Hypothalamic unresponsiveness to an
estradiol surge appears to be an underlying cause of large follicle anovular condition (
follicular cysts), but
progesterone exposure for 7 days resolves this condition. In this study, dairy cows with induced (Experiment 1) or naturally occurring (Experiment 2)
follicular cysts were treated for different times with
progesterone. In Experiment 1, 16 of 26 cows (62%) were induced into
anovulation by causing a
GnRH/LH surge when no ovulatory follicle was on the ovary. Anovular cows (n = 16) were assigned to one of four treatment groups ( 0, 1, 3, or 7 days of
progesterone treatment) using an intravaginal,
progesterone-releasing implant (CIDR). All anovular cows had low circulating
progesterone concentrations before controlled internal
drug releasing (CIDR) and greater concentrations that reached steady state (1.3 +/- 0.1 ng/mL
progesterone) by 3 h after CIDR insertion. Circulating
progesterone decreased to basal concentrations by 4 h after CIDR removal. Cows were treated with 5mg
estradiol benzoate (EB) 12 h after CIDR removal. None (n = 4) of the control cows (0 day) had an LH surge after EB. All of the 3 days (5/5) and 7 days (4/4) CIDR-treated cows had an LH surge following EB, but only one of the 1 day (1/3) CIDR-treated cows. Magnitude of the LH peak was similar in the 3 and 7 days cows. All cows treated for 7 days ovulated (4/4), whereas, ovulation occurred in only 3/5, 1/3, and 0/4 of the cows treated for 3, 1, and 0 day, respectively. The two cows in the 3 days group that did not ovulate had a normal LH surge, but these two cows had a smaller maximal follicle size than cows that ovulated. In Experiment 2, naturally anovular lactating dairy cows (24 of 248) were identified using weekly ultrasonography. All anovular cows grew follicles to >12 mm, with 54% (13 of 24) having follicles larger than ovular size (15-24 mm) and 33% (8 of 24) having follicles that would be considered cystic (>25 mm). Anovular cows were randomly assigned to CIDR treatment for 0, 1, or 3 days. All (7/7) of 3 days, 33% (3/9) of 1 day, and 25% (2/8) of control (0 day) cows ovulated by 1 week after CIDR removal. Thus, 3 days but not 1 day of
progesterone exposure appears to be sufficient to reinitiate
estradiol responsiveness of the hypothalamus.