Adjusting seeding rate across soybean planting date and maturity in the Southeast United States

Abstract

Soybeans [Glycine max (L.) Merr.] are planted across a wide range of planting dates (PDs) (March to early August) in the Southeast United States, resulting in a wide range of growing conditions and, consequently, soybean production practices used. Current seeding rate (SR) recommendations should be revisited to reflect the range of PDs and other management practices used in the Southeast United States. Studies were conducted across 15 North Carolina environments from 2019 to 2022 to determine the agronomically optimal seeding rate (AOSR) and economically optimal seeding rate (EOSR) required for the PDs and maturity groups (MGs) used by soybean producers in the Southeast United States. Main plot treatments included PD (mid-March through mid-July), sub-plot included MGs (2–7), and sub-subplot included SR (185,329–432,434 seeds ha⁻¹). Early PDs generally resulted in lower plant populations due to environmental conditions such as cooler soil temperatures. Higher SRs resulted in higher plant populations across environments. PD, MG, and SR interacted to impact soybean yield (p = 0.02) and revenue (p = 0.02). Earlier PDs, March to April 10 (day of year [DOY] 80–100), resulted in lower yields and revenues compared to a more moderate full-season PD, April 30–May 20 (DOY 120–140), and delayed planting required higher AOSR and EOSR to maximize yield and revenue. Variations in MGs also impacted optimal SRs, with MGs (2–4) generally requiring higher AOSR and EOSR than MGs (5–8). AOSR and EOSR analyses reveal a positive correlation between SR, yield, and revenue up to a certain threshold, beyond which increasing SR does not significantly improve yield or revenue. Soybean producers should adjust their SR based on PD and MG selection.