Effects of oestradiol and some of its esters on gonadotrophin release and ovarian follicular dynamics in CIDR-treated beef cattle

Abstract Three experiments were conducted to: (1) compare the effect of three oestradiol formulations on gonadotrophin release in ovariectomised cows; (2) compare the effects of either oestradiol-17β (E-17β) or oestradiol benzoate (EB), given at two doses, on the synchrony of ovarian follicular wave emergence in CIDR-treated beef cattle; and (3) determine the timing of ovulation of the dominant follicle of a synchronised follicular wave following administration of E-17β or EB 24 h after progesterone withdrawal. In Experiment 1, ovariectomised cows ( n = 16) received a once-used CIDR on Day 0 (beginning of the experiment) and were allocated randomly to receive 5 mg of E-17β, EB or oestradiol valerate (EV) plus 100 mg progesterone i.m. The CIDR inserts were removed on Day 7. There were effects of time, and a treatment-by-time interaction ( P P P P n = 29) received a new CIDR on Day 0 (beginning of the experiment), and were assigned randomly to receive 1 or 5 mg of E-17β or EB i.m. on Day 1. On Day 8, CIDR were removed and PGF was given. Transrectal ultrasonography was done once daily from 2 days before CIDR insertion to 2 days after CIDR removal, and then twice-daily to ovulation. Although there was no difference among groups in the interval from oestradiol treatment to follicular wave emergence (4.2 ± 0.3 days; P = 0.5), 5 mg of E-17β resulted in the least variable interval to wave emergence ( P P = 0.1). For the interval from CIDR removal to ovulation, there were no differences among groups for either means ( P = 0.5) or variances ( P = 0.1). In Experiment 3, beef heifers ( n = 32) received a once-used CIDR on Day 0 (beginning of the experiment) plus 100 mg progesterone i.m. and were assigned randomly to receive 5 mg E-17β or 1 mg EB i.m. On Day 7, CIDR were removed and all heifers received PGF. On Day 8 (24 h after CIDR removal), each group was subdivided randomly to receive 1 mg of either E-17β or EB i.m. There was no effect of oestradiol formulation on interval from treatment to follicular wave emergence (4.1 ± 0.2 days; P = 0.7) or on the median interval (76.6 h; P = 0.7) or range (72–120 h; P = 0.08) from CIDR removal to ovulation. In summary, oestradiol treatments suppressed FSH in ovariectomised cows, with the duration of suppression dependent on the oestradiol formulation. Both E-17β and EB effectively synchronised ovarian follicular wave emergence and ovulation in CIDR-treated cattle, and the interval from CIDR removal to ovulation did not differ in heifers given either E-17β or EB 24 h after CIDR removal.