Methane Mitigation in Livestock Series - Part Three: Diet

Awareness around an animal’s gut microbiome leading to its health is at an all-time high. This is as true for humans as much as it is for livestock species. In recent years, a gut’s microbiome population has been linked not only to health and disease of the gastrointestinal tract, but also to the health of organ systems distant from the gastrointestinal tract (Vijay and Valdes, 2022).

A gut microbiome’s profile is determined by the diet as much as any other factor (e.g. immune status, anatomical features, the microbiome of the mother). Therefore, the constituent microbes (bacteria, fungi and protozoa) will be those that can make best use of the ingested diet as a source of nutrients for their own proliferation. In the context of methane formation in the forestomach of ruminant livestock it is important to note the bacteria responsible for methane biosynthesis (i.e., methanogens) are a taxonomically distinct group from the bacteria that most people would be familiar (e.g., E.coli), instead belonging to the Archaea taxonomic domain. Nonetheless, the methanogenic bacteria are reliant on the digestion of the feed by other resident microbes to produce the precursors necessary for methane formation. Methanogens make use of pools of free hydrogen (H2) formed in the breakdown the animal’s feed in reducing CO2 to form the relatively simple methane molecules, CH4.

The importance of diet to the health and productivity of livestock has long been established. In addition, both the scientific and farming communities have investigated the role diet can play in mitigating methane emissions from ruminants (Beauchemin et al., 2009; Eckard et al., 2010; Martin et al., 2010; van Gastelen ?et al., 2019), long before the awareness and concern of the general public regarding the contribution of ruminant methane emissions to the overall anthropogenic greenhouse gas emissions.

As mentioned in a previous instalment of this series of posts, the main sources of energy for the ruminant are the volatile fatty acids (VFAs), formed from the feed by the resident bacteria. These VFAs include acetate, butyrate and propionate. Importantly, the relative concentrations of each of these VFAs in the rumen will be determined by both the composition of the diet and the rumen microbiome. Higher concentrations of acetate and butyrate will lead to higher concentrations of residual H2 and as a consequence more CH4. In contrast, if both the diet and microbiome can be re-directed to the formation of relatively higher concentrations of propionate, there will be a smaller pool of residual H2 and so less CH4 formed. This is because in the process of forming propionate much of the residual H2 is consumed.

Some general comments can be made with respect to feed intake and the diet of a ruminant, and how that might impact the levels of CH4 formed in the rumen. Firstly, before considering the composition of the diet, the volume of the rumen and the retention time and rate of passage of the feed through the gastrointestinal tract will determine the amounts of CH4 produced. The larger the volume of the rumen and the slower the passage of feed will allow the methanogenic bacteria to accumulate more CH4. Broadly speaking, forage-based diets lead to higher levels of CH4 produced than grain-based diets. These differences are due to the composition of the respective diets with structural and soluble carbohydrates of forage diets leading to the production of VFAs and H2 more conducive to CH4 production, while the starch, protein and oils found in many grains less conducive to CH4 production. Within these two broad groups of feed, more nuanced differences in CH4 emissions can be seen. For example, the proportion of fibrous carbohydrates as opposed to soluble carbohydrates in forage-based diets will influence the levels of CH4 emissions; the more fibrous carbohydrates the higher the level of emissions.

In assessing which diets lead to the best outcomes with respect to CH4 emissions, it should not be forgotten that the greenhouse gas emissions generated during the production of the forage or grain-based diets need to be included in the overall life cycle analysis of all farm activities.

REFERENCES:

Beauchemin, K.A., McAllister, T.A. and McGinn, S.M. (2009) Dietary mitigation of enteric methane from cattle. CAB Reviews: Perspectives in Agriculture, Veterinary Science, Nutrition and Natural Resources 4: No. 035. doi: 10.1079/PAVSNNR20094035

Eckard, R.J., Grainger, C., and de Kelin, C.A.M. (2010) Options for the abatement of methane and nitrous oxide from ruminant production: A review. Livestock Science 130: 47-56. doi:10.1016/j.livsci.2010.02.010

Martin, C., Morgavi D.P., and Doreau, M. (2010) Methane mitigation in ruminants: from microbe to the farm scale. Animal 4: 351-365. doi:10.1017/S1751731109990620

van Gastelen, S., Dijkstra, J., and Bannink, A. (2019) Are dietary strategies to mitigate enteric methane emission equally effective across dairy cattle, beef cattle, and sheep. Journal of Dairy Science 102: 6109-6130. https://doi.org/10.3168/jds.2018-15785

Vijay, A., and Valdes, A.M. (2022) Role of the gut microbiome in chronic diseases: a narrative review. European Journal of Clinical Nutrition 76: 489-501. https://doi.org/10.1038/s41430-021-00991-6