Higher Soybean Planting Rates Increase Weed Suppression, Yields, and Profitability in Cover Crop-Based, Organic, No-till Soybean.

Organic soybean (Glycine max) that is no-till planted into rolled-crimped winter cereal cover crops has been shown to perform reasonably well. Previous research in Pennsylvania and Maryland on increasing cereal rye mulch and soybean density showed a synergistic interaction in weed suppression between these management tactics. However, mulch was hand applied in this research and the economic impact of increasing soybean planting rates was not assessed. We expanded on this research and conducted farm-scale field experiments in 2014 in central and eastern New York to assess how soybean planting rate affects weed suppression, crop yield, and profitability in the cover crop-based, organic, rotational no-till system. Cover crop mixtures of cereal rye (Secale cereale) and triticale (x Triticosecale) were terminated with a roller-crimper in early-June, and organic clear hilum soybean ‘IA 2053’ (2.1 relative maturity) was no-till planted in 72-cm rows immediately after rolling-crimping. Soybean was no-till planted at five rates (198,000; 420,000; 618,000; 778,000; and 914,000 seed ha^-1), and we quantified weed suppression, soybean yield, and profitability. Canopy closure was attained earlier and average weed biomass decreased from more than 1,300 to less than 300 kg ha^-1 (P < 0.05) as planting rate increased across both sites. Soybean yield ranged from 1,823 kg ha^-1 at the 198,000 seed ha^-1 planting rate to 3,427 kg ha^-1 at the 778,000 seed ha^-1 rate (P < 0.01) across both sites. Despite being approximately double the recommended planting rate of conventional soybean, the 778,000 seed ha^-1 rate was the most profitable of the five rates when soybean yield and organic market value were taken into account. In contrast to the recent trend of using lower planting rates to reduce seed costs in conventional soybean production, our results indicate that farmers should consider high planting rates for organic soybeans that are no-till planted into rolled-crimped winter cereal cover crops.