Canopy nitrogen reserves: Impact on soybean yield and seed quality traits in northern latitudes

Soybean [Glycine max (L.) Merr.] seed produced in the upper Midwest of the United States is lower in protein concentration than soybean produced throughout much of the Corn Belt, the southern United States, and Brazil. As protein comprises on average more than 400 g kg^-1 of the soybean seed, yields in northern latitudes may be limited by seed protein accrual. Our objective was to examine the role of canopy N reserves at maximum vegetative growth (R5.5) on seed yield, and seed protein, oil, and sulfur concentrations in the upper Midwest. Six commercial cultivars with similar yield potentials and maturities were grown in 10 central Minnesota environments. Mean cultivar seed yields ranged from 3.1 to 3.2 Mg ha^-1, while mean environment yields ranged from 1.7 to 5.0 Mg ha^-1. High-yielding environments produced soybean plants with greater R5.5 canopy N reserves and total dry matter (DM) and seed with low protein and S concentration. Mean R5.5 canopy N quantities ranged from 117 to 309 kg ha^-1. These quantities were sufficient to satisfy seed N requirements which ranged from 43 to 91 kg ha^-1. While R5.5 canopy N and DM may have a positive influence on seed yields in these upper Midwestern environments, seed protein concentration effects appear to be mediated through effects on seed yield. A positive correlation between yield and seed N to S ratio noted here indicates that sulfur may be more limited than protein at high yield levels. Oil concentration was not affected by yield.