Methods to improve reproductive efficiency in beef cattle

Five experiments were conducted to discover methods to increase reproductive efficiency in beef cattle. In Exp. 1 prepubertal beef heifers were used to compare the effects of administering gonadotropin-releasing hormone (GnRH) and human chorionic gonadotropin (hCG; 500 or 1,000 IU). A greater proportion of heifers treated with hCG ovulated and subsequently had greater concentrations of progesterone compared to heifers treated with GnRH.;In Exp. 2, a fixed-time artificial insemination (TAI) protocol was evaluated in beef heifers that used hCG 14 d prior to, or in place of the first injection of GnRH in a CO-Synch+CIDR estrous synchronization protocol. A greater proportion of heifers treated with hCG at CIDR insertion ovulated and subsequently had greater concentrations of progesterone compared to heifers treated with hCG. However, treatment with hCG 14 d prior to CIDR insertion failed to enhance pregnancy rates and replacing GnRH with hCG at CIDR insertion resulted in decreased pregnancy rates. In addition, heifers that experienced an ovulation to the initial injection of the CO-Synch+CIDR protocol had smaller follicles at the time of CIDR removal, greater concentrations of progesterone at the time of breeding, but pregnancy rates similar were compared to heifers that did not ovulate.;In Exp. 3, hCG was administered to suckled beef cows 7 d after TAI. Concentrations of progesterone were increased 14 and 33 d after TAI and volume of luteal tissue on d 14 was increased in cows treated with hCG. In addition, treatment with hCG tended to increase pregnancy rates at 5 of 6 locations (average = 10.2%).;Attempts to increase fertility and number of calves born by transferring an embryo 7 d after TAI to suckled cows (Exp. 4) and transferring two demi-embryos into a single heifer recipient (Exp. 5) were evaluated. Transferring an embryo into a cow after TAI or transferring two demi-embryos into a single recipient did not alter the proportion of females calving, but did improve total fetal pregnancy rates and total number of calves born. While no difference was present in cows, pregnancy loss in heifers gestating twin pregnancies occurred at rates greater than those of single-fetus pregnancies.