Soybean response to plant population at early and late planting dates in the Mid-South

The widespread adoption of glyphosate [N-(phosphonylmethyl)-glycine]- resistant soybean [Glycine max (L.) Merr.] and the increased cost of soybean seed have generated interest in determining the minimum plant population needed for maximum yield. The objective of this study was to determine yield and economic return responses to plant population for normal and late planting dates. Cultivars with relative maturities of 2.8 to 4.9 were planted at five seeding rates (43,000 to 560,000 seeds ha^-1) in May and/or June in 38-cm rows during 2003 to 2005. The effect of plant population on both yield and economic return was explained with a variation of a Mitscherlich equation. Optimum plant population (OPP) and economically optimum plant population (EOPP) were defined as those resulting in 95% of the estimated yield or estimated economic return, respectively, at the maximum plant population. Optimum plant population ranged from 108,000 to 232,000 plants ha^-1 for May planting dates and 238,000 to 282,000 plants ha^-1 for June planting dates. Economically optimum plant populations were 7 to 33% less than OPPs. Complete canopy cover at R1 produced maximum yield in 8 of 10 comparisons. These results suggest that seeding rates below those that are currently recommended could lower seed costs without reducing yield.