Establishment and persistence of legumes in switchgrass biomass and forage/biomass production systems

Switchgrass, Panicum virgatum, is being developed as an economically and ecologically sustainable biomass crop. Nitrogen is considered one of the most limiting inputs of switchgrass. Alternatives to synthetic nitrogen fertilization may be nitrogen-fixing legumes interseeded into switchgrass. The objectives of this research were: (1) develop efficient legume management strategies for switchgrass production systems, (2) evaluate and identify cool and warm-season legumes that can be grown compatibly with switchgrass, (3) determine whether switchgrass yields are increased by legume N-fixation, and (4) determine N-fixation of common (Vicia sativa) and hairy vetch (Vicia villosa).

This study examined the establishment and persistence of ten different legume species in ‘Alamo’, a lowland variety of switchgrass in two switchgrass production systems: a one-cut biomass harvest and a two-cut forage/biomass harvest. Cool-season legumes were alfalfa (Medicago sativa), arrowleaf clover (Trifolium vesiculosum), common vetch, crown vetch (Securigera varia), red clover (Trifolium pretense), hairy vetch, and crimson clover (Trifolium incarnatum). Warm-season legumes were Illinois bundle flower (Desmanthus illinoensis), trailing wild bean (Strophostyles helvula), and partridge pea (Chamaechrista fasciculata). Red clover showed the highest plant densities and increase in switchgrass yields when interseeded into existing switchgrass stands in both harvest systems. Crude protein levels were highest in the 135 kg N ha^-1 treatment in the forage cut of the two-cut harvest system. Arrowleaf clover, crimson clover, and red clover had high stand densities with annual reseeding. A combination of cool-season legumes, crimson clover and common vetch, in combination with warm-season partridge pea, were established in existing switchgrass stands after one year.

Common vetch was evaluated for its nitrogen fixing capacity, seed germination, establishment, and effects on yield of switchgrass. Scarification by sulfuric acid had higher seed germination than other scarification treatments, except 100 grit sandpaper treatment for one minute at 0.7 kg of pressure. Common and hairy vetch nitrogen contributions were 59.3 and 43.3 kg N ha^-1 respectively at seeding rates of 6.7 kg PLS ha^-1. Switchgrass yields can increase with common and hairy vetch seeding rates of 7.6 and 10.4 kg PLS ha^-1 to achieve 67 kg N ha^-1, the recommended rate of N-fertilization for switchgrass stands.