Efficacy of the 5-day CO-Synch ovulation synchronization protocol with or without the inclusion of exogenous progesterone in beef cows

Fixed timed-AI (TAI) pregnancy rates were compared in suckled beef cows (n = 883; 5 replications) synchronized using the 5-d CO-Synch protocol with (CDR) or without (NCDR) the inclusion of exogenous progesterone (EAZI-BREED CIDR insert; CIDR). Cows were assigned to either the CDR (n = 445) or NCDR (n = 438) treatment by breed, parity, and days postpartum. Blood samples were collected 10 d before and immediately before CIDR insertion (d 0) to determine cyclic status. On d 0 all cows received gonadotropin-releasing hormone (100 μg), and cows in the CDR treatment received a CIDR. On d 5, CIDR were removed in CDR treatment and all cows received 2 separate doses of prostaglandin F2α (25 mg/dose). Cows were TAI 72 h after CIDR removal (d 8), concurrent with gonadotropin-releasing hormone (100 μg). The proportion of females determined to be cyclic before estrous synchronization was 87.1% (757/869). Timed-AI pregnancy rates were greater (P = 0.05) in CDR (62.3%, n = 438) than NCDR (50.7%, n = 436) treatment. However, a treatment × parity interaction (P = 0.08) was noted relative to TAI. Although CDR obtained greater (P ≤ 0.01) TAI pregnancy rates than NCDR in both primiparous and multiparous groups, difference between treatments was greater in primiparous females (69.0 vs. 46.3% for CDR and NCDR, respectively) than multiparous females (60.7 vs. 51.7% for CDR and NCDR, respectively). To optimize TAI pregnancy rates in beef cows synchronized with the 5-d CO-Synch protocol, inclusion of a CIDR is recommended.