Submitted to: Journal of Animal Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/1/2001
Publication Date: 8/1/2001
Citation: GEARY, T.W., SALVERSON, R.R., WHITTIER, J.C. SYNCHRONIZATION OF OVULATION USING GNRH OR HCG WITH THE CO-SYNCH PROTOCOL IN SUCKLED BEEF COWS. JOURNAL OF ANIMAL SCIENCE. 2001. v. 79. p. 2536-1541. Interpretive Summary: While hCG may be less expensive than GnRH, and is often used as a replacement for GnRH to treat cystic ovaries, it is not a suitable replacement for GnRH to synchronize ovulation with the current CO-Synch protocol. Further studies with hCG in 2 yr old cows are needed to confirm the present findings that it is better than GnRH among cows of this age group. Over all age groups, use of hCG resulted in numerically lower estrous responses and ovulation rates, numerically higher progesterone levels at the time of insemination, and a higher frequency of short estrous cycles that resulted in lower pregnancy rates. In addition, 48-h calf removal did not improve the pregnancy rates of cows receiving either GnRH or hCG using the CO-Synch protocol in this study.
Technical Abstract: This study was conducted to evaluate hCG in place of GnRH and the effects of 48-h calf removal (CR) on pregnancy rates of cows synchronized with the CO-Synch protocol. Suckled beef cows (n = 467) within two locations received either GnRH +/- CR or hCG +/- CR using the CO-Synch protocol. At one location, blood samples were collected from all cows on d -14, -7, 0, 7, 9, and 16 for analysis of progesterone concentration. Calf removal was from d 7 to 9 of the CO-Synch protocol. Cows that exhibited estrus between were bred 12 h later, and those not observed in heat by d 9 were mass mated and received a second injection of GnRH or hCG. The AI pregnancy rate for GnRH-treated cows with and without CR and hCG-treated cows with and without CR was 46, 49, 35, and 34%, respectively (P=.44). Pregnancy rates of cows differed between treatments by age (P=.07), hormone (P=.09), and hormone by age (P=.01), but not CR (P=.66) or CR by age (P=.33). Pregnancy rates were higher for 2 yr old cows that received hCG without CR than other treatments. Two year old cows had higher pregnancy rates to the hCG treatments than GnRH treatments independent of CR, whereas the opposite was true for cows 3 yr old and older. Overall, GnRH-treated cows (48%) had a higher (P=.09) pregnancy rate than hCG-treated cows (34%). Both GnRH and hCG were similar (P=.40) in their ability to induce ovulation and corpus luteum formation among anestrous cows. More (P=.001) hCG-treated cows exhibited short estrous cycles following timed AI, and there tended to be more (P=.20) hCG-treated cows with elevated progesterone at the timed AI. We conclude hCG is not a suitable replacement for GnRH to synchronize ovulation with the CO-Synch protocol.