Seeding Rate Effects on Soybean Height, Yield, and Economic Return

The high cost of soybean [Glycine max (L.) Merr.] seed has increased the importance of determining the optimum seeding rates for maximum yield and economic return. Previous research has evaluated vegetation indices of soybean cultivars in five maturity groups (MG IV, V, VI, VII, and VIII) at six seeding rates (68,000; 136,000; 204,000; 272,000; 340,000; and 408,000 seeds ha−1) under dryland conditions on a loamy sand soil in the southeastern Coastal Plain from 2007 to 2009. This study evaluated the effects of soybean seeding rate in 96-cm rows on mature plant height, grain yield, and economic return, and determined the optimum seeding rates. Plant height increased with increasing seeding rate for MGs IV, VII, and VIII in 2 of 3 yr and for MGs V and VI in all 3 yr. For MG V in 2009, and VII and VIII in 2008, optimum seeding rates were 255,200; 228,200; and 342,500 seeds ha−1, respectively, at maximum yield, and 230,600; 179,100; and 325,900 seeds ha−1, respectively, at maximum economic return. For MG IV in 2008 and VI in 2009, economic return decreased linearly with seeding rate. Seeding rates that produced economic returns within $2.50 ha−1 of the maximum across years were 136,000; 208,600 to 252,600; 136,000; ≥ 172,300; and 308,400 to 343,200 seeds ha−1 for soybean cultivars in MG IV, V, VI, VII, and VIII, respectively. These results suggest that optimum seeding rate varies for cultivars in different MGs and could be lower than current recommendations for MGs IV, VI, and VII, but higher for MG VIII.