Growth and Yield of Irrigated and Rainfed Soybean with Late Nitrogen Fertilization

Nitrogen fertilization and supplemental irrigation during soybean [Glycine max (L.) Merr.] reproductive stages have gained interest to increase soybean yields. We assessed the effect of N fertilizer (0 and 60 kg N ha(-1)) applied at the beginning of bloom (R1) and full pod (R4) combined with rainfed (NIrr) and irrigated (Irr) conditions during reproductive stages on crop growth and yield in the southeast of Buenos Aires Province, Argentina. The NIrr treatments experienced a severe drought (30 d) in 2002/2003, and a moderate but longer drought (46 d) in 2003/2004. At the beginning of seed filling (R5), aboveground biomass and plant N accumulation were unaffected by the addition of N fertilizer. Aboveground biomass at R5 was 16% greater under Irr as compared to NIrr. Average soybean yields were 4.24 and 3.39 Mg ha(-1) for Irr and NIrr treatments, respectively, and were not affected by N fertilization. Application of N fertilizer neither reduced the anticipated plant senescence nor increased plant N accumulation under water stress conditions. Our results suggest addition of N fertilizer during soybean reproductive stages was not an effective management practice to increase yields of irrigated or rainfed soybean plants. Current rainfed soybean yields in the region can be increased significantly by maintaining soil water level at or above 60% plant available water during beginning of pod (R3.5) to full seed (R6) period.