Effect of soybean on subsequent maize grain yield in the guinea savanna zone of West Africa

Two varieties of soybean (Glycine max (L.) Merr.) and a maize (Zea mays L.) control crop were grown in replicated trials at 10 sites in the Guinea savana of Nigeria in 1993 followed by a test crop of maize in 1994 with 20 or 60 kg N ha-1 to test effect of soybean on subsequent maize grain yield. Maize grain yield increase following soybean was variable but the main effect of previous soybean crop was positive even though the soybean was not inoculated with rhizobia and the aboveground soybean residues, except litter fallen before harvest, were exported from the fields, following the present farmer practice. The yield increase following the medium duration soybean variety (TGx1660-19F) was similar to that from 40 kg N ha-1 applied 4 weeks after planting to maize preceded by maize. The increase following the early soybean variety (TGx1456-2E) was significantly (P