LEGUMES AS DRY SEASON FALLOW IN UPLAND RICE-BASED SYSTEMS OF WEST-AFRICA

Declining fallow length in traditional upland rice-based cropping syst ems in West Africa results in a significant yield reduction due mainly to increased weed pressure and declining soil fertility. Promising cr opping system alternatives include the use of weed-suppressing legumes as short duration fallows. N accumulation, N derived from the atmosph ere (Ndfa), weed suppression, and the effects on rice yield were evalu ated in 50 legumes, grown at four sites in Cote d'Ivoire with contrast ing climate, soils, and rice production systems. The sites were locate d in the derived and the Guinea savanna and in the bimodal and the mon omodal rainfall forest zones. Legume and weed biomass during the fello w were determined at bimonthly intervals. Percent Ndfa by biological N fixation was determined by N-15 natural abundance. Fallow vegetation was cleared and rice seeded according to the practice of local farmers and the cropping calendar. Weed biomass and species composition were monitored at monthly intervals. Legume fellows appear to offer the pot ential to sustain rice yields under intensified cropping. Biomass was in most instances significantly greater in the legume fallow than in t he ''weedy'' fellow control, and several legume species suppressed wee d growth. N accumulation by legumes varied between 1-270 kg N ha(-1) w ith 30-90% Ndfa. Across sites, Mucuna spp., Canavalia spp., and Stylos anthes guianensis showed consistently high N accumulation. Grain yield s of rice which had been preceded by a legume fallow were on average 0 .2 Mg ha(-1) or about 30% greater than that preceded by a natural weed y fallow control. At. the savanna sites where fallow vegetation was in corporated. Mucuna spp. and Canavalia in ensiformis significantly incr eased rice yield. In the bimodal forest zone. the highest rice yield a nd lowest weed biomass were obtained with Crotalaria anagyroides. In g eneral, the effects of legume fallows on rice yield were most signific ant in environments with favourable soil and hydrological conditions.