Effects of dietary protein level on milk production and efficiency of N utilization in dairy cows.

The objective of this experiment was to evaluate the effect of the reduction of dietary crude protein (CP), to strictly cover the metabolizable protein requirement, on milk production and efficiency of N utilization. The study involved 28 mid-lactating Italian Friesian dairy cows and was conducted as a cross-over design comprising two periods of 28 d each. The cows were assigned (14 per treatment) to one of 2 isoenergetic dietary treatments (TMR based on corn silage): basal diet (B) and Low Protein diet (LP) containing 15.5 and 14.2 % of CP (DM basis), respectively. Average daily DMI and N intake of each treatment was calculated on a weekly basis. Individual milk yield was recorded daily, and milk samples were weekly collected and analyzed. Average daily secretion of N in milk (g/d) was calculated on a weekly basis. Equation based on urea clearance rate was used to estimate urinary N excretion on a weekly basis. Blood samples were collected at the end of each period and analyzed. Data were processed using mixed procedure for repeated measures. Neither milk yield nor milk protein content were negatively affected by the dietary CP. The urea content in milk and blood (22.4 vs 28.5 and 23.8 vs 30.7 mg/100 mL, in LP and B respectively; P<0.001) was affected by the dietary treatment. The estimated N urinary excretion (both as g d-1 and as percentage of N intake) was lower in LP than in B; conversely, the overall average efficiency of N utilization was greater in LP (33.8 vs 31.1% of N output with milk in percentage of N intake in LP and B respectively; P=0.05). In conclusion results highlighted that a dietary CP lower than the practice standard feeding does not reduce milk yield of mid-lactating cows, improves the efficiency of N utilization, and reduces N urinary excretion.