Decreasing enteric methane (CH 4 ) emissions from ruminants without altering animal production is desirable both as a strategy to reduce global greenhouse gas (GHG) emissions and as a means of improving feed conversion efficiency. The aim of this paper is to provide an update on a selection of proved and potential strategies to mitigate enteric CH 4 production by ruminants. Various biotechnologies are currently being explored with mixed results. Approaches to control methanogens through vaccination or the use of bacteriocins highlight the difficulty to modulate the rumen microbial ecosystem durably. The use of probiotics, i.e. acetogens and live yeasts, remains a potentially interesting approach, but results have been either unsatisfactory, not conclusive, or have yet to be confirmed in vivo. Elimination of the rumen protozoa to mitigate methanogenesis is promising, but this option should be carefully evaluated in terms of livestock performances. In addition, on-farm defaunation techniques are not available up to now. Several feed additives such as ionophores, organic acids and plant extracts have also been assayed. The potential use of plant extracts to reduce CH 4 is receiving a renewed interest as they are seen as a natural alternative to chemical additives and are well perceived by consumers. The response to tannin-and saponincontaining plant extracts is highly variable and more research is needed to assess the effectiveness and eventual presence of undesirable residues in animal products. Nutritional strategies to mitigate CH 4 emissions from ruminants are, without doubt, the most developed and ready to be applied in the field. Approaches presented in this paper involve interventions on the nature and amount of energy-based concentrates and forages, which constitute the main component of diets as well as the use of lipid supplements. The possible selection of animals based on low CH 4 production and more likely on their high efficiency of digestive processes is also addressed. Whatever the approach proposed, however, before practical solutions are applied in the field, the sustainability of CH 4 suppressing strategies is an important issue that has to be considered. The evaluation of different strategies, in terms of total GHG emissions for a given production system, is discussed.Keywords: methane, greenhouse gases, ruminant, mitigation strategies Implications Methane (CH 4 ) mitigation in ruminants is possible through various strategies. Today, the feeding management approach is the most developed. Other strategies (biotechnologies, additives) are promising but the diversity and plasticity of functions of the rumen bacterial and methanogenic communities may be a limiting factor for their successful application. A possible selection of animals on CH 4 production and more likely on digestive processes is evocated. In any case, before practical solutions are proposed for field application more research in vivo is needed. The sustainability of CH 4 -suppressing strategies is also an important issue and they mi...

Ruminant production is under increased public scrutiny in terms of the importance of cattle and other ruminants as major producers of the greenhouse gas methane. Methanogenesis is performed by methanogenic archaea, a specialised group of microbes present in several anaerobic environments including the rumen. In the rumen, methanogens utilise predominantly H 2 and CO 2 as substrates to produce methane, filling an important functional niche in the ecosystem. However, in addition to methanogens, other microbes also have an influence on methane production either because they are involved in hydrogen (H 2 ) metabolism or because they affect the numbers of methanogens or other members of the microbiota. This study explores the relationship between some of these microbes and methanogenesis and highlights some functional groups that could play a role in decreasing methane emissions. Dihydrogen ('H 2 ' from this point on) is the key element that drives methane production in the rumen. Among H 2 producers, protozoa have a prominent position, which is strengthened by their close physical association with methanogens, which favours H 2 transfer from one to the other. A strong positive interaction was found between protozoal numbers and methane emissions, and because this group is possibly not essential for rumen function, protozoa might be a target for methane mitigation. An important function that is associated with production of H 2 is the degradation of fibrous plant material. However, not all members of the rumen fibrolytic community produce H 2 . Increasing the proportion of non-H 2 producing fibrolytic microorganisms might decrease methane production without affecting forage degradability. Alternative pathways that use electron acceptors other than CO 2 to oxidise H 2 also exist in the rumen. Bacteria with this type of metabolism normally occupy a distinct ecological niche and are not dominant members of the microbiota; however, their numbers can increase if the right potential electron acceptor is present in the diet. Nitrate is an alternative electron sinks that can promote the growth of particular bacteria able to compete with methanogens. Because of the toxicity of the intermediate product, nitrite, the use of nitrate has not been fully explored, but in adapted animals, nitrite does not accumulate and nitrate supplementation may be an alternative under some dietary conditions that deserves to be further studied. In conclusion, methanogens in the rumen co-exist with other microbes, which have contrasting activities. A better understanding of these populations and the pathways that compete with methanogenesis may provide novel targets for emissions abatement in ruminant production.

The rumen is a complex ecosystem composed of anaerobic bacteria, protozoa, fungi, methanogenic archaea and phages. These microbes interact closely to breakdown plant material that cannot be digested by humans, whilst providing metabolic energy to the host and, in the case of archaea, producing methane. Consequently, ruminants produce meat and milk, which are rich in high-quality protein, vitamins and minerals, and therefore contribute to food security. As the world population is predicted to reach approximately 9.7 billion by 2050, an increase in ruminant production to satisfy global protein demand is necessary, despite limited land availability, and whilst ensuring environmental impact is minimized. Although challenging, these goals can be met, but depend on our understanding of the rumen microbiome. Attempts to manipulate the rumen microbiome to benefit global agricultural challenges have been ongoing for decades with limited success, mostly due to the lack of a detailed understanding of this microbiome and our limited ability to culture most of these microbes outside the rumen. The potential to manipulate the rumen microbiome and meet global livestock challenges through animal breeding and introduction of dietary interventions during early life have recently emerged as promising new technologies. Our inability to phenotype ruminants in a high-throughput manner has also hampered progress, although the recent increase in “omic” data may allow further development of mathematical models and rumen microbial gene biomarkers as proxies. Advances in computational tools, high-throughput sequencing technologies and cultivation-independent “omics” approaches continue to revolutionize our understanding of the rumen microbiome. This will ultimately provide the knowledge framework needed to solve current and future ruminant livestock challenges.