Invited Review: Enteric methane in dairy cattle production: Quantifying the opportunities and impact of reducing emissions

Many opportunities exist to reduce enteric methane (CH₄) and other greenhouse gas (GHG) emissions per unit of product from ruminant livestock. Research over the past century in genetics, animal health, microbiology, nutrition, and physiology has led to improvements in dairy production where intensively managed farms have GHG emissions as low as 1 kg of CO₂ equivalents (CO₂e)/kg of energy-corrected milk (ECM), compared with >7 kg of CO₂e/kg of ECM in extensive systems. The objectives of this review are to evaluate options that have been demonstrated to mitigate enteric CH₄ emissions per unit of ECM (CH₄/ECM) from dairy cattle on a quantitative basis and in a sustained manner and to integrate approaches in genetics, feeding and nutrition, physiology, and health to emphasize why herd productivity, not individual animal productivity, is important to environmental sustainability. A nutrition model based on carbohydrate digestion was used to evaluate the effect of feeding and nutrition strategies on CH₄/ECM, and a meta-analysis was conducted to quantify the effects of lipid supplementation on CH₄/ECM. A second model combining herd structure dynamics and production level was used to estimate the effect of genetic and management strategies that increase milk yield and reduce culling on CH₄/ECM. Some of these approaches discussed require further research, but many could be implemented now. Past efforts in CH₄ mitigation have largely focused on identifying and evaluating CH₄ mitigation approaches based on nutrition, feeding, and modifications of rumen function. Nutrition and feeding approaches may be able to reduce CH₄/ECM by 2.5 to 15%, whereas rumen modifiers have had very little success in terms of sustained CH₄ reductions without compromising milk production. More significant reductions of 15 to 30% CH₄/ECM can be achieved by combinations of genetic and management approaches, including improvements in heat abatement, disease and fertility management, performance-enhancing technologies, and facility design to increase feed efficiency and life-time productivity of individual animals and herds. Many of the approaches discussed are only partially additive, and all approaches to reducing enteric CH₄ emissions should consider the economic impacts on farm profitability and the relationships between enteric CH₄ and other GHG.

Knapp, J. R., G. L. Laur, P. A. Vadas, W. P. Weiss, & J. M. Tricarico (2014) Invited Review: Enteric methane in dairy cattle production: Quantifying the opportunities and impact of reducing emissions, Journal of Dairy Science 97(6):3231-3261.