Synchronization of ovulation using GnRH or hCG with the CO-Synch protocol in suckled beef cows

The objectives of this study were to evaluate replacing GnRH with hCG and t he effects of 48-h calf removal (CR) on pregnancy rates of cows synchronize d with the CO-Synch protocol. Suckled beef cows (n = 467) at two locations were assigned to treatment by breed, age, and calving date. Treatment inclu ded either GnRH with (n = 121) or without CR (n = 117) or hCG with (n = 115 ) or without CR (n = 114) using the CO-Synch protocol. On d 0 and 9, cows r eceived either hCG (2,500 IU, i.m.) or GnRH (100 mug, i.m.), and on d 7 all cows received PGF(2 alpha) (25 mg). At one location, blood samples were co llected from all cows (n = 203) on d -14, -7, 0, 7, 9, and 16. Calves were removed on d 7 and returned on d 9 (48 h) from approximately half of the co ws that received GnRH or hCG. Cows that were detected in estrus between d 6 and 9 were bred approximately 12 h later and received no further injection s. Cows not observed in estrus by d 9 received a second injection of either GnRH or hCG and were timed-inseminated. The AI pregnancy rates for GnRH-tr eated cows with or without CR and hCG-treated cows with or without CR were 46, 49, 35, and 34%, respectively (P = 0.44). Pregnancy rates of cows diffe red by treatment x age interaction (P = 0.07), hormone (P = 0.09), and horm one x age (P = 0.01) but not by CR (P = 0.66) or CR x age (P = 0.33). Among 2-yr-olds, pregnancy rates were higher for cows treated with hCG without C R than for cows that received GnRH with calf removal, whereas cows treated with hCG with CR and GnRH without CR were intermediate. In addition hCG-tre ated 2-yr-olds had higher pregnancy rates than GnRH-treated 2-yr-olds regar dless of calf presence, but the reverse was true for older cows. Overall, G nRH-treated cows (48%) had a higher (P = 0.09) pregnancy rate than hCG-trea ted cows (34%). Among anestrous cows, GnRH and hCG were similar (P = 0.40) in their ability to induce ovulation and corpus luteum formation after the first and second injections of GnRH (31 and 76%, respectively) or hCG (39 a nd 61%, respectively). More (P = 0.001) hCG-treated cows exhibited short es trous cycles following timed AL We conclude that hCG is not a suitable repl acement for GnRH to synchronize ovulation with the CO-Synch protocol in mul tiparous cows, although further evaluation among primiparous cows is warran ted using hCG with the CO-Synch protocol.