Effects of recombinant bovine somatotropin on milk production, body composition, and physiological parameters.

Twenty-eight multiparous Holstein cows were utilized to determine effects of long-term administration of recombinant bovine somatotropin on lactational performance, body condition, body composition, hormones, blood constituents, and physiological parameters. Treatments were 0 (control), 10.3, 20.6, and 41.2 mg recombinant bovine somatotropin administered daily as subcutaneous injections beginning 4 to 5 wk postpartum and continuing for 38 wk. A total mixed diet of 28% corn silage, 22% alfalfa hay cubes, and 50% corn-soybean meal-based grain mixture was fed. Fat-corrected milk yields were increased 12 to 25% for cows treated with somatotropin as compared with controls (29.9 kg/d). Milk composition was similar among treatments. Cows receiving somatotropin consumed 4 to 10% more feed and were 11 to 17% more efficient than controls for conversion of feed to milk. Body weight changes were not significantly different among treatments, but cows receiving somatotropin gained 4 to 10% less weight than controls. Body fat (kg) of cows receiving 20.6 and 41.2 mg/d was less than that of cows receiving 10.3 mg/d or no somatotropin. Estimated weights of body protein and mineral, most blood constituents, respiratory rates, and body temperature were not affected by somatotropin administration. Plasma fatty acids were elevated and hematocrit values were reduced. Plasma insulin, serum somatotropin, and heart rate increased concurrently with somatotropin administration.