Interest in alternatives to synthetic nitrogen (N) fertilizer sources for switchgrass (Panicum virgatum L.) production, such as interseeding with N- fixing legumes continues to increase. Common vetch (Vicia sativa) is a N-fixing legume that occurs naturally throughout the U.S. and has less hard seed than hairy vetch (Vicia villosa), making it potentially less invasive, and it may fix N at similar rates to that of hairy vetch. However, little data exist to substantiate this. In this study, N-fixation rates via the N-difference method were determined to be 59.3 and 43.3 kg N ha-1 for common and hairy vetch, respectively, when seeded at 6.7 kg ha-1. At these rates, neither common nor hairy vetch significantly affected switchgrass yields. Based on the N-fixation rates and vetch plant masses, we estimate that minimum seeding rates of 7.6 and 10.4 kg PLS ha-1 of common and hairy vetch, respectively are required to obtain plant stands needed to fix the current recommended rate of N for switchgrass biomass production
