Interseeded Cover Crop Species and Timing on Biomass Production and Soybean (Glycine max) Yield in Eastern South Dakota.

Interseeded Cover Crop Species and Timing on Biomass Production and Soybean (Glycine max) Yield in Eastern South Dakota S.A. Clay, D.E. Clay, G.W. Reicks, and J. Chang Integrating cover crops into South Dakota soybean production system after harvest poses many challenges, due to cold dry autumn conditions that lead to poor or no seed germination and, if emerged, limited time for growth. Interseeding cover crops into a wide-row soybean crop may be an alternative to either surface broadcasting just prior to leaf drop or drilling after soybean harvest. A study was performed near Beresford and Aurora in the SE and East-Central parts of the state, respectively. In mid-May, soybeans with a 2.9 and 2.0 maturity group rating were planted in 76 cm rows at 65,000 seeds ha-1 near Beresford and Brookings, respectively. The warm season mix included cowpea (Vigna unguiculata) and grain sorghum (Sorghum bicolor) at 37.8 kg ha-1 and 4.1 kg ha-1, respectively. The cool season mix included forage radish (Raphanus sativus) and crimson clover (Trifolium incarnatum) at 4.7 and 16.5 kg ha-1, respectively. Warm and cool season seeds were either mixed and planted either in a single furrow half-way between two soybean rows at approximately 1.3 cm deep with a hand-push drill or kept separate and planted in two furrows, each spaced approximately 19 cm apart. Seeds were planted at R1 (July 10, 50% canopy closure), R2 (July 24, canopy closed), and R7 (September 12, just prior to leaf drop). At R7, seeds of the entire mix, or divided into warm and cool season plants was also broadcast applied. Cover crop biomass samples were harvested just before soybean harvest (October 13) and soybean was harvested for grain yield. When averaged over both sites, seeding time influenced soybean yield (p=0.01) but species composition did not. Soybean yield increased by 200 kg ha-1 (6% increase) when cover crops were planted at R1 compared to other treatments, which were not significantly different. Delaying planting from R1 to R2 resulted in a total reduction of cover crop biomass by 50% (67.5 vs 33.5 kg ha-1; p =0.01). Cowpea had the most biomass, comprising 79 and 60% of total biomass at R1 and R2, respectively. Cover crop seeding at R7 reduced biomass production to only 11.0 and 1.3 kg ha-1 for drilled and broadcast treatments, respectively. These small amounts of biomass from the R7 planting were attributed to cowpea, which did not grow when planted this late in the South Dakota growing season. Unlike cowpea, forage radish and crimson clover did not follow any biomass production trends. For example, maximum forage radish biomass averaged 13.2 kg ha-1 when drilled at R2 near Aurora and at R7 near Beresford. Maximum crimson clover biomass was 14.7 kg ha-1 when seeded at R1 near Aurora. Even the maximum biomass produced by a single cool season species in 2014 may not be great enough to consider for field-wide planting. Planting these species with other cool season crops may increase total biomass. Care must be taken that the species have a low growth habit, or be killed with frost before soybean harvest, so that these species do not interfere with the low cutting of soybean plants at harvest.