Context. Genotype by environment interaction or sire re-ranking between measurements of methane emission in different environments or from using different measurement protocols can affect the efficiency of selection strategies to abate methane emission. Aim. This study tested the hypothesis that measurements of methane emission from grazing sheep under field conditions, where the feed intake is unknown, are genetically correlated to measurements in a controlled environment where feed intake is known. Methods. Data on emission of methane and carbon dioxide and uptake of oxygen were measured using portable accumulation chambers from 499 animals in a controlled environment in New South Wales and 1382 animals in a grazing environment in Western Australia were analysed. Genetic linkage between both environments was provided by 140 sires with progeny in both environments. Multi-variate animal models were used to estimate genetic parameters for the three gas traits corrected for liveweight. Genetic groups were fitted in the models to account for breed differences. Genetic correlations between the field and controlled environments for the three traits were estimated using bivariate models. Key results. Animals in the controlled environment had higher methane emission compared to the animals in the field environment (37.0 ± s.d 9.3 and 35.3 ± s.d 9.4 for two protocols vs 12.9 ± s.d 5.1 and 14.6 ± s.d 4.8 mL/min for lambs and ewes (±s.d); P < 0.05) but carbon dioxide emission and oxygen uptake did not significantly differ. The heritability estimates for methane emission, carbon dioxide emission and oxygen uptake were 0.15, 0.06 and 0.11 for the controlled environment and 0.17, 0.27 and 0.35 for the field environment. The repeatability for the traits in the controlled environment ranged from 0.51 to 0.59 and from 0.24 to 0.38 in the field environment. Genetic correlations were high (0.85–0.99) but with high standard errors. Conclusion. Methane emission phenotypes measured using portable accumulation chambers in grazing sheep can be used in genetic evaluation to estimate breeding values for genetic improvement of emission related traits. The combined measurement protocol-environment did not lead to re-ranking of sires. Implication. These results suggest that both phenotypes could be used in selection for reduced methane emission in grazing sheep. However, this needs to be consolidated using a larger number of animals and sires with larger progeny groups in different environments.

Ruminant livestock contribute to atmospheric methane (CH₄) from enteric microbial fermentation of feed in the reticulo-rumen. Our research aimed to increase understanding of how digestive characteristics and rumen anatomy of the host animal contribute to variation in CH₄ emissions between individual sheep. In total, 64 ewes were used in an incomplete block experiment with four experimental test periods (blocks). Ewes were chosen to represent the diversity of phenotypic variation in CH₄ emissions: there were at least 10 offspring from each of four sires and a range of liveweights. Throughout the experiment, the ewes were fed equal parts of lucerne and oaten chaff, twice daily, at 1.5 times the maintenance requirements. Daily CH₄ emission (g/day) increased significantly (P < 0.001) with an increasing dry-matter intake (DMI) and reticulo-rumen volume (P < 0.001). Lower methane yield (g CH₄/kg DMI) was associated with shorter mean retention times of liquid (r = 0.59; P < 0.05) and particle (r = 0.63; P < 0.05) phases of the digesta in the rumen. Significant between sire variation was observed in CH₄ emissions and in rumen volume (P = 0.02), the masses of liquids (P = 0.009) and particles (P < 0.03) in the rumen and the proportion of gas in the dorsal sac of the rumen (P = 0.008). The best predictors of variation in CH₄ emissions due to the host were DMI, CO₂ emissions, rumen volume, liveweight, mean retention time of particles in the rumen, dorsal papillae density and the proportion of liquid in the contents of the rumen compartments.

Livestock produce 10% of the total CO₂-equivalent greenhouse gases in Australia, predominantly as methane from rumen fermentation. Genetic selection has the potential to reduce emissions and be adopted in Australian grazing systems. Developing a breeding objective for reduced methane emissions requires information about heritability, genetic relationships, when best to measure the trait and knowledge of the annual production of methane. Among- and within-animal variation in methane production, methane yield and associated traits were investigated, so as to determine the optimal time of measurement and the relationship between that measurement and the total production of methane. The present study measured 96 ewes for methane production, liveweight, feed intake, rumen volume and components, and volatile fatty acid (VFA) production and composition. Measurements were recorded at three ages and different physiological states, including growing (12 months), dry and pregnant (21 months) and dry (non-pregnant, non-lactating; 28 months of age). The single biggest determinant of methane production was feed intake, but there were additional effects of age, proportion of propionate to (acetate+butyrate) in rumen VFA, total VFA concentration and CO₂ flux. Rumen volume and pregnancy status also significantly affected methane production. Methane production, CO₂ flux, liveweight, feed intake and rumen volume had high repeatability (>65%), but repeatability of methane yield and VFA traits were low (<20%). There were no interactions between sire and age (or pregnancy status) for methane traits. This suggests that methane could be measured at any time in the production cycle. However, because MY is reduced during pregnancy, it might be best to measure methane traits in dry ewes (neither pregnant nor lactating).