PROGESTIN-INDUCED PUBERTY AND SECRETION OF LUTEINIZING-HORMONE IN HEIFERS

Two experiments were designed to investigate the endocrine mechanisms by which progestin administration induces puberty in heifers. In exper iment 1, prepubertal heifers were randomly assigned by age to receive either a single norgestomet implant for 10 days (NORG; n = 15) or to s erve as unimplanted controls (CONT; n = 14). Serial blood samples were obtained on Days -0.5, 8.5, and 10.5 (Day 1 = day of implant insertio n) and were analyzed for concentrations of LH. On Days 9 and 11, 4 hei fers in each treatment were slaughtered, and the reproductive tracts w ere obtained. Weekly progesterone analyses were performed to estimate the day of puberty in heifers not slaughtered. Puberty was induced in 6 of 7 heifers in the NORG treatment, resulting in an earlier (p < 0.0 5) day of puberty in the NORG than in the CONT treatment. The frequenc y of LH pulses was higher (p < 0.05) on Days 8.5 and 10.5 in the NORG as compared to the CONT treatment. Although no difference (p > 0.10) w as observed between treatments in follicular development, uterine weig ht was greater (p < 0.05) in NORG than in CONT heifers on Day 11. In e xperiment 2, prepubertal heifers (n = 47) were administered either 1 ( 1NORG; n = 16), 3 (3NORG; n = 16), or 0 (CONT; n = 15) norgestomet imp lants for 10 days, and serial blood samples were obtained as described for experiment 1. Transrectal ultrasonography was used to determine t he diameter of the largest follicle on Day 9. Plasma samples were obta ined after each serial sample collection period and were analyzed for estradiol concentrations, Puberty was induced in 75% (12 of 26) and 81 % (13 of 16) of heifers in the 1NORG and 3NORG treatments, respectivel y. Four heifers in the 1NORG treatment, from which serial blood sample s were collected, ovulated before removal of the progestin implant, an d the LH data for this treatment were deleted, In the 3NORG treatment, LH pulse frequency was suppressed (p < 0.05) on Day 8.5, but was grea ter (p < 0.05) 12 h after removal of the progestin (Day 10.5) than in the CONT treatment. We conclude that progestin administration hastens puberty by increasing LH secretion after progestin withdrawal and prop ose that progestin administration induces puberty by accelerating the peripubertal decrease of estradiol negative feedback on LH secretion.