title: The effect of pasture pregrazing herbage mass on methane emissions, ruminal fermentation, and average daily gain of grazing beef heifers.
abstract: The objective of this study was to determine the effect of pregrazing pasture herbage mass (HM) on CH4 emissions, ruminal fermentation, and ADG of grazing beef heifers at 2 stages of the grazing season. Thirty Limousin cross heifers were allocated to 1 of 2 target pregrazing HM treatments [a low HM (LHM) or high HM (HHM) treatment] for 126 d in a randomized block design experiment. Pasture herbage and heifer rumen fluid samples were collected, and enteric CH4 emissions were determined using an SF6 tracer technique during two 5-d measurement periods [MP; MP 1 (25 to 29 May) and MP 2 (6 to 10 September)]. Both DMI and GE intake (GEI) were measured during MP 2, and ADG of the heifers was measured every 14 d throughout the 126-d grazing period. Mean HM for the LHM and HHM treatments were 1,300 and 2,000 kg DM/ha, respectively, during MP 1 and 2,800 and 3,200 kg DM/ha, respectively, during MP 2. The CP concentration of the offered herbage was greater (P < 0.01) for the LHM treatment during MP 1 and tended (P < 0.1) to be greater for the LHM herbage during MP 2. No difference (P > 0.10) in the NDF concentration of the herbage was found between the HM treatments during MP 1 or 2. There was no effect (P > 0.10) of HM treatment on total CH4 emissions (g/d) for either MP [mean value across HM treatments of 121 (SED 5.4) g/d during MP 1 and 132 (8.8) g/d during MP 2], but CH4 emissions (g) per kilogram of ADG were reduced (P < 0.05) from heifers fed the LHM treatment during MP 1 and 2 [mean values for LHM and HHM of 135 and 163 (SED 9.5) g/kg, respectively, during MP 1 and corresponding values of 150 and 194 (9.9) g/kg during MP 2]. Heifers fed the LHM treatment had greater (P < 0.001) ADG throughout the grazing period [mean value across the 126-d grazing period of 0.88 (SEM 0.032) kg/d] than those fed the HHM treatment [corresponding value of 0.73 (0.034)]. For MP 2, CH4 emissions per kilogram of DMI (g CH4/kg DMI) and per megajoule of GEI (MJ CH4/MJ GEI) tended (P ≤ 0.08) to be less for heifers fed the LHM [19.3 (0.08) g/kg and 0.056 (0.0020) MJ/MJ, respectively] than for the HHM (21.1 g/kg and 0.061 MJ/MJ) treatment, and there were no differences (P > 0.10) in DMI or GEI of the heifers between the HM treatments. The results of this study suggest that offering a low pregrazing HM sward will reduce enteric CH4 emissions relative to ADG throughout the grazing season because of increased ADG.
authors.affiliation: School of Agriculture and Food Science, College of Agriculture, Food Science and Veterinary Medicine, University College Dublin Lyons Research Farm, Newcastle, Co. Dublin, Ireland. tommy.boland@ucd.ie
authors.forename: T M
authors.initials: TM
authors.lastname: Boland
authors.forename: C
authors.initials: C
authors.lastname: Quinlan
authors.forename: K M
authors.initials: KM
authors.lastname: Pierce
authors.forename: M B
authors.initials: MB
authors.lastname: Lynch
authors.forename: D A
authors.initials: DA
authors.lastname: Kenny
authors.forename: A K
authors.initials: AK
authors.lastname: Kelly
authors.forename: P J
authors.initials: PJ
authors.lastname: Purcell
mesh_terms: D000821:Animal Feed / Q000032:analysis; D000822:Animal Husbandry; D000824:Animal Nutritional Physiological Phenomena; D000818:Animals; D002417:Cattle / Q000502:physiology*; D004032:Diet / Q000662:veterinary; D005260:Female; D005285:Fermentation; D008129:Lolium / Q000502:physiology*; D008697:Methane / Q000737:chemistry / Q000378:metabolism*; D012417:Rumen / Q000502:physiology*; D015430:Weight Gain / Q000502:physiology*
chemical_list: D008697:Methane