The Effect of Varying Levels of Metabolizable Protein on Milk Yield and on Blood Metabolites in Holstein Cows

The aim followed in the present study was to investigate the effects of different Metabolizable Protein (MP) levels on Holstein cows، milk yield and composition، and as well on concentration of plasma metabolites in the cow،s early lactation. Twenty - four Holstein cows (BW 580±23، DIM 30±7، parity 2. 4±0. 5) were randomly assigned to each one of the 4 dietary treatments containing 1) 10. 60% (diet A)، 2) 11. 07 (diet B)، 3) 11. 54 (diet C)، and 4)12. 00 % (diet D) of MP on a dry matter basis. The experiment lasted for 55 days. The results revealed that none of the factors of: dry matter intake، body weight change، milk fat yield، or non-fat solids were affected by the diets. Fat corrected milk yield and milk protein yield increased as the MP level increased up to 11. 54% of dry matter and more. Rumen metabolites، namely concentration of total volatile fatty acids، proportion of individual volatile fatty acids، pH and concentration of ammonia were not affected by dietary treatments. Concentration of blood glucose، cholesterol، and non-esterified fatty acids were not affected by different MP levels. Concentrations of blood urea nitrogen and milk urea nitrogen increased with incrase in the MP level.