Impact of pasture type on methane production by lactating beef cows

In order to determine the quantity of methane (CH4) produced by lactating b eef cows on pasture, 16 Hereford-Simmental first-calf heifers with a mean w eight of 511.2 +/- 5.8 kg were randomly selected from a larger group of cow s (n = 60) on a grazing management experiment and used to evaluate the effe cts of pasture type on ruminal CH4 production using the sulfur hexafluoride (SF6) tracer-gas technique. Pasture treatments consisted of two pasture ty pes, alfalfa-grass [78% alfalfa (Medicago sativa L.) - 22% meadow bromegras s (Bromus biebersteinii Roem and Schult.)] or 100% meadow bromegrass at eac h of two fertility levels (either unfertilized or soil test recommended fer tilizer levels) with two replications of each pasture treatment (8 pastures ). Cows were managed using a "put and take" stocking system to leave equal residual herbage mass on all treatments following the grazing period in eac h paddock. During the 69-d grazing season, two cows from each pasture were sampled to determine daily forage intake and CH4 production on four occasio ns. The chemical composition of diets differed between pasture types and sa mpling periods. Dry matter intake was greater for cows grazing alfalfa-gras s pastures than for cows grazing grass-only pastures (11.4 vs. 9.7 kg DM d( -1); P < 0.018). However, methane production was greater for cows grazing a lfalfa-grass pastures than for cows grazing grass-only pastures (373.8 vs. 411.0 L CH4 d(-1); P < 0.008). Consequently, energy lost through eructation of CH4 was less for cows grazing alfalfa-grass pastures than it was for co ws grazing grass-only pastures (7.1 vs. 9.5% of GEI; P < 0.001).