FROST-SEEDING LEGUMES INTO ESTABLISHED SWITCHGRASS - FORAGE YIELD ANDBOTANICAL COMPOSITION OF THE STRATIFIED CANOPY

Grasses need adequate N for optimum herbage yield, Legumes could be gr own in mixed swards with switchgrass (Panicum virgatum L.) to provide symbiotic N and replace the need for N fertilization, extend the growi ng season because C-3 species begin growth earlier and continue growth later into the growing season, and increase dry matter (DM) yield, Th e objective of this held study on a Webster-Nicollet soil (fine-loamy, mixed, mesic, Typic Haplaquoll-Aquic Hapludoll) at Ames, IA, was to c ompare herbage DM yield and botanical composition for legume-renovated switchgrass with that of N-fertilized switchgrass. Five inoculated fo rage legumes and a legume mixture were frost-seeded into an excellent stand of established 'Cave-in-Rock' switchgrass in mid-March of 1991 a nd 1992, Grass-legume DM yield was compared with N fertilization (0, 6 0, 120, and 240 kg ha(-1)) in the seeding year (YR 1) and the second y ear (YR 2). Harvested samples were separated into grass and legume com ponents, and further divided into upper and basal canopy, Nitrogen fer tilization provided greatest upper and basal canopy yields in both Jun e and July of YR 1. By June YR 2, legume renovation increased upper-ca nopy yields more than for 240 N, Upper-canopy YR 2 yields of grass-leg ume mixtures seeded in 1991 remained greater than for 240 N in July, a nd were similar to 120 N in August. During a wetter and cooler than no rmal 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 unfe rtilized and N-fertilized switchgrass by August, We concluded that yie lds of legume-renovated switchgrass were generally greater than for mi d to high levels of N fertilization during YR 2. Because of relatively low herbage production during YR 1, producers should consider frost-s eeding legumes into only part of existing switchgrass pastures, while fertilizing the remaining nonrenovated pasture with N to maintain high forage supply and pasture productivity.