EFFECTS OF RECOMBINANT BOVINE SOMATOTROPIN AND DIETARY ENERGY-INTAKE ON GROWTH, SECRETION OF LUTEINIZING-HORMONE, FOLLICULAR DEVELOPMENT, AND ONSET OF PUBERTY IN BEEF HEIFERS

The effects of dietary energy and recombinant bovine somatotropin (bST ) on pattern of LH release, follicular development, and onset of puber ty were studied in 40 Angus heifers. At 7 mo of age, heifers were assi gned to a 2 x 2 factorial experiment; the main effects were dietary en ergy (high [HDE]: 14.15 Meal of ME/d or moderate [MDE]: 10.84 Meal of ME/d) and somatotropin (bST; 350 mg every 2 wk or vehicle). Beginning at 9 mo of age, heifers were observed twice daily for estrous activity . From 10.5 to 12 mo of age, five heifers from each treatment group we re selected for weekly ultrasound. examination of ovarian structures a nd biweekly sequential blood sampling to determine concentrations of L H. Somatotropin treatment altered intermediary metabolism in a manner consistent with enhanced accretion of lean tissue and decreased deposi tion of fat; The HDE heifers were younger (P < .001) at puberty than t he MDE heifers, but BW at puberty was not different among treatment gr oups. Age and body weight at puberty were not affected by bST. Frequen cy of LH pulses increased within the 10.5 to 12 mo of age sampling win dow in HDE-treated heifers but not in MDE heifers (dietary energy x mo nth of age; P < .02). Secretion of LH was unaffected by bST. Ovaries o f bST-treated heifers tended (P < .09) to have fewer follicles > 5 mm in diameter than those of vehicle-treated heifers. We conclude that ch ronic treatment with bST did not alter age at puberty or pattern of LH release in heifers and that even modest differences in energy intake influence the timing of the prepubertal increase in pulsatile LH relea se.