Effects of dietary crude protein concentration and degradability on milk production responses of early, mid, and late lactation diary cows

Three experiments were conducted to investigate the effect of crude protein (CP) concentration and ruminally undegraded protein (RUP) concentration on milk production and composition of dairy cows at three different stages of lactation. Experiments 1, 2, and 3 using 39, 40, and 39 Holstein cows were conducted for cows in early (wk 4 to 14 postpartum), mid (wk 19 to 29), and late (wk 34 to 44) lactation, respectively. Cows were assigned to one of four corn-based diets: high CP, medium RUP (control); low CP, low RUP; low CP, medium RUP; and low CP, high RUP. Percentages of CP in the high and low CP diets were, respectively, 17.4 and 15.2 for Experiment 1, 15.3 and 13.3 for Experiment 2, and 14.2 and 12.6 for Experiment 3. The RUP concentrations (percentages of CP) for low, medium, and high diets averaged 35.5, 41.4, and 46.5%, respectively. For Experiment 1, production of milk, 4% fat-corrected milk, milk fat, and milk protein was increased by the high protein diets versus the low protein diets. Production of milk and fat-corrected milk increased linearly as RUP in the diet increased. During Experiment 2, lactational responses were not affected by treatment. During Experiment 3, dry matter intake, body weight, and body weight change increased for cows fed the high protein diets versus those same measurements for cows fed the low protein diets. Milk fat and milk protein percentage decreased linearly as RUP in the diet increased. Because there was no effect of diet on milk production, decreasing CP in diets fed to cows in mid or late lactation can reduce the cost of the diet and waste N excreted from the cow.