GROWTH, REPRODUCTIVE-PERFORMANCE, MAMMARY DEVELOPMENT, AND MILK-PRODUCTION OF BEEF HEIFERS AS INFLUENCED BY PREPUBERTAL DIETARY ENERGY AND ADMINISTRATION OF BOVINE SOMATOTROPIN

Three trials with 156 crossbred heifers were used to determine the eff ects of dietary energy and bovine somatotropin administration on subse quent heifer productivity. In Trial 1, heifer calves were weaned from their dams (n = 28; 113 +/- 13 d of age) and assigned to a 2 x 2 facto rial arrangement of moderate (MDE) or high dietary energy (HDE) and in jections of vehicle (VEH) or 250 mg of bovine somatotropin (bST) every 14 d. Heifer calves in Trial 2 (n = 28; 123 +/- 20 d of age) and Tria l 3 (n 100; 134 +/- 22 d of age) nursed their dams while grazing pastu re and received no creep feed (MDE) or ad libitum access to creep feed (HDE) and received VEH or bST as in Trial 1. Treatments were administ ered for 112 d and heifers within trial were managed alike after that time. There were no dietary treatment x bST treatment interactions for any of the variables analyzed (P > .05). Dietary energy did not affec t daily gain in Trial 1; however, heifers receiving HDE in Trials 2 an d 3 had greater weight, hip height growth, and fat thickness (P < .01) by the end of the treatment period. Treatment with bST increased gain in Trials 1 and 2 (P < .01) but did not affect growth in Trial 3. The re were trends in all trials for HDE to reduce age at puberty (P < .15 ), but there were no consistent effects of bST on reproductive measure s. Mammary gland composition and milk production were unaffected by tr eatment in Trial 1. In Trial 2, HDE decreased subsequent milk producti on, calf weaning weight, and mammary dry fat free tissue and DNA(P < . 05). Treatment with bST in Trial 3 tended (P = .08) to result in great er milk production and increased calf weaning weights (P < .05). We co nclude that bST treatment may enhance growth with no effect on reprodu ction. Treatment with bST did not overcome deleterious effects of HDE on milk production but may play an important role in mammogenesis.