Integrating Legumes as Cover Crops and Intercrops into Grain Sorghum Production Systems

Core Ideas Intercropping cowpea with grain sorghum dramatically decreased sorghum grain yield. Clover green manure only increased sorghum grain yield 1 out of 3 yr. Fertilizer effects likely masked by mineralized organic matter from plowing of perennial pasture. Crop residue decomposition was mostly unaffected by treatments. Crop residue decomposition was largely driven by residue C/N ratios.Legumes fix N, reduce fertilizer inputs for primary crops, and contribute C to soil organic matter (SOM), which benefits soil productivity and sustainability. Objectives of this study were to (i) maintain grain sorghum [ Sorghum bicolor (L.) Moench] yields while decreasing N fertilizer requirements and build SOM using a temperate legume cover crop and tropical legume intercrop; (ii) quantify soil water usage between legume treatments and N fertilizer rates; and (iii) evaluate treatment effects on residue decomposition of each crop. In Overton, TX, crimson clover ( Trifolium incarnatum L.) and cowpea [ Vigna unguiculata L. (Walp)] were used as green manure and an intercrop, respectively, with grain sorghum. Four N fertilizer rates of 0, 45, 90, and 135 kg ha −1 were randomly assigned within a four replicate split‐split plot design on a Lilbert loamy fine sand. From 2010 through 2012, intercropped cowpea decreased sorghum grain yield between 77 and 93% compared to monocropped sorghum. Clover green manure increased sorghum grain yield 21% over winter fallow in 2012, but not significantly ( P = 0.22). After 3 yr, soil organic C (12.4 g kg −1 ) and soil N (1.1 g kg −1 ) were 18% ( P = 0.01) and 21% ( P = 0.01) higher, respectively, for clover green manure compared to fallow at 0 to 15‐cm depth. Crop residue decomposition was relatively stable across years and only intercropping showed small increases in decomposition of grain sorghum residue. Overall, clover green manure proved to be a more sustainable choice than intercropping cowpea from both a yield and soil perspective.