Legumes incorporated into established switchgrass (Panicum virgatum L.) could provide symbiotic N, improve herbage quality, and extend the grazing season. Almost no information exists for legume renovation of switchgrass by frost‐seeding. The objective of this study was to identify cool‐season legumes that can be established into switchgrass by frostseeding, yet not provide excessive competition to the associated grass. Six legumes [biennial ‘Polara’ white‐flowered sweetclover (Melilotus alba Medik.), biennial ‘Madrid’ yellow‐flowered sweetclover (Melilotus officinalis Lam.), ‘Norcen’ birdsfoot trefoil (Lotus corniculatus L.), ‘Redland II’ medium red clover (Trifolium pratense L.), ‘Apollo Supreme’ alfalfa (Medicago sativa L.), and a 50:50 mixture of Redland II red clover and Norcen birdsfoot trefoil] were inoculated with rhizobia and frost‐seeded into established ‘Cave‐in‐Rock’ switchgrass in mid‐March of 1991 and 1992. These mixtures were compared with switchgrass fertilized at four N levels (0, 60, 120, and 240 kg ha−1). Successful legume establishment was observed, with an average of 24 and 25% of viable seed producing seedlings by June for 1991 and 1992 seedings, respectively. Mean legume density was 160 and 170 plants m−2 by June of the seeding year (YR 1) and 95 and 55 plants m−2 by June of the second year (YR 2) for 1991 and 1992 seedings, respectively. Legume renovation of switchgrass stands did not affect grass stem density by July YR 1. Mean forage stand composition (based on stem counts) was 87, 67, and 45% legume in June, July, and August YR 2, respectively, for 1991 seedings, and 54, 29, and 23% legume during the same time periods for 1992 seedings. Legumes did not seriously reduce switchgrass stem density, although red clover, birdsfoot trefoil, and their mixture were more competitive than others. Legumes can be successfully introduced into established Cave‐in‐Rock switchgrass by frost‐seeding and they will persist for at least 2 yr with favorable weather.

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.

Little information is available for renovation of established switch‐grass (Panicum virgatum L.) with cool‐season forage legumes. Field experiments were conducted to determine seeding success, persistence, and impact on grass when 10 forage legumes and a legume mixture were no‐till interseeded into established ‘Cave‐in‐Rock’ switchgrass near Ames, IA. Legumes interseeded in early April 1991 and 1992 were biennial ‘Polara’ white‐flowered sweetclover (Melilotus alba Medik.) and ‘Madrid’ yellow‐flowered sweetclover (‘Melilotus officinalis’ Lam.), ‘Norcen’ and ‘Fergus’ birdsfoot trefoil (Lotus corniculatus L.), ‘Apollo Supreme’ and ‘Alfagraze’ alfalfa (‘Medicago saliva’ L.), Mammoth and ‘Redland II’ red clover (Trifolium pratense L.), ‘Emerald’ crownvetch (Coronilla varia L.), common hairy vetch (Vicia villosa Roth.), and a 50:50 mixture of Norcen trefoil and medium red clover. Legume renovation was compared with grass fertilized with N at 0, 60, 120, and 240 kg ha−1. Excellent legume establishment was observed in June, about 2.5 mo after interseedings, with mean legume plant density of 195 and 163 plants m−2 for 1991 and 1992 seedings, respectively. Both alfalfa cultivars and hairy vetch had highest percentage establishment. Average legume persistence to June of the second year was nearly 50% for 1991 and 30% for 1992 seedings (less in 1992 because of greater winter losses). Grass stem density during the establishment year was not affected by legume renovation. Decline in grass stems during the second year was greatest for trefoil and the trefoil‐red clover mixture, and intermediate for alfalfa and red clover, with minimal to no effect for the sweetclovers, crownvetch, and hairy vetch. Hence, legumes can be successfully established into switchgrass and can be maintained into the second year

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 C3 species begin growth earlier and continue growth later into the growing season, and increase dry matter (DM) yield. The objective of this field 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.

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