Rumen methanogenic archaea and their correlation with enteric methane emission in ruminant animals: A systematic review and meta‐analysis

Abstract Background Livestock, especially ruminants, are a major source of global methane emissions, primarily produced by methanogenic archaea during enteric fermentation. We performed a systematic review and meta‐analysis to account for factors that could influence the relationship between ruminal methanogenic populations and methane emissions, such as animal species, diet composition, and method of study (i.e., in vitro vs. in vivo , primer sets). Results The meta‐analysis showed a moderate correlation between methane production and archaeal populations in cattle ( r = 0.47; p < 0.001), but a strong correlation was observed for studies conducted in vitro ( r = 0.64; p < 0.001). Similar correlations were found for high ( r = 0.46; p = 0.0012) and low forage diets ( r = 0.58; p = 0.0012) and for archaea quantification using the mcrA gene ( r = 0.71; p < 0.001). Methane production was strongly correlated ( r = 0.88; p < 0.001) with the abundance of ruminal archaea in buffaloes. Rumen protozoa were weakly correlated with methane emissions ( r = 0.29; p = 0.02), but were strongly correlated with archaeal abundance based on the mcrA gene ( r = 0.65; p < 0.001). In contrast, the abundance of some species of fibrolytic bacteria (i.e., Ruminococcuss spp.) was negatively correlated with methane production and the archaeal population. No association was detected between volatile fatty acids and methane or archaea, except for a weak correlation with the acetate‐to‐propionate ratio ( r = −0.29; p = 0.026). Dry matter digestibility correlated with archaea ( r = 0.48; p = 0.002), but not with methane. Conclusions These findings reveal significant differences between archaeal populations and methane production across ruminant species and diets and highlight the influence of methods of study and quantification approaches in estimating associations between methanogenic populations and enteric methane emissions.