Effects of estradiol cypionate (ECP) on ovarian follicular dynamics, synchrony of ovulation, and fertility in CIDR-based, fixed-time AI programs in beef heifers

Estradiol cypionate (ECP) was used in beef heifers receiving a controlled internal drug release (CIDR: insertion = Day 0) device for fixed-time AI (FTAI) in four experiments. In Experiment 1, heifers (n = 24) received 1 mg ECP or 1 mg ECP plus 50 mg commercial progesterone (CP) preparation i.m. on Day 0. Eight or 9 days later, CIDR were removed, PGF was administered and heifers were allocated to receive 0.5 mg ECP i.m. concurrently (ECP0) or 24 It later (ECP24). There was no effect of treatment (P = 0.6) on mean ( S.E.M.) day of follicular wave emergence (3.9 +/- 0.4 days). Interval from CIDR removal to ovulation was affected (P < 0.05) only by duration of CIDR treatment (88.3 +/- 3.8 h versus 76.4 +/- 4.1 h; 8 days versus 9 days, respectively). In Experiment 2, 58 heifers received 100 mg progesterone and either 5 mg estradiol- 17beta or 1 mg ECP i.m. (E-17beta and ECP groups, respectively) on Day 0. Seven (E-17beta group) or 9 days (ECP group) later, CIDR were removed, PGF was administered and heifers received ECP (as in Experiment 1) or 1 mg EB 24 h after CIDR removal, with FTAI 58-60 h after CIDR removal. Follicular wave emergence was later (P < 0.02) and more variable (P < 0.002) in heifers given ECP than in those given E-17beta (4.1 +/- 0.4 days versus 3.3 +/- 0.1 days), but pregnancy rate was unaffected (overall, 69%; P = 0.2). In Experiment 3, 30 heifers received a CIDR device and 5 mg E-17beta, with or without 100 mg progesterone (P) i.m. on Day 0. On Day 7, CIDR were removed and heifers received ECP as described in Experiment 1 or no estradiol (Control). Intervals from CIDR removal to ovulation were shorter (P < 0.05) in ECP0 (81.6 +/- 5.0 h) and ECP24 (86.4 +/- 3.5 h) groups than in the Control group (98.4 +/- 5.6 h). In Experiment 4, heifers (n = 300) received a CIDR device, E- 17beta, P, and PGF (as in Experiment 3) and after CIDR removal were allocated to three groups (as in Experiment 2), with FTAI 54-56 h (ECP0) or 56-58 h (ECP24 and EB24) after CIDR removal. Pregnancy rate did not differ among groups (overall, 63.6%, P = 0.96). In summary, although 1 mg ECP (with or without progesterone) was less efficacious than 5 mg E-17beta plus 100 mg progesterone for synchronizing follicular wave emergence, 0.5 mg ECP (at CIDR removal or 24 h later) induced a synchronous ovulation with an acceptable pregnancy rate to fixed-time AI. (C) 2003 Elsevier Science Inc. All rights reserved.