Influence of bovine somatotropin and nutrition on production and composition of milk from dairy cows.

Forty-eight multiparous Holstein cows were used in an experiment with a 2 x 2 factorial arrangement of treatments to study interactions of bST and nutrient density during wk 5 through 20 postpartum. Main effects were bST (0 or 10.3 mg/d) and diet (control or high protein and energy). The control diet was formulated to contain 16.3% CP and 1.65 Mcal of NEL/kg of DM; the high protein and energy diet was formulated to contain 18% CP and 1.74 Mcal of NEL/kg of DM with additional energy as fat from extruded soybeans and calcium salts of fatty acids. Diet ratios were 50:25:25 for concentrate:alfalfa hay:corn silage (DM basis). Bovine somatotropin increased milk production (40.9 vs. 37.9 kg/d) and production of 4% FCM (35.7 vs. 32.3 kg/d). The high protein and energy diet increased milk production (40.3 vs. 38.5 kg/d); 4% FCM production was higher for high protein and energy plus bST (36.0 kg) than without bST (33.6 kg). Percentage of milk fat was not affected by treatment. Percentage of milk protein was reduced with the high protein and energy diet, presumably because of the added fat, but this reduction was less with high protein and energy plus bST. Dry matter intake was not significantly elevated by bST (23.1 vs. 22.8 kg/d) but was lower for high protein and energy (22.0 vs. 23.8 kg/d). Increased dietary protein and energy with bST augmented the total milk production response.