Early planting dates maximize soybean yield in Kentucky

In Kentucky, the optimum planting window for soybean [Glycine max (L.) Merr.] was mid-May to mid-June for more than 30 years. This is despite recent work in the United States indicating that April to May planting dates maximize seed yield. To determine the effect of planting date on seedling emergence and harvested plant population, seed yield, and yield loss/d when soybean are planted outside the optimum planting window a maturity group (MG) II and a MG IV cultivar were planted between mid-April and early-July from 2006 to 2011. For the MG II cultivar, April planting dates required the most days to seedling emergence and had the greatest seed yield; the harvested plant population was similar across all planting dates. For the MG IV cultivar, April planting dates required the most days to seedling emergence, had the least number of plants/acre at harvest and the greatest seed yield. Average stand loss was approximately 20% for all planting dates of the MG II cultivar. For MG IV, average stand loss ranged from 15 to 35% with greatest loss occurring with the earliest planting dates. The relative yield loss/d was 0.50%, beginning on 9 May, for MG IV cultivars. These findings indicate that late-April to early-May planting dates maximize soybean seed yield in Kentucky assuming greater seeding rates are utilized to ensure at least 100,000 plants are harvested/acre.