Switchgrass (Panicum virgatum L.) has a relatively high N requirement for high yields of quality forage. It is not clear what role legumes can play in supplying this N and in improving herbage yield when grown in association with Switchgrass. To evaluate cool-season legume renovation vs. N fertilization of established Switchgrass, 10 forage legumes and a legume mixture were compared with 0, 60, 120, and 240 kg N ha−1. Forage yield and botanical composition of basal (<20 cm) and upper (>20 cm) canopy were compared at Ames, IA, on a Webster silty clay loam (mesic Typic Haplaquoll). Legumes were no-till interseeded in early April; N was applied before mid-May. Legume renovation did not affect June yield during the establishment year (Yl), but produced 9% greater yields than 0-N grass in July. N fertilization increased uppercanopy grass yield 2.4-fold compared with 0 N and legume renovation during Yl. During the second year (Y2) of 1991 seedings, all legume treatments except crownvetch (Coronilla varia L.) produced more totalseason upper-canopy yield than grass fertilized with 240 kg N. For 1992 seedings, birdsfoot trefoil (Lotus corniculatus L.), Mammoth red clover (Trifolium pratense L.), and trefoil-red clover mixture had Y2 yields that equaled or exceeded yield for 240 kg N. Mean legume composition of Y2 upper canopy for June, July, and August was 84, 70, and 51%, respectively, in 1991 seedings and 53,28, and 27% in weather-damaged stands of 1992 seedings. Y2 yields for interseeded legumes provided significant improvement over 120 or 240 kg ha−1 N, so cool-season legumes can substitute for N fertilization after the seeding year. Adequate defoliation in early June is important to minimize legume competition to established Switchgrass. Livestock producers should renovate only a portion of Switchgrass pastures in a single year, because of a shortfall in forage supply during legume establishment compared with that of N-fertilized grass.
Joint contribution of Iowa State Univ. and the U.S. Dairy Forage Res. Ctr., USDA-ARS. Journal Paper no. J-16100 of the Iowa Agric. and Home Econ. Exp. Stn., Ames, IA; Project no. 2899. Supported in part by the Leopold Center for Sustainable Agriculture.