Recombinant bovine somatotropin increases milk yield and calf gain in diverse breeds of beef cattle: associated changes in hormones and indices of metabolism.

In Exp. 1, Angus (A, n = 30), Charolais (C, n = 37), and Simmental (S, n = 30) multiparous cows received (s.c.) recombinantly derived bovine somatotropin (bST; sometribove, 500 mg) or vehicle (VEH) at 2-wk intervals from 124 to 228 d postpartum (DPP). Calves were weaned at 228 DPP. Bovine somatotropin increased (P < .01) milk yield and percentage of milk fat similarly in A, C, and S cows. Calf weaning weight was greater (P < .05) in cows treated with bST than in those given VEH. Administration of bST decreased deposition of fat and increased concentrations of IGF-I, insulin, glucose, and nonesterified fatty acids. In Exp. 2, we compared effects of bST initiated before or after the breeding season. Charolais (n = 33) and S (n = 40) cows were administered (at 2-wk intervals) VEH or bST beginning at 28 DPP (B-bST) or bST beginning at 105 DPP (A-bST). Calves were weaned at 243 DPP. Administration of bST before or after the breeding season increased milk yield on DPP 136 and 194; however, yields were greater in A-bST than in B-bST cows. Milk yields were similar in all cows at 236 DPP, corresponding to decreased forage availability. Calf body weight was greater (P < .05) in A-bST than VEH; B-bST calves were similar to VEH. Fat depth was greater in VEH than in bST-treated cows in C but not in S cows. Serum IGF-I was greater in A- and B-bST than in VEH cows. Mean days from calving to serum progesterone > 1 ng/mL and pregnancy rates were similar in VEH, A-, and B-bST cows. Administration of bST increased cow milk yield and subsequent calf weaning weight when initiated after 100 d postpartum. As anticipated, bST increased IGF-I, insulin, glucose, and nonesterified fatty acids. Administration of bST before and during the breeding season did not affect reproductive performance.