Enteric methane emissions in grazing dairy systems

Abstract

Approximately 80% of agricultural CH₄ comes from livestock systems, with 90% of that derived from enteric CH₄ production by ruminants. Grazing systems are used worldwide to feed dairy cattle. Although quantifying enteric CH₄ emissions in grazing systems has unique challenges, emerging technologies have made gaseous data collection more feasible and less laborious. Nevertheless, robust data sets on enteric CH₄ emissions under various grazing conditions, as well as effective and economic strategies to mitigate CH₄ emissions in grazing dairy cows, are still in high demand because data collection, feeding management, and milk market regulations (e.g., organic certification, grassfed) impose more challenges to grazing than confinement dairy systems. This review will cover management strategies to mitigate enteric CH₄ emissions and applicability to pastoral dairy systems. The effects of enteric CH₄ in the broader context of whole-system assessments will be discussed, which are key to assess the overall environmental impact of grazing dairies.

Summary: Quantifying and mitigating enteric CH₄ from pastoral dairy systems is challenging. A variety of dietary- and husbandry-management strategies are being evaluated to mitigate enteric CH₄ emissions in dairy cattle. Some strategies may not be applicable to pastoral dairy systems. Further research is needed to identify protracted strategies for reducing CH₄ emissions that do not require frequent vaccination or constant feeding, particularly for grazed herds that do not receive supplementation. Breeding animals for reduced CH₄ emissions shows promise, as long as the purchase of such animals is not cost-prohibitive and animal production is not impaired. More robust data sets are needed to develop life cycle assessments that account for all inputs and outputs on the farm that affect carbon and nitrogen cycling.