Mid‐South Soybean Yield and Net Return as Affected by Plant Population and Row Spacing

Abstract

Traditionally grown soybean [Glycine max (L.) Merr.] Maturity Groups V and VI are subject to late‐season drought in the US Mid‐South, resulting in yield reductions when planted in mid‐May. Earlier maturing soybean, such as the more recently adapted Maturity Group III cultivars, have generated interest among farmers as a way to avoid the effects of late‐season drought. We investigated economically optimal plant population density for soybean considering seeding rate, row spacing, seed and soybean prices, and weather for Maturity Groups V, IV, and III grown on the rainfed soils in the rolling uplands region of the US Mid‐South. Three separate experiments were conducted for Maturity Groups V, IV, and III in 2005 through 2007. Soybean was planted in mid‐May in narrow (38‐cm) and wide (76‐cm) rows at seeding rates of 60 to 593 (× 103) seeds ha−1. Three key implications resulted from this study. First, yield did not respond to plant population density, and an extended yield plateau was found in each maturity group experiment, implying an economic incentive to reduce seed costs to achieve the plant population density at the minimum point on the yield plateau. Second, yield and net‐return responses to row spacing were inconsistent and primarily influenced by weather, but if Maturity Group III soybean were planted, narrow row spacing may provide higher expected yields and net returns. Third, planting Maturity Group III soybean may not overcome the susceptibility of Maturity Groups V and IV soybean to the late‐season drought conditions in the rolling uplands region of the Mid‐South.