Effect of Zinc Methionine Supplementation on Productive Performance of Lactating Friesian Cows

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 calving in a complete switch-back design. The cows were fed a basal ration of 35% concentrate feed mixture with 40% berseem and 25% rice straw (on a dry matter (DM) basis) without supplementation (G1) or supplemented with 5 and 10 g zinc methionine per head per day (G2 and G3, respectively). Nutrient digestibility coefficients and nutritive values were significantly increased (P<0.05) by zinc methionine supplementation. Cows in G2 had the highest feed intake of total digestible nutrients (TDN) and digestible crude protein (DCP), followed by G3 and G1, respectively (P<0.05). The ruminal total volatile fatty acid concentration increased and ammonia nitrogen decreased significantly (P<0.05) by zinc methionine supplementation. Cows in G3 showed the highest plasma total protein, albumin, and globulin concentrations (P<0.05). Zinc methionine supplementation 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 G2 (2.54 kg/d or 18.1%) and G3 (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 G2 and G3, respectively. Cows in G2 had the highest fat, total protein, and total solids contents, and the yield of all milk constituents and animals in G3 showed the highest lactose, solids-not-fat, and ash contents. Zinc methionine supplementation improved feed conversion, reducing the quantities of DM, TDN, and DCP required to produce 1 kg of 4% FCM (P<0.05). The average daily 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.