Response of Soybean to Stand Reduction at Different Growth Stages.

The cost of soybean seed has dramatically increased in the last fifteen years due to available genetic technologies and improved breeding techniques. Subsequently, the economics of these changes are causing a reduction in seeding rates. A reduction in seeding rates also changes the expectations of crop growth rates, canopy closure to aid weed suppression, and the ability to compensate yield if plant stands are reduced during the growing season. The objective of this experiment was to determine the response of soybean when plant stands were thinned by 172,900 plants ha^-1 at different growth stages. This experiment was a randomized complete block with a factorial arrangement of treatments and four replications. Factors were two soybean cultivars resistant to glyphosate (AG3803 and AG3705), three population densities (518,700; 245,800; 172,900 plants ha^-1), and five thinning timings (V3, V5, V7, V9, and R2). The experimental area was planted on 30 May 2009 and 6 May 2010. Soybean seeds were placed 2.5-cm deep in 38-cm rows at a rate of 555,750 seeds ha^-1. Plots were 9m long and 3m wide. Initial soybean stands of 518,700, 245,800, or 172,900 evenly spaced plants ha^-1 were established at the V1 growth stage by removing excess plants by hand. Thereafter, soybean stands were thinned by 172,900 plants ha^-1 or left at the initial V1 population at the later thinning timings. In 2009, there was no difference in yield with 4720 and 4780 kg ha^-1 for the 172,900 and 518,700 plants ha^-1 rates, respectively. The largest yield reduction was 11% when 172,900 plants ha^-1 were removed from 245,800 plants ha^-1 at the V9 growth stage. At the V9 growth stage, leaf area plant^-1 was similar at 890 cm² and 700 cm² for the 245,800 plants ha^-1 and 518,700 plants ha^-1 densities, respectively, however, leaf area averaged 1300 cm² plant^-1 in the 172,900 plants ha^-1 density treatments.  Individual plant growth rates are greater when plant densities are lower starting early in the season.