Winter Annual Cover Crops Interseeded into Soybean in Eastern Virginia: Influence on Soil Nitrogen and Corn Yield.

Fertilizer nitrogen (FN) demand in corn (Zea mays L.) production is expected to increase with global population growth, and alternative sources for plant available nitrogen (PAN) will need to be optimized.  Hairy vetch (Vicia villosa Roth) utilized as a winter annual cover crop (WCC) in the United States has been shown to supply subsequent corn with substantial PAN.  Corn-soybean (Glycine max L. Merr.) rotations leave fields fallow following soybeans.  Our objective is to evaluate hairy vetch’s capacity to supply PAN to subsequent corn following soybeans in the mid-Atlantic coastal plain region of the United States.  Eight sites were seeded into standing soybeans with rye (Secale cereale L.), hairy vetch, and a mixture of the two in the fall of study years.  The experimental design was split-plot with WCC species as the main plot and ten FN rates as subplots.  Soil samples were collected at WCC termination and at corn growth stage V4 and analyzed for residual soil nitrate NO₃-N) and ammonium (NH₄-N).  Winter cover crop tissue samples were collected and analyzed at WCC termination to determine WCC N supplying capacity.  Corn grain yield were measured and response to WCC treatment and FN rate were determined.  At WCC termination average biomass dry matter (DM) yield for cereal rye, cereal rye + hairy vetch mix (RV mix), and hairy vetch was 2356, 2000, and 1864 kg ha^-1 in 2013; and 2055, 2701, and 692 kg ha^-1 2014, respectively.  Average N uptake for cereal rye was 35 and 40 kg N ha^-1 in years 2013 and 2014, respectively.  Significant differences for residual soil nitrogen were most apparent for residual soil NO₃-N during WCC termination and corn GS V4 of both years.  During WCC termination significant differences were mostly observed at lower depths (15-30 and 30-60 cm) while at corn GS V4 significant differences were mostly observed in the upper 0-15 cm, with hairy vetch treatments generally having highest residual soil NO₃-N concentrations.  Average corn grain maximum yield following hairy vetch WCC was 11.4 and 11.6 Mg ha^-1 and 10.7 and 11.1 following cereal rye WCC at zero and 45 kg ha^-1 preplant FN, respectively.  Average agronomic optimum N rates (AONR) were 26 and 9 kg ha^-1 lower following hairy vetch than cereal rye WCC at zero and 45 kg ha^-1 preplant FN, respectively.  At location BRM1, hairy vetch WCC achieved high aboveground N content of 146 kg N ha^-1 and the maximum yield of subsequent corn was 2.9 Mg ha^-1 higher than when following cereal rye.  Additionally at BRM1, corn following hairy vetch without FN yielded similarly to corn following cereal rye at AONR.  The results from this study indicate that hairy vetch interseeded into soybean has the potential to provide subsequent corn with adequate PAN to achieve yields similar as corn following cereal rye at AONR.  However, as noted in other studies, hairy vetch did not reduce AONR of subsequent corn but only increased yield potential.