International approach to reduce greenhouse gas emissions from sheep

Reduction of greenhouse gas (GHG) emissions, and particularly enteric methane (EM) emissions, from ruminant livestock is a global problem faced by all producers. In 2015 there were 1.18 billion sheep in the world emitting 158 Mt of carbon dioxide (CO₂) equivalents of EM, or 6% of the total EM emissions (FAO 2022). Animal breeding is a highly cost-effective strategy to achieve reductions in EM and has already been included in national plans and Nationally Determined Contributions (NDCS) to achieve mitigation targets in several countries.

An international project (‘Grass To Gas’, 2019-2023) combined expertise and generated new knowledge towards the reduction of methane (CH₄ ), a potent GHG, from sheep. Proxy measurements were investigated including individual animal feed intake and N-alkane measures indoors and at pasture, rumen microbiome-generated data, nuclear magnetic resonance (NMR) spectroscopy with plasma and rumen samples, and rumen volume by Computer-Tomography (CT), amongst others, in native, improved and unimproved sheep breeds and with different feed and forage qualities. Direct measurements of CH₄, and CO₂ from ewes and lambs were compared using portable accumulation chambers (PACs), sheep and Greenfeed to enable animal ranking and first breeding values for GHG emissions for sheep. Heritabilities (with s.e.) for PAC raw CH₄ (g/day) were reported as being between 0.26(0.03) and 0.34(0.09) from 4 countries, with methane intensity CH₄ / (CH₄+CO₂) moles/day having lower values [0.21(0.03) to 0.29(0.06)]. Estimates for residual feed intake (RFI) were higher at between 0.37(0.08) to 0.45(0.08). As expected, genetic correlations between RFI and feed intake were moderate to strong (0.41(0.14) to 0.79(0.09), and low or close to zero with body weight or growth. Comparing low vs high RFI progeny of Romane sires showed that after 4 generations, the dry matter intake difference between RFI lines reached 123 g/day of concentrate and 80 g/day of forage in favour of the efficient line. A larger difference of 20% in feed intake was reported when comparing Australian Merino lambs contrasting in RFI.

The links between RFI, feed intake and CH₄ emissions are more complex and require further investigation. However, high CH₄ was associated with higher levels of feed intake with preliminary genetic correlations reported as being between 0.33(0.17) and 0.43(0.19) and those between CH₄ and metabolic body weight being 0.58(0.15) to 0.68(0.11). The use of routine CT scans enabled retrospective computation of rumen volume which has been shown to be moderately heritable. The genetic correlation between rumen volume and CH₄ is yet to be determined although larger reticulo-rumen volumes, as measured by CT scanning, were associated with increased methane emissions but not with RFI at the phenotypic level.