Genetic gain × management interactions in soybean: II. nitrogen utilization

Soybean [Glycine max (L.) Merr.] grain yield has annually increased nearly 23 kg ha^-1, but the interaction of genetic advancement and improved agronomic practices has not been well quantified, including N utilization and fertilization. A field study with soybean cultivars released from 1923 to 2008 in maturity group (MG) II and MG III was conducted in multiple environments with a nonlimiting supply of fertilizer N to examine the main effects and interactions of N supply and release year on grain yield and seed quality. We hypothesized that grain yield and seed quality would be improved with the nonlimiting supply of N, especially for the modern cultivars. Supplemental N totaled 560 kg N ha^-1 with 40% applied at planting and 60% applied at V5. Grain yield increased with release year in MG II (17.2 kg ha^-1 yr^-1). Application of N to MG II cultivars increased seed protein by 10 to 19.5 g kg^-1 across all release years, but grain yield and seed oil was not affected. Grain yield gains of MG III cultivars fertilized with N was 27.4 kg ha^-1 yr^-1, which was 20% better than unfertilized (22.8 kg ha^-1 yr^-1). Application of N to MG III cultivars increased seed mass (11%) across release years with no changes in seed protein and oil. The nonlimiting supply of N increased seed protein across all release years in MG II cultivars, and the N supply from the soil and biological N fixation was insufficient to maximize grain yield in modern, MG III cultivars in the tested environments.