Intervals of Five or Seven Days Between Controlled Internal Drug-Release Insertion, Gonadotropin-Releasing Hormone, and Prostaglandin F2α Injections: Effects on Pregnancy Rate and Follicular Size

Angus-based cows (n = 509) were used to evaluate the effects of a 5- or 7-d controlled internal drug-release (CIDR) insert and 5- or 7-d intervals between the first gonadotropin-releasing hormone (GnRH) and prostaglandin (PG) injection on follicle size and AI pregnancy rate. Cows were randomized into 3 treatment groups. Group 1 (5,5) received both CIDR and GnRH, followed by CIDR removal and PG 5 d later. Group 2 (7,7) received CIDR and GnRH with CIDR removal and PG 7 d later. Group 3 (7,5) received CIDR and GnRH 2 d after CIDR insertion, with CIDR removal and PG 5 d after GnRH. Cows were observed for estrus twice daily for 72 h after PG injection; cows detected in estrus were artificially inseminated 12 h later (EAI), and cows not observed by 72 h were timed artificially inseminated (TAI) at 78 h after PG and given GnRH. Diameter of the largest follicle was determined by transrectal ultrasonography at AI. Pregnancy rate was greater (P < 0.01) in 7,5 (69.0%) than 5,5 cows (56.3%) with 7,7 cows being intermediate (63.0%). More (P < 0.05) 7,5 cows (40.8%) than 5,5 cows were bred by EAI. Cows in the 7,5 treatment had a greater (P < 0.05) conception rate to TAI (64.2%) than 5,5 cows (46.8%) or 7,7 cows (45.1%). Follicle size at breeding was 13.1 mm and did not differ among treatments, but was larger (P < 0.05) for primiparous (13.4 ± 0.4 mm) than for multiparous cows (12.5 ± 0.2). It was concluded that increased AI pregnancy rate can be obtained by shortening the interval between GnRH and PG injections in the presence of CIDR.