Effects of specific essential oil compounds on the ruminal environment, milk production and milk composition of lactating dairy cows at pasture

Sixty multiparous, lactating Holstein cows (57 ± 23.1 d in milk at the start of the experiment) were used in a completely randomized design to examine effects of adding incremental levels of dietary essential oil compounds (EO; 0, 200, 400 and 600 mg/d) on milk production and composition. Cows were allowed to graze on winter oats for 8 h/d with a daily herbage allowance of 15 kg dry matter (DM)/cow, and then received supplemental corn silage and sunflower meal in confinement for the remainder of the day. The EO were fed individually at milking times (0600 and 1600 h), mixed with 0.86 kg DM of dry rolled corn grain. In addition, 4 ruminally cannulated lactating Holstein cows in mid lactation were used in a 4×4 Latin Square design with 14 d periods to study effects of EO on ruminal fermentation characteristics and ruminal in sacco DM, crude protein (CP), and neutral detergent fiber (aNDF) degradability. Milk production, which ranged from 18.8 to 20.2 kg/d, and milk composition were not affected by EO. In general, ruminal fermentation characteristics were not affected by EO addition at any level, except for a 13% increase in butyrate concentrations with all EO levels compared to the control. Ruminal ammonia N concentration was high in all treatments (51.5 ± 5.75 mg/100 ml) and tended (P=0.09) to increase when 200 and 400 mg/d of EO were added. In addition, 200 mg/d of EO marginally decreased the potentially ruminally degradable fraction of the CP of the complete ration. Results using dairy cows in mid lactation that grazed 8 h/d on lush pasture showed limited effects of this EO complex on ruminal fermentation, milk production and milk composition.