Recent advances in feed additives with the potential to mitigate enteric methane emissions from ruminant livestock

Livestock production represents a significant (14.5%) source of anthropogenic greenhouse gas (GHG) emissions. A large share of the emissions from livestock production is due to enteric fermentation from ruminants, which produces methane (CH4), a potent GHG. Nevertheless, livestock production remains essential for nutrition, sustainability, and food security globally. In addition to atmospheric effects, CH4 emissions represent a direct loss of dietary energy from the animal. It is, therefore, imperative that solutions are developed and implemented to mitigate enteric CH4 emissions from ruminants. Methane is produced as a result of feed fermentation in the rumen, as carbohydrates are broken down to form energy in the form of volatile fatty acids, and carbon dioxide (CO2) and hydrogen (H2) are produced as byproducts. Carbon dioxide and H2 are then utilized by methanogenic archaea to form CH4 via the hydrogenotrophic pathway. One proposed solution for mitigating enteric CH4 emissions are feed additives. Feed additives have the potential to decrease CH4 emissions while sustaining animal production parameters, the latter a necessary condition for incorpora-tion as a regular part of the diet. To decrease CH4 emissions, feed additives can either directly or indirectly inhibit methanogenic archaea. Additives that directly inhibit methanogenesis include 3-nitrooxypropanol (3NOP) and halogenated CH4 analogs that naturally occur in some species of macroalgae. These additives work by interfering with the enzyme that cata-lyzes the final step of the methanogenesis pathway. Both 3NOP and halogenated CH4 analogs show great potential, demonstrating up to a 76% and 98% reduction in CH4 yield (g kg-1 dry matter intake), respectively. Nitrates (NO3-), ionophores, plant secondary compounds, and direct fed microbials are all feed additives that indirectly inhibit methanogenesis by altering the rumen environment, primarily through the reduction in substrate availability for meth-anogenic archaea. These additives, however, show more variability in their CH4 reduction potential (with the exception of NO3-) due to inconsistencies in composition. In order to present the most promising and immediate solutions to mitigate enteric CH4 emissions it is necessary to focus on recent advancements as feed additive research is rapidly evolving. Thus, this analysis aims to review feed additives with the potential to reduce enteric CH4 emissions that have been studied in vivo from 2018 to 2022.