Response of the Ruminal Microbial Environment to a High‐Dose Supplementation of Blended Essential Oils

ABSTRACT We conducted two experiments to investigate the effects of a high‐dose blend of essential oils (BEO) on rumen microbial community structure and methane (CH 4 ) production. The first was an in vitro study that measured gas production, methane emissions, and microbial composition when two BEO levels (0.1% and 1.0% of the substrate) were added to a forage‐based substrate incubated with rumen fluid from dry cows. The second was a feeding trial using a crossover design with four Japanese native goats to evaluate the effects of BEO at 1 g/DM feed on rumen fermentation characteristics and microbial composition. Both experiments demonstrated that BEO supplementation reduced CH 4 production and influenced rumen fermentation parameters. However, BEO altered microbial composition and carbohydrate fermentation patterns differently between the two experiments. In in vitro study, many of the major bacterial groups exhibited population shifts, whereas only one family, Acidoaminococcaceae, decreased in the rumen community of goats fed BEO for 3 weeks. Overall, the results suggest inconsistencies in the microbial response, which may underlie variations in methane emissions and rumen microbiota composition. Our findings imply that BEO broadly suppresses microbial activity, shifting fermentation toward the utilization of readily digestible carbohydrates and leading to reduced methane production.