A high-estrogen environment during late pregnancy is suspected to cause postpartum
silent ovulation, and progesterone (P4) is suggested to recover estrus.
However, few attempts have been undertaken to elucidate the influence of these steroids on
estrus by analyzing hormonal profiles. We investigated estrus and luteinizing hormone (LH)
surges in ovariectomized goats (n=6) assigned to three treatments in a cross-over design.
In groups 1 and 2, 200 µg/kg body weight/day estradiol benzoate (Dose-200 E2B)
was administered for 14 days concurrent with P4 for 11 days, while in the
control, saline solution and P4 were administered likewise. Ten days after the
final administration of Dose-200 E2B, group 2 was treated with P4
for 8 days, and all groups were treated with 2 µg/kg body weight E2B (Dose-2
E2B) 20 days after the final administration of Dose-200 E2B (or
saline solution). The proportion of cases expressing estrus after the administration of
Dose-2 E2B was smaller (P<0.01) in group 1 than in the control (1/6, 3/6 and
6/6; groups 1 and 2 and the control, respectively). The proportions of cases generating LH
surges did not differ (P>0.1) among the groups (5/6, 5/6 and 6/6; groups 1 and 2 and
the control, respectively), but the peak concentrations in groups 1 and 2 (26.2 ± 14.7 and
11.3 ± 6.7 ng/ml) were lower (P<0.01) than those in the control (67.8 ± 19.4 ng/ml).
These results demonstrated that elevation of plasma estrogen mimicking late pregnancy
inhibits the subsequent estrus induced by estrogen simulating the follicular phase.