The nitrogen (N) fertilizer equivalencies and effects of the N2-fixing legume crops, cowpea (Vigna unguiculata (L.) Walp.), groundnut (Arachis hypogea (L.)) and soybean (Glycine max (L.) Merr.) on succeeding sorghum (Sorghum bicolour (L.) Moench) yields were studied during two years (2000 and 2001) at Farakô-Ba (4° 20’ West, 11° 6’ North and 405 m above sea level) in the Guinean Savannah zone of Burkina Faso. Sorghum was cultivated in rotation with maize (Zea mays (L.)) and the three legumes. The two cereal-cereal rotations (Sorghum-Sorghum and Maize-Sorghum) were used as controls. The optimum yields of sorghum increased from 14, 20, and 35% when it was cultivated after soybean, groundnut or cowpea, respectively. Only 74% of the potential agronomic yield was achieved with cereal-cereal rotations. But 84, 89, and 100%, respectively, of the potential agronomic yields of sorghum were achieved when soybean, groundnut and cowpea were used as previous crops. Soybean, groundnut, and cowpea had N fertility equivalencies of 30, 35, and 42 kg N ha-1. Considering the N fertilizer equivalents (NFE) of the legumes, the recommended doses of N fertilizer for sorghum were 37, 38, 44, 50, and 53 kg N ha-1 when the previous crops were maize, soybean, sorghum, groundnut or cowpea
Key words: Legume, fertilizer, nitrogen, crop rotation, sorghum.
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