Double-Crop Soybean Growth and Yield Response to Starter Nitrogen and Seed-Applied Inoculant.

Double-crop soybean typically exhibit reduced vegetative growth and decreased leaf area, which negatively influences seed production.  Increasing early-season growth may improve photosynthetic efficiency, while conserving soil moisture through canopy closure.  Field experiments were conducted in eastern Virginia in 2012 and 2013 to evaluate seed-applied inoculant combined with starter nitrogen (N) applied at planting on soybean vegetative response, total N uptake (TNU), seed yield and quality, and yield components produced in a wheat-soybean double-crop system.  Measurements included plant height, TNU, normalized difference vegetative index (NDVI), seed yield and quality, and yield components.  Canopy NDVI during early-pod fill was greater with starter N application.  Node production was maximized at 47 kg N ha^-1.  Starter N increased seed yield by 6 kg ha^-1 per kg N applied until yield plateaued from 16 to 31 kg N ha^-1 and decreased thereafterward.  Yield increased by 5% from 4055 to 4257 kg ha^-1 with 16 kg N ha^-1 and increased net profit by $67 ha^-1, compared to 0 N.  Total dry matter, pods m^-2 and seed weight m^-2 were greater with 16 kg N ha^-1 and comparable to 63 kg N ha^-1.  Seed size had a linear response to increasing N rates.  Although inoculant did not increase growth or yield, it increased 100 seed weight and provided a return on investment of $8 ha^-1.  These data demonstrate that starter N at low rates may increase double-crop soybean yield, which can be partially attributed to increased biomass, nodes, pods, and seed size.