Seeding Rates for Full-Season and Double-Crop Soybean.

Soybean [Glycine max (L.) Merr.] seed cost has been increased dramatically over the last two decades from $27 ha^-1 in 1996 to $150 ha^-1 in 2015 due to the introduction of glyphosate resistant cultivars. Determination of site-specific economically-optimum seeding rate continues to be a major agronomic objective. We evaluated full-season and double-crop soybean yield responses to multiple seeding rates across seven locations in Virginia during 2003 to 2011 and determined the optimum seeding rates under three different yield conditions i.e. low (1350 to 2700 kg ha^-1), medium (2701 to 3700 kg ha^-1), and high (3701 to 4700 kg ha^-1). In general, soybean yield was a positive linear-plateau or quadratic function of seeding rate. Seeding rate explained 41-67% of the variability of relative yield for full-season soybean, 43-58% for double-crop soybean followed by wheat (Triticum aestivum L.), and 30-69% for double-crop soybean followed by barley (Hordeum vulgare L.). For full-season soybean, the optimum seeding rate ranged from 250,000 to 350,000 seeds ha^-1 for low yielding soils, 270,000 to 280,000 seeds ha^-1 for medium yielding soils, and 240,000 to 270,000 seeds ha^-1 for high yielding soils. For double-crop soybean planted in early-June following barley harvest, the optimum seeding rates were 220,000 to 360,000 seeds ha^-1, 227,000 to 353,000 seeds ha^-1, and 330,000 to 395,000 seeds ha^-1 for low, medium, and high yielding soils, respectively. Double-crop soybean planted in late-June to early-July following wheat harvest required 540,000 to 556,000 seeds ha^-1 to maximize yield on low yielding soils and 495,000 to 550,000 seeds ha^-1 on medium yielding soils; there was no data for high yielding soils. Late-planted soybean requires more seeds to produce desired leaf area and maximize yield due to a shorter growing season. Results suggest that optimum soybean seeding rates may depend on yield potential, offering potential for variable-rate seeding.