Responses of Canopy Reflectance, Light Interception, and Soybean Seed Yield to Replanting Suboptimal Stands.

Soybean [Glycine max (L.) Merr.] planting date trends have steadily shifted earlier within the Northern Corn Belt while inclement weather, insect pressure, and disease pressure associated with spring planting can results in replanting some years.  However, limited published literature exists about soybean replant thresholds for sub-optimal plant stands and the effects of seed treatments on this decision.  This study evaluated three planting dates, three seed treatments, and twelve seeding rate – replant combinations to determine replant thresholds in terms of maximizing seed yield, the effects of seed treatments on these thresholds, and the relationship of cumulative intercepted photosynthetically active radiation (CIPAR) and cumulative normalized difference vegetative index (CumNDVI) on seed yield.  Trials were conducted during 2012 and 2013 in southern Wisconsin. Soybean planting in early May increased seed yield by 358 kg ha^-1 compared to late May and by 805 kg ha^-1 compared to mid-June. Seed yield reductions were due to decreased seeds m^-2 while CIPAR and CumNDVI measurements may partially explain this planting date effect.  When initial plant stands were below threshold (<247,000 plants ha^-1), filling in these stands with enough seed to bring the final plant stand above the threshold increased yield, while using tillage and replanting the entire stand only increased yield when initial plant stands were extremely low (<91,000 plants ha^-1).  Yield was related to both CIPAR (R² = 0.54) and CumNDVI (R² = 0.49) and the latter two variables were highly correlated (R² = 0.89).  We observed linear yield increases through 700 MJ m^-2 of CIPAR; therefore, management practices to increase CIPAR should be used. This study showed that early May planting with a fungicide/insecticide seed treatment (CruiserMaxx) and generating adequate plant stands (>247,000 plants ha^-1) maximized CIPAR.