Soybean Cultivar Response to Planting Date and Seeding Rate under Organic Management

Delayed planting is a common strategy for enhancing weed control in organic soybean [Glycine max (L.) Merr.] production, but it has the potential to reduce yield. Increasing soybean seeding rate may be a way to enhance weed control with early planting, and to mitigate potential yield losses due to delayed planting. From 2006 to 2008, experiments were conducted in southern and central Minnesota in organically managed systems with tillage for weed control to determine agronomic responses of soybean and weed density to planting date and seeding rate for feed-and food-grade soybean cultivars. Increasing seeding rate from 395,400 to 543,600 seeds ha(-1) did not enhance weed control or soybean yield, and this was consistent across planting dates and cultivars. Total weed density at harvest and soybean yield were greatest when planting occurred in mid-or late May, and were reduced by 51 and 21% when planting was delayed until mid-June, respectively. Yield of feed-grade cultivars was 21% higher than food-grade cultivars, which was consistent across planting dates. Total weed density at harvest differed among cultivars, and was lowest with two of the three highest-yielding cultivars. These results demonstrate that in organic cropping systems with tillage for weed control in the Upper Midwest, planting in mid-to late May rather than mid-June is critical for maximizing yield, while high seeding rates are not likely to improve yield or weed control.