Dose-response of Asparagopsis extract and diet quality effects enteric methane emissions for sheep fed high-forage diets

Asparagopsis spp. (Asparagopsis) receives considerable attention, due to the antimethanogenic properties it exhibits when included in diets of ruminant livestock. However, only one previous study has investigated the use of Asparagopsis in sheep in vivo, offered as part of a modified total mixed ration, reporting up to 80 % enteric methane (CH₄) abatement at an inclusion of 3 % (organic-matter basis). The objective of this study was to investigate the interaction of bromoform (CHBr₃) dose and basal diet quality on CH₄ abatement in sheep, when fed a high forage basal diet with supplementary pellets containing CHBr₃ extracted from Asparagopsis armata in oil (Asp-oil). Sixty merino wethers were used in a split plot design of five supplementary pellet treatments (100 g/d) of increasing CHBr₃ concentrations (A0, 0 mg CHBr₃/kg DM; A1, 36 mg CHBr₃/kg DM; A2, 72 mg CHBr₃/kg DM; A3, 108 mg CHBr₃/kg DM; and A4, 144 mg CHBr₃/kg DM) and fed either a high-quality (neutral detergent fibre (NDF) 40 %) or low-quality (NDF 55 %) forage basal diet. Wethers were housed in individual pens for 117 days to allow for measurements of feed intake and production parameters. Five measurements of CH₄ production (MP, g CH₄/d), 14 days apart, were conducted with open circuit respiration chambers. Wool growth (g/d), intake of basal diet and supplementary pellets (kg DMI/d), and average daily gain (kg/d) were production parameters. There was no significant interaction between diet × Asp-oil level on CH₄ yield (MY, g CH₄/kg DMI), but independently each had a significant effect. In comparison to their respective control (A0), A4 resulted in a 12 % and 19 % MY abatement for high-quality and low-quality diets, respectively. Average daily gain was significantly affected by diet (P < 0.001) but not Asp-oil inclusion. Wool growth rate was affected by diet (P = 0.05) only. This study has demonstrated that feeding sheep Asp-oil can reduced MP, regardless of diet quality. However, basal diet continues to influence MY. Future grazing trials are necessary to validate the results of this animal house experiment.