Assessing alternative metrics of methane output measured in a multi-breed, pasture-based sheep population

Reducing methane emissions from ruminant livestock is a global priority, yet no consensus exists on the optimal trait definition for methane emissions. This study compared absolute, ratio-based and residual methane emission metrics in order to quantify their phenotypic inter-relationships and their associations with key production traits in sheep. Gaseous emissions, including methane, and performance data were collected from 15,385 records on 8,182 sheep, including both growing animals and mature ewes, using portable accumulation chambers. Additional data available included live weight, metabolic body weight (MBW), dry matter intake (DMI), slaughter data (carcass weight and days to slaughter), and computed tomography measurements such as rumen volume, predicted kill-out percentage and kg of muscle/fat mass. Statistical analysis assessed phenotypic correlations, repeatability, and animal ranking differences across methane metrics. The repeatability of methane emissions was moderate (26% in growing animals; 34% in ewes), while body weight was highly repeatable (74% in growing animals; 68% in ewes). Males emitted 0.55-1.36 g/day more than females (P < 0.01). Absolute methane emissions were strongly correlated with intensity metrics such as methane per metabolic body weight (0.83 ± 0.01 in growing animals; 0.96 ± 0.004 in ewes), while methane yield (g/kg DMI) was weakly correlated (0.28 ± 0.01 to 0.30 ± 0.01), highlighting its dependence on feed intake variability. Residual methane traits (e.g., residual methane adjusted for metabolic body weight, RMTMBW) accounted for performance-related differences and captured individual biological variation (e.g., range -16.6 to 13.4 g/day in ewes), with moderate correlations to absolute methane emissions (r = 0.54-0.68). Animals selected for low methane yield or residual traits tended to have higher DMI and daily emissions, while absolute trait selection favoured smaller, less productive animals. These findings demonstrate that methane metrics capture distinct biological processes and are not interchangeable. Trait choice must align with breeding and production objectives, whether to reduce total emissions, improve efficiency, or identify inherently low emitters.