1111 Comparison of long- versus short-term CIDR-based protocols to synchronize estrus before fixed-time AI in primiparous 2-yr-old beef cows

This experiment was designed to compare the 14-d CIDR-PG (14-d) and 7-d CO-Synch + CIDR (7-d) protocols on the basis of estrous response, pregnancy rates resulting from fixed-time AI (FTAI), and final pregnancy rates at the end of the breeding season in primiparous 2-yr-old beef cows. Cows assigned to the 14-d treatment (n = 355) received a CIDR insert on d 0 with removal on d 14. Cows assigned to the 7-d treatment (n = 349) received GnRH and CIDR inserts on d 23. On d 30 CIDRs were removed from cows assigned to the 7-d treatment and PGF2α was administered to cows in both treatments. On d 33 GnRH was administered concurrent with FTAI at 66 and 72 h after PGF2α for 7-d and 14-d treated cows, respectively. Estrus response at FTAI was higher for 7-d compared to 14-d treated cows (7-d = 74%; 14-d = 43%; P < 0.0001); however, pregnancy rates resulting from FTAI were similar between treatments (7-d = 64%; 14-d = 63%; P = 0.52). Furthermore, 87% and 88% of 14-d and 7-d treated cows conceived within the first 30 d of the breeding season, and final pregnancy rates at the end of the breeding season did not differ between treatments (7-d = 96%; 14-d = 95%; P = 0.93). To understand similarities between treatments in pregnancy rates resulting from FTAI, despite differences in estrous response rates before FTAI, ovaries were mapped and serum estradiol-17-β (E2) concentrations were evaluated among a subset of cows in each treatment. The 14-d treated cows had smaller diameter dominant follicles at PGF2α (P = 0.04) and FTAI (P = 0.002) compared to 7-d treated cows (10.9 ± 0.3; 13.0 ± 0.3; vs. 11.9 ± 0.4; 14.5 ± 0.3); however, serum E2 concentrations at PGF2α(P = 0.06) and FTAI (P = 0.001) were greater for 14-d vs. 7-d treated cows (3.7 ± 0.4; 8.0 ± 0.7 vs. 2.5 ± 0.4; 4.2 ± 0.8). These differences suggest that dominant follicles of 14-d treated cows remain in an active growth stage at the time FTAI is performed compared to 7-d treated cows in which growth of dominant follicles may have plateaued. This theory is supported from previous studies that report decreased aromatase activity in granulosa cells when growth of dominant follicles plateau compared to actively growing follicles. In summary, these data suggest that the 14-d CIDR-PG and 7-d CO-Synch + CIDR protocols may be used to effectively synchronize estrus before FTAI in primiparous 2-yr-old beef cows.