The impact of methane inhibitors on ruminants: A systematic review and meta-analysis.

Knowledge about methane production in the rumen still demonstrates many uncertainties and some information is not irrevocable or not one directional. The status of current results from recent publications referenced below are summarized.

In Reference 1, the impact of methane inhibitors on ruminant performance and rumen microbial community composition was reviewed.

The outcomes revealed the following: (1) Methane inhibitors will reduce feed intake marginally and digestibility significantly. (2) The energy contained in the reduced methane cannot be simply redistributed and produced, which means that adding methane inhibitors will not improve animal performance, which implies there will be a cost factor which will not be recovered. (3) Methane inhibitors will lead to the accumulation of H2 (hydrogen) in the rumen, and high concentrations of H2 could inhibit microbial dehydrogenase activity, thereby explaining the decreased feed intake and digestibility. (4) The VFA in the rumen are also influenced by the partial pressure of H2 gas. When the partial pressure is high, it favours the production of propionate, and when low the production of acetate. (5) Lower than normal rumen pH values will result in a reduction in methane emissions in dairy cows. (6) The main fibre-degrading bacteria in the rumen, Fibrobacter succinogenes, Ruminococcus flavefaciens and Ruminococcus albus may be inhibited due to the accumulation of hydrogen, whereas for archaea (bacteria without nuclei), suppressing methane production will reduce their abundance and composition. The review also observed a decrease in archaeal α-diversity and an increase in β-diversity. (7) Protozoa provide the necessary hydrogen for methane production, and studies have shown that even in cases of hydrogen accumulation in the rumen, methane inhibitors do not lead to a decrease in protozoa numbers, although there are also results to the contrary. (8) Different methane inhibitors reduce methane emissions in different ways, and there are also differences in the effects of different doses of methane inhibitors. This may be due to their action on different mechanisms within the gut microbiota. For example, 3-NOP, as a methyl-CoM analog, inhibits methane production and simultaneously reduces the relative abundance of Methanobacteria. Saponins can bind to sterol-like substances on the surface of protozoa, causing cell lysis, whereas essential oils not only inhibit the growth of protozoa but also increase the abundance of Bacteroides spp. and Succinivibrio spp. Furthermore, the dose of these inhibitors will affect the extent of microbial activity suppression. Higher doses may lead to more pronounced effects but could also result in potential side effects, such as changes in feed intake or overall animal health. In contrast, lower doses may be less effective at reducing methane emissions but carry fewer risks to the animals. These findings suggest that optimizing the type and dose of methane inhibitors is crucial for achieving effective and sustainable methane reduction in livestock production. 

In Reference 2, the aim was to assess the relationships of enteric methane (CH4) yield (g/kg of DMI) with immune response, feed efficiency (ECM/DMI), and rumen microbiome in dairy cows, both in early and late lactation. For that purpose, cows were divided into a low methane yield (LMY) group and a high methane yield (HMY) group.

The results in early lactation showed that CH4 yields in LMY and HMY cows were (LSM) 18.7 and 25.3 g/kg of DMI, respectively, whereas in late lactation, CH4 yields in LMY and HMY cows were 22.8 and 26.8 g/kg of DMI, respectively. In terms of immune response, in early lactation whole blood and isolated peripheral blood mononuclear cells from LMY compared with HMY animals, were less responsive to stimulants in vitro. In addition, feed conversion efficiency was lower in LMY than HMY cows, and the relative abundance of the archaeal genus Methanosphaera and the bacterial genus Marvinbryantia were higher. In late lactation, there were no differences in immune response and feed conversion efficiency between LMY and HMY cows. Rumen bacterial diversity was also affected: in LMY cows several bacterial genera including Prevotella 7, Ruminococcus gauvreauii group, and Shuttleworthia were enriched, whereas in HMY cows Methanobrevibacter, Veillonellaceae UCG-001, Succinivibrionaceae UCG- 002, Rikenellaceae RC9, and CAG-352 were enriched.

These results indicate that in early lactation the cows with low CH4 yield reach energy balance faster, at the expense of an inadequate immune response. In the high CH4 yield cows, the increased CH4 yield in early lactation may reflect higher rumen fermentation activity, fostering feed efficiency and energy availability for supporting immune function.

In Reference 3, since genetic selection for feed efficiency in dairy cattle is a promising strategy to mitigate environmental emissions, genetic selection for residual feed intake (RFI) was evaluated as a tool to improve feed efficiency and reduce GHG emissions. A life cycle assessment approach was used to quantify emissions from feed production, enteric fermentation, and manure management, using three RFI selection scenarios: (a) baseline (average genomic RFI [gRFI]), (b) one standard deviation (1-SD) improvement in the gRFI for heifers and cows, and (c) 3-SD improvement in gRFI for heifers and cows.

Selection for improved gRFI led to enhanced feed efficiency, as expected. The group with a 1-SD improvement in gRFI consumed 2.73% less feed over their lifetime, whereas those with a 3-SD improvement consumed 8.2% less, with no impact on productivity. These improvements in feed efficiency translated into a 2.42% reduction in lifetime CO2 equivalent (CO2e) emissions in the 1-SD group, and a 7.31% reduction in the 3-SD group. Enteric CH4 emissions were the largest contributor to the lifetime carbon footprint, accounting for 38.9% of total emissions in the baseline scenario, highlighting the importance of genetic selection for methane mitigation. Feed production and manure management accounted for 17.5% and 32.5% of total emissions, respectively.

These findings suggest that genetic selection for RFI will significantly reduce the carbon intensity of milk production through improved lifetime feed efficiency and subsequently reduced feed intake per unit of milk production, establishing it as a key strategy for reducing GHG emissions in the dairy sector.

General conclusions: (1) The development and further influence of enteric methane production is still not well predictable; (2) although methane inhibitors will lower enteric methane, it may be associated with poorer feed efficiency because of a reduction in feed intake and digestibility, resulting in extra costs; (3) normal selection of cows within a herd with lower enteric methane production may be negative to immune function, thereby compromising animal health and welfare; (4) if selection (genomic or otherwise) for lower enteric methane is to be effective, it should be done through a RFI program which will improve feed efficiency, thereby accounting for all risk and productivity factors, and also reducing enteric methane because of lower feed intake.

References:

1. G. Hu, J. Gao, V. Padmakumar, N. Joshi, W. Zhu, & Y. Cheng, 2026. The impact of methane inhibitors on ruminants: A systematic review and meta-analysis. J. Dairy Sci. 109, 3697–3709. https://doi.org/10.3168/jds.2025-27479
2. H. Nur Cahyo, P. Niu, P. B. Pope, U. Gimsa, B. Kuhla, & A. Schwarm, 2026. Methane category, immune response, feed efficiency, and rumen microbial community in lactating dairy cows. J. Dairy Sci. 109, 3710–3724. https://doi.org/10.3168/jds.2025-26925
3. K. R. G. Lucas, J. R. R. Johnson, P. Khanal, N. J. Deeb, P. Ross, & E. Kebreab, 2026. Improving feed efficiency with the EcoFeed index reduces greenhouse gas emissions in dairy cattle. J. Dairy Sci. 109, 4087–4097. https://doi.org/10.3168/jds.2025-27149