Modulation of milking performance, methane emissions, and rumen microbiome on dairy cows by dietary supplementation of a blend of essential oils

•Feeding 1 g/d of a blend of essential oils (Agolin Ruminant) in rations with no silages:•Alters rumen microbial population.•Reduces daily methane emissions on absolute terms.•Reduces methane emissions per kilogram of feed consumed.•Has no consequences on milk production or composition but reduces feed efficiency slightly. Cattle represent a high contribution of the livestock's greenhouse gas emissions, mainly in the form of methane. Essential oils are a group of plant secondary metabolites obtained from volatile fractions of plants that have been shown to exert changes in the rumen fermentation and may alter feed efficiency and to reduce methane production. The objective of this study was to investigate the effect on rumen microbial population, CH4 emissions and milking performance of a mixture of essential oils (Agolin Ruminant, Switzerland) incorporated daily in the ration of dairy cattle. Forty Holstein cows (644 ± 63.5 kg of BW producing 41.2 ± 6.44 kg/d of milk with 190 ± 28.3 DIM) were divided into two treatments (n = 20) for 13 wk and housed in a single pen equipped with electronic feeding gates to control access to feed and monitor individual DM intake (DMI) on a daily basis. Treatments consisted of no supplementation (Control) or supplementation of 1 g/d of a blend of essential oils (BEOs) fed in the TMR. Individual milk production was recorded using electronic milk meters on a daily basis. Methane emissions were recorded using sniffers at the exit of the milking parlour. At day 64 of the study, a sample of rumen fluid was collected from 12 cows per treatment after the morning feeding using a stomach tube. There were no differences in DMI, milk yield, or milk composition between the two treatments. However, cows on BEO exhaled less CH4 (444 ± 12.5 l/d) than cows on Control (479 ± 12.5 l/d), and exhaled less (P