YIELD AND BOTANICAL COMPOSITION OF LEGUME-INTERSEEDED VS NITROGEN-FERTILIZED SWITCHGRASS

Switchgrass (Panicum virgatum L.) has a relatively high N requirement for high yields of quality forage. It is not clear what role Legumes c an play in supplying this N and in improving herbage yield when grown in association with switchgrass. To evaluate cool-season legume renova tion 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 loa m (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 (Y1), but produced 9% greater yields than 0-N grass in July. N fertilization increased upper-canopy grass yield ZA-fold compared with 0 N and legume renovation during Y1 . During the second year (Y2) of 1991 seedings, all legume treatments except crownvetch (Coronilla varia L.) produced more total-season uppe r-canopy yield than grass fertilized with 240 kg N. For 1992 seedings, birdsfoot trefoil (Lotus corniculatus L.), Mammoth red clover (Trifol ium pratense L.), and trefoil-red clover mixture had Y2 yields that eq ualed or exceeded yield for 240 kg N. Mean legume composition of Y2 up per canopy for June, July, and August was 84, 70, and 51%, respectivel y, in 1991 seedings and 53, 28, and 27% in weather-damaged stands of 1 992 seedings. Y2 yields for interseeded legumes provided significant i mprovement over 120 or 240 kg ha(-1) N, so cool-season legumes can sub stitute for N fertilization after the seeding year. Adequate defoliati on in early June is important to minimize legume competition to establ ished switchgrass. Livestock producers should renovate only a portion of switchgrass pastures in a single year, because of a shortfall in fo rage supply during legume establishment compared with that of N-fertil ized grass.