Soybean seed yield response to planting date and seeding rate in the upper Midwest
Planting date and seeding rate are agronomic decisions that producers can use to maximize soybean [Glycine max (L) Merr.] seed yield and economic return. Current information on the response to planting date and seeding rate may underestimate the yield penalty for delayed planting in northern climates and overestimate the seeding rate required to optimize yield. The objective was to determine the effect of planting date and seeding rate on soybean seed yield at environments with different yield potentials in Iowa. Field experiments were conducted in six locations between 2003 and 2006 for a total of 13 environments. Four seeding rates were planted at four dates between late April and the middle of June. No yield difference existed between the late April and early May planting dates and the yield decline rate between each date was consistent among the six locations. Harvest plant populations were not influenced by planting date indicating that plant establishment does not pose a limitation to late April planting. Harvest plant populations required to reach 95% of the maximum yield were as great as 290,800 or as low as 194,000 plants ha–1, for locations seeded in 38-cm row spacing and 157,300 to 211,800 plants ha–1 for locations seeded in 76-cm row spacing. Planting in late April or early May increased economic return ha–1 but there was no difference between seeding rates of 185,300 and 556,000 seeds ha–1, accounting for additional seed costs. In the cool spring climate of the upper Midwest soybean producers can increase yield by planting soybean 1 to 2 wk earlier than current planting dates. These data also suggest that the optimal seeding rate for most locations is less than current seeding rates.