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

• The essential oil combinations markedly decreased methane and ammonia production.
• Only one of the essential oil combinations did not affect feed digestion.
• The essential oil combinations reduced the abundance of archaea and protozoa.
• These combinations reduced the abundance of some hyper-ammonia producing bacteria.
• Some combinations modified the structure of the bacterial and archaeal communities.

Various essential oils (EO) have been individually evaluated to mitigate methane and ammonia production by rumen microbiota. Interactions between EO can affect their potency but such interactions largely remain unexplored. In the present study, EO from oregano, rosemary, Ceylon cinnamon, cinnamon leaves, cinnamon bark, dill seeds, and eucalyptus were chemically characterized and then evaluated in vitro, both individually (at 1.125 ml/L culture) and in three-way EO combinations (at total EO 0.8 ml/L, equal ratio), for their 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 Ceylon cinnamon-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) but only oregano EO reduced the abundance of archaea (P < 0.001). The EO combinations significantly decreased the abundances of archaea (P < 0.001), protozoa (P < 0.001), and select groups or species of different rumen bacteria to different extents. Changes in bacterial and archaeal communities in response to several EO combinations were also shown by DGGE analyses. Combination of EO from Ceylon cinnamon, dill seeds, eucalyptus, and probably others, at low concentrations may be a practical approach to mitigate methane emission and nitrogen excretion from ruminant without adverse effect on feed digestion or fermentation.