Switchgrass Yield and Stand Dynamics from Legume Intercropping Based on Seeding Rate and Harvest Management.

Intercropping legumes may reduce inputs and enhance sustainability of forage and feedstock production, especially on marginal soils. This approach is largely untested for switchgrass (Panicum virgatum L.) production, yet producer acceptance should be high given the traditional use of legumes in forage/agricultural systems. Our objectives were to evaluate three cool-season and two warm-season legumes, and their required densities to influence yield and supply nitrogen (N) compared to three inorganic-N levels (0, 33, and 66 kg N ha^-1; 0, 30, and 60  lbs ac^-1) at three locations in Tennessee (Knoxville [Sequatchie Silt Loam], Crossville [Lilly Loam]; and, Milan [Loring B2 Series]). Fall 2010 seeded, cool-season legumes (red clover [Trifolium pretense L.], hairy vetch [Vicia sativa L.], ladino clover [Trifolium repens L.]) and spring 2011 seeded, warm-season legumes (partridge pea [Chamaecrista fasciculate L.], and arrowleaf clover [Trifolium vesiculosum L.]) were interseeded into switchgrass at three (high, medium, and low) seeding rates each in two experiments. Harvest treatments were annual single, post-dormancy biofuel (Experiment One) or integrated forage-biofuel (pre-anthesis and post-dormancy; Experiment Two). Year one yield impacts were minimal. During the second harvest year, legumes increased yield versus Yr-1; in general, yields for 33 and 67 kg N ha^-1did not differ from those for red clover, hairy vetch, ladino clover, or partridge pea (P<0.05). Arrowleaf clover yields were not different than 0 kg N ha^-1. Forage biomass yields were generally more responsive to legumes (P<0.05) than the biomass regime. Legume persistence after 3-yrs was generally greatest for ladino clover and partridge pea. Forage quality (switchgrass only) in some cases was positively influenced by legume treatments, notably hairy vetch and partridge pea (P<0.05). Intercropping selected legumes in switchgrass may enhance forage quality and yield while reducing non-renewable inputs, fertilizer costs, and emissions/runoff to air and groundwater.