Interseeded Cover Crop Time of Sowing Impacts on Biomass Production, Soybean (Glycine max) Yield, and 100-Seed Weight in East-Central South Dakota.

Cover crop implementation into South Dakota soybean production systems is difficult due to short growing seasons, limited planting time, and cool and dry fall seasons.  Interseeding a cover crop into soybeans mid-season has potential to provide necessary establishment time and environmental conditions, and late-season ground cover.  A 1-year field study was conducted in order to determine effects and interactions between cover crop sowing times and biomass production on soybean yield and 100-seed weight.  There was a combination of three cover crop sowing times [second trifoliate (V2), full flower (R2), and first leaf drop (LFD)] and two interseeding techniques [drill (DRL) and broadcast (BRD)].  The cover crop mixture planted consisted of cowpea (Vigna unguiulata) and grain sorghum [Sorghum bicolor (L.) Moench ssp. bicolor] which were sown at rates of 7.6 and 1.4 kg pure live seed ha^-1, respectively.  Cover crops had emerged 4 to 13 d (days) after interseeding, with cowpea being more prevalent at emergence.  The V2 interseeded cover crop mixture produced about 93 to 99% more dry matter (DM) compared to the R2 and LFD treatments.  Furthermore, the V2 treatment reduced soybean yield and 100-seed weight by about 32 and 4% compared to the control.  The R2 and LFD treatments had no impacts on soybean yield or 100-seed weight, with exceptions to the R2 treatment which increased the 100-seed weight by 8.3% (1.6 grams) compared to the control, respectively.  These results indicate that cover crops could be established into soybeans after the R2 growth stage with no adverse impacts on soybean yield and could potentially increase seed weight.  Therefore, interseeding a cover crop mixture into soybeans may be a feasible alternative management strategy for establishing a cover crop into South Dakota soybean production systems.