Effects of Legume Rotations on Grain Sorghum Growth and Grain Yield.

Legume cover crops are important components of sustainable cropping systems through reducing soil erosion, increasing soil carbon, and providing a source of nitrogen (N) to subsequent crops. Winter treatments of fallow or crimson clover (Trifolium incarnatum L.) cover crop, and previously grown cowpea (Vigna unguiculata L. Walp.), for previous 3 yr, as a summer green manure were included in a split-split plot design with N fertilizer treatments to evaluate grain sorghum (Sorghum bicolor L.) plant growth and grain yields. There were more (P=0.006) sorghum plants in winter fallowed compared to clover plots due to reduced soil moisture. Initial sorghum height was greater (P=0.005) in fallow (26 cm) vs clover cover plots (23 cm). However by mid-season and final growth status, sorghum plants were taller in clover plots (P=0.005) and N treatments (P=0.05) at 116 vs 112 cm. Main effects of winter cover, cowpea green manure, and N rate had no impact on sorghum grain yield. There was an interaction between winter cover treatment and summer green manure for sorghum grain yield (P=0.04). In plots with clover cover and incorporated in spring, the cowpea green manure treatment had no effect on grain yield that averaged 732 kg ha^-1.  However in winter fallowed plots and with cowpea incorporated in the previous fall, grain yields were higher (P=0.06) at 1350 kg ha^-1 compared to winter fallow with no cowpea, at 838 kg ha^-1.  During this study-season with below normal spring rainfall, crop rotations with cowpeas were effective in improving grain sorghum yields, but winter legume cover crops did not improve grain sorghum seed yields. Although a winter cover crop of crimson clover has significant, positive applications for sustained cropping systems, management strategies for earlier incorporation of winter legumes are directly related to success of summer grain crops.