Genetic Gain x Management Interactions in Soybean: III. Nitrogen Utilization.

Soybean yields in the U.S. increased annually at a rate of 23.5 kg ha^-1 since 1924. Improved genetics and agronomic practices contributed to this yield gain with documentation of each individually, but not collectively. As production potential of newer cultivars increases annually, biological N fixation (BNF) may become a limiting factor. We applied a non-limiting supply of nitrogen (N) fertilizer to soybean cultivars released from 1923 to 2008 in maturity group (MG) II and MG III to determine (1) the yield potential, (2) the effects on seed quality, and (3) if BNF met the N needs of newer, high yielding cultivars. A field study was conducted at WI, MN, IL, and IN in 2010 and 2011. Fifty-nine MG II cultivars were used in WI and MN, and 57 MG III cultivars were used in IL and IN. Nitrogen was supplied to the cultivars from two sources: soil N + BNF vs. soil N + fertilizer N. Fertilizer N totaled 560 kg N ha^-1 in a split application at planting and V5. Biomass accumulation increased with cultivar modernization in both MGs, and N fertilization further increased biomass production in MG III cultivars. Yield increases of MG III cultivars from N fertilization were likely linked to increased N accumulation of vegetative biomass. Yield of MG II cultivars was not affected by N fertilization, but grain protein concentration increased in MG II cultivars with N fertilization. Nitrogen fertilization increased grain N concentration of MG II cultivars; however, total N accumulation did not improve. Changes in aggressiveness of N redistribution under non-limiting N supply could explain grain quality increases in MG II cultivars.  This suggests N supplied from BNF was insufficient to realize grain protein potential in MG II cultivars and yield potential in MG III cultivars released over the past nine decades.