Impact of pasture type on methane production by lactating beef cows

In order to determine the quantity of methane (CH4) produced by lactating beef cows on pasture, 16 Hereford-Simmental first-calf heifers with a mean weight of 511.2 +/- 5.8 kg were randomly selected from a larger group of cows (n = 60) on a grazing management experiment and used to evaluate the effects of pasture type on ruminal CH4 production using the sulfur hexafluoride (SF6) tracer-gas technique. Pasture treatments consisted of two pasture types, alfalfa-grass [78% alfalfa (Medicago sativa L.) - 22% meadow bromegrass (Bromus biebersteinii Roem and Schult.)] or 100% meadow bromegrass at each of two fertility levels (either unfertilized or soil test recommended fertilizer 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 each paddock. During the 69-d grazing season, two cows from each pasture were sampled to determine daily forage intake and CH4 production on four occasions. The chemical composition of diets differed between pasture types and sampling periods. Dry matter intake was greater for cows grazing alfalfa-grass 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 alfalfa-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 cows grazing grass-only pastures (7.1 vs. 9.5% of GEI; P < 0.001).