In vitro screening of the ruminal methane and ammonia mitigating potential of mixtures of either chestnut or quebracho tannins with blends of essential oils as feed additives

Tannins and essential oils (EOs) have been previously described for their properties to mitigate ruminal methane and ammonia. Their combination might be even more efficient, as they have different modes of action on rumen pathways. This study aimed to screen in vitro the mitigating properties of variously combining tannins with EO in a total of 48 treatments: 12 single additives, 10 mg of EO or 20 mg of tannins/g diet, to establish their basal efficiency; 36 combinations of 20 mg of tannins/g + 10–15 mg of EO. Quebracho (Q) and chestnut (C) tannins defined C, Q and C/Q groups of mixtures with EO blends, formulated with oregano, thyme and clove EO, citrus peel, carvacrol, thymol, eugenol, α-pinene, and bornyl acetate. Supplements were added to a control diet, which was also incubated alone as a basis for comparisons with supplemented treatments, in a total of six runs. Effects on rumen pH, protozoal count, and proportions of individual volatile fatty acids (VFA) were limited. The tannins extracts seemed to cause most of the mitigating effects by suppressing ammonia by up to 31% and methane yield by up to 15%, with the highest reductions obtained with tannins-based supplements. However, this occurred by contemporary affecting the feeding value of the substrate, as indicated by reductions in total VFA and in vitro organic matter digestibility. Overall, six mixtures of C and Q groups were the most efficient and they need further studies to understand the mechanisms of actions and the synergistic effects occurring among compounds.Highlights Combinations of tannins and essential oil compounds were screened in vitro Some combinations lowered methane yield and ammonia formation by up to 15 and 31% Essential oil compounds enhanced the mitigating properties of the tannins.