This review examines the current state of knowledge regarding the effectiveness of different dietary ruminant enteric methane mitigation strategies and their modes of action together with the issues discussed regarding the potential harms/risks and applicability of such strategies. By investigating these strategies, we can enhance our understanding of the mechanisms by which they influence methane production and identify promising approaches for sustainable mitigation of methane emissions. Out of all nutritional strategies, the use of 3-nitrooxypropanol, red seaweed, tannins, saponins, essential oils, nitrates, and sulfates demonstrates the potential to reduce emissions and receives a lot of attention from the scientific community. The use of certain additives as pure compounds is challenging under certain conditions, such as pasture-based systems, so the potential use of forages with sufficient amounts of plant secondary metabolites is also explored. Additionally, improved forage quality (maturity and nutrient composition) might help to further reduce emissions. Red seaweed, although proven to be very effective in reducing emissions, raises some questions regarding the volatility of the main active compound, bromoform, and challenges regarding the cultivation of the seaweed. Other relatively new methods of mitigation, such as the use of cyanogenic glycosides, are also discussed in this article. Together with nitrates, cyanogenic glycosides pose serious risks to animal health, but research has proven their efficacy and safety when control measures are taken. Furthermore, the risks of nitrate use can be minimized by using probiotics. Some of the discussed strategies, namely monensin or halogenated hydrocarbons (as pure compounds), demonstrate efficacy but are unlikely to be implemented widely because of legal restrictions.

Essential oil-based feed additives have been proposed as a practical strategy to mitigate enteric methane emissions in ruminants, but their effects are not always consistent. The objective of this study was to evaluate the effects of dietary supplementation with an essential oil blend on in vitro methane production, rumen fermentation, nutrient digestibility, growth performance, carcass traits, and meat quality in Lithuanian Blackface lambs. We hypothesized that supplementation would induce measurable changes in in vitro methane production and selected rumen fermentation variables, while growth performance and technological meat quality would remain comparable between treatments. Sixty Lithuanian Blackface lambs were allocated to control (C) and treatment (T) groups (30 lambs per group). The C group received a basal diet, and the T group received the same diet supplemented with an essential oil blend, Agolin Ruminant, at a dose rate of 0.1 g/animal/day, consisting of linalool, eugenol, geranyl acetate, and geraniol. An in vitro rumen fermentation assay was performed using rumen fluid pooled within both dietary groups from multiple lambs and incubated as a single batch with four replicate fermentation flasks per treatment (n = 4 fermenters per group) to quantify methane production and in vitro nutrient digestibility. In vivo apparent nutrient digestibility was evaluated in a dedicated sub-trial (n = 6 animals per group). Growth performance in the main trial was analyzed using the pen as the experimental unit (n = 3 pens per group), and slaughter-based measurements—including slaughter and carcass traits, rumen volatile fatty acids and protozoal counts, and Longissimus dorsi meat quality and intramuscular fatty acids—were determined in 10 lambs per treatment (n = 10 animals per group). In vitro methane production did not differ between groups (p = 0.366); in vitro crude fiber digestibility showed a tendency to increase with supplementation (p = 0.066). Fermentation end-products were largely unchanged, although propionate tended to be higher (p = 0.063), and the acetate:propionate ratio was lower (p = 0.043) in the supplemented group; protozoal counts were not different between groups. In vivo apparent nutrient digestibility was comparable between treatments. Growth performance was lower in the supplemented group, resulting in an overall mean ADG 19.0% lower. Slaughter and carcass traits were comparable between treatments. Meat proximate composition, cholesterol concentration, pH, color, water losses, and instrumental texture/shear parameters were not affected by supplementation. Intramuscular lauric (C12:0), myristic (C14:0), and pentadecanoic (C15:0) fatty acids were lower (p < 0.05), while C14:1 n-7 tended to decrease (p = 0.050); however, total saturated, monounsaturated, and polyunsaturated fatty acids and nutritional ratios were unchanged. Overall, under the study conditions and dose used, the essential oil blend did not significantly reduce in vitro methane production and elicited only limited rumen fermentation responses; ADG was 19.0% lower in the supplemented group, whereas carcass traits and technological meat quality were unaffected, and only specific intramuscular fatty acids were altered.