|| Gettle, R. M., George, J. R., Blanchet, K. M., Buxton, D. R., Moore, K. J.
||Grasses need adequate N for optimum herbage yield, Legumes could be grown in mixed swards with switchgrass (Panicum virgatum L.) to provide symbiotic N and replace the need for N fertilization, extend the growing season because C-3 species begin growth earlier and continue growth later into the growing season, and increase dry matter (DM) yield, The objective of this held study on a Webster-Nicollet soil (fine-loamy, mixed, mesic, Typic Haplaquoll-Aquic Hapludoll) at Ames, IA, was to compare herbage DM yield and botanical composition for legume-renovated switchgrass with that of N-fertilized switchgrass. Five inoculated forage legumes and a legume mixture were frost-seeded into an excellent stand of established ’Cave-in-Rock’ switchgrass in mid-March of 1991 and 1992, Grass-legume DM yield was compared with N fertilization (0, 60, 120, and 240 kg ha(-1)) in the seeding year (YR 1) and the second year (YR 2). Harvested samples were separated into grass and legume components, and further divided into upper and basal canopy, Nitrogen fertilization provided greatest upper and basal canopy yields in both June and July of YR 1. By June YR 2, legume renovation increased upper-canopy yields more than for 240 N, Upper-canopy YR 2 yields of grass-legume mixtures seeded in 1991 remained greater than for 240 N in July, and were similar to 120 N in August. During a wetter and cooler than normal summer, YR 2 yields of upper-canopy grass-legume mixtures seeded in 1992 were similar to 60 and 120 N by July, and similar to both unfertilized and N-fertilized switchgrass by August, We concluded that yields of legume-renovated switchgrass were generally greater than for mid to high levels of N fertilization during YR 2. Because of relatively low herbage production during YR 1, producers should consider frost-seeding legumes into only part of existing switchgrass pastures, while fertilizing the remaining nonrenovated pasture with N to maintain high forage supply and pasture productivity.