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

Abstract

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 h 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-17β or 1 mg ECP i.m. (E-17β and ECP groups, respectively) on Day 0. Seven (E-17β 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-17β (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-17β, 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-17β, 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-17β 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.