Enteric methane conversion factor for dairy and beef cattle: Effects of feed digestibility and intake level

The objectives of this study were to identify the sources of variations in reported methane (CH4) conversion factor (Ym) for dairy and beef cattle through a meta-Analytical approach, and to specifically investigate the effects of feed digestibility and intake level on Ym, as well as the alternative expression of methane conversion factor on a digestible energy basis (Dm). Results from 89 peer-reviewed journal articles published from 1992 to 2015 were compiled, and data across studies were analyzed statistically using the MIXED procedures of SAS. For housed cattle, Ym increased significantly with forage-To-concentrate ratio of feed and was also affected by breed. For grazing cattle, Ym varied by geographic region. The observed effect of forage-To-concentrate ratio of feed for housed cattle and the effect of geographic region for grazing cattle on enteric CH4 emissions could both be related to the effect of feed digestibility. Dm was affected significantly by both energy intake level and energy digestibility of feed as well as their interaction. Higher energy digestibility of feed and higher energy intake level of cattle resulted in a lower percentage of digestible energy intake being converted to CH4. Results of this study improved estimation of CH4 conversion factor according to animal husbandry practices. Compared with the Ym approach, the use of CH4 conversion factor on a digestible energy basis can better represent the large variation among diets and the effects of different dietary emission mitigation strategies.