Plant Population and Latitude Effects on Cumulative Light Interception and Yield In Soybean.

In recent years, soybean [Glycine max (L.) Merr.] seeding cost have increased as a result of technology traits and seed treatments marketed in conjunction with the seed.  In order to maximize producer’s profits, it is critical for farmers to plant the minimum seeding rate that results in optimal yield.  The objective of this research is to determine the optimum planting density for any given latitude in the USA.  To accomplish this objective, collaborators in six states (MN, IA, MI, AR, KY, LA) covering latitudes from 47.8°N to 30.5°N initiated a three year experiment in 2009 to evaluate seeding densities ranging from 61,750 to 679,250 seeds ha^-1.  Two varieties were planted at each location: an adapted cultivar and a cultivar approximately one maturity group earlier than adapted.  The cumulative amount of photosynthetically active solar radiation (PAR) intercepted by each plant population and cultivar combination was determined based on light interception measurements made from emergence to R6.  Biomass accumulation was measured at the R5.5 stage of growth while grain yield and yield components were measured at harvest maturity.  Yield was measured at a minimum of three locations per state and light interception measurements were made at a minimum of one location per state.  In 2009, the relationship between yield and total intercepted PAR was curvilinear in that yield increased as the amount of cumulative PAR increased up to approximately 450 MJ m^-2, where greater amounts of intercepted PAR beyond this threshold had little effect on yield.  While there was variability in the optimal seeding density across the different locations, at five locations yield was not significantly greater for seeding rates higher than 185,250 seeds ha^-1, and ten locations did not show yield increases for seeding rates greater than 308,750 seeds ha^-1.  Data from 2009 and 2010 will be presented.