Dosage of the synthetic progestin, norgestomet, influences luteinizing hormone pulse frequency and endogenous secretion of 17 beta-estradiol in heifers.

The objective of this study was to determine whether there were doses at which the synthetic progestin, norgestomet, could mimic midluteal phase concentrations of progesterone in regulating the secretion of LH and 17 beta-estradiol in bovine females. Heifers were randomly assigned to one of five groups to receive: 1) one (1Norg, n = 5), 2) two (2Norg, n = 5), 3) four (4Norg, n = 5), or 4) eight (8Norg, n = 5) norgestomet implants or to serve as untreated control heifers (control, n = 5). On Day 7 (Day 0 = behavioral estrus), implants containing norgestomet were inserted, and they remained in place for 10 days. All heifers implanted with norgestomet were treated with 25 mg prostaglandin F2 alpha (PGF2 alpha) on Days 7 and 8 to lyse the CL. Controls were treated with 25 mg PGF2 alpha at the time norgestomet implants were removed from heifers of the other treatment groups. Blood samples were collected every 15 min for 24 h on Days 10 and 16 to determine the frequency of LH pulses. Beginning 24 h after removal of implants, samples of blood were collected at 4-h intervals for 96 h to determine the time of the preovulatory surge of LH. Daily blood samples were collected from Day 2 to Day 48 to determine concentrations of progesterone, and samples collected between Days 2 and 17 were used to determine concentrations of 17 beta-estradiol. Ultrasonography was performed daily from Day 2 until Day 23 to evaluate ovarian follicular development.(ABSTRACT TRUNCATED AT 250 WORDS)