Selection for increased seedling vigor might also contribute to improving Kura clover establishment (De-The quantification of symbiotic N 2 fixation by legumes is essential Haan et al., 1997). Delayed and limited nodulation to determine their impact on N budgets. Kura clover (Trifolium ambiguum M.B.) and birdsfoot trefoil (Lotus corniculatus L.) are two (Hely, 1963;Zorin et al., 1976; Patrick et al., 1994) and promising pasture legumes for which such estimates are scarce. Dini-poor N 2 fixation in the seeding year (Watson et al., 1996) trogen fixation by 2-and 3-yr-old stands of these species was determay also contribute to establishment problems. These mined at two locations in Minnesota using the 15 N isotope dilution limitations apparently do not restrict the growth of Kura method. Species were evaluated for forage and N yields, the percent clover once it is established, but there is currently no of N derived from the atmosphere (%Ndfa), and the amount of fixed information available on the rates of N 2 fixation in estab-N 2 at each of four harvests. The results for each parameter varied lished Kura clover. with location, legume species, and harvest, as demonstrated by a three-Birdsfoot trefoil is another perennial forage legume way interaction among these terms. Across environments, both speciesthat is adapted to a wide variety of soil types and proproduced the greatest amount of forage, total N, and fixed N yields duces high quality, nonbloating forage. However, it is at the first harvest. The yields decreased with successive forage harvests at one location but stabilized at the other. The %Ndfa for Kura characterized by a slow recovery rate after harvesting, clover was stable across harvests and environments. The %Ndfa for and like Kura clover, a slow and difficult establishment birdsfoot trefoil varied during the season, with a peak at the end of (Grant and Marten, 1985). Under field conditions, the the season. The yearly total forage, total N, and fixed N yields of rates of N 2 fixation in birdsfoot trefoil vary between 50 Kura clover were stable across environments, and the total N and and 205 kg ha Ϫ1 yr Ϫ1 , with a %Ndfa varying between fixed N yields were greater than those of birdsfoot trefoil at one 30 and 100 (Heichel et al., 1985; Mallarino et al., 1990; location. Overall, Kura clover fixed about 155 kg N ha Ϫ1 yr Ϫ1 , with Farnham and George, 1994). The median values are 92 a %Ndfa of 57; birdsfoot trefoil had amounts of fixed N 2 that varied kg ha Ϫ1 yr Ϫ1 of fixed N 2 and a %Ndfa of 55 (Vance, with locations (avg. of 145 kg N ha Ϫ1 yr Ϫ1 ), and it obtained about 1998). These estimates are generally lower than those 62% of its herbage N from fixation.of other temperate forage legumes. This is attributable, in part, to the fact that birdsfoot trefoil has determinate Abbreviation: %Ndfa, percent of N derived from the atmosphere. Published in Agron.