Ovarian response and pregnancy rates to timed artificial insemination in beef heifers after synchronization of follicular waves and ovulation via a 14-day controlled internal drug-release insert estrus synchronization protocol

The effects of extended progestin exposure, gonadotropin-releasing hormone (GnRH) versus prostaglandin (PG) F2α at controlled internal drug-release (CIDR) insertion, and GnRH on d 7 versus 9 of a 14-d CIDR on ovarian response and timed AI (TAI) pregnancy rate were evaluated. In Exp. 1 beef heifers (n = 319) received 1) GnRH and CIDR on d 0, GnRH on d 9, and 50 mg of PG on d 14 at CIDR removal (14-d GnRH-9); 2) 14-d GnRH-9 but with PG versus GnRH on d 0 (14-d PG GnRH-9); or 3) GnRH and CIDR on d 9 and 25 mg of PG at CIDR removal on d 14 (5-d CO-Synch+CIDR). Heifers (n = 452) in Exp. 2 received 1) 14-d GnRH-9, 2) 14-d GnRH-9 with GnRH on d 7 versus 9 and 25 versus 50 mg of PG on d 14 (14-d GnRH-7), or 3) GnRH and CIDR on d 7 and 25 mg of PG at CIDR removal on d 14 (7-d CO-Synch+CIDR). All heifers received GnRH at TAI. Pregnancy rate was not different (P ≥ 0.66) among treatments in either experiment. In Exp. 1, heifers in d 6 to 10 of the estrous cycle at protocol initiation had a greater (P < 0.05) TAI pregnancy rate than did heifers in d 11 to 16 and d 17 to 21 but did not differ (P > 0.10) from d 0 to 5. Results indicate extended progestin exposure, PG at CIDR insertion, and GnRH timing did not affect pregnancy rate to TAI.