摘要Twelve lactating Friesian Cows with body weight of 450 to 550 kg and in the second to fifth lactating season were used after 8 weeks of cal-ving in a complete switch-back design. The cows were fed a basal ration of 35% concentrate feed mix-ture with 40% berseem and 25% rice straw(on a dry matter(DM)basis)without supplementation(G<,1>)or supplemented with 5 and 10 g zinc methionine per head per day(G<,2> and G<,3>, respectively). Nutrient di-gestibility coefficients and nutritive values were signif-icantly increased(P < 0.05)by zinc methionine sup-plementation. Cows in G<,2> had the highest feed intake of total digestible nutrients(TDN)and digestible crude protein(DCP), followed by G<,3> and G<,1>, re-spectively(P <0.05). The ruminal total volatile fat-ty acid concentration increased and ammonia nitrogen decreased significantly(P < 0.05)by zinc methio-nine supplementation. Cows in Ga showed the highest plasma total protein, albumin, and globulin concen-trations(P < 0.05). Zinc methionine supplementa-tion led to significant decreases(P < 0.05)in the activities of aspartate aminotransferase and alanine aminotransferase. A significant increase(P < 0.05)was observed in the milk yield for G<,2>(2.54 kg/d or 18.1%)and G<,3>(1.95 kg/d or 14.2%)compared with the control group. The corresponding increase in 4% fat-corrected milk(FCM)was 2.85 and 2.03 kg/d (21.63% and 15.74%)for G<,2> and G<,3>, respectively.Cows in G<,2> had the highest fat, total protein, and to-tal solids contents, and the yield of all milk constitu-ents and animals in G<,3> showed the highest lactose,solids-not-fat, and ash contents. Zinc methionine sup-plementation improved feed conversion, reducing the quantities of DM, TDN, and DCP required to pro-duce 1 kg of 4% FCM(P<0.05). The average dai-ly feed cost per kilogram of 4% FCM decreased and the average income from milk production increased with zinc methionine supplementation(P < 0.05).Quadratic analysis showed that the optimal level of zinc methionine supplementation is 6 g per head per day in terms of milk yield and net revenue.