Effects of estradiol on gonadotrophin release, estrus and ovulation in CIDR-treated beef cattle

The effects of estradiol-17β (E-17β) or estradiol benzoate (EB) on gonadotrophin release, estrus and ovulation in beef cattle were evaluated in two experiments. In experiment 1, 16 ovariectomized cows received a previously used CIDR insert from days 0 to 7 and 1 mg of EB on day 8; they also received 5 mg of E-17β on days 0 or 1, or 5 mg of E-17β + 100 mg of progesterone on day 0. There was only an effect of time (P < 0.0001) on plasma concentrations of progesterone, estradiol, FSH, and LH. Following treatment with E-17β, plasma FSH concentrations were suppressed for approximately 36 h, whereas plasma LH concentrations were reduced (P < 0.05) for 6 h, but surged within 24 h. Injecting 1 mg of EB 24 h after CIDR removal decreased (P < 0.02) plasma LH concentrations for 6 h, followed by an LH surge at 18 h. In experiment 2, ovary-intact heifers (n = 40) received a used CIDR and 5 mg of E-17β + 100 mg of progesterone on day 0. On day 7, CIDR were removed, PGF given, and heifers received nothing (control) or 1 mg of EB 12, 24, or 36 h later. In these groups, plasma LH peaked (mean ± SEM) 78.0 ± 23.0, 37.8 ± 8.5, 44.4 ± 10.3, and 51.0 ± 5.1 h after CIDR removal (means, P < 0.001; variances, P < 0.001) and intervals from CIDR removal to ovulation were 102.0 ± 6.7, 63.6 ± 3.6, 81.6 ± 3.5, and 78.0 ± 4.1 h (P < 0.05). The interval from CIDR removal to ovulation was shorter and less variable in EB-treated groups; the interval from EB to ovulation was shortest (P < 0.05) in the 12-h group. In summary, E-17β or EB decreased both FSH and LH, but LH increased after 6 h (despite elevated progesterone concentrations). Following CIDR removal, 1 mg of EB effectively synchronized LH release, and ovulation (in intact cattle), but the interval from CIDR removal to EB treatment affected the time of ovulation.