Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/26/2000
Publication Date: N/A
Interpretive Summary: Dairy farmers and other livestock farmers need more sustainable ways to produce meat and milk. The introduction of forage legumes in pastures is beneficial for improving forage quality and animal production, providing more equal forage yield over the growing season, and reducing the need for commercial nitrogen fertilizer. This latter characteristic is due to the unique ability of legumes to obtain nitrogen from the air by a symbiosis with bacteria known as rhizobia. Forage legumes currently used in pastures, like red and white clover and birdsfoot trefoil, are known to fix large amounts of nitrogen, but they often do not persist in northern climates over the long term. Kura clover is a relatively new forage legume species to the USA. It is very persistent and could help alleviate problems associated with other forage legume species. We made the first estimates of nitrogen fixation by this species in North America. This research showed that Kura clover had nitrogen fixation levels similar to those reported fo other commonly grown forage legumes, which is roughly equal to the amount of fertilizer nitrogen farmers apply to productive grass pastures. Because of this amount of nitrogen fixation and Kura clover's persistence, we showed that this new forage legume will help make livestock grazing systems more sustainable and productive.
Technical Abstract: Quantification of symbiotic dinitrogen fixation by legumes is essential to determine their impact on N budgets. Kura clover (Trifolium ambiguum M.B.) and birdsfoot trefoil (Lotus corniculatus L.) are two promising pasture legumes for which such estimates are scarce. Dinitrogen fixation by these species in their first and second production years was determined at two locations in Minnesota using the 15N isotope dilution method. Species were evaluated for forage and N yields, percent of N derived from the atmosphere (%Ndfa), and N2 fixed at each of four harvests. Results for each parameter varied with location, legume species, and harvest, as demonstrated by a three-way interaction among these terms. Across environments, both species produced greatest amount of forage, total N, and fixed N yields at the first harvest. Yields decreased with successive forage harvest at one location but stabilized at the other. Kura clover %Ndfa was stable across harvests and environments. Birdsfoot trefoil %Ndfa varied during the season, with a peak at the end of the season. Yearly total forage, total N, and fixed N yields of Kura clover were stable across environments, and total N and fixed N yields were greater than those of birdsfoot trefoil at one location. Overall, Kura clover fixed about 155 kg N/ha/yr with a %Ndfa of 57%; birdsfoot trefoil had amounts of N2 fixed varying with locations (ave. of 145 kg N/ha/yr), and obtained about 62% of herbage N from fixation.