Pod removal, shade, and defoliation effects on soybean yield, protein, and oil

Soybean [Glycine max (L.) Merr.] yield formation in field environments can be either source-limited or sink-limited, depending on the assimilatory capacity of the mother plant relative to the assimilate demands of the developing seeds. Our objective was to evaluate yield and seed quality relationships under source-limited and sink-limited conditions during the seed-filling period, as protein and oil comprise on average more than 600 g kg^-1 of the soybean seed. Field experiments were conducted in five central Minnesota environments to examine the effects of multiple pod removal, shade, and defoliation treatments on soybean yield, seed size, seed protein concentration, and seed oil concentration. Pod removal, shade, and defoliation treatments each resulted in decreased yield and altered protein and oil concentrations relative to the control. Averaged across all levels for each treatment, pod removal increased seed size 19% over the control, while defoliation and shade resulted in respective seed size reductions of 7.7 and 15% when compared to the control. Despite differential treatment effects on individual seed growth, pod removal and shade treatments each resulted in increased seed protein concentration and decreased seed oil concentration. In contrast, defoliation resulted in seeds with lower protein concentration and higher oil concentration than pod removal and shade. Pod removal and shade effects on seed quality differed primarily in magnitude, as the average oil concentration across all shade treatments was 10 g kg^-1 lower than the average oil concentration across all pod removal treatments. No individual treatment resulted in increased oil concentration relative to the control.