The animal Article of the Month for November is entitled ‘Influence of rumen protozoa on methane emission in ruminants: a meta-analysis approach’

Livestock farming is responsible for 14.5% of total anthropogenic greenhouse gas emissions, with half of them represented by methane emissions, mostly produced by ruminants. In addition, enteric methane production by ruminants represents up to 6% of energy loss for the animal. Reduction of methanogenesis is then important to limit the negative environmental impacts of ruminants and to improve their feed efficiency.

In the rumen, methane is mainly produced by methanogenic archaea from carbon dioxide and hydrogen released during feed fermentation by bacteria, protozoa and fungi. One methane mitigation strategy consists in reducing protozoa in the rumen, which are important hydrogen producers in close interaction with methanogens. However, the relationship between protozoa concentration in the rumen and methane emissions has never been quantified. In this objective, we carried out a quantitative analysis of the literature which also highlighted factors affecting this relationship.

A database was built from 76 in vivo experiments, which recorded methane production and rumen protozoa concentration measured on the same animals. The meta-design was well balanced for animal species, with half of experiments represented by dairy and beef cattle and half of experiments represented by sheep and goats. No difference between animal species was observed for methane emissions and protozoa concentrations, which averaged 18.9 g/kg dry matter intake and 5.4 log10 protozoa cells/ml. Most experiments which reported a concomitant reduction of protozoa concentration and methane emission tested lipid addition as methane mitigation strategy. Inversely, experiments that used chemical compounds as antimethanogenic treatments reported a reduction in methane emission without change in protozoa numbers, confirming that methanogenesis can be regulated by other mechanisms than protozoa population.

The relationship between methane emission and protozoa concentration was studied with a variance− covariance model. To understand the effect of a variation of protozoa concentration on methane emissions, only experiments having an important within-experiment variation of protozoa concentration were included in the analysis as fixed effects. We observed a linear and positive relationship between methane emission and protozoa concentration. A reduction of 0.12 log10 protozoa cells/ml induced a reduction of 1 g methane/kg dry matter intake. In addition, to detect potential factors affecting this relationship, the influence of several qualitative and quantitative secondary factors was extensively tested on slopes, intercept and lsmeans. The proportion of butyrate in the rumen VFA positively influenced the relationship: for a similar level of protozoa, methane emission was enhanced with a higher proportion of butyrate.

By a comprehensive meta-analysis approach allowing dissociation between within- and inter- experiment variability, we quantified the importance of protozoa, as hydrogen supplier, on methanogenesis. This work supports methane mitigating strategies based on reduction of protozoa concentration in the rumen, such as partial defaunation induced by lipids and/or plant extracts addition. Further studies are required to demonstrate the interest of associating these dietary treatments acting on hydrogen production to strategies acting on hydrogen utilization such as hydrogen sinks (nitrate, sulfate…).