Conclusions
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 lev-els of dietary essential oil compounds (EO; 0, 200, 400 and 600 mg/d) on milk productionand composition. Cows were allowed to graze on winter oats for 8 h/d with a daily herbageallowance of 15 kg dry matter (DM)/cow, and then received supplemental corn silage andsunflower meal in confinement for the remainder of the day. The EO were fed individuallyat milking times (0600 and 1600 h), mixed with 0.86 kg DM of dry rolled corn grain. Inaddition, 4 ruminally cannulated lactating Holstein cows in mid lactation were used in a4×4 Latin Square design with 14 d periods to study effects of EO on ruminal fermentationcharacteristics 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 com-position were not affected by EO. In general, ruminal fermentation characteristics were notaffected by EO addition at any level, except for a 13% increase in butyrate concentrationswith all EO levels compared to the control. Ruminal ammonia N concentration was highin all treatments (51.5 ± 5.75 mg/100 ml) and tended (P=0.09) to increase when 200 and400 mg/d of EO were added. In addition, 200 mg/d of EO marginally decreased the poten-tially ruminally degradable fraction of the CP of the complete ration. Results using dairycows in mid lactation that grazed 8 h/d on lush pasture showed limited effects of this EOcomplex on ruminal fermentation, milk production and milk composition.