Nitrogen Equivalent of Legume Species in the Production of Sand Bluestem Forage.

Over-seeding legumes into grasses can provide a sustainable nitrogen source for plant growth; but, many environmental factors may limit their effectiveness to consistently provide a benefit.  An experiment was conducted in northwest Oklahoma to determine the nitrogen equivalents provided by five legume species inter-seeded into a 4-yr-old stand of sand bluestem (Andropogon hallii) as a split-plot experiment with five replications.  The whole-plot treatments were legume species or nitrogen fertilization. Except for alfalfa (Medicago sativa), legume were sown annually in September and urea was applied in April.  The split-plot treatments were 0, 1X, 2X, and 3X rates of seeds per 30 cm of drill row and nitrogen fertilization rates (kg ha^-1) of urea.  For hairy vetch (Vicia villosa) and winter pea (Lathyrus hirsutus), X=2; for alfalfa, crimson (Trifolium incarnatum) and red (T. pratense) clover, X=10; and for urea, X=40. Sand bluestem was harvested in July (2007-2009). A regression equation was used to estimate nitrogen equivalents for legume species and seeding rates. In 2009, the sand bluestem yield prediction equation for urea was: yield = 3009.1 + 44.95x (r ²=0.75), where x is the nitrogen fertilization rate.  Alfalfa and hairy vetch provided nitrogen equivalents to sand bluestem forage production similar to that of a nitrogen fertilization rate of 40 kg ha^-1.  Crimson clover, red clover, and winter pea did not provide a nitrogen benefit and no differences (P>0.05) were found among seeding rates.  In conclusion, alfalfa- and hairy vetch-Rhizobium associations can provide a source of nitrogen for the production of sand bluestem forage and in a forage production system hairy vetch could provide additional forage; in that, it was the only legume species to provide harvestable forage (a single May harvest of 2,650 kg ha^-1).