Using Legumes to Improve Sustainability of Grain Sorghum Cropping Systems.

Legumes can contribute substantial amounts of N to a cropping system and reduce subsequent fertilizer applications to non-legume crops. By incorporating legumes into a grain sorghum (Sorghum bicolor L.) (GSO) cropping system, we hope to not only reduce N fertilizer requirements, but also increase soil organic matter and improve soil quality over time. This split-split plot study was conducted at the Texas AgriLife Research Center in Overton, TX and assigned a cool-season legume cover crop treatment (‘Dixie’ crimson clover, Trifolium incarnatum L.) (CLO) as the main split and a warm-season legume intercrop (‘Iron and Clay’ cowpea, (Vigna unguiculata L. [Walp])) (COP) as the sub-split. Four N fertilizer rates of 0, 45, 90, and 135 kg ha^-1 were randomly assigned within the four replications on a Lilbert loamy fine sand (Loamy, siliceous, semiactive, thermic Arenic Plinthic Paleudult). In the first year, CLO contributed 108 kg ha^-1 of N when incorporated, however biomass yield of GSO (6.5 Mt ha^-1) was unaffected. An interaction showed a significant increase in sole-cropped GSO ‘Pioneer 84G62’ grain yield (2007 kg ha^-1) when following a CLO cover crop and 80 kg ha^-1 N fertilizer. However, CLO did not influence soil N (0.11%) or C (1.32%) in the initial year. COP significantly reduced GSO grain and biomass yield by 76% and 65%, respectively. Dry conditions in the first year likely caused moisture competition and reduction in yield. It was evident that COP had a competitive advantage once it began to vine and grain yield benefits may be best realized when planted in rotation rather than intercropped with GSO.