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 puberty were studied in 40 Angus heifers. At 7 mo of age, heifers were assigned to a 2 x 2 factorial experiment; the main effects were dietary energy (high [HDE]: 14.15 Mcal of ME/d or moderate [MDE]: 10.84 Mcal 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 were selected for weekly ultrasound examination of ovarian structures and biweekly sequential blood sampling to determine concentrations of LH. Somatotropin treatment altered intermediary metabolism in a manner consistent with enhanced accretion of lean tissue and decreased deposition of fat. The HDE heifers were younger (P < .001) at puberty than the MDE heifers, but BW at puberty was not different among treatment groups. Age and body weight at puberty were not affected by bST. Frequency of LH pulses increased within the 10.5 to 12 mo of age sampling window in HDE-treated heifers but not in MDE heifers (dietary energy x month of age; P < .02). Secretion of LH was unaffected by bST. Ovaries of bST-treated heifers tended (P < .09) to have fewer follicles > 5 mm in diameter than those of vehicle-treated heifers. We conclude that chronic 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 release.