Effect of Maturity Group and Seed Rate on Double-Crop Soybean Grain Yield in the Ohio River Valley Region.

When raising double-crop soybean in the Midwestern U.S., timely wheat harvest (i.e., planting date of soybean) and appropriate soybean maturity selection are some of the most important decisions a grower can make. The objective of this research is to determine the agronomic and economic optimum seeding rates of early, medium, and full season maturing soybean varieties across the Ohio River Valley Region for double-crop soybean. Six field trials were established in 2016 in Indiana, Illinois, Kentucky, Ohio, and Missouri using representative varieties of ‘early’, ‘medium’, and ‘full’ maturity groups at five seed rates from 123,000 to 617,000 seeds ha^-1 on increments of 123,000 seeds ha^-1. The 15 treatments (3 maturity groups x 5 seed rates) were arranged in a RCBD with a minimum of five replications. Grain yield was regressed against seed rate, early season plant population, or harvest plant population, depending on the data obtained from each location. SAS was used to fit linear, quadratic, and quadratic plateau models to determine the best fitting model, as well as the agronomic optimum seeding rate for each maturity group at each location. Maximum partial economic returns for the early, medium, and full maturities ranged from $1,274; $1,470; and $1,433 ha^-1 in Indiana to $1,083; $1,129; and $1,147 in Missouri. Averaged across locations, populations of 353,000; 308,000; and 248,000 plants ha^-1 of the early, medium, and full season maturities achieved maximum percent relative grain yield. These results suggest that higher populations may be required for earlier maturing varieties to produce equivalent yields to full season cultivars, thus reducing overall economic returns. This study is being repeated in 2017.