187 Impact of prolonged CIDR insertion on follicular dynamics, estrus timing, and pregnancy outcomes in beef cattle submitted to estrous synchronization.

Abstract The conventional progestin-based estrous synchronization program for AI, CO-Synch, uses an intravaginal device containing progesterone (CIDR) for 5 to 7d to control follicular growth. About 50% of cattle submitted to a synchronization protocol fail to become pregnant due to artificial insemination, highlighting opportunities for improvement in traditional synchronization protocols. Prolonging the length of progestin treatment can result in a tighter synchrony and improved estrous response, due to the extended time for follicular growth. This study aimed to evaluate the effect of extending CIDR insertion from 7d to 8d and 7d to 9d on estrous response and distribution, dominant follicle size at CIDR removal, and pregnancy per AI in beef cattle synchronized with the CO-Synch protocol. Thus, cows and heifers from MU agricultural experiment stations (n=273) and beef producers (n=184) were submitted to one of three CO-Synch treatments with varying CIDR lengths: (CIDR7, n= 154; CIDR8, n= 150; and CIDR9, n= 153) and AI based on estrus for 3d followed by TAI for non-estrus cows/heifers. Lengthening time of CIDR treatment increased the diameter of the largest dominant follicle at CIDR removal (CIDR7: 10.39mma,b ± 0.4176 vs. CIDR8: 10.81mma ± 0.4137 vs. CIDR9: 10.77mm b0.422; P< 0.10). Despite the difference on follicle diameter, CIDR treatment did not affect estrous response (CIDR7: 69.53% ± 4.43 vs. CIDR8: 68.83 % ± 4.40 vs. CIDR9: 61.10% ± 4.47; P >0.10). However, cows/heifers from the CIDR9 group had 1.19 times the hazard (adjusted hazard ratio: 1.191 [95% CI: 0.893–1.587]) to experience estrus during the 3d period than the cows/heifers from CIDR7 treatment (P=0.2338). Size of follicle at CIDR removal affected (P< 0.0001) interval to estrus, as every 1 mm increase in the diameter of the largest dominant follicle increased hazard of estrus by 16.6% (adjusted hazard ratio: 1.169 [95% CI: 1.114 - 1.220). There was no treatment by estrous response interaction effect on pregnancy per AI of cows/heifers (estrus: 52.69% 13.70 ± vs. non-estrus: 25.75% ± 11.82; P >0.10). Prolonging CIDR treatment nevertheless decreased pregnancy per AI (CIDR7: 51.79% a ± 12.67, CIDR8: 45.81%a,b ± 12.66, CIDR9: 38.14% b ± 12.69; P< 0.10). Extending the period of CIDR insertion in the CO-Synch program from 7d to 8d or 9d overall increased dominant follicle size but did not improve estrus response nor pregnancy per AI. While longer progestin exposure enhances estrous synchrony, timing remains critical, as follicles aged 10 days or more exhibit reduced fertility. The compromised pregnancy per AI observed in the 8d and 9d CIDR treatments may be attributed to animals that did not respond to GnRH at CIDR insertion. These findings emphasize the importance of refining synchronization protocols to optimize follicular development while minimizing impairment of fertility in beef cattle.