Abstract

The objectives were to determine the effects of presynchronization and eCG on fertility for a GnRH-based timed-AI (TAI) protocol in beef cattle. Previously autoclaved once-used CIDR inserts (Colazo et al. 2004 Anim. Reprod. Sci. 81, 25-34) were used for experimental purposes so that all cattle had equivalent exogenous progesterone after Day 7 in Experiment 1, and to induce GnRH-responsive follicles in Experiment 2. In Experiment 1, 12-15 month old beef heifers (n = 447) were used (three locations, A, B, and C). On Day 0, half received a new CIDR insert (Pfizer Animal Health, Montreal, Quebec, Canada) and 500 µg of cloprostenol i.m. (PGF: Estrumate; Schering-Plough Animal Health, Pointe-Claire, Quebec, Canada). On Day 7, the remainder received an autoclaved once-used CIDR and all received 100 µg of GnRH i.m. (Cystorelin; Merial Canada, Inc., Victoriaville, Quebec, Canada). On Day 14, CIDR inserts were removed, PGF was given to all heifers, and heifers were given either 300 IU eCG i.m. (Pregnacol; Bioniche Animal Health, Belleville, Ontario, Canada) or no treatment (2 × 2 factorial design). On Day 16 (54-56 h after CIDR removal and PGF administration), heifers were given 100 µg of GnRH i.m., concurrent with TAI. Transrectal ultrasonography was done on Day 0 to assess ovarian structures, and on approximately Day 50 (range, 44-69) to confirm pregnancy. Overall, 72.7% of heifers were puberal on Day 0 (150/150, 5/37, and 170/260 for locations A, B, and C, respectively; P < 0.001). Pregnancy rate was affected (P < 0.001) by location (27.3, 62.2, and 51.1% for the three locations, respectively) but was not affected (P = 0.3) by puberal status at locations B and C. Pregnancy rate was not affected (P = 0.5) by presynchronization, but tended (P = 0.1) to be reduced by eCG treatment (40.4 vs. 47.7%, respectively). In Experiment 2, lactating crossbred beef cows (n = 411), 2-12 years of age were randomly allocated to receive either a previously used, autoclaved CIDR for 7 days and 25 mg of dinoprost i.m. (PGF: Lutalyse; Pfizer Animal Health) or no treatment (control; Day 0). At CIDR removal (Day 7), all cows received 100 µg of GnRH. On Day 14, all cows received PGF and were allocated to receive either 400 IU of eCG i.m. or no treatment (2 × 2 factorial design). On Day 16 (54-56 h after PGF), cows were given 100 µg of GnRH concurrent with TAI. Transrectal ultrasonographic examinations were done on approximately 65 cows in each group on Days 0, 7, and 14 to assess ovarian structures and on all cows on Day 42 to confirm pregnancy. Overall, 88.8% of cows had a corpus luteum (CL) on Day 0. Presynchronization increased both the proportion of cows that ovulated to GnRH treatment on Day 7 (76.7 vs. 55.0%; P < 0.001) and pregnancy rate (58.2 vs. 45.4%; P = 0.03) for cows that had received a CIDR vs. control cows. However, eCG treatment did not affect pregnancy rate (P = 0.3) in either group. In summary, presynchronization with a used CIDR and PGF prior to a Cosynch protocol increased ovulation rate to the first GnRH treatment and pregnancy rate in cows but not in heifers. In a GnRH-based TAI protocol, treatment with eCG had no significant effect on fertility in cows, but tended to decrease fertility in heifers.