Response of complex grass-legume mixtures to nitrogen fertilization under cattle grazing

Complex grass-legume mixtures have been studied in recent years to identify the best species for high dry matter (DM) yield in forage systems for grazing livestock but little is known of their N requirements. Our objective was to evaluate the effect of N fertilization on the DM yield of complex grass-legume pasture mixtures. Two grass mixtures (GM1: timothy, meadow fescue, reed canary grass, Kentucky bluegrass, and GM2: tall fescue, meadow bromegrass, reed canarygrass, and Kentucky bluegrass) were seeded in 2013 with alfalfa (cv. CRS-1001) or birdsfoot trefoil (cv. AC Langille) at New-Liskeard (ON). These mixtures were evaluated under three N rates (0, 60, and 120 kg N ha-1 year-1) with half of the N applied in the spring and the second half applied after the second grazing cycle. Nitrogen fertilization did not significantly affect DM yield (3.0 to 3.9 t ha-1) in the first post-seeding year but it significantly increased DM yield by nearly 1 t ha-1 in the second post-seeding year. The contribution of the two legume species to DM yield ranged from 18 to 49 % in the first post-seeding year and from 1 to 6% in the second post-seeding year. The DM yield of the GM1-based mixture was the same with both legume species, while the GM2-based mixture yielded more with birdsfoot trefoil than with alfalfa in first post-seeding year (4.0 vs 3.1 t ha-1) and in second post seeding year (10.5 vs 10.0 t ha-1). In the second post-seeding year, tall fescue contributed more than 65% of the DM yield of the GM2-based mixture, while timothy and meadow fescue contributed more than 42% of the DM yield of the GM1-based mixture. Our results demonstrate the importance of legume species for the N nutrition of grass-legume pasture mixtures.