Breeding ruminants for low methane (CH4) emissions is an attractive mitigation approach for this greenhouse gas. During the last three years research has progressed at AgResearch to accurately identify individual sheep with contrasting low (Low) and high (High) methane yields. A study was conducted using these animal resources in order to explore the animal factors responsible for the observed differences between rankings. For this, the sheep were fed first on fresh pasture forage (pasture) and then on pellets containing lucerne hay and wheat grain (pellet). Methane emissions were measured in respiration chambers over two consecutive days. Feed dry matter digestibility (DMD) was estimated from a total faecal collection, whereas ruminal retention times of feed particles (PRRT) and solutes (SRRT) were estimated by single dosing of external markers. Methane yields from the High sheep were higher (P >0.05) than from their Low counterparts both the pasture and pellet diets. High emission ranking sheep had higher (P <0.05) DMD and SRRT than their Low counterparts, whereas PRRT was only numerically higher for the High sheep than for the Low sheep. Results of this study align with previous findings that retention time of digesta is implicated in animal differences in CH4 emission.

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