Predicting the effects of diet and longevity on lifetime methane production and intensity in dairy ewes

Methane intensity (MeI), defined as the grams of enteric methane emitted (MeP) per kilogram of milk yield (MY), is a widely used metric to assess the GHG efficiency of dairy production. Although dairy sheep produce less milk globally than cattle, they emit more methane per unit of milk, making MeI a critical metric for these systems. However, most studies on enteric MeI focus on the lactation period only, overlooking nonproductive phases like growth, gestation, and dry periods. Such short-term measures can be misleading when evaluating sustainability and efficiency and raise the need for a more comprehensive metric. This study employed a lifetime energy-allocation model to predict how 2 agroecological strategies in dairy sheep systems, reducing dietary concentrate levels in the diet and extending longevity, affect lifetime MeI in Lacaune dairy ewes. The model simulated a single average ewe across her lifespan, with outputs of MeP and MY aggregated across all physiological stages. These stage-specific outputs were then scaled to the herd level using real demographic structures to estimate average annual MeP and MY across the lifetime, as well as lifetime MeI, per ewe. Results showed that reducing dietary concentrate reduced both lifetime MY and MeP, with similar magnitudes of decline resulting in stable MeI values. In contrast, increasing longevity improved lifetime MeI because the associated gain in milk output exceeded the supplementary methane emissions of a longer life. These findings underscore the importance of assessing methane intensity at the lifetime scale. They also demonstrate that agroecological strategies can align with mitigation strategies.