Effects of Rumen-Protected Methionine on Dairy Performance and Amino Acid Metabolism in Lactating Cows

Problem statement: Free Met as one of the most limiting AA in dairy cows would be mostly degraded in the rumen. This study was to determine the effect of different levels of Rumen-Protected Met (RPMet) on dairy performance and serum amino acid metabolism. Approach: Thirty-six Holstein cows in similar condition were randomly assigned to six experimental treatments with six replicates each. Levels of RPMet in six treatments were 0(control), 14, 28, 42, 56 and 70 g day^-1 per cow, respectively. Results: Treatment had no effect on percentage of milk protein, lactose and SNF. However, milk yield of cows fed 42 g day^-1 RPMet was significantly higher than that of the control group and milk fat percentage was significantly increased with 56 g day^-1 RPMet supplementation. There was the trend to decrease the concentration of serum amino acids except Met and Arg with the supplementation of RPMet. Serum EAA contents of the group supplementation of 42 g day^-1 RPMet were lowest although there were no significant differences among all treatments. Serum BCAA concentrations of cows fed 28 g RPMet were significantly lower than that of the control group. Supplementation of 42 g RPMet could significantly decrease the concentration of NEAA and TAA compared to the control group. Conclusion/Recommendations: Supplementation of rumen-protected methionine improved dairy performance and promoted amino acid utilization in lactating cows in the present experiment. The optimal level of RPMet in the diet was 42 g per cow day^-1.