Current Perspectives on Achieving Pronounced Enteric Methane Mitigation From Ruminant Production

Abstract

Limiting global warming to 1.5°C above pre-industrial levels by 2050 requires achieving net zero emissions of greenhouse gases by 2050 and a strong decrease in methane (CH4) emissions. Our aim was to connect the global need for mitigation of the emissions of greenhouse gases and enteric CH4 from ruminant production to basic research on the biological consequences of inhibiting rumen methanogenesis in order to better design strategies for pronounced mitigation of enteric CH4 production without negative impacts on animal productivity or economic returns. Ruminant production worldwide has the challenge of decreasing its emissions of greenhouse gases while increasing the production of meat and milk to meet consumers demand. Production intensification decreases the emissions of greenhouse gases per unit of product, and in some instances has decreased total emissions, but in other instances has resulted in increased total emissions of greenhouse gases. We propose that decreasing total emission of greenhouse gases from ruminants in the next decades while simultaneously increasing meat and milk production will require strong inhibition of rumen methanogenesis. An aggressive approach to pronounced inhibition of enteric CH4 emissions is technically possible through the use of chemical compounds and/or bromoform-containing algae, but aspects such as safety, availability, government approval, consumer acceptance, and impacts on productivity and economic returns must be satisfactorily addressed. Feeding these additives will increase the cost of ruminant diets, which can discourage their adoption. On the other hand, inhibiting rumen methanogenesis potentially saves energy for the host animal and causes profound changes in rumen fermentation and post-absorptive metabolism. Understanding the biological consequences of methanogenesis inhibition could allow designing strategies to optimize the intervention. We conducted meta-regressions using published studies with at least one treatment with >50% inhibition of CH4 production to elucidate the responses of key rumen metabolites and animal variables to methanogenesis inhibition, and understand possible consequences on post-absorptive metabolism. We propose possible avenues, attainable through the understanding of biological consequences of the methanogenesis inhibition intervention, to increase animal productivity or decrease feed costs when inhibiting methanogenesis.

Opportunities and Hurdles to the Adoption and Enhanced Efficacy of Feed Additives Towards Pronounced Mitigation of Enteric Methane Emissions

“…Alterations in both catabolic and anabolic processes may result in increased absorption of some nutrients and may open opportunities to decrease the needs for them to be supplied by the basal diet. Inhibiting rumen methanogenesis decreases the acetate to propionate concentration ratio, as predicted by the elevation of H2 concentration [88] and confirmed in metaanalysis of in vivo experiments [16]. Although propionate concentration adjusted by DMI did not respond to methanogenesis inhibition [16], it is still possible that if increases in propionate production occur they may have gone unnoticed when measuring propionate concentration, because of compensatory changes occurring in propionate absorption.…”

“…~30%), there have been various studies in which the inhibition of CH4 production was considerably more profound (i.e. ~80% or more) [16]. Tables 2 and 3 summarize experiments in which the utilization of chemical inhibitors of methanogenesis or Asparagopsis spp.…”

“…6 Daily average, estimated from graph. With regards to the first point, i.e., extent of inhibition, the experiments and treatments resulting in pronounced inhibition of rumen methanogenesis presented in Tables 2 and 3 and Supplementary Table 1, and in Ungerfeld et al [16], do not constitute a majority of studies of chemical inhibition of methanogenesis. Importantly however, meta-analyses also show that the extent of inhibition of methanogenesis by 3-NOP is positively related to dose [28][29]31].…”

“…It has been suggested that the moderate inhibition of CH4 production observed in most studies may not translate into large enough savings of net energy to elicit significant differences in feed efficiency or animal productivity [16][17]. Experiments with much larger number of animals would be thus needed to detect significant differences in feed efficiency; in that regard, Alemu et al [73] working with 4048 steers in a commercial feedlot, reported a tendency towards a 2.5% increase in feed efficiency when CH4 production was moderately inhibited by 26%.…”

“…It should be noted, however, that VFA concentration does not necessarily reflect VFA production. Changes in VFA production might be compensated by changes in rates of VFA absorption or passage, incorporation into microbial biomass, and changes in rumen volume; effects of inhibiting methanogenesis on actual production of VFA have not been determined [16,37].…”