Evaluation of human chorionic gonadotropin as a replacement for gonadotropin-releasing hormone in ovulation-synchronization protocols before fixed timed artificial insemination in beef cattle

Two experiments were conducted during 2 yr to evaluate differences in ovulation potential and fertility in response to GnRH or hCG. In Exp. 1, 46 beef cows were given 100 μg of GnRH or 500, 1,000, 2,000, or 3,000 IU of hCG. Ovulation incidence was not differ- ent between GnRH and any of the hCG doses, indicat- ing that ovulatory capacity of at least 500 IU of hCG was equivalent to GnRH. In Exp. 2, beef cows (n = 676) at 6 locations were assigned randomly to a 2 × 3 facto- rial arrangement of treatments. Main effects were: 1) pre-timed AI (TAI) treatment (GnRH or hCG) and 2) post-TAI treatment (saline, GnRH, or hCG) to initiate resynchronization of ovulation in previously insemi- nated cattle. Blood samples were collected (d −21 and −10) to determine progesterone concentrations and as- sess cyclicity. Cattle were treated with a progesterone insert on d −10 and with 100 μg of GnRH or 1,000 IU of hCG. A PGF2α injection was given at insert removal on d −3. Cows were inseminated 62 h (d 0) after insert re- moval. On d 26 after first TAI, cows of unknown preg- nancy status were treated with saline, GnRH, or hCG to initiate a CO-Synch protocol. Pregnancy was diag- nosed 33 d after first TAI to determine pregnancies per AI (P/AI). Nonpregnant cows at 6 locations in yr 1 and 1 location in yr 2 were given PGF2α and inseminated 56 h later, concurrent with a GnRH injection. Five weeks later, pregnancy diagnosis was conducted to determine pregnancy loss after first TAI and pregnancy outcome of the second TAI. Injection of pre-TAI hCG reduced (P < 0.001) P/AI compared with GnRH, with a greater reduction in cycling cows. Post-TAI treatments had no negative effect on P/AI resulting from the first TAI. Serum progesterone was greater (P = 0.06) 7 d after pre-TAI hCG than after GnRH and greater (P < 0.05) after post-TAI hCG on d 26 compared with saline 7 d after treatment in association with greater frequency of multiple corpora lutea. Compared with saline, injec- tions of post-TAI GnRH and hCG did not increase sec- ond insemination P/AI, and inconsistent results were detected among locations. Use of hCG in lieu of GnRH is contraindicated in a CO-Synch + progesterone in- sert protocol. Compared with a breeding season having only 1 TAI and longer exposure to cleanup bulls, total breeding season pregnancy rate was reduced by one- third, subsequent calving distribution was altered, and 50% more AI-sired calves were obtained by applying 2 TAI during the breeding season.