is maintained and the more useful species retained within the ®eld or ®eld margin. Those weeds with high food potential or which have pesticidal or medicinal properties might be deliberately encouraged within the crop or ®eld margins. Certain weed species may harbour important pests or diseases of local crops and therefore should be selectively removed. The paper reviews and discusses the literature on the bene®cial and deleterious effects of weeds and argues for a weed management strategy which balances the effects of weed competition on crop production with the ethnobotanical and pest control attributes of individual weed species and weed communities