Abstract: This experiment was conducted to determine the effects of different combinations of oregano oil (Oo) and rumen-protected methionine (RPMet) on lactation performance and nitrogen excretion of dairy cows. Forty lactating holstein dairy cows of similar age, body weight, parity, milk yield, milk composition and lactation status [(90±15) days post-calving] were divided into 10 groups with 4 cows per group. A Control group was fed a basal diet, and experimental groups were supplemented with different levels of Oo [11.5 (L), 13 (M) or 14.5 (H) g·(d·head)^-1] and RPMet [22.5 (L), 25 (M) or 27.5 (H) g·(d·head)^-1] to make nine treatment combinations, which were designated LL, ML, HL, LM, MM, HM, LH, MH and HH (i.e. the first letter indicates Oo supplementation rate, and the second letter indicates RPMet supplementation rate). Following a pre-experiment adjustment period of 15 days, the experiment lasted for 60 days. It was found that: The milk yield of LL, HL, HM, MH and HH groups was significantly higher than in the Control group (P<0.01), with MH being the highest. In terms of milk components, the milk fat percentages of MH and HM groups were significantly higher than the Control group (P<0.01), and the milk protein percentages of HM and HH groups were significantly higher than the Control group (P<0.01). Milk somatic cell of the experimental groups were significantly lower than the Control group (P<0.01). Nitrogen excretion rate of LL, ML, LM, MM, HM, LH, MH and HH groups was significantly lower than the Control group (P<0.01), with the MH group being the lowest. Under the conditions of the experiment, the MH group [Oo 13 g·(d·head)^-1, RPMet 27.5 g·(d·head)^-1] was the best treatment combination.

Key words: oregano oil, rumen-protected methionine, lactation performance, nitrogen excretion