Administration of a GnRH analog on day 9 of a 14-day controlled internal drug release insert with timed artificial insemination in lactating beef cows.

摘要

Many estrus synchronization protocols aim to induce a new follicular wave to improve response and enhance pregnancy rate. Our objectives were to determine the effectiveness of GnRH analog administered d 0 and 9 during an extended controlled internal drug release (CIDR) protocol to produce 2 follicular waves, induce cyclicity in anestrus cows, and evaluate the efficacy of a single 50-mg dose of PGF_{2 alpha} to initiate luteal regression on CIDR removal. Lactating beef cows (n=779) at 3 locations (n=247, location 1; n=395, location 2; n=137, location (3)) were randomly assigned to 1 of 3 treatments. Cows in the 14-d 50 PG treatment received a CIDR (1.38 g progesterone) with 100 micro g GnRH analog intramuscularly (i.m.) on d 0, 100 micro g GnRH analog i.m. on d 9, and CIDR removal concurrent with 50 mg PGF_{2 alpha} i.m. on d 14. Cows in the 14-d 6-h PG treatment were assigned the same protocol as the 14-d 50 PG treatment except that 25 mg PGF_{2 alpha} i.m. was given on d 14 plus 25 mg PGF2 alpha i.m. 6+or-1 h later. Cows in the control treatment, 5-d CO-Synch+CIDR (5-d CO-Synch), received a CIDR concurrent with 100 micro g GnRH analog i.m. on d 9, CIDR removal concurrent with 25 mg PGF_{2 alpha} i.m. on d 14, and 25 mg PGF_{2 alpha} i.m. 6+or-1 h after first F2 alpha injection. Cows in all treatments received 100 micro g GnRH analog i.m. and timed AI (TAI) 72+or-3 h after CIDR removal. Pregnancy status to TAI was determined by ultrasonography 37 to 40 d after TAI. Averaged over all locations, pregnancy rates to TAI for 14-d 50 PG, 14-d 6-h PG, and 5-d CO-Synch treatments were 58.2%, 46.8%, and 41.9%, respectively. Pregnancy rates to TAI were greater (P<0.05) in 14-d 50 PG treatment than 14-d 6-h PGF_{2 alpha} and 5-d CO-Synch treatments. Cycling status at 2 locations (n=243, location 1; n=391, location 2) was determined from blood collected on d -7 and 0; cows with serum progesterone concentrations >1 ng/mL at either (or both) bleeding date were considered cyclic. Averaged over the 2 locations, there was a tendency (P=0.06) for a greater number of cyclic animals to become pregnant to TAI in the 14-d 50 PG treatment (64.4%) than 5-d CO-Synch treatment (50.2%). The 14-d CIDR with GnRH analog on d 0 and 9 and a single 50-mg dose of PG i.m. at CIDR removal was a more efficacious protocol to maximize TAI pregnancy rates than the standard 5-d CO-Synch.