Methane is a greenhouse gas (GHG) far less abundant than CO₂ but with a global warming potential 28-times more powerful on a 100-year scale (Jackson et al. 2020Jackson, R.B., Saunois, M., Bousquet, P., Canadell, J.G., Poulter, B., Stavert, A R., Bergamaschi, P., Niwa, Y., Segers, A. & Tsuruta, A. 2020. "Increasing anthropogenic methane emissions arise equally from agricultural and fossil fuel sources". Environmental Research Letters, 15: 071002, ISSN: 1748-9326. https://doi.org/10.1088/1748-9326/ab9ed2.). The more abundant methane sources include anthropogenic emissions from agriculture, waste management, fossil fuels, and natural emissions from wetlands, freshwater systems, and geological sources (Saunois et al. 2016Saunois, M., Jackson, R.B., Bousquet, P., Poulter, B. & Canadell, J.G. 2016. "The growing role of methane in anthropogenic climate change". Environmental Research Letters, 11: 120207, ISSN: 1748-9326. https://doi.org/10.1088/1748-9326/11/12/120207.). Agriculture contributes with a percentage varying from 8 to 18 % of total anthropogenic GHG emissions and the ruminants account for about 81 % of GHG from the livestock which involves enteric fermentation (around 90 %) and manure management (Hristov et al. 2013Hristov, A.N., Oh, J., Firkins, J.L., Dijkstra, J., Kebreab, E., Waghorn, G., Makkar, H.P.S., Adesogan, A.T., Yang, W., Lee, C., Gerber, P.J., Henderson, B. & Tricarico, J.M. 2013. "Special topics—Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options". Journal of Animal Science, 91(11): 5045-5069, ISSN: 1525-3163. https://doi.org/10.2527/jas.2013-6583.). Among ruminants-related direct emissions, cattle are responsible for 65 %, buffaloes for 8 %, and sheep and goats for 7 % (figure 1) (Steinfeld et al. 2019Steinfeld, H. Opio, C., Chara, J., Davis, K.F., Tomlin, P. & Gunter, S. 2019. Overview paper: Livestock, Climate and Natural Resource Use, http://www.livestockdialogue.org/fileadmin/templates/res_livestock/docs/2019_Sept_Kansas/4_Climate_and_Natural_Resource_Use_-_Online_consultation.pdf [Consulted: August 10, 2022]). Based on 2010 GHG emissions, to limit global warming to 1.5 °C, agricultural emissions should be decreased by 11-30 % by 2030 and by 24-47 % by 2050 (Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119.).

Livestock sustains the livelihood of millions of people in the world (up to 12 %), both in developing and developed countries. The world’s population has been estimated to reach 9.7 billion in 2050 and 10.4 billion in 2100 (UN 2022United Nations (UN). 2022. Available: https://www.un.org/en/desa/world-population-projected-reach-98-billion-2050-and-112-billion-2100 [Consulted: August 13, 2022]), particularly in Low- and Middle-income Countries (LMC) along with increasing production and demand for milk and meat products by 35 % (1168 Mt) and 44 % (373 Mt), respectively (IFCN 2018International Federation of Clinical Neurophysiology (IFCN). 2018. Available: https://ifcndairy.org/wp-content/uploads/2018/06/IFCN-Dairy-Outlook-2030-Brochure.pdf. [Consulted: August 13, 2022]).

There is a growing concern that the demand for animal products, associated with population growth, prolonged lifespan, and improved economic welfare in developing countries, will put an unsustainable call on the environment (Salter 2017Salter, A.M. 2017. "Improving the sustainability of global meat and milk production". Proceedings of the Nutrition Society, 76(1): 22-27, ISSN: 1475-2719. https://doi.org/10.1017/S0029665116000276.). Nevertheless, ruminants, especially when fed with feedstuff produced on land not suitable for primary cropping or by-products from agro-industrial, can be a net contributor to the global supply of human edible food, maintaining and enhancing the provision of protein and essential micronutrients (zinc, calcium, Vit.B₁₂, and riboflavin) (Scollan et al. 2011Scollan, N.D., Hocquette, J.F., Richardson, R.I., Kim E.J., Wood, J.D. & Rowlings C. 2011. Raising the nutritional value of beef and beef products to add value in beef production. Nutrition and climate change: major issues confronting the meat industry (ed. JD Wood and C Rowlings) : 79-104.).

A massive worldwide research effort has investigated various mitigation strategies that can be summarized into three categories: changes in animal and feed management, diet formulation, and rumen manipulation (Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119.). All the strategies potentially involve changes in the rumen microbiome (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). Rumen methane production also represents a loss of energy (from 2 to 12 % of gross energy intake) for animal growth and production (Johnson and Johnson 1995Johnson, K.A. & Johnson, D.E. 1995. "Methane emissions from cattle". Journal of Animal Science, 73(8): 2483-2492, ISSN: 1525-3163. https://doi.org/10.2527/1995.7382483x.). Thus, lowering CH₄ emissions would benefit the environment and eventually the livestock production efficiency.

Rumen microbial community and methanogenesis. Ruminants live on plant matter using their specialized digestive system with a well-adapted symbiotic web of microorganisms (Cammack et al. 2018Cammack, K.M., Austin, K.J., Lamberson, W.R., Conant, G.C. & Cunningham, H.C. 2018. "Ruminant nutrition symposium: tiny but mighty: the role of the rumen microbes in livestock production". Journal of Animal Science, 96(2): 752-770, ISSN: 1525-3163. https://doi.org/10.1093/jas/skx053.) which includes ciliate protozoa, anaerobic fungi, bacteria, and archaea that have co-evolved with their host (Henderson et al. 2015Henderson, G. Cox, F., Ganesh, S., Jonker, A., Young, W., Global Rumen Census collaboration & Janssen, P.H. 2015. "Rumen microbial community composition varies with diet and host, but a core microbiome is found across a wide geographical range". Scientific Reports, 5: 14567, ISSN: 2045-2322. https://doi.org/10.1038/srep14567., Sasson et al. 2017Sasson, G., Kruger Ben-Shabat, Sh., Seroussi, E., Doron-Faigenboim, A., Shterzer, N., Yaacoby Sh., Berg Miller, M.E., White, B.A., Halperin, E. & Mizrahi, I. 2017. "Heritable bovine rumen bacteria are phylogenetically related and correlated with the cow’s capacity to harvest energy from its feed". mBio, 8(4): e00703-17, ISSN: 2150-7511. https://doi.org/10.1128/mBio.00703-17. and Huws et al. 2018Huws, Sh.A., Creevey, Ch.J., Oyama, L.B., Mizrahi, I., Denman, S.E., Popova, M., Muñoz-Tamayo, R., Forano, E., Waters, S.M., Hess, M., Tapio, I., Smidt, H., Krizsan, S.J., Yáñez-Ruiz, D.R., Belanche, A., Guan, L., Gruninger, R.J., McAllister, T.A., Newbold, C.J., Roehe, R., Dewhurst, R.J., Snelling, T.J., Watson, M., Suen, G., Hart, E.H., Kingston-Smith, A.H., Scollan N.D., M do Prado, R., Pilau, E.J., Mantovani, H.C., Attwood, G.T., Edwards, J.E., McEwan, N.R., Morrisson, S., Mayorga, O.L., Elliott, Ch. & Morgavi, D.P. 2018. "Addressing global ruminant agricultural challenges through understanding the rumen microbiome: Past, Present, and Future". Frontiers Microbiology, 9: 2161, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2018.02161.). Protozoa can be up to half of the rumen biomass (Hungate 1966Hungate, R.E. 1966. The rumen and its microbes. New York: Academic Press, 533 pp. Book ISBN: 9781483263625. and Newbold et al. 2015Newbold, C.J., De la Fuente, G., Belanche, A., Ramos-Morales, E. & McEwan, N.R. 2015. "The role of ciliate protozoa in the rumen". Frontiers in Microbiology, 6: 1313, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2015.01313.), fungi that may reach 20 % (i.e., sheep, Rezaeian et al. 2004Rezaeian, M., Beakes, G.W. & Parker, D.S. 2004. "Distribution and estimation of anaerobic zoosporic fungi along the digestive tracts of sheep". Mycological Research, 108: 1227–1233, ISSN: 1469-8102. https://doi.org/10.1017/S0953756204000929.), archaea between 0.3-4 % (Janssen and Kirs 2008Janssen, P.H. & Kirs, M. 2008. "Structure of the archaeal community of the rumen". Applied Environmental Microbiology, 74(12): 3619-3625, ISSN: 1098-5536. https://doi.org/10.1128/AEM.02812-07.) and the bacteria as the largest group.

Microbial fermentations in the rumen play an essential role in the ability of ruminants to utilize lignocellulosic materials to produce volatile fatty acids (VFAs) and to convert non-protein nitrogen into microbial protein, which is an essential source of energy and protein for the host, while the rumen provides the microbes a suitable environment to thrive and grow (Cammack et al. 2018Cammack, K.M., Austin, K.J., Lamberson, W.R., Conant, G.C. & Cunningham, H.C. 2018. "Ruminant nutrition symposium: tiny but mighty: the role of the rumen microbes in livestock production". Journal of Animal Science, 96(2): 752-770, ISSN: 1525-3163. https://doi.org/10.1093/jas/skx053.). Nevertheless, microbes also have potential environmental detrimental effects through the emission of GHGs and excessive N excretions in feces and urine.

Rumen methanogenesis carried out by archaea follows two main pathways (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). The hydrogenotrophic route converts H₂ and CO₂ produced by protozoa, fungi, and bacteria in CH₄ thus reducing the metabolic (Kittelmann et al. 2013Kittelmann, S., Seedorf, H., Walters, W.A., Clemente, J.C., Knight, R., Gordon, J.I. & Janssen, P.H. 2013. "Simultaneous amplicon sequencing to explore co-occurrence patterns of bacterial, archaeal and eukaryotic microorganisms in rumen microbial communities". Plos One, 8(2): e47879, ISSN: 1932-6203. https://doi.org/10.1371/journal.pone.0047879. and Poulsen et al. 2013Poulsen, M., Schwab, C., Borg Jensen, B., Engberg, R.M., Spang, A., Canibe, N., Højberg, O., Milinovich, G., Fragner, L., Schleper, C. & Weckwerth, W. 2013. "Methylotrophic methanogenic Thermoplasmata implicated in reduced methane emissions from bovine rumen". Nature communications, 4(1): 1428, ISSN: 2041-1723. https://doi.org/10.1038/ncomms2432.). Formate, which can be used by all the most abundant archaea, is considered equivalent and is included in the hydrogenotrophic category (Janssen and Kirs 2008Janssen, P.H. & Kirs, M. 2008. "Structure of the archaeal community of the rumen". Applied Environmental Microbiology, 74(12): 3619-3625, ISSN: 1098-5536. https://doi.org/10.1128/AEM.02812-07. and Janssen 2010Janssen, P.H. 2010. "Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics". Animal Feed Science and Technology, 160(1-2): 1–22, ISSN: 1873-2216. https://doi.org/10.1016/j.anifeedsci.2010.07.002.). The second pathway uses methyl groups as substrates, such as those present in methylamines and methanol (Poulsen et al. 2013Poulsen, M., Schwab, C., Borg Jensen, B., Engberg, R.M., Spang, A., Canibe, N., Højberg, O., Milinovich, G., Fragner, L., Schleper, C. & Weckwerth, W. 2013. "Methylotrophic methanogenic Thermoplasmata implicated in reduced methane emissions from bovine rumen". Nature communications, 4(1): 1428, ISSN: 2041-1723. https://doi.org/10.1038/ncomms2432. and De la Fuente et al. 2019De la Fuente, G., Yañez-Ruiz, D.R., Seradj, A.R., Balcells, J. & Belanche, A. 2019. "Methanogenesis in animals with foregut and hindgut fermentation: a review". Animal Production Science, 59(12): 2109-2122, ISSN: 1836-5787. https://doi.org/10.1071/AN17701.). If H₂ accumulates in the rumen NADH re-oxidation, microbial growth, forage digestion, and associated production of acetate, propionate, and butyrate are inhibited, so any mitigation strategy that reduces methanogens populations must include some pathway for H₂ removal from the rumen (Eckard et al. 2010Eckard, R.J., Grainger, C. & De Klein, C.A.M. 2010. "Options for the abatement of methane and nitrous oxide from ruminant production: A review". Livestock Science, 130 (1-3): 47-56, ISSN: 1871-1413. https://doi.org/10.1016/j.livsci.2010.02.010.).

Propionate production is the second major H₂ sink in the rumen plus other minor such as nitrate/nitrite reduction, reductive acetogenesis, and unsaturated fatty acid biohydrogenation (Mitsumori and Sung 2008Mitsumori, M. & Sun, W. 2008. "Control of rumen microbial fermentation for mitigating methane emissions from the rumen". Asian-Australasian Journal of Animal Sciences, 21(1): 144-154, ISSN: 1976-5517. http://dx.doi.org/10.5713/AJAS.2008.R01. and Kobayashi 2010Kobayashi, Y. 2010. "Abatement of Methane Production from Ruminants: Trends in the Manipulation of Rumen Fermentation". Asian-Australasian Journal of Animal Science, 23(3): 410-416, ISSN: 1976-5517. https://doi.org/5713/AJAS.2010.R.01.). Therefore, for optimum rumen function, the methane reduction strategy must be paralleled by the enhancement of propionate production without compromising feed digestion, stimulating H₂ utilizing pathways, and inhibiting the population and activity of methanogens (Martin et al. 2010Martin, C., Morgavi, D.P. & Doreau, M. 2010. "Methane mitigation in ruminants: from microbe to the farm scale". Animal, 4(3): 351–365, ISSN: 1751-732X. https://doi.org/10.1017/S1751731109990620.).

So, methanogens in the gastrointestinal tract produce methane as a by-product of anaerobic fermentation (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). As the sole producer, it would be reasonable to consider the increase in number to be associated with the greater production of CH₄. Nonetheless, it would seem that methanogens community composition rather than its size is correlated to methane production and that this diversity is influenced by H₂ availability and interactions within and between H₂ producing microbes in the rumen (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0., Abbot et al. 2020Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432. and Pitta et al. 2021Pitta, D.W., Melgar, A., Hristov, A.N., Indugu, N., Narayan, K. S., Pappalardo, C., Hennessy, M.L., Vecchiarelli, B., Kaplan-Shabtai, V., Kindermann, M. & Walker, N. 2021. "Temporal changes in total and metabolically active ruminal methanogens in dairy cows supplemented with 3-nitrooxypropanol". Journal of Dairy Science, 104(8): 8721–8735, ISSN: 1525-3198. https://doi.org/10.3168/jds.2020-19862.).

Culture-independent next-generation sequencing together with “omics” approaches, developed in recent years, have become powerful tools to understand which microorganisms are in the rumen, which role they play in methanogenesis, and what is the effect of mitigation strategies. Reports from Söllinger et al. (2018)Söllinger, A., Tveit, A.T., Poulsen, M., Noel, S.J., Bengtsson, M., Bernhardt, J., Frydendahl Hellwing, A.L., Lund, P., Riedel, K., Schleper, Ch., Højberg, O. & Urich, T. 2018. "Holistic assessment of rumen microbiome dynamics through quantitative metatranscriptomics reveals multifunctional redundancy during key steps of anaerobic feed degradation". mSystems, 3(4): e00038-18, ISSN: 2379-5077. https://doi.org/10.1128/mSystems.00038-18. and Söllinger and Urich (2019)Söllinger, A. & Urich, T. 2019. "Methylotrophic methanogens everywhere—Physiology and ecology of novel players in global methane cycling". Biochemical Society Transactions, 47(6): 1895–1907, ISSN: 1470-8752. https://doi.org/10.1042/BST20180565. have found that less abundant methanogenic lineages may have a more significant role in CH₄ formation than the most represented rumen methanogens. Methanogens are less diverse than ruminal bacteria, and the type and abundance variation is due to host genetics as well as dietary, environmental, and ruminal factors (i.e., H₂ concentrations, pH, and interactions with other fermenting microbes). A deeper understanding of methanogens diversity under different environmental conditions and the mechanistic basis of methanogenesis are necessary to develop targeted and effective enteric methane mitigation strategies (Pitta et al. 2022Pitta, D., Indugu, N., Narayan, K. & Hennessy, M. 2022. "Symposium review: Understanding the role of the rumen microbiome in enteric methane mitigation and productivity in dairy cows". Journal of Dairy Science, 105(10): 8569-8585, ISSN: 1525-3198. https://doi.org/10.3168/jds.2021-21466.).

Mitigation strategies. Several reviews indicate the three main roads for mitigation are: animal and feed management, diet formulation, and rumen manipulation (Hristov et al. 2013Hristov, A.N., Oh, J., Firkins, J.L., Dijkstra, J., Kebreab, E., Waghorn, G., Makkar, H.P.S., Adesogan, A.T., Yang, W., Lee, C., Gerber, P.J., Henderson, B. & Tricarico, J.M. 2013. "Special topics—Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options". Journal of Animal Science, 91(11): 5045-5069, ISSN: 1525-3163. https://doi.org/10.2527/jas.2013-6583., Veneman et al. 2016Veneman, J.B., Saetnan, E.R., Clare, A.J. & Newbold, C.J. 2016. "MitiGate; an online meta-analysis database for quantification of mitigation strategies for enteric methane emissions". Science of the Total Environment, 572: 1166-1174, ISSN: 1879-1026. https://doi.org/10.1016/j.scitotenv.2016.08.029., Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119. and Tseten et al. 2022Tseten, T., Sanjorjo, R.A., Kwon, M. & Kim, S-W. 2022. "Strategies to Mitigate Enteric Methane Emissions from Ruminant Animals". Journal of Microbiology and Biotechnology, 32(3): 269-277, ISSN: 1738-8872. https://doi.org/10.4014/jmb.2202.02019.). Nevertheless, according to Arndt et al. (2022)Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119. methane yield is not the only relevant measure, other CH₄ emissions and animal performance metrics should be considered to estimate the feasibility of mitigation strategies. In this paper, only the nutritional strategies (diet formulation and rumen manipulation) will be evaluated (figure 2).

Diet formulation. Dietary manipulation by changing the feed composition and quality is a simple approach that may enhance animal productivity and reduces GHG emission (Khusro et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.). This strategy alone could obtain interesting results depending on the method or nature of the nutritional intervention (Mosier et al. 1998Mosier, A.R., Duxbury, J.M., Freney, J.R., Heinemeyer, O., Minami K. & Johnson, D.E. 1998. "Mitigating agricultural emissions of methane". Climatic Change, 40: 39-80, ISSN: 1573-1480. and Benchaar et al. 2001Benchaar, C., Pomar, C. & Chiquette, J. 2001. "Evaluation of dietary strategies to reduce methane production in ruminants: a modelling approach". Canadian Journal of Animal Science, 81: 563-574, ISSN: 1918-1825. https://doi.org/10.4141/A00-119.). The predominant approach is to improve forage quality or change the forage type or proportion or add supplements such as probiotics, oils, and enzymes that either reduce methanogenesis or alter the metabolic pathways leading to the H₂ reduction as a useful substrate.

Forage quality. CH₄ production might be reduced by improving forage quality, feeding less-mature plants, switching from C₄ to C₃ grasses, or even grazing on less-mature pastures (Ulyatt et al. 2002Ulyatt, M.J., Lassey, K.R., Shelton, I.D. & Walker, C.F. 2002. "Methane emission from dairy cows and wether sheep fed subtropical grass‐dominant pastures in midsummer in New Zealand". New Zealand Journal of Agricultural Research, 45(4): 227-234, ISSN: 1175-8775. https://doi.org/10.1080/00288233.2002.9513513. and Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). These forages contain higher amounts of easily fermentable carbohydrates and less NDF, leading to a higher digestibility and faster passage rate in the rumen. In contrast, more mature forage induces a higher CH₄ yield mainly due to an increased C:N ratio, which decreases the digestibility.

Methane production per unit of cellulose digested is three times that of hemicellulose (Moe and Tyrrell 1979Moe, P.W. & Tyrell, H.F. 1979. "Methane production in dairy cows". Journal of Dairy Science, 62(10): 1583-1586, ISSN: 1525-3198. http://dx.doi.org/10.3168/jds.S0022-0302(79)83465-7.). Cellulose and hemicellulose ferment more slowly than non-structural carbohydrates, thus yielding more CH₄ per unit of the digested substrate (McAllister et al. 1996McAllister, T.A., Cheng, K.J., Okine E.K. & Mathison, G.W. 1996. "Dietary, environmental and microbiological aspects of methane production in ruminants". Canadian Journal of Animal Science, 76: 231-243, ISSN: 1918-1825. http://dx.doi.org/10.4141/CJAS96-035.). Consequently, the addition of grain to the diet increases starch and reduces fibre intake, reducing the rumen pH and favouring the production of propionate rather than acetate in the rumen (McAllister and Newbold 2008McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218. and Hills et al. 2015Hills, J.L., Wales, W.J., Dunshea, F.R., Garcia, S.C. & Roche, J.R. 2015. "Invited review: an evaluation of the likely effects of individualized feeding of concentrate supplements to pasture-based dairy cows". Journal of Dairy Science, 98: 1363-1401, ISSN: 1525-3198. https://doi.org/10.3168/jds.2014-8475.). Improving forage quality also tends to increase the DM intake and reduce the retention time in the rumen, promoting energetically more efficient post-ruminal digestion and reducing the proportion of energy converted to CH₄ (Blaxter and Clapperton 1965Blaxter, K.L. & Clapperton, J.L. 1965. "Prediction of the amount of methane produced by ruminants". British Journal of Nutrition, 19(4): 511-522, ISSN: 1475-2662. https://doi.org/10.1079/bjn19650046.). Methane emissions are also commonly lower with higher proportions of forage legumes in the diet, partly because of the lower fibre content, the faster retention time, and in some cases, the presence of condensed tannins (Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.).

Improving forage quality can both improve animal performance and reduce CH₄ production, but it can also improve efficiency by reducing CH₄ emissions per unit of animal product (Beauchemin et al. 2009Beauchemin, K.A., McAllister, T.A. & McGinn, S.M. 2009. "Dietary mitigation of enteric methane from cattle". CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 4(35): 1-8, ISSN: 1749-8848. https://doi.org/10.1079/PAVSNNR20094035.). However, many of these strategies may also provide the farmer with an opportunity to increase the stocking rate, leading to a no net change or even a net increase in CH₄ production. Similarly, the addition of more grain to the diet will incur additional N₂O emissions and transport during the grain production processes.

Forage processing and preservation also affect methane emissions (Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). Chopping or pelleting forages reduces the feed size and consequently less degradation in the rumen as well as CH₄ emissions per kg DM intake (Boadi et al. 2004Boadi, D., Benchaar, C., Chiquette, J. & Massé, D. 2004. "Mitigation strategies to reduce enteric methane emissions from dairy cows: an updated review". Canadian Journal of Animal Science, 84: 319-335, ISSN: 1918-1825. https://doi.org/10.4141/A03-109.).

Therefore, further research and modelling are required to understand the likely relationships between improvements in diet quality and voluntary intake, stocking rates, and net CH₄ production in various production systems.

Lipids. The efficacy of fat supplementation depends on the fat source, quantity, fatty acid profile, the form in which the fat is added (refined oil/full-fat oilseeds), and the diet (Bauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). Fats supplementation effect could be summarized as: reduction of fibre digestion (mainly in long-chain fatty acids); decreased DM intake (if total dietary fat exceeds 6-7 %); decreased organic matter fermentation; reduction of activities of different microbes including methanogens and hydrogen producing microorganisms; reduction of rumen protozoa number; and to a limited extent biohydrogenation of unsaturated fatty acids which serve as a hydrogen sink, although only 1-2 % of the metabolic hydrogen in the rumen is used for this purpose (Bauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.,Eckard et al. 2010Eckard, R.J., Grainger, C. & De Klein, C.A.M. 2010. "Options for the abatement of methane and nitrous oxide from ruminant production: A review". Livestock Science, 130 (1-3): 47-56, ISSN: 1871-1413. https://doi.org/10.1016/j.livsci.2010.02.010. and Samal and Dash 2022Samal, L. & Dash, S.K. 2022. Nutritional Interventions to Reduce Methane Emissions in Ruminants', in A. K. Patra (ed.), Animal Feed Science and Nutrition - Production, Health and Environment, IntechOpen, London. 10.5772/intechopen.101763.).

The addition of different vegetable oils (soybean, coconut, canola, rapeseed, sunflower, linseed) to ruminant diets has been shown to reduce CH₄production. Moreover, fats are not metabolized in the rumen and therefore do not contribute to methanogenesis (Johnson and Johnson 1995Johnson, K.A. & Johnson, D.E. 1995. "Methane emissions from cattle". Journal of Animal Science, 73(8): 2483-2492, ISSN: 1525-3163. https://doi.org/10.2527/1995.7382483x.). Seen the substantial body of literature, lipids addition to the diet is considered a promising technique.

Essential oils and plant metabolites. Supplements from biological sources have been investigated recently as feed ingredients and additives to mitigate emissions (Salem et al. 2014Salem, A.Z.M., Kholif, A.E. & Elghandour, M.M. 2014. "Effect of increasing levels of seven tree species extracts added to a high concentrate diet on in vitro rumen gas output". Animal Science Journal, 85: 853–860, ISSN: 1740-0929. https://doi.org/10.1111/asj.12218. and Bayat et al. 2018Bayat, A.R., Tapio, I., Vilkki, J., Shingfield, K.J. & Leskinen, H. 2018. "Plant oil supplements reduce methane emissions and improve milk fatty acid composition in dairy cows fed grass silage-based diets without affecting milk yield". Journal of Dairy Science, 101(2): 1136–1151, ISSN: 1525-3198. https://doi.org/10.3168/jds.2017-13545.).

Tekippe et al. (2012)Tekippe, J.A., Hristov, A.N., Heyler, K.S., Zheljazkov, V.D., Ferreira, J.F.S., Cantrell, C.L. & Varga, G.A., 2012. "Effects of plants and essential oils on ruminal in vitro batch culture methane production and fermentation". Canadian Journal of Animal Science, 92(3): 395-408, ISSN: 1918-1825. https://doi.org/10.4141/CJAS2012-059. tested 100 essential oils (EO) and plants for their ability to reduce methanogenesis. Essential oils are volatile and aromatic oily liquids extracted from plant materials such as flowers, seeds, buds, leaves, herbs, wood, fruits, twigs, and roots (Burt 2004Burt, S. 2004. "Essential oils: their antibacterial properties and potential applications in foods, a review". International Journal of Food Microbiology, 94(3): 223-253, ISSN: 1879-3460. https://doi.org/10.1016/j.ijfoodmicro.2004.03.022.). They demonstrate broad-spectrum antimicrobial properties, inhibit rumen archaea, alter the rumen fermentation path by inhibiting fibrolytic bacteria (Cobellis et al. 2016Cobellis, G., Trabalza-Marinucci, M. & Marcotullio, M.C. 2016. "Evaluation of different essential oils in modulating methane and ammonia production, rumen fermentation, and rumen bacteria in vitro". Animal Feed Science and Technology, 215: 25–36, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2016.02.008.), and are generally considered safe for human and animal consumption (Davoodi et al. 2019Davoodi, S.M., Mesgaran, M.D., Vakili, A.R., Valizadeh, R. & Pirbalouti, A.G. 2019. "In vitro effect of essential oils on rumen fermentation and microbial nitrogen yield of high concentrate dairy cow diet". Biosciences, Biotechnology Research Asia, 16(2): 333-341, ISSN: 0973-1245. https://doi.org/10.13005/bbra/2749.). Some inhibit the growth of protozoa indirectly or by biohydrogenation of unsaturated fatty acids limiting the hydrogen availability for methanogens (Iqbal et al. 2008Iqbal, M.F., Cheng, Y.F., Zhu, W.Y. & Zeshan, B. 2008. "Mitigation of ruminant methane production: current strategies, constraints and future options". World Journal of Microbiology & Biotechnology, 24(12): 2747-2755, ISSN:1573-0972. https://doi.org/10.1007/s11274-008-9819-y. and Toprak 2015Toprak, N.N. 2015. "Do fats reduce methane emissions by ruminants? - A review". Animal Science Papers and Report, 33(4): 305-321, ISSN: 2300-8342.). Nevertheless, they produce a scarce effect in vivo, probably due to the rumen adaptation mechanism. Moreover, the reduction of fibre digestibility is another issue as it reduces animal performance (Benchaar and Greathead 2011Benchaar, C. & Greathead, H. 2011. "Essential oils and opportunities to mitigate enteric methane emissions from ruminants". Animal Feed Science and Technology, 166: 338-355, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2011.04.024.).

Numerous researches evaluated the efficacy of plant secondary metabolites as a mitigation strategy, (including saponins, flavonoids, tannins, and other terpenoids), mostly in vitro and with inconsistent results. Hydrolyzable tannins inhibit rumen methanogens bacteria, while condensed ones inhibit fibre digestion (Khusro et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.). Saponins decrease protein degradation and favour microbial and biomass synthesis (Makkar and Beker 1996Makkar, H.P.S. & Becker, K. 1996. "Effect of pH, temperature, and time on inactivation of tannins and possible implications in detannification studies". Journal of Agricultural and Food Chemistry, 44(5): 1291–1295, ISSN: 0021-8561. https://doi.org/10.1021/jf9506287.), two processes that reduce hydrogen availability (Dijkstra et al. 2007Dijkstra, J., Bannink, A., France, J. & Kebreab, E. 2007. "Nutritional control to reduce environmental impacts of intensive dairy cattle systems". In Proceedings of the VII International Symposium on the Nutrition of Herbivores (ed. QX Meng, LP Ren and ZJ Cao). China Agricultural University Press, Beijing, China. pp. 411–435.). However, the saponins effect seems related to their anti-protozoal effect (Newbold and Rode 2006Newbold, C.J. & Rode, L.M. 2006. Dietary additives to control methanogenesis in the rumen. In Greenhouse gases and animal agriculture: an update (ed. CR Soliva, J Takahashi and M Kreuzer), Elsevier International Congress Series 1293, pp. 138–147. Elsevier, Amsterdam, The Netherlands).

Additional Organic Additives Biochar has also been tested in the last decade because of its effect on growth, egg yield, blood profiles, inhibitory effects against the growth of rumen pathogens, and the reduction of enteric methane emission (Leng et al. 2012Leng, R.A., Preston, T.R. & Inthapanya, S. 2012. "Biochar reduces enteric methane and improves growth and feed conversion in local “Yellow” cattle-fed cassava root chips and fresh cassava foliage". Livestock Research Rural Development, 24(11): 199, ISSN: 2521-9952. Available: http://www.lrrd.org/lrrd24/11/leng24199.htm. and Man et al. 2021Man, K.Y., Chow, K.L., Man, Y.B., Mo, W.Y. & Wong, M.H. 2021. "Use of biochar as feed supplements for animal farming". Critical Reviews in Environmental Science and Technology, 51(2): 187-217, ISSN: 1547-6537. https://doi.org/10.1080/10643389.2020.1721980.).

Seaweeds. Seaweeds known as macroalgae, including brown (Phaeophyta), red (Rhodophyta), and green (Chlorophyta) seaweeds are rich in bioactive compounds including proteins, carbohydrates, and to a lesser extent, lipids, saponins, alkaloids, and peptides. These bioactive could also play a role as feed ingredients to decrease enteric CH₄ (Abbott et al. 2020Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432.). The reduction is largely attributed to the compound bromoform which is found in several seaweed species especially red seaweeds like Asparagopsis spp. and is known to inhibit the CH₄ biosynthetic pathway within methanogens (Machado et al. 2015Machado, L., Kinley, R.D., Magnusson, M., de Nys, R. & Tomkins, N.W. 2015. "The potential of macroalgae for beef production systems in Northern Australia". Journal of Applied Phycology, 27(5): 2001–2005, ISSN: 1573-5176. https://doi.org/10.1007/s10811-014-0439-7.).

Several in vitro studies of seaweed supplements have been carried out, but gaps remain in current knowledge regarding the efficacy of seaweeds to tackle climate change both as a diet supplement and feed for livestock. The potential positive and negative environmental and economic impacts of seaweed farming on a large scale are still to clarify (Abbott et al. 2020Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432.).

Additives. Several additives consisting of either inorganic or organic compounds or direct-fed probiotics have been added to feed to reduce methane emissions in ruminants. These additives either directly inhibit methanogens or alter the metabolic pathways leading to a reduction of the substrate for it (Halmemies-Beauchet-Filleau et al. 2018Halmemies-Beauchet-Filleau, A., Rinne, M., Lamminen, M., Mapato, C., Ampapon, T., Wanapat, M. & Vanhatalo, A. 2018. "Review: Alternative and novel feed for ruminants: nutritive value, product quality and environmental aspects". Animal, 12(s2): s295-s309, ISSN: 1751-732X. https://doi.org/10.1017/S1751731118002252. and Haque 2018Haque, M.N. 2018. "Dietary manipulation: a sustainable way to mitigate methane emissions from ruminants". Journal of Animal Science and Technology, 60: 15, ISSN: 2055-0391. https://doi.org/10.1186/s40781-018-0175-7.).

Exogenous enzymes. Cellulase, xylanase, and hemicellulase have been used in ruminant diets as feed additives. These enzymes can improve fibre digestibility and animal productivity (Beauchemin et al. 2003Beauchemin, K.A., Colombatto, D., Morgavi, D.P. & Yang, W.Z. 2003. "Use of exogenous fibrolytic enzymes to improve feed utilization by ruminants". Journal of Animal Science, 81: E37–E47, ISSN: 1525-3163.). They also decrease the acetate/propionate ratio in the rumen, thus reducing CH₄ production (Eun and Beauchemin 2007Eun, J.S. & Beauchemin, K.A. 2007. "Assessment of the efficacy of varying experimental exogenous fibrolytic enzymes using in vitro fermentation characteristics". Animal Feed Science and Technology, 132 (3-4): 298-315, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2006.02.014.). However, the supplementation of exogenous enzymes at the farm level is very limited (Khusro et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.).

Ionophores. Commercially available ionophores such as monensin, lasalocid, salinomycin, and laidlomycin are widely applied as feed additives to dairy cows’ diets in many countries and have been used to increase milk production, improve feed efficiency and prevent metabolic disorders (McGuffey et al.2001McGuffey, R.K., Richardson, L.F. & Wilkinson, J.I. 2001. "Ionophores for dairy cattle: current status and future outlook". Journal of Dairy Science, 84: E194-E203, ISSN: 1525-3198. http://dx.doi.org/10.3168/jds.S0022-0302(01)70218-4.). They benefit animal metabolism by enhancing the efficiency of energy metabolism, improving ruminal nitrogen metabolism while modulating intake, optimizing fermentation routes, and reducing the rates of digestive disorders (Duffield et al. 2012Duffield, T.F., Merrill, J.K. & Bagg, R.N. 2012. "A Meta-analysis of the effects of monensin in beef cattle on feed efficiency, body weight gain, and dry matter intake". Journal of Animal Science, 90: 4583–4592, ISSN: 1525-3163. https://doi.org/10.3168/jds.2007-0607.). Ionophores also act as antimicrobials, preferentially inhibiting gram-positive bacteria that produce lactate, acetate, butyrate, formate, and hydrogen as end products, resulting in a propionate increased and an acetate-reduced concentration (Marques and Cooke 2021Marques, R.D. & Cooke, R.F. 2021. "Effects of ionophores on the ruminal function of beef cattle". Animals, 11(10): 2871, ISSN: 2076-2615. https://doi.org/10.3390/ani11102871.). They also affect protozoa (Guan et al. 2006Guan, H., Wittenberg, K.M., Ominski, K.H. & Krause, D.O. 2006. "Efficacy of ionophores in cattle diets for mitigation of enteric methane". Journal of Animal Science, 84: 1896-1906, ISSN: 1525-3163. https://doi.org/10.2527/jas.2005-652.).

The pressure to reduce the use of antimicrobials in livestock production suggests that is not a long-term solution. Furthermore, this family of additives is not permitted in many countries, Europe included.

Organic acids. The addition of organic acids like fumarate, malate, and acrylate, precursors to propionate production in the rumen, can be an alternative H₂ sink, reducing methanogenesis. McAllister and Newbold (2008)McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218. reviewed studies that showed 0 % - 75 % reductions in CH₄ achieved by feeding fumaric acid. Organic acid supplementation has mostly been tested for CH₄ production in vitro, producing inconsistent results. Moreover, at the relatively high doses required, dicarboxylic acids are prohibitively expensive as an abatement strategy.

Rumen manipulation. Manipulating microbial populations in the rumen either by chemical means or by introducing competitive or predatory microbes, or with vaccination approaches, can reduce CH₄ production. Biological control strategies, such as bacteriophages or bacteriocins, could prove effective in directly inhibiting methanogens and redirecting H₂ to other reductive rumen bacteria, such as propionate producers or acetogens (McAllister and Newbold 2008McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218.). However, they still require significant research for a prolonged period to deliver commercially viable vaccines or biological control options that might be useful in different production systems and areas.

Vaccination. Methane reduction in ruminants could be obtained by vaccination and the strategy has been considered promising by many authors. Numerous trials are reported to reduce methane emissions by vaccination with extremely variable results (from a 20 % increase to a 69 % reduction in methane production, Baca-Gonzalez et al. 2020Baca-Gonzalez, V., Asensio-Calavia, P., Gonzalez-Acosta, S., Perez de la Lastra, J.M. & Morales de la Nuez, A. 2020. "Are vaccines the solution for methane emissions from ruminants? A systematic review". Vaccines 8(3): 460, ISSN: 2076-393X. https://doi.org/10.3390/vaccines8030460.). Some of the causes of vaccination failures in reducing methane output are methanogens diversity, different animal-rearing conditions, and rumen adaptation (Williams et al. 2009Williams, Y.J., Popovski, S., Rea, S.M., Skillman, L.C., Toovey, A.F., Northwood, K.S. & Wright, A.D.G. 2009. "A vaccine against rumen methanogens can alter the composition of archaeal populations". Applied and Environment Microbiology, 75: 1860–1866, ISSN: 1098-5336. https://doi.org/10.1128/AEM.02453-08.). Nevertheless, it is complicated to evaluate the real effectiveness of this strategy as few studies have directly assessed the complete approach, i.e., from vaccination to enteric animal CH₄ emission measurement. Therefore, for successful vaccination against methanogens, a much more broad-spectrum approach is required with a greater understanding rumen methanogen population (Mir and Begun 2022Mir, N.A. & Begum, J. 2022. "Rumen microbial system, methanogenesis, and methane mitigation strategies in ruminants: Methanogenesis in ruminants". Letters in Animal Biology, 2(1): 12-22.).

Defaunation. Defaunation is the protozoa removal from the rumen. It has been reported to reduce methane emissions by as high as 50 % depending on the diet (Hegarty 1999Hegarty, R.S. 1999. "Reducing rumen methane emissions through the elimination of rumen protozoa". Australian Journal of Agricultural Research, 50: 1321–1327, ISSN: 1467-8489. https://doi.org/10.1071/AR99008.). The protozoa are associated with methanogens and are large producers of H₂ in the rumen so favouring the process of methane production by methanogens.

In defaunated animals, the lower methane production was sustained for more than two years which indicates a stable change induced by defaunating agents (Morgavi et al. 2008Morgavi, D.P., Jouany, J.P., Martin, C. 2008. "Changes in methane emission and rumen fermentation parameters induced by refaunation in sheep". Australian Journal of Experimental Agriculture, 48(2): 69–72, ISSN: 1446-5574. https://doi.org/10.1071/EA07236.). However, in some cases, this reduction in methane production is not consistent (Hegarty et al. 2008Hegarty, R.S., Bird, S.H., Vanselow, B.A. & Woodgate, R. 2008. "Effects of the absence of protozoa from birth or from weaning on the growth and methane production of lambs". British Journal of Nutrition, 100(6): 1220–1227, ISSN: 1475-2662. https://doi.org/10.1017/S0007114508981435.). Moreover, it may negatively affect the normal rumen functions and in turn the animals’ performance (Mir and Begun 2022Mir, N.A. & Begum, J. 2022. "Rumen microbial system, methanogenesis, and methane mitigation strategies in ruminants: Methanogenesis in ruminants". Letters in Animal Biology, 2(1): 12-22.).

Direct-fed microbials. Direct-fed microbials (DFM) is defined as a single or mixed culture of live organisms, which promotes desirable rumen microflora and provide beneficial effects when fed to animals (Krehbiel et al. 2003Krehbiel, C.R., Rust, S.R., Zhang, G. & Gilliland, S.E. 2003. "Bacterial direct-fed microbial in ruminant diets: performance response and mode of action". Journal of Animal Science, 81(4): E120-132, ISSN: 1525-3163. https://doi.org/10.2527/2003.8114_suppl_2E120x.). Various rumen bacteria are thought to compete with methanogens for the hydrogen supply by promoting propionogenesis, acetogenesis, and nitrate/nitrite or sulfate reduction which can serve as an alternative H₂ sink. This redirects the metabolic flow of rumen hydrogen toward VFAs production which could otherwise be used for methanogenesis (Ungerfeld 2015Ungerfeld, E.M. 2015. "Shifts in metabolic hydrogen sink in the methanogenesis-inhibited ruminal fermentation: a meta-analysis". Frontiers in Microbiology, 6: 37, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2015.00037.).

Since H₂ is a limiting substrate for methane production, the addition of propionate-forming bacteria might help in lowering methane production (Jeyanathan et al. 2014Jeyanathan, J., Martin, C. & Morgavi, D.P. 2014. "The use of direct-fed microbial for mitigation of ruminant methane emissions: a review". Animal, 8(2): 250-261, ISSN: 1751-732X. https://doi.org/10.1017/S1751731113002085.). However in vivo, Propionibacteria spp. do not last in the rumen of cattle when a starch-rich diet is administered. High starch fermentation results in an increased molar proportion of propionate reducing their efficacy (Jeyanathan et al. 2019Jeyanathan, J., Martin, C., Eugène, M., Ferlay, A., Popova, M. & Morgavi, D.P. 2019. "Bacterial direct fed microbial fail to reduce methane emissions in primiparous lactating dairy cows". Journal of Animal Science and Biotechnology, 10: 41, ISSN: 2049-1891. https://doi.org/10.1186/s40104-019-0342-9.).

Acetogens. Homoacetogens are a group of bacteria producing acetate (Drake et al. 2008Drake, H.L., Gößner, A.S. & Daniel, S.L. 2008. "Old acetogens, new light". Annals of the New York Academy of Sciences, 1125: 100-128, ISSN: 1749-6632. https://doi.org/10.1196/annals.1419.016.). In vitro studies have also suggested that acetogenesis could serve as an alternative to methanogenesis in eliminating H₂ from the rumen (Morvan et al. 1996Morvan, B., Bonnemoy, F., Fonty, G., Gouet, P. 1996. "Quantitative determination of H2-utilizing acetogenic and sulfate-reducing bacteria and methanogenic archaea from the digestive tract of different mammals". Current Microbiology, 32(3): 129-133, ISSN: 1432-0991. https://doi.org/10.1007/s002849900023.). Nevertheless, Lopez et al. (1999)Lopez, S., McIntosh, F.M, Wallace, R.J. & Newbold, C.J. 1999. "Effect of adding acetogenic bacteria on methane production by mixed rumen microorganisms". Animal Feed Science and Technology, 78(1-2): 1-9, ISSN: 1873-2216. https://doi.org/10.1016/S0377-8401(98)00273-9. reported that high concentrations of acetogenic bacteria cannot compete against methanogens for H₂ disposal, making it unclear whether homoacetogens could play a pivotal role in the ruminal ecosystem (Henderson et al. 2010Henderson, G., Naylor, G.E., Leahy, S.C. & Janssen, P.H. 2010. "Presence of novel, potentially homoacetogenic bacteria in the rumen as determined by analysis of formyltetrahydrofolate synthetase sequences from ruminants". Applied and Environmental Microbiology, 76: 2058-2066, ISSN: 1098-5336. https://doi.org/10.1128/AEM.02580-09.).

Methane Oxidizing Bacteria (MOB) is a class of bacteria that can grow on methane as a sole carbon and energy source. However, in vivo studies using MOB as probiotics are scarce and need to expand to verify its probiotic potential.

El metano es un gas de efecto invernadero (GEI) mucho menos abundante que el CO₂, pero con un potencial de calentamiento global 28 veces mayor en una escala de 100 años (Jackson et al. 2020Jackson, R.B., Saunois, M., Bousquet, P., Canadell, J.G., Poulter, B., Stavert, A R., Bergamaschi, P., Niwa, Y., Segers, A. & Tsuruta, A. 2020. "Increasing anthropogenic methane emissions arise equally from agricultural and fossil fuel sources". Environmental Research Letters, 15: 071002, ISSN: 1748-9326. https://doi.org/10.1088/1748-9326/ab9ed2.). Las fuentes de metano más abundantes incluyen las emisiones antropogénicas de la agricultura, el manejo de desechos, combustibles fósiles y las emisiones naturales de humedales, sistemas de agua dulce y fuentes geológicas (Saunois et al. 2016Saunois, M., Jackson, R.B., Bousquet, P., Poulter, B. & Canadell, J.G. 2016. "The growing role of methane in anthropogenic climate change". Environmental Research Letters, 11: 120207, ISSN: 1748-9326. https://doi.org/10.1088/1748-9326/11/12/120207.). La agricultura contribuye con un porcentaje que varía del 8 al 18 % de las emisiones antropógenas totales de GEI y los rumiantes representan alrededor del 81 % de los GEI del ganado, lo que incluye la fermentación entérica (alrededor del 90 %) y el manejo del estiércol (Hristov et al. 2013Hristov, A.N., Oh, J., Firkins, J.L., Dijkstra, J., Kebreab, E., Waghorn, G., Makkar, H.P.S., Adesogan, A.T., Yang, W., Lee, C., Gerber, P.J., Henderson, B. & Tricarico, J.M. 2013. "Special topics—Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options". Journal of Animal Science, 91(11): 5045-5069, ISSN: 1525-3163. https://doi.org/10.2527/jas.2013-6583.). Entre las emisiones directas relacionadas con rumiantes, el ganado bovino es responsable del 65 %, los búfalos del 8 % y las ovejas y cabras del 7 % (figura 1) (Steinfeld et al. 2019Steinfeld, H. Opio, C., Chara, J., Davis, K.F., Tomlin, P. & Gunter, S. 2019. Overview paper: Livestock, Climate and Natural Resource Use, http://www.livestockdialogue.org/fileadmin/templates/res_livestock/docs/2019_Sept_Kansas/4_Climate_and_Natural_Resource_Use_-_Online_consultation.pdf [Consulted: August 10, 2022]). Según las emisiones de GEI de 2010, para limitar el calentamiento global a 1.5 °C, las emisiones agrícolas deberían reducirse entre 11 % y 30 % para 2030 y entre 24 % y 47 % para 2050 (Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119.).

El ganado sustenta a millones de personas en el mundo (hasta el 12 %), tanto en países en desarrollo como desarrollados. Se ha estimado que la población mundial alcanzará los 9700 millones en 2050 y los 10400 millones en 2100 (ONU 2022United Nations (UN). 2022. Available: https://www.un.org/en/desa/world-population-projected-reach-98-billion-2050-and-112-billion-2100 [Consulted: August 13, 2022]), particularmente en los países de bajos y medianos ingresos (LMC), junto con la creciente producción y demanda de productos lácteos y cárnicos en 35 % (1168 Mt) y 44 % (373 Mt), respectivamente (IFCN 2018International Federation of Clinical Neurophysiology (IFCN). 2018. Available: https://ifcndairy.org/wp-content/uploads/2018/06/IFCN-Dairy-Outlook-2030-Brochure.pdf. [Consulted: August 13, 2022]).

Existe una preocupación creciente de que la demanda de productos animales, relacionada con el crecimiento de la población, la esperanza de vida prolongada y la mejora del bienestar económico en los países en desarrollo, suponga un llamado insostenible para el medio ambiente (Salter 2017Salter, A.M. 2017. "Improving the sustainability of global meat and milk production". Proceedings of the Nutrition Society, 76(1): 22-27, ISSN: 1475-2719. https://doi.org/10.1017/S0029665116000276.). Sin embargo, los rumiantes, especialmente cuando consumen alimentos producidos en tierras no aptas para cultivos primarios o subproductos de la agroindustria, pueden ser un contribuyente neto al abastecimiento mundial de alimentos para humanos, manteniendo y mejorando el suministro de proteínas y micronutrientes esenciales (zinc, calcio, Vit. B₁₂ y riboflavina) (Scollan et al. 2011Scollan, N.D., Hocquette, J.F., Richardson, R.I., Kim E.J., Wood, J.D. & Rowlings C. 2011. Raising the nutritional value of beef and beef products to add value in beef production. Nutrition and climate change: major issues confronting the meat industry (ed. JD Wood and C Rowlings) : 79-104.).

Un esfuerzo mundial de investigación masiva ha analizado varias estrategias de mitigación que se pueden agrupar en tres categorías: cambios en el manejo de animales y alimentos, formulación de dietas y manipulación del rumen (Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119.). Todas las estrategias involucran cambios potenciales en el microbioma del rumen (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). La producción de metano en el rumen también representa una pérdida de energía (de 2 a 12 % del consumo de energía bruta) para el crecimiento y la producción animal (Johnson y Johnson 1995Johnson, K.A. & Johnson, D.E. 1995. "Methane emissions from cattle". Journal of Animal Science, 73(8): 2483-2492, ISSN: 1525-3163. https://doi.org/10.2527/1995.7382483x.). Por lo tanto, la reducción de las emisiones de CH₄ beneficiaría al medio ambiente y eventualmente a la eficiencia de la producción ganadera.

Metanogénesis y comunidad microbiana del rumen. Los rumiantes viven de materia vegetal utilizando su sistema digestivo especializado con una red simbiótica bien adaptada de microorganismos (Cammack et al. 2018Cammack, K.M., Austin, K.J., Lamberson, W.R., Conant, G.C. & Cunningham, H.C. 2018. "Ruminant nutrition symposium: tiny but mighty: the role of the rumen microbes in livestock production". Journal of Animal Science, 96(2): 752-770, ISSN: 1525-3163. https://doi.org/10.1093/jas/skx053.) que incluye protozoos ciliados, hongos anaerobios, bacterias y arqueas que han co-evolucionado con su huésped (Huws et al. 2018Huws, Sh.A., Creevey, Ch.J., Oyama, L.B., Mizrahi, I., Denman, S.E., Popova, M., Muñoz-Tamayo, R., Forano, E., Waters, S.M., Hess, M., Tapio, I., Smidt, H., Krizsan, S.J., Yáñez-Ruiz, D.R., Belanche, A., Guan, L., Gruninger, R.J., McAllister, T.A., Newbold, C.J., Roehe, R., Dewhurst, R.J., Snelling, T.J., Watson, M., Suen, G., Hart, E.H., Kingston-Smith, A.H., Scollan N.D., M do Prado, R., Pilau, E.J., Mantovani, H.C., Attwood, G.T., Edwards, J.E., McEwan, N.R., Morrisson, S., Mayorga, O.L., Elliott, Ch. & Morgavi, D.P. 2018. "Addressing global ruminant agricultural challenges through understanding the rumen microbiome: Past, Present, and Future". Frontiers Microbiology, 9: 2161, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2018.02161., Henderson et al. 2015Henderson, G. Cox, F., Ganesh, S., Jonker, A., Young, W., Global Rumen Census collaboration & Janssen, P.H. 2015. "Rumen microbial community composition varies with diet and host, but a core microbiome is found across a wide geographical range". Scientific Reports, 5: 14567, ISSN: 2045-2322. https://doi.org/10.1038/srep14567. y Sasson et al. 2017Sasson, G., Kruger Ben-Shabat, Sh., Seroussi, E., Doron-Faigenboim, A., Shterzer, N., Yaacoby Sh., Berg Miller, M.E., White, B.A., Halperin, E. & Mizrahi, I. 2017. "Heritable bovine rumen bacteria are phylogenetically related and correlated with the cow’s capacity to harvest energy from its feed". mBio, 8(4): e00703-17, ISSN: 2150-7511. https://doi.org/10.1128/mBio.00703-17.). Los protozoos pueden representar hasta la mitad de la biomasa ruminal (Hungate 1966Hungate, R.E. 1966. The rumen and its microbes. New York: Academic Press, 533 pp. Book ISBN: 9781483263625. y Newbold et al. 2015Newbold, C.J., De la Fuente, G., Belanche, A., Ramos-Morales, E. & McEwan, N.R. 2015. "The role of ciliate protozoa in the rumen". Frontiers in Microbiology, 6: 1313, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2015.01313.), los hongos pueden llegar al 20 % (ovinos, Rezaeian et al. 2004Rezaeian, M., Beakes, G.W. & Parker, D.S. 2004. "Distribution and estimation of anaerobic zoosporic fungi along the digestive tracts of sheep". Mycological Research, 108: 1227–1233, ISSN: 1469-8102. https://doi.org/10.1017/S0953756204000929.), arqueas entre 0.3 y 4 % (Janssen y Kirs 2008Janssen, P.H. & Kirs, M. 2008. "Structure of the archaeal community of the rumen". Applied Environmental Microbiology, 74(12): 3619-3625, ISSN: 1098-5536. https://doi.org/10.1128/AEM.02812-07.) y las bacterias como grupo mayoritario.

Las fermentaciones microbianas en el rumen tienen una función esencial en la capacidad de los rumiantes para utilizar materiales lignocelulósicos para producir ácidos grasos volátiles (AGV) y convertir el nitrógeno no proteico en proteína microbiana, que es una fuente esencial de energía y proteína para el huésped, mientras que el rumen proporciona a los microorganismos un entorno adecuado para crecer y prosperar (Cammack et al. 2018Cammack, K.M., Austin, K.J., Lamberson, W.R., Conant, G.C. & Cunningham, H.C. 2018. "Ruminant nutrition symposium: tiny but mighty: the role of the rumen microbes in livestock production". Journal of Animal Science, 96(2): 752-770, ISSN: 1525-3163. https://doi.org/10.1093/jas/skx053.). No obstante, los microorganismos también tienen efectos perjudiciales para el medio ambiente a través de la emisión de GEI y excreciones excesivas de N en las heces y la orina.

La metanogénesis ruminal de las arqueas sigue dos vías principales (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). La ruta hidrogenotrófica convierte el H₂ y CO₂ producido por protozoos, hongos y bacterias en CH₄, lo que reduce el metabolismo (Kittelman et al. 2013Kittelmann, S., Seedorf, H., Walters, W.A., Clemente, J.C., Knight, R., Gordon, J.I. & Janssen, P.H. 2013. "Simultaneous amplicon sequencing to explore co-occurrence patterns of bacterial, archaeal and eukaryotic microorganisms in rumen microbial communities". Plos One, 8(2): e47879, ISSN: 1932-6203. https://doi.org/10.1371/journal.pone.0047879. y Poulsen et al. 2013Poulsen, M., Schwab, C., Borg Jensen, B., Engberg, R.M., Spang, A., Canibe, N., Højberg, O., Milinovich, G., Fragner, L., Schleper, C. & Weckwerth, W. 2013. "Methylotrophic methanogenic Thermoplasmata implicated in reduced methane emissions from bovine rumen". Nature communications, 4(1): 1428, ISSN: 2041-1723. https://doi.org/10.1038/ncomms2432.). El formiato, que puede utilizarse por todas las arqueas más abundantes, se considera equivalente y se incluye en la categoría hidrogenotrófica (Janssen y Kirs 2008Janssen, P.H. & Kirs, M. 2008. "Structure of the archaeal community of the rumen". Applied Environmental Microbiology, 74(12): 3619-3625, ISSN: 1098-5536. https://doi.org/10.1128/AEM.02812-07. y Janssen 2010Janssen, P.H. 2010. "Influence of hydrogen on rumen methane formation and fermentation balances through microbial growth kinetics and fermentation thermodynamics". Animal Feed Science and Technology, 160(1-2): 1–22, ISSN: 1873-2216. https://doi.org/10.1016/j.anifeedsci.2010.07.002.). La segunda vía utiliza grupos metilo como sustratos, como los presentes en las metilaminas y el metanol (Poulsen et al. 2013Poulsen, M., Schwab, C., Borg Jensen, B., Engberg, R.M., Spang, A., Canibe, N., Højberg, O., Milinovich, G., Fragner, L., Schleper, C. & Weckwerth, W. 2013. "Methylotrophic methanogenic Thermoplasmata implicated in reduced methane emissions from bovine rumen". Nature communications, 4(1): 1428, ISSN: 2041-1723. https://doi.org/10.1038/ncomms2432. y De la Fuente et al. 2019De la Fuente, G., Yañez-Ruiz, D.R., Seradj, A.R., Balcells, J. & Belanche, A. 2019. "Methanogenesis in animals with foregut and hindgut fermentation: a review". Animal Production Science, 59(12): 2109-2122, ISSN: 1836-5787. https://doi.org/10.1071/AN17701.). Si se acumula H₂ en el rumen, se inhibe la reoxidación de NADH, el crecimiento microbiano, la digestión del forraje y la producción asociada de acetato, propionato y butirato, por lo que cualquier estrategia de mitigación que reduzca las poblaciones de metanógenos debe incluir alguna manera para la eliminar H₂ del rumen (Eckard et al. 2010Eckard, R.J., Grainger, C. & De Klein, C.A.M. 2010. "Options for the abatement of methane and nitrous oxide from ruminant production: A review". Livestock Science, 130 (1-3): 47-56, ISSN: 1871-1413. https://doi.org/10.1016/j.livsci.2010.02.010.).

La producción de propionato es el segundo sumidero principal de H₂ en el rumen, además de otros menores, como la reducción de nitrato/nitrito, la acetogénesis reductora y la biohidrogenación de ácidos grasos insaturados (Mitsumori y Sung 2008Mitsumori, M. & Sun, W. 2008. "Control of rumen microbial fermentation for mitigating methane emissions from the rumen". Asian-Australasian Journal of Animal Sciences, 21(1): 144-154, ISSN: 1976-5517. http://dx.doi.org/10.5713/AJAS.2008.R01. y Kobayashi 2010Kobayashi, Y. 2010. "Abatement of Methane Production from Ruminants: Trends in the Manipulation of Rumen Fermentation". Asian-Australasian Journal of Animal Science, 23(3): 410-416, ISSN: 1976-5517. https://doi.org/5713/AJAS.2010.R.01.). Por lo tanto, para una función ruminal óptima, la estrategia de reducción de metano debe ir acompañada de la mejora de la producción de propionato sin comprometer la digestión del alimento, estimulando las vías de utilización de H₂ e inhibiendo la población y la actividad de los metanógenos (Martin et al. 2010Martin, C., Morgavi, D.P. & Doreau, M. 2010. "Methane mitigation in ruminants: from microbe to the farm scale". Animal, 4(3): 351–365, ISSN: 1751-732X. https://doi.org/10.1017/S1751731109990620.).

Entonces, los metanógenos del tracto gastrointestinal producen metano como subproducto de la fermentación anaeróbica (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0.). Como único productor, sería razonable considerar que el incremento en número estaría asociado a una mayor producción de CH₄. No obstante, parecería que la composición de la comunidad de metanógenos, más que su tamaño, está correlacionada con la producción de metano y que la disponibilidad de H₂ y las interacciones dentro y entre los microorganismos productores de H₂ en el rumen influye en esta diversidad (Tapio et al. 2017Tapio, I., Snelling, T.J., Strozzi, F. & Wallace, R.J. 2017. "The ruminal microbiome associated with methane emissions from ruminant livestock". Journal of Animal Science and Biotechnology, 8: 7, ISSN: 2049-1891. https://doi.org/10.1186/s40104-017-0141-0., Abbot et al. 2020Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432. y Pitta et al. 2021Pitta, D.W., Melgar, A., Hristov, A.N., Indugu, N., Narayan, K. S., Pappalardo, C., Hennessy, M.L., Vecchiarelli, B., Kaplan-Shabtai, V., Kindermann, M. & Walker, N. 2021. "Temporal changes in total and metabolically active ruminal methanogens in dairy cows supplemented with 3-nitrooxypropanol". Journal of Dairy Science, 104(8): 8721–8735, ISSN: 1525-3198. https://doi.org/10.3168/jds.2020-19862.).

La secuenciación de próxima generación independiente del cultivo junto con los enfoques "ómicos", desarrollados en los últimos años, se han convertido en herramientas poderosas para comprender qué microorganismos se encuentran en el rumen, qué función desempeñan en la metanogénesis y cuál es el efecto de las estrategias de mitigación. Informes de Söllinger et al. (2018)Söllinger, A., Tveit, A.T., Poulsen, M., Noel, S.J., Bengtsson, M., Bernhardt, J., Frydendahl Hellwing, A.L., Lund, P., Riedel, K., Schleper, Ch., Højberg, O. & Urich, T. 2018. "Holistic assessment of rumen microbiome dynamics through quantitative metatranscriptomics reveals multifunctional redundancy during key steps of anaerobic feed degradation". mSystems, 3(4): e00038-18, ISSN: 2379-5077. https://doi.org/10.1128/mSystems.00038-18. y Söllinger y Urich (2019)Söllinger, A. & Urich, T. 2019. "Methylotrophic methanogens everywhere—Physiology and ecology of novel players in global methane cycling". Biochemical Society Transactions, 47(6): 1895–1907, ISSN: 1470-8752. https://doi.org/10.1042/BST20180565. encontraron que los linajes metanogénicos menos abundantes pueden tener un papel más significativo en la formación de CH₄ que los metanógenos ruminales más representados. Los metanógenos son menos diversos que las bacterias ruminales, y la variación de tipo y abundancia se debe a la genética del huésped, así como a factores dietéticos, ambientales y ruminales (es decir, concentraciones de H₂, pH e interacciones con otros microorganismos en fermentación). Se necesita una comprensión más profunda de la diversidad de metanógenos en diferentes condiciones ambientales y de la base mecánica de la metanogénesis, para desarrollar estrategias específicas y efectivas de mitigación de metano entérico (Pitta et al. 2022Pitta, D., Indugu, N., Narayan, K. & Hennessy, M. 2022. "Symposium review: Understanding the role of the rumen microbiome in enteric methane mitigation and productivity in dairy cows". Journal of Dairy Science, 105(10): 8569-8585, ISSN: 1525-3198. https://doi.org/10.3168/jds.2021-21466.).

Estrategias de mitigación. Varias referencias indican que las tres formas principales para la mitigación son el manejo de animales y alimentos, la formulación de dietas y la manipulación del rumen (Hristov et al. 2013Hristov, A.N., Oh, J., Firkins, J.L., Dijkstra, J., Kebreab, E., Waghorn, G., Makkar, H.P.S., Adesogan, A.T., Yang, W., Lee, C., Gerber, P.J., Henderson, B. & Tricarico, J.M. 2013. "Special topics—Mitigation of methane and nitrous oxide emissions from animal operations: I. A review of enteric methane mitigation options". Journal of Animal Science, 91(11): 5045-5069, ISSN: 1525-3163. https://doi.org/10.2527/jas.2013-6583., Veneman et al. 2016Veneman, J.B., Saetnan, E.R., Clare, A.J. & Newbold, C.J. 2016. "MitiGate; an online meta-analysis database for quantification of mitigation strategies for enteric methane emissions". Science of the Total Environment, 572: 1166-1174, ISSN: 1879-1026. https://doi.org/10.1016/j.scitotenv.2016.08.029., Arndt et al. 2022Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119. y Tseten et al. 2022Tseten, T., Sanjorjo, R.A., Kwon, M. & Kim, S-W. 2022. "Strategies to Mitigate Enteric Methane Emissions from Ruminant Animals". Journal of Microbiology and Biotechnology, 32(3): 269-277, ISSN: 1738-8872. https://doi.org/10.4014/jmb.2202.02019.). Sin embargo, según Arndt et al. (2022)Arndt, C., Hristov, A.N., Price, W.J., McClelland, S.C., Pelaez, A.M., Cueva, S.F., Oh, J., Dijkstra, J., Bannink, A., Bayat, A.R., Crompton, L.A., Eugène, M.A., Enahoro, D., Kebreab, E., Kreuzer, M., McGee, M., Martin, C., Newbold, Ch.J., Reynolds, Ch.K., Schwarm, A., Shingfield, K.J., Veneman, J.B., Yáñez-Ruiz, D.R. & Yu, Zh. 2022. "Full adoption of the most effective strategies to mitigate methane emissions by ruminants can help meet the 1.5° C target by 2030 but not 2050". Proceedings of the National Academy of Sciences, 119 (20): 2111294119, ISSN: 1091-6490. https://doi.org/10.1073/pnas.2111294119., el rendimiento de metano no es la única medida relevante, y que otras emisiones de CH₄ y medidas de rendimiento animal se deben tener en cuenta para estimar la viabilidad de las estrategias de mitigación. El presente estudio solo evaluará las estrategias nutricionales (formulación de la dieta y manipulación del rumen) (figura 2).

Formulación de la dieta. La manipulación de la dieta mediante el cambio de composición y calidad del alimento es un enfoque simple que puede mejorar la productividad animal y reducir las emisiones de GEI (Khusro et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.). Esta estrategia por sí sola podría tener resultados interesantes según el método o la naturaleza de la intervención nutricional (Mosier et al. 1998Mosier, A.R., Duxbury, J.M., Freney, J.R., Heinemeyer, O., Minami K. & Johnson, D.E. 1998. "Mitigating agricultural emissions of methane". Climatic Change, 40: 39-80, ISSN: 1573-1480. y Benchaar et al. 2001Benchaar, C., Pomar, C. & Chiquette, J. 2001. "Evaluation of dietary strategies to reduce methane production in ruminants: a modelling approach". Canadian Journal of Animal Science, 81: 563-574, ISSN: 1918-1825. https://doi.org/10.4141/A00-119.). El enfoque predominante es mejorar la calidad del forraje o cambiar la variedad de este o la proporción, o agregar suplementos como probióticos, aceites y enzimas que pueden reducir la metanogénesis o alterar las vías metabólicas que conducen a la reducción de H₂ como sustrato útil.

Calidad del forraje. La producción de CH₄ podría reducirse con el aumento de la calidad del forraje, la alimentación con plantas menos maduras, el cambio de pastos C₄ a C₃ o incluso con pastos menos maduros (Ulyatt et al. 2002Ulyatt, M.J., Lassey, K.R., Shelton, I.D. & Walker, C.F. 2002. "Methane emission from dairy cows and wether sheep fed subtropical grass‐dominant pastures in midsummer in New Zealand". New Zealand Journal of Agricultural Research, 45(4): 227-234, ISSN: 1175-8775. https://doi.org/10.1080/00288233.2002.9513513. y Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). Estos forrajes contienen mayor cantidad de carbohidratos fácilmente fermentables y menos de FDN, lo que conduce a una mayor digestibilidad y una tasa de pasaje más rápida en el rumen. Por el contrario, un forraje más maduro provoca un mayor rendimiento de CH₄ debido principalmente a una mayor relación C:N, lo que disminuye la digestibilidad.

La producción de metano por unidad de celulosa digerida es tres veces mayor que la de hemicelulosa (Moe y Tyrrell 1979Moe, P.W. & Tyrell, H.F. 1979. "Methane production in dairy cows". Journal of Dairy Science, 62(10): 1583-1586, ISSN: 1525-3198. http://dx.doi.org/10.3168/jds.S0022-0302(79)83465-7.). La celulosa y la hemicelulosa fermentan más lentamente que los carbohidratos no estructurales, produciendo así más CH₄ por unidad de sustrato digerido (McAllister et al.1996McAllister, T.A., Cheng, K.J., Okine E.K. & Mathison, G.W. 1996. "Dietary, environmental and microbiological aspects of methane production in ruminants". Canadian Journal of Animal Science, 76: 231-243, ISSN: 1918-1825. http://dx.doi.org/10.4141/CJAS96-035.). En consecuencia, la adición de granos a la dieta aumenta el almidón y disminuye el consumo de fibra, lo que reduce el pH del rumen y favorece la producción de propionato en lugar de acetato en el rumen (McAllister y Newbold 2008McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218. y Hills et al. 2015Hills, J.L., Wales, W.J., Dunshea, F.R., Garcia, S.C. & Roche, J.R. 2015. "Invited review: an evaluation of the likely effects of individualized feeding of concentrate supplements to pasture-based dairy cows". Journal of Dairy Science, 98: 1363-1401, ISSN: 1525-3198. https://doi.org/10.3168/jds.2014-8475.). Mejorar la calidad del forraje también tiende a aumentar el consumo de MS y reducir el tiempo de retención en el rumen, lo cual promueve una digestión post-ruminal energéticamente más eficiente y reduce la proporción de energía que se convierte en CH₄ (Blaxter y Clapperton 1965Blaxter, K.L. & Clapperton, J.L. 1965. "Prediction of the amount of methane produced by ruminants". British Journal of Nutrition, 19(4): 511-522, ISSN: 1475-2662. https://doi.org/10.1079/bjn19650046.). Las emisiones de metano también son comúnmente más bajas con proporciones más altas de leguminosas forrajeras en la dieta, en parte debido al menor contenido de fibra, el tiempo de retención más rápido y, en algunos casos, la presencia de taninos condensados (Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.).

Mejorar la calidad del forraje puede optimizar el desempeño animal y reducir la producción de CH₄, pero también puede aumentar la eficiencia al reducir las emisiones de CH₄ por unidad de producto animal (Beauchemin et al. 2009Beauchemin, K.A., McAllister, T.A. & McGinn, S.M. 2009. "Dietary mitigation of enteric methane from cattle". CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources, 4(35): 1-8, ISSN: 1749-8848. https://doi.org/10.1079/PAVSNNR20094035.). Sin embargo, muchas de estas estrategias también pueden brindarle al agricultor la oportunidad de aumentar la carga ganadera, lo que lleva a un cambio neto nulo o incluso a un aumento neto en la producción de CH₄. De manera similar, la adición de más granos a la dieta generará emisiones adicionales de N₂O y transporte durante los procesos de producción de granos.

El procesamiento y conservación del forraje también afectan las emisiones de metano (Beauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). Cortar o granular forrajes reduce el tamaño del alimento y, en consecuencia, reduce la degradación en el rumen, así como las emisiones de CH₄ por kg de MS consumido (Boadi et al. 2004Boadi, D., Benchaar, C., Chiquette, J. & Massé, D. 2004. "Mitigation strategies to reduce enteric methane emissions from dairy cows: an updated review". Canadian Journal of Animal Science, 84: 319-335, ISSN: 1918-1825. https://doi.org/10.4141/A03-109.).

Por lo tanto, se requieren más investigaciones y modelos para comprender las posibles relaciones entre las mejoras en la calidad de la dieta y el consumo voluntario, la carga animal y la producción neta de CH₄ en varios sistemas de producción.

Lípidos. La eficacia de la suplementación con grasas depende de la fuente, cantidad, el perfil de ácidos grasos, la forma en que se agrega la grasa (aceite refinado/semillas oleaginosas enteras) y la dieta (Bauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199.). El efecto de la suplementación con grasas podría resumirse como reducción de la digestión de la fibra (principalmente en ácidos grasos de cadena larga), descenso del consumo de MS (si la grasa dietética total supera el 6-7 %), disminución de la fermentación de materia orgánica, reducción de actividades de diferentes microorganismos incluyendo metanógenos y otros productores de hidrógeno, decrecimiento del número de protozoos en el rumen y, hasta cierto punto, la biohidrogenación de ácidos grasos insaturados que sirven como sumidero de hidrógeno, aunque solo el 1-2 % del hidrógeno metabólico en el rumen se utiliza para este propósito (Bauchemin et al. 2008Beauchemin, K.A., Kreuzer, M., O’Mara, F. & McAllister, T.A. 2008. "Nutritional management for enteric methane abatement: a review". Australian Journal of Experimental Agriculture, 48(2): 21-27, ISSN: 1446-5574. https://doi.org/10.1071/EA07199., Eckard et al. 2010Eckard, R.J., Grainger, C. & De Klein, C.A.M. 2010. "Options for the abatement of methane and nitrous oxide from ruminant production: A review". Livestock Science, 130 (1-3): 47-56, ISSN: 1871-1413. https://doi.org/10.1016/j.livsci.2010.02.010. y Samal y Dash 2022Samal, L. & Dash, S.K. 2022. Nutritional Interventions to Reduce Methane Emissions in Ruminants', in A. K. Patra (ed.), Animal Feed Science and Nutrition - Production, Health and Environment, IntechOpen, London. 10.5772/intechopen.101763.).

Se ha demostrado que la adición de diferentes aceites vegetales (soya, coco, canola, colza, girasol, linaza) a las dietas para rumiantes reduce la producción de CH₄. Además, las grasas no se metabolizan en el rumen y, por lo tanto, no contribuyen a la metanogénesis (Johnson y Johnson 1995Johnson, K.A. & Johnson, D.E. 1995. "Methane emissions from cattle". Journal of Animal Science, 73(8): 2483-2492, ISSN: 1525-3163. https://doi.org/10.2527/1995.7382483x.). Después de consultar mucha literatura, la adición de lípidos a la dieta se considera una técnica prometedora.

Aceites esenciales y metabolitos vegetales. Los suplementos de fuentes biológicas se han investigado recientemente como ingredientes para piensos y aditivos para mitigar las emisiones (Salem et al. 2014Salem, A.Z.M., Kholif, A.E. & Elghandour, M.M. 2014. "Effect of increasing levels of seven tree species extracts added to a high concentrate diet on in vitro rumen gas output". Animal Science Journal, 85: 853–860, ISSN: 1740-0929. https://doi.org/10.1111/asj.12218. y Bayat et al. 2018Bayat, A.R., Tapio, I., Vilkki, J., Shingfield, K.J. & Leskinen, H. 2018. "Plant oil supplements reduce methane emissions and improve milk fatty acid composition in dairy cows fed grass silage-based diets without affecting milk yield". Journal of Dairy Science, 101(2): 1136–1151, ISSN: 1525-3198. https://doi.org/10.3168/jds.2017-13545.).

Tekippe et al. (2012)Tekippe, J.A., Hristov, A.N., Heyler, K.S., Zheljazkov, V.D., Ferreira, J.F.S., Cantrell, C.L. & Varga, G.A., 2012. "Effects of plants and essential oils on ruminal in vitro batch culture methane production and fermentation". Canadian Journal of Animal Science, 92(3): 395-408, ISSN: 1918-1825. https://doi.org/10.4141/CJAS2012-059. analizaron 100 aceites esenciales (AE) y plantas por su capacidad para reducir la metanogénesis. Los aceites esenciales son líquidos oleaginosos volátiles y aromáticos extraídos de materiales vegetales como flores, semillas, capullos, hojas, hierbas, madera, frutas, ramitas y raíces (Burt 2004Burt, S. 2004. "Essential oils: their antibacterial properties and potential applications in foods, a review". International Journal of Food Microbiology, 94(3): 223-253, ISSN: 1879-3460. https://doi.org/10.1016/j.ijfoodmicro.2004.03.022.). Demuestran propiedades antimicrobianas de amplio espectro, inhiben las arqueas del rumen, alteran la ruta de fermentación ruminal al inhibir las bacterias fibrolíticas (Cobellis et al. 2016Cobellis, G., Trabalza-Marinucci, M. & Marcotullio, M.C. 2016. "Evaluation of different essential oils in modulating methane and ammonia production, rumen fermentation, and rumen bacteria in vitro". Animal Feed Science and Technology, 215: 25–36, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2016.02.008.) y, en general, se consideran seguras para el consumo humano y animal (Davoodi et al. 2019Davoodi, S.M., Mesgaran, M.D., Vakili, A.R., Valizadeh, R. & Pirbalouti, A.G. 2019. "In vitro effect of essential oils on rumen fermentation and microbial nitrogen yield of high concentrate dairy cow diet". Biosciences, Biotechnology Research Asia, 16(2): 333-341, ISSN: 0973-1245. https://doi.org/10.13005/bbra/2749.). Algunos inhiben el crecimiento de protozoos indirectamente o por biohidrogenación de ácidos grasos insaturados, lo que limita la disponibilidad de hidrógeno para los metanógenos (Iqbal et al. 2008Iqbal, M.F., Cheng, Y.F., Zhu, W.Y. & Zeshan, B. 2008. "Mitigation of ruminant methane production: current strategies, constraints and future options". World Journal of Microbiology & Biotechnology, 24(12): 2747-2755, ISSN:1573-0972. https://doi.org/10.1007/s11274-008-9819-y. y Toprak 2015Toprak, N.N. 2015. "Do fats reduce methane emissions by ruminants? - A review". Animal Science Papers and Report, 33(4): 305-321, ISSN: 2300-8342.). Sin embargo, producen un escaso efecto in vivo, probablemente debido al mecanismo de adaptación del rumen. Además, la reducción de la digestibilidad de la fibra es otro problema, ya que reduce el desempeño animal (Benchaar y Greathead 2011Benchaar, C. & Greathead, H. 2011. "Essential oils and opportunities to mitigate enteric methane emissions from ruminants". Animal Feed Science and Technology, 166: 338-355, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2011.04.024.).

Numerosas investigaciones evaluaron la eficacia de los metabolitos secundarios de plantas como estrategia de mitigación (incluyendo saponinas, flavonoides, taninos y otros terpenoides), principalmente in vitro, y con resultados inconsistentes. Los taninos hidrolizables inhiben las bacterias metanogénicas del rumen, mientras que los condensados inhiben la digestión de la fibra (Khurso et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.). Las saponinas disminuyen la degradación de proteínas y favorecen la síntesis microbiana y de biomasa (Makkar y Beker 1996Makkar, H.P.S. & Becker, K. 1996. "Effect of pH, temperature, and time on inactivation of tannins and possible implications in detannification studies". Journal of Agricultural and Food Chemistry, 44(5): 1291–1295, ISSN: 0021-8561. https://doi.org/10.1021/jf9506287.), dos procesos que reducen la disponibilidad de hidrógeno (Dijkstra et al. 2007Dijkstra, J., Bannink, A., France, J. & Kebreab, E. 2007. "Nutritional control to reduce environmental impacts of intensive dairy cattle systems". In Proceedings of the VII International Symposium on the Nutrition of Herbivores (ed. QX Meng, LP Ren and ZJ Cao). China Agricultural University Press, Beijing, China. pp. 411–435.). Sin embargo, el efecto de las saponinas parece estar relacionado con su efecto antiprotozoario (Newbold y Rode 2006Newbold, C.J. & Rode, L.M. 2006. Dietary additives to control methanogenesis in the rumen. In Greenhouse gases and animal agriculture: an update (ed. CR Soliva, J Takahashi and M Kreuzer), Elsevier International Congress Series 1293, pp. 138–147. Elsevier, Amsterdam, The Netherlands).

Aditivos orgánicos adicionales Biochar también se ha probado en la última década debido a su efecto en el crecimiento, el rendimiento de huevos, los perfiles sanguíneos, los efectos inhibitorios contra el crecimiento de patógenos ruminales y la reducción de la emisión de metano entérico. (Leng et al. 2012Leng, R.A., Preston, T.R. & Inthapanya, S. 2012. "Biochar reduces enteric methane and improves growth and feed conversion in local “Yellow” cattle-fed cassava root chips and fresh cassava foliage". Livestock Research Rural Development, 24(11): 199, ISSN: 2521-9952. Available: http://www.lrrd.org/lrrd24/11/leng24199.htm. y Man et al. 2021Man, K.Y., Chow, K.L., Man, Y.B., Mo, W.Y. & Wong, M.H. 2021. "Use of biochar as feed supplements for animal farming". Critical Reviews in Environmental Science and Technology, 51(2): 187-217, ISSN: 1547-6537. https://doi.org/10.1080/10643389.2020.1721980.).

Algas marinas. Las algas marinas conocidas como macroalgas, incluidas las algas marrones (Phaeophyta), rojas (Rhodophyta) y verdes (Chlorophyta), son ricas en compuestos bioactivos que incluyen proteínas, carbohidratos y, en menor medida, lípidos, saponinas, alcaloides y péptidos. Estos bioactivos también podrían desempeñar un papel como ingredientes de alimentos para disminuir el CH₄ entérico (Abbott et al. 2021Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432.). La reducción se atribuye en gran medida al compuesto bromoformo que se encuentra en varias especies de algas marinas, especialmente algas rojas como Asparagopsis spp. y se conoce que inhibe la vía biosintética de CH₄ en los metanógenos (Machado et al. 2015Machado, L., Kinley, R.D., Magnusson, M., de Nys, R. & Tomkins, N.W. 2015. "The potential of macroalgae for beef production systems in Northern Australia". Journal of Applied Phycology, 27(5): 2001–2005, ISSN: 1573-5176. https://doi.org/10.1007/s10811-014-0439-7.).

Se han llevado a cabo varios estudios in vitro con suplementos de algas marinas, pero aún quedan lagunas en el conocimiento actual sobre la eficacia de las macroalgas para enfrentar el cambio climático, tanto como suplemento dietético como alimento para el ganado. Los posibles impactos ambientales y económicos positivos y negativos del cultivo de algas marinas a gran escala aún deben estudiarse (Abbott et al. 2020Abbott, D.W., Aasen, I.M., Beauchemin, K.A., Grondahl, F., Gruninger, R., Hayes, M., Huws, Sh., Kenny, D.A., Krizsan, S.J., Kirwan, S.F., Lind, V., Meyer, U., Ramin, M., Theodoridou, K., von Soosten, D., Walsh, P.J., Waters, S. & Xing, X. 2020. "Seaweed and seaweed bioactives for mitigation of enteric methane: challenges and opportunities". Animals, 10(12): 2432, ISSN: 2076-2615. https://doi.org/10.3390/ani10122432.).

Aditivos. Varios aditivos de compuestos inorgánicos u orgánicos o probióticos de alimentación directa se han añadido para reducir las emisiones de metano en los rumiantes. Estos aditivos inhiben directamente los metanógenos o alteran las vías metabólicas que conducen a una reducción del sustrato (Halmemies-Beauchet-Filleau et al. 2018Halmemies-Beauchet-Filleau, A., Rinne, M., Lamminen, M., Mapato, C., Ampapon, T., Wanapat, M. & Vanhatalo, A. 2018. "Review: Alternative and novel feed for ruminants: nutritive value, product quality and environmental aspects". Animal, 12(s2): s295-s309, ISSN: 1751-732X. https://doi.org/10.1017/S1751731118002252. y Haque 2018Haque, M.N. 2018. "Dietary manipulation: a sustainable way to mitigate methane emissions from ruminants". Journal of Animal Science and Technology, 60: 15, ISSN: 2055-0391. https://doi.org/10.1186/s40781-018-0175-7.).

Enzimas exógenas. La celulasa, la xilanasa y la hemicelulasa se han utilizado en dietas para rumiantes como aditivos alimenticios. Estas enzimas pueden mejorar la digestibilidad de la fibra y la productividad animal (Beauchemin et al. 2003Beauchemin, K.A., Colombatto, D., Morgavi, D.P. & Yang, W.Z. 2003. "Use of exogenous fibrolytic enzymes to improve feed utilization by ruminants". Journal of Animal Science, 81: E37–E47, ISSN: 1525-3163.). También disminuyen la relación acetato/propionato en el rumen, reduciendo así la producción de CH₄ (Eun y Beauchemin 2007Eun, J.S. & Beauchemin, K.A. 2007. "Assessment of the efficacy of varying experimental exogenous fibrolytic enzymes using in vitro fermentation characteristics". Animal Feed Science and Technology, 132 (3-4): 298-315, ISSN: 0377-8401. https://doi.org/10.1016/j.anifeedsci.2006.02.014.). Sin embargo, la suplementación de enzimas exógenas a nivel de granja es muy limitada (Khurso et al. 2021Khusro, A., Aarti, C., Elghandour, M.M., Adegbeye, M.J., Mellado, M., Barbabosa-Pliego, A., Rivas-Caceres, R.R & Salem, A.Z.M. 2021. Dietary Manipulation to Mitigate Greenhouse Gas Emission from Livestock. In: Lackner, M., Sajjadi, B., Chen, WY. (eds) Handbook of Climate Change Mitigation and Adaptation. Springer, New York, NY. pp. 2537-2575. https://doi.org/10.1007/978-1-4614-6431-0_131-1.).

Ionóforos. Los ionóforos disponibles comercialmente, como monensina, lasalocida, salinomicina y laidlomicina, se aplican ampliamente como aditivos alimenticios en las dietas de vacas lecheras en muchos países y se han utilizado para aumentar la producción de leche, mejorar la eficiencia alimenticia y prevenir trastornos metabólicos (McGuffey et al. 2001McGuffey, R.K., Richardson, L.F. & Wilkinson, J.I. 2001. "Ionophores for dairy cattle: current status and future outlook". Journal of Dairy Science, 84: E194-E203, ISSN: 1525-3198. http://dx.doi.org/10.3168/jds.S0022-0302(01)70218-4.). Estas benefician el metabolismo animal al aumentar la eficiencia del metabolismo energético, mejorar el metabolismo del nitrógeno ruminal mientras modulan la ingesta, optimizar las rutas de fermentación y reducir las tasas de trastornos digestivos (Duffield et al. 2012Duffield, T.F., Merrill, J.K. & Bagg, R.N. 2012. "A Meta-analysis of the effects of monensin in beef cattle on feed efficiency, body weight gain, and dry matter intake". Journal of Animal Science, 90: 4583–4592, ISSN: 1525-3163. https://doi.org/10.3168/jds.2007-0607.). Los ionóforos también actúan como antimicrobianos, inhibiendo preferentemente las bacterias gram positivas que producen lactato, acetato, butirato, formiato e hidrógeno como productos finales, lo que resulta en un aumento de propionato y una concentración reducida de acetato (Marques y Cooke 2021Marques, R.D. & Cooke, R.F. 2021. "Effects of ionophores on the ruminal function of beef cattle". Animals, 11(10): 2871, ISSN: 2076-2615. https://doi.org/10.3390/ani11102871.). También afectan a los protozoos (Guan et al. 2006Guan, H., Wittenberg, K.M., Ominski, K.H. & Krause, D.O. 2006. "Efficacy of ionophores in cattle diets for mitigation of enteric methane". Journal of Animal Science, 84: 1896-1906, ISSN: 1525-3163. https://doi.org/10.2527/jas.2005-652.).

La presión para reducir el uso de antimicrobianos en la producción ganadera sugiere que no es una solución a largo plazo. Además, esta familia de aditivos no está permitida en muchos países, incluido Europa.

Ácidos orgánicos . La adición de ácidos orgánicos como fumarato, malato y acrilato, precursores de la producción de propionato en el rumen, puede ser un sumidero alternativo de H₂, con la reducción de la metanogénesis. McAllister y Newbold (2008)McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218. encontraron reducciones de 0 % a 75 % en CH₄ logradas mediante la alimentación con ácido fumárico. La suplementación con ácidos orgánicos se ha probado principalmente para la producción in vitro de CH₄, con resultados inconsistentes. Además, con las dosis relativamente altas que se requieren, los ácidos dicarboxílicos son prohibitivamente caros como estrategia de reducción.

Manipulación ruminal. La manipulación de las poblaciones microbianas en el rumen, ya sea por medios químicos o mediante la introducción de microorganismos competitivos o depredadores, o con vacunación, puede reducir la producción de CH₄. Las estrategias de control biológico, como bacteriófagos o bacteriocinas, podrían resultar eficaces para inhibir directamente los metanógenos y redirigir el H₂ a otras bacterias reductoras del rumen, como los productores de propionato o los acetógenos (McAllister y Newbold 2008McAllister, T.A. & Newbold, C.J., 2008. "Redirecting rumen fermentation to reduce methanogenesis". Australian Journal of Experimental Agriculture, 48(2): 7–13, ISSN: 1446-5574. http://dx.doi.org/10.1071/EA07218.). Sin embargo, aún se requiere investigar por un mayor período de tiempo para lograr opciones de control biológico o vacunas comercialmente viables que pudieran ser útiles en diferentes áreas y sistemas de producción.

Vacunación. La reducción de metano en rumiantes podría obtenerse mediante vacunación y muchos autores han considerado esta estrategia como muy prometedora. Se han realizado numerosos estudios para reducir las emisiones de metano mediante la vacunación, con resultados extremadamente variables (desde el aumento del 20 % hasta una reducción del 69 % en la producción de metano, Baca-Gonzalez et al. 2020Baca-Gonzalez, V., Asensio-Calavia, P., Gonzalez-Acosta, S., Perez de la Lastra, J.M. & Morales de la Nuez, A. 2020. "Are vaccines the solution for methane emissions from ruminants? A systematic review". Vaccines 8(3): 460, ISSN: 2076-393X. https://doi.org/10.3390/vaccines8030460.). Algunas de las causas de los fracasos de la vacunación para reducir la producción de metano son la diversidad de metanógenos, las condiciones diferentes de crianza y la adaptación del rumen (Williams et al. 2009Williams, Y.J., Popovski, S., Rea, S.M., Skillman, L.C., Toovey, A.F., Northwood, K.S. & Wright, A.D.G. 2009. "A vaccine against rumen methanogens can alter the composition of archaeal populations". Applied and Environment Microbiology, 75: 1860–1866, ISSN: 1098-5336. https://doi.org/10.1128/AEM.02453-08.). Sin embargo, es complicado evaluar la efectividad real de esta estrategia ya que pocos estudios han evaluado directamente el enfoque completo, es decir, desde la vacunación hasta la medición de las emisiones de CH₄ entérico de los animales. Por lo tanto, para lograr una vacunación exitosa contra metanógenos, se requiere un enfoque de espectro mucho más amplio con una mayor comprensión de la población de metanógenos en el rumen (Mir y Begun 2022Mir, N.A. & Begum, J. 2022. "Rumen microbial system, methanogenesis, and methane mitigation strategies in ruminants: Methanogenesis in ruminants". Letters in Animal Biology, 2(1): 12-22.).

Defaunación. La defaunación es la eliminación de protozoos del rumen. Se ha informado que reduce las emisiones de metano hasta en un 50 % dependiendo de la dieta (Hegarty 1999Hegarty, R.S. 1999. "Reducing rumen methane emissions through the elimination of rumen protozoa". Australian Journal of Agricultural Research, 50: 1321–1327, ISSN: 1467-8489. https://doi.org/10.1071/AR99008.). Los protozoos están asociados a los metanógenos y son grandes productores de H₂ en el rumen favoreciendo así el proceso de producción de metano por parte de los metanógenos.

En los animales defaunados, se mantuvo una producción mínima de metano durante más de dos años, lo que indica un cambio estable inducido por los agentes defaunadores (Morgavi et al. 2008Morgavi, D.P., Jouany, J.P., Martin, C. 2008. "Changes in methane emission and rumen fermentation parameters induced by refaunation in sheep". Australian Journal of Experimental Agriculture, 48(2): 69–72, ISSN: 1446-5574. https://doi.org/10.1071/EA07236.). Sin embargo, en algunos casos, esta reducción de la producción de metano no es consistente (Hegarty et al. 2008Hegarty, R.S., Bird, S.H., Vanselow, B.A. & Woodgate, R. 2008. "Effects of the absence of protozoa from birth or from weaning on the growth and methane production of lambs". British Journal of Nutrition, 100(6): 1220–1227, ISSN: 1475-2662. https://doi.org/10.1017/S0007114508981435.). Además, puede afectar negativamente las funciones ruminales normales y, a su vez, el rendimiento de los animales (Mir y Begun 2022Mir, N.A. & Begum, J. 2022. "Rumen microbial system, methanogenesis, and methane mitigation strategies in ruminants: Methanogenesis in ruminants". Letters in Animal Biology, 2(1): 12-22.).

Microorganismos de alimentación directa. Los microorganismos de alimentación directa (DFM) se definen como un cultivo único o mixto de organismos vivos, que promueve la microflora ruminal deseada y proporciona efectos beneficiosos cuando se alimenta a los animales (Krehbiel et al. 2003Krehbiel, C.R., Rust, S.R., Zhang, G. & Gilliland, S.E. 2003. "Bacterial direct-fed microbial in ruminant diets: performance response and mode of action". Journal of Animal Science, 81(4): E120-132, ISSN: 1525-3163. https://doi.org/10.2527/2003.8114_suppl_2E120x.). Se cree que varias bacterias del rumen compiten con los metanógenos por el suministro de hidrógeno mediante la promoción de la propionogénesis, la acetogénesis y la reducción de nitrato/nitrito o sulfato, lo que puede servir como sumidero alternativo de H₂. Esto redirige el flujo metabólico de hidrógeno del rumen hacia la producción de AGV que podrían usarse, de lo contrario, para la metanogénesis (Ungerfeld 2015Ungerfeld, E.M. 2015. "Shifts in metabolic hydrogen sink in the methanogenesis-inhibited ruminal fermentation: a meta-analysis". Frontiers in Microbiology, 6: 37, ISSN: 1664-302X. https://doi.org/10.3389/fmicb.2015.00037.).

Debido a que el H₂ es un sustrato limitante para la producción de metano, la adición de bacterias formadoras de propionato podría ayudar a disminuir la producción de metano (Jeyanathan et al. 2014Jeyanathan, J., Martin, C. & Morgavi, D.P. 2014. "The use of direct-fed microbial for mitigation of ruminant methane emissions: a review". Animal, 8(2): 250-261, ISSN: 1751-732X. https://doi.org/10.1017/S1751731113002085.). Sin embargo, in vivo, Propionibacteria spp. no perdura en el rumen del ganado cuando se administra una dieta rica en almidón. Una alta fermentación de almidón resulta en una mayor proporción molar de propionato que reduce su eficiencia (Jeyanathan et al. 2019Jeyanathan, J., Martin, C., Eugène, M., Ferlay, A., Popova, M. & Morgavi, D.P. 2019. "Bacterial direct fed microbial fail to reduce methane emissions in primiparous lactating dairy cows". Journal of Animal Science and Biotechnology, 10: 41, ISSN: 2049-1891. https://doi.org/10.1186/s40104-019-0342-9.).

Acetógenos. Los homoacetógenos son un grupo de bacterias que producen acetato (Drake et al. 2008Drake, H.L., Gößner, A.S. & Daniel, S.L. 2008. "Old acetogens, new light". Annals of the New York Academy of Sciences, 1125: 100-128, ISSN: 1749-6632. https://doi.org/10.1196/annals.1419.016.). Los estudios in vitro también han sugerido que la acetogénesis podría servir como una alternativa a la metanogénesis para eliminar H₂ del rumen (Morvan et al. 1996Morvan, B., Bonnemoy, F., Fonty, G., Gouet, P. 1996. "Quantitative determination of H2-utilizing acetogenic and sulfate-reducing bacteria and methanogenic archaea from the digestive tract of different mammals". Current Microbiology, 32(3): 129-133, ISSN: 1432-0991. https://doi.org/10.1007/s002849900023.). Sin embargo, López et al. (1999)Lopez, S., McIntosh, F.M, Wallace, R.J. & Newbold, C.J. 1999. "Effect of adding acetogenic bacteria on methane production by mixed rumen microorganisms". Animal Feed Science and Technology, 78(1-2): 1-9, ISSN: 1873-2216. https://doi.org/10.1016/S0377-8401(98)00273-9. informaron que las altas concentraciones de bacterias acetogénicas no pueden competir con los metanógenos por la eliminación de H₂, por lo que no queda claro si los homoacetógenos podrían desempeñar un papel fundamental en el ecosistema ruminal (Henderson et al. 2010Henderson, G., Naylor, G.E., Leahy, S.C. & Janssen, P.H. 2010. "Presence of novel, potentially homoacetogenic bacteria in the rumen as determined by analysis of formyltetrahydrofolate synthetase sequences from ruminants". Applied and Environmental Microbiology, 76: 2058-2066, ISSN: 1098-5336. https://doi.org/10.1128/AEM.02580-09.).

Las bacterias oxidantes del metano (MOB) son bacterias que pueden crecer en el metano como única fuente de carbono y energía. Sin embargo, los estudios in vivo que utilizan MOB como probióticos son escasos y necesitan ampliarse para verificar su potencial probiótico.