Administration of human chorionic gonadotropin 7 days after fixed-time artificial insemination of suckled beef cows

We determined the effects of administering hCG 7 d after a fixed-time AI (TAI) on ovarian response, concentrations of progesterone, and pregnancy rates in postpartum suckled beef cows. Cows at 6 locations received 100 microg of GnRH (Fertagyl, Intervet Animal Health, Millsboro, DE) and a controlled internal drug release (CIDR) device (CIDR EAZI-Breed, Pfizer Animal Health, New York, NY), followed in 7 d by 25 mg of PGF(2alpha) (Lutalyse, Pfizer Animal Health) and CIDR removal. At 64 h after CIDR removal, cows received an injection of GnRH and AI (d 0), and then were stratified by days postpartum and parity and assigned randomly to 2 treatments administered 7 d after TAI: 1) 1 mL of saline (saline; n = 252); or 2) 1,000 IU of hCG (Chorulon, Intervet Animal Health; n = 254). Blood samples were collected on d -21, -10, and 33 relative to TAI (d 0) at all locations, on d 7 and 68 at 5 locations, and on d 14 at 1 location to determine concentrations of progesterone. Transrectal ultrasonography was used to determine pregnancy status on d 33 and 68 at all locations, to monitor response of follicles and corpora lutea (CL) in response to treatment at 1 location (n = 106) on d 7 and 14, and to determine the number of CL present in pregnant cows on d 33 in 3 locations (n = 130). Pregnant cows had greater (P < 0.05) concentrations of progesterone at the time of treatment (d 7) compared with nonpregnant cows (3.7 +/- 0.1 vs. 2.6 +/- 0.2 ng/mL, respectively). On d 14, hCG-treated cows had a greater (P < 0.05) volume of luteal tissue (12.1 +/- 0.5 vs. 7.3 +/- 0.5 cm(3), respectively) and greater concentrations of progesterone (6.8 +/- 0.4 vs. 5.4 +/- 0.5 ng/mL, respectively) compared with saline-treated cows. A greater (P < 0.01) percentage of hCG-treated cows (90.6%) had multiple CL on d 14 compared with saline-treated cows (0%), and a greater percentage of pregnant cows treated with hCG (74.6%) had multiple CL on d 33 compared with saline-treated cows (3.0%). Pregnancy rates of hCG-treated cows (56.3%) tended (P = 0.07) to differ from those of saline-treated cows (50.0%). Concentrations of progesterone in pregnant hCG-treated cows were greater (P < 0.05; 7.7 +/- 0.3 vs. 5.8 +/- 0.3 ng/mL, respectively) on d 33 than for pregnant saline-treated cows, but were similar between treatments on d 68 (7.2 +/- 0.3 vs. 6.7 +/- 0.4 ng/mL, respectively). We conclude that treatment with hCG increased the volume of luteal tissue on d 14 and concentrations of progesterone on d 14 and 33 after TAI. Treatment with hCG tended to increase pregnancy rates at 5 of 6 locations from 1.1 to 27 percentage points (average = 10.2) compared with saline, but cumulative pregnancy rates determined on d 68 after TAI were similar between treatments.