Microbial composition, rumen fermentation parameters, enteric methane emissions, and lactational performance of phenotypically high and low methane-emitting dairy cows

Methane is a greenhouse gas which because of climate change expectations should be mitigated. Its production in the rumen also results in 4 to 12% loss in potential energy to the cow, and therefore mitigation may improve production performance, at least theoretically. There are several feeding and management strategies to exploit methane mitigation, and more recently genetic variability, which is the subject of this investigation, has also been explored. The authors cited, designed an experiment to investigate the relationship of high and low methane yielding phenotypes with body weight (BW), dry matter intake (DMI), lactation performance (energy corrected milk – ECM), enteric methane emissions, and various rumen fermentation parameters in lactating dairy cows.

A total of 130 Holstein cows were screened for enteric methane emissions using the GreenFeed system, and five each identified as high and low methane emitters. The cows in the low group produced on average 346 g methane per day (15.5 g per kg DMI, and 13.2 g per kg ECM), whereas the cows in the high group produced on average 439 g methane per day (20.4 g per kg DMI and 17.0 g per kg ECM). The cows were blocked by lactation number, days in milk and milk production and were used in a five week randomized complete block design experiment.                                                                                                                                 

Enteric emissions of CO₂ and H₂ did not differ between the low and high methane cows. Milk production and composition, and BW were also not different. The concentration of volatile fatty acids did not differ either, but the low group had a lower proportion of acetate and a higher proportion of propionate. Consistently, the 16S cDNA analysis revealed a higher abundance of Succinivibrionaceae and unclassified Veillonellaceae in the low group compared to the high group, bacteria that are positively correlated with ruminal propionate production. Additionally, the low methane cows possessed fewer transcripts of a gene encoding for methyl-COM reductase enzyme compared with the high methane cows.

Overall, the cows with the low methane-emission phenotype were smaller in stature, and showed lower total-tract digestibility of dietary fibre and organic matter. They also had lower acetate and higher concentrations of propionate in the rumen fluid. These differences were accompanied by differences in the genes and transcripts that encode the MCR enzyme, and notably, there were differences in the bacteria-to-archaea ratio, bacteria-archaea networks and functional pathways leading to propionate formation between the low and high methane cows. These findings imply that the low methane genotype exert the selective potential for certain bacteria-archaea networks which then govern the energy-partitioning mechanisms to divert H₂ away from methanogenesis to e.g. succinate and propionate production. Propionate is more favourable for milk production as it is a direct precursor of glucose, which acetate is not.