The role of legume fallows in intensified upland rice-based systems of West Africa

Traditional upland rice-based cropping systems in West Africa rely on perio ds of fallow to restore soil fertility and prevent the build-up of insect p ests and weeds. Demographic growth and increased demand for land is forcing many farmers to intensify their rice production systems. Declining fallow length and increasing number of crops before leaving the land to extended f allow result in a significant yield reduction. Promising cropping system al ternatives include the use of site specific, weed-suppressing, multi-purpos e cover legumes as short duration fallows. Constraints to rice production r elated to intensification were determined in 209 farmers' fields in four ag ro-ecological zones during 1994 and 1995. Nitrogen accumulation and weed su ppression were evaluated in 54 legume accessions, grown for six months duri ng the dry season, under a range of hydrological and soil conditions in 199 4/95. Their effect on the yield of upland rice was determined in 1995. To i ncrease benefits from improved fallow technology, the timing of legume esta blishment in relation to rice and the effect on crop and weed growth of rem oving, burning, mulching, or incorporating allow residues prior to the rice crop were determined. Intensified land use resulted in a significant plot- level yield reduction that was highest in the derived savanna and the bimod al forest zones where it was associated with a doubling of the weed biomass in rice and a significant reduction in soil N supply. Legume fallows appea r to offer the potential to sustain rice yields under intensified cropping. Legume biomass was in most instances significantly greater than in the wee dy fallow control and several legume species suppressed weed growth. Nitrog en accumulation by legumes varied between 1-200 kg N ha(-1) with 30-90% Ndf a. Rice grain yield following legume fallows increased by an average of 0.2 mg ha(-1) or 29% above the weedy fallow control. Relay establishment subst antially increased legume biomass. However, seeding of the legume at 28 day s or earlier significantly reduced grain yield due to interspecific competi tion. Incorporating or mulching of fallow residues provided no significant yield advantage as compared to burning. Absolute effects varied as a functi on of site, legume species, and management practice.