ENDOCRINE MECHANISMS OF PUBERTY IN HEIFERS - ESTRADIOL NEGATIVE FEEDBACK-REGULATION OF LUTEINIZING-HORMONE SECRETION

The hypothesis that luteinizing hormone (LH) secretion in prepubertal females is responsive to estradiol negative feedback and that decreased feedback occurs as puberty approaches was tested in heifers. In the 1st experiment, 7 heifers were maintained prepubertal by dietary energy restriction until 508 days of age (Day0). All heifers were placed on a high-energy diet on Day 0 at which time they received no additional treatment (CONT), were ovariectomized (OVX) or were ovariectomized and s.c. implanted with estradiol-17.beta. (OVX-E2). This feeding regimen was used to synchronize reproductive state in all heifers. A 2nd experiment was performed with 16 prepubertal heifers using the same treatments at 266 days (Day 0) of age (CONT, OVX and OVX-E2) but no dietary intake manipulation. In both experiments, LH secretion increased rapidly following ovariectomy in OVX heifers. In the initial experiment, LH secretion was maintained at a low level in OVX-E2 heifers until a synchronous rapid increase was noted coincidental with puberty in the CONT heifer. In the 2nd experiment, LH secretion increased gradually in OVX-E2 heifers and attained castrate levels coincidental with puberty in CONT heifers. A gradual increase in LH secretion occurred as puberty approached in CONT heifers. LH secretion in prepubertal heifers evidently is responsive to estradiol negative feedback. Estradiol negative feedback decreases during the prepubertal period in beef heifers.