Effects of diet and injection of bovine somatotropin on prepubertal growthand first-lactation milk yields of Holstein cows

To determine effects of rapid prepubertal growth on first-lactation milk pr oduction, Holstein heifers were randomly assigned to one of three treatment s. Thirty-five heifers were fed a standard diet to meet NRC recommendations and produce 0.8 kg of body weight (BW) gain/d (standard). Thirty-five heif ers were fed a diet with higher energy (2.8 Mcal of metabolizable energy/kg ) and protein (19.7% crude protein; high diet) to produce 1.2 kg of BW gain /d (high). Thirty-five heifers were fed the high diet and injected daily wi th bovine somatotropin (bST) (25 mu g/kg of BW; high-bST). Diets were fed a nd bST was injected from 135 kg of BW until pregnancy was confirmed. Heifer s were inseminated after BW exceeded 363 kg. Pregnant heifers were commingl ed and fed similar diets through gestation, parturition, and lactation. Hig h and high-bST heifers had greater prebreeding average BW gains than standa rd heifers. Conversely, standard heifers had a greater average BW gain duri ng gestation than high and high-bST heifers. High and high-bST heifers were approximately 90 d younger than standard heifers at first insemination and first parturition. Postpartum BW, body condition scores, and withers heigh ts at parturition, and calving ease scores were not different among treatme nts. Standard heifers produced 14% more milk than high heifers but not more than high-bST heifers. The high-protein, high-energy diet decreased age at first parturition and first-lactation milk production, but did not affect reproduction. Injection of bST during the prepubertal growth period combine d with the high diet decreased age at first parturition without reducing mi lk production.