An assessment of global ruminant methane-emission measurements shows bias relative to contributions of farmed species, populations and among continents

Abstract

Global ruminant methane (CH4)-mitigation strategies rely on data from in vivo CH4-emission measurements. This survey of 415 peer-reviewed studies of in vivo enteric-CH4 measurements from farmed ruminants details research objectives, diets, and methodology as well as groups within ruminant species. The survey results have been evaluated in relation to ruminant population data and the contributions of each species to CH4 emissions. Despite the highest estimated total CH4 emissions from ruminants in Asia, South America and Africa (accounting for 37%, 23% and 17% of total enteric-CH4 emissions respectively), the number of in vivo studies of CH4 measurements were 15%, 9% and 1% of global studies respectively. Globally, the most studied species were cattle (64%) and sheep (22%), whereas goats and buffalo accounted for 7% and 5% of studies respectively. These species account for 75%, 7%, 5% and 12% of total enteric-CH4 emissions respectively. Most cattle studies were with Bos taurus and only 12% of the cattle studies were with Bos indicus. Respiration chambers have been used in 51% of studies and, despite the development of other methodologies, they remain the dominant technique for measurement of enteric-CH4 production. Most studies involved animals fed high-forage diets; these were 56% of the studies with cattle, 73% with sheep, 47% for goats, but only 15% of studies with buffalo. The evaluation of diets as a mitigation strategy was the prime objective of all regions. The number of studies that have measured CH4 from cattle aligns with their contribution to enteric emissions; however, buffalo, Bos indicus cattle and mature beef cows were under-represented relative to their global populations and contribution to global emissions. Dominance of measurements from cattle was evident in all continents.