What are proxies for methane and for what are they used for?

This was the main subject of a recent invited review paper published on the Journal of dairy science and authored by scientists of the METHAGENE-consortium.The paper is titled “Invited review: Large-scale indirect measurements for enteric methane emissions in dairy cattle: A review of proxies and their potential for use in management and breeding decisions”. In its introduction the paper highlighted that efforts to reduce the carbon footprint of milk production through selection and management require accurate and large-scale measurements of methane (CH₄) emissions from individual cows. Although so far several techniques have been developed for the measurement of CH₄ emissions from ruminants, routine individual animal measurements on a large scale (a requisite for genetic selection) are still proving to be difficult and expensive. Therefore, identifying proxies (i.e. indicators or indirect traits) that are correlated to CH₄ emission, but which are easy and relatively low-cost to record on a large scale, is a much needed alternative. Therefore this timely review, i) systematically describes the biological basis of current potential CH₄ proxies for dairy cattle; ii) assesses the accuracy and predictive power of single proxies and determines the added value of combining proxies; iii) provides a critical evaluation of the relative merit of the main proxies in terms of their simplicity, cost, accuracy, invasiveness and throughput; and iv) discusses their suitability as selection traits.