Increased Partitioning to Reproductive Organs with Early Maturities in Double Crop Soybean.

Full season soybean cultivars are often recommended for double crop soybean planted in June or early July. We hypothesized that under no water limitation, relatively early maturity groups (MG) for a region will reach reproductive stages under better environmental conditions and increase yield potential compared to later MG cultivars. Irrigated trials were conducted at two locations in Kentucky in 2017, and one location in 2016. Eight soybean cultivars from late MG 2 to 4 were planted in late June or early July at two planting densities (40 and 54 seed m-²). A partitioning coefficient to reproductive organs was estimated as the product of seed number by individual seed growth rate divided by crop growth rate. Results from 2016 indicate that MG 3 cultivars (4500 kg/ha) and the high population density (4200 kg/ha) increased yields by 15 and 7%, respectively compared with MG 2 and 4. The crop growth rate during the seed set period was similar across MG and planting density treatments. The high planting density increased yield as a result of increased seed number. MG 2 cultivars showed the highest partitioning coefficient to reproductive organs. In MG 4 cultivars, a lower final seed number combined with a smaller individual seed growth rate resulted in a partitioning coefficient that was 38% lower compared with MG 2. The effective seed-fill period increased from 32 days in MG 2 cultivars to 49 days in MG 4 cultivars. However, the increased seed-fill period in MG 4 cultivars did not sufficiently compensate for the reduced seed number in this treatment. Results from the first year confirm our hypothesis that early MGs can maximize yield potential for double crop soybean.