Minimum Photosynthetically Active Solar Radiation Requirements for Optimum Soybean Yields.

The cumulative amount of photosynthetically active solar (PAR) radiation intercepted by a crop canopy is an integrated measure of the environment where the crop is grown and the production practices employed by a producer (i.e. genotype maturity, planting date, seeding density, row spacing, etc.). Previous research indicates that soybean yield response to cumulative PAR interception is curvilinear; yield increases rapidly until the crop accumulates a critical quantity of PAR, where additional PAR accumulation does not result in greater yield. The objective of this research is to establish a minimum threshold quantity of solar energy (PAR) required by a soybean crop.  Ultimately, development of a PAR threshold will be useful when making soybean management decisions.  To accomplish our objective, collaborators in Minnesota, Michigan, Iowa, Kentucky, Arkansas, and Louisiana (47.8°N to 30.5°N latitude) began a three-year study in 2009 in which accumulated PAR is measured across  six seeding densities (61,750 to 679,250 seeds ha^-1) and 2 maturity groups.   Grain yield and yield components were measured at maturity.  For 2009 and 2010, yields were generally maximized with an adapted maturity at a seeding density of 308,750 seeds ha^-1which corresponds to approximately 500 MJ m^-2 of intercepted PAR.