Evaluation of different essential oils in modulating methane and ammonia production, rumen fermentation, and rumen bacteria in vitro

tVarious essential oils (EO) have been individually evaluated to mitigate methane andammonia production by rumen microbiota. Interactions between EO can affect theirpotency but such interactions largely remain unexplored. In the present study, EO fromoregano, rosemary, Ceylon cinnamon, cinnamon leaves, cinnamon bark, dill seeds, andeucalyptus were chemically characterized and then evaluated in vitro, both individually (at1.125 ml/L culture) and in three-way EO combinations (at total EO 0.8 ml/L, equal ratio), fortheir effects on fermentation, methanogenesis, ammoniagenesis, and bacteria and archaea.All the EO and their combinations decreased production of total gas (P < 0.001), methane(P < 0.001), and ammonia (except eucalyptus EO; P < 0.001), but they (except the Ceyloncinnamon-dill seeds-eucalyptus EO combination) also decreased dry matter digestibility(P < 0.001). The EO individually decreased the abundances of Prevotella spp. (P < 0.001) butonly oregano EO reduced the abundance of archaea (P < 0.001). The EO combinations sig-nificantly decreased the abundances of archaea (P < 0.001), protozoa (P < 0.001), and selectgroups or species of different rumen bacteria to different extents. Changes in bacterial andarchaeal communities in response to several EO combinations were also shown by DGGEanalyses. Combination of EO from Ceylon cinnamon, dill seeds, eucalyptus, and probablyothers, at low concentrations may be a practical approach to mitigate methane emission andnitrogen excretion from ruminant without adverse effect on feed digestion or fermentation.