Enteric methane emissions and mitigation strategies within pasture-based dairy systems.

Enteric methane is the dominant GHG emitted from pasture-based dairy systems, defined for this review as dairy systems in which grazed pasture makes up the majority of feed intake. This review summarizes the main drivers of methane emissions and the mitigation options available in these systems. Evidence indicates that cows offered high-nutritive-value perennial ryegrass pasture, characterized by high OM digestibility and ME, and moderate NDF content, have lower methane conversion factors (CH4 as a percentage of gross energy intake) and methane yields (g CH4/kg DMI) than values currently used by the Intergovernmental Panel on Climate Change and national inventories. Potential methane mitigation strategies for these pasture-based systems fall into 3 broad categories: technological interventions (such as feed additives or vaccines), genetics, and nutritional approaches. Some mitigations show promise; however, their effectiveness under grazing conditions is often limited by delivery constraints, variable responses, and uncertainty around cost, persistence, and whole-farm impacts such as economics and GHG life cycle assessment outcomes. Overall, evidence indicates that well-managed pasture-based dairy systems have lower baseline methane emissions than current inventory estimates. However, beyond this lower baseline, no mitigation strategies have yet demonstrated cost-effective, scalable, and consistent emissions reductions across these systems.