Impact of supplemental nitrogen application at different reproductive stages on soybean yield and seed composition

Declining soybean seed protein concentrations in the United States have raised concerns about the quality of raw soybean and soybean meal. This concern has increased interest in alternative ways to enhance protein content in seeds. Previous studies have suggested that nitrogen limitation during the seed filling stage is one of the main reasons for reduced seed protein concentration and have reported benefits of nitrogen supplementation in high yielding or favorable environment. However, those studies have evaluated split or repeated N applications rather than a single application. In this study, a two-year field experiment evaluated whether a single supplemental N application at different reproductive growth stages could improve soybean growth, yield and seed composition. A maturity group IV soybean cultivar was grown in a Randomized Complete Block Design with six replications and six treatments: no N control and 80 kg N ha −1 applied once at R1, R3, R5, R5+1 week, or R5+2 weeks. Plant growth and physiological traits were measured during the season at regular intervals and final grain yield, 100-seed weight and seed protein and oil concentrations were also recorded. Nitrogen application did not significantly affect growth, physiology, grain yield, seed weight and seed composition. Overall, under rainfed, drought affected and low yielding conditions, a single 80 kg N ha −1 application during reproductive stages did not provide a consistent benefit to soybean yield or seed composition. The findings are limited to conditions tested in the experiment and do not rule out the potential response under high-yielding management systems or split nitrogen application.