Effects of Delayed Insemination on Pregnancy Rates of Suckled Beef Cows Enrolled in the 7-d CO-Synch+Cidr Estrus Synchronization Protocol and That Were Not Detected in Estrus By the Time of Fixed-Time AI.

We determined the effects on pregnancy rates of delaying the time of AI of beef cows enrolled in an estrus synchronization protocol that were not detected in estrus by the time of fixed-time AI. At five locations, a total of 546 suckled beef cows were enrolled in the 7-d CO-Synch+CIDR estrus synchronization protocol. Briefly, 100 μg injection of GnRH and CIDR (controlled internal drug release device) insertion (d -10); 25 mg injection of PGF2α, CIDR removal and estrus alert patch attachment (d -3); on d 0 (approximately 66 h after CIDR removal) cows with activated estrus alert patches received AI, but no injection of GnRH (TAI; n = 295); and cows with non-activated patches were randomly assigned to receive either 100 μg injection of GnRH and AI immediately (GnRH+TAI; n = 130); or 100 μg injection of GnRH and delayed AI 8 h later (GnRH+8AI; n = 121). Pregnancy status was determined by transrectal ultrasonography between d 35 to 45. Pregnancy data were analyzed using GLIMMIX procedure of SAS with cows as the experimental unit, including the fixed effects of treatment, location, age of cow, days post-partum (DPP), BCS, and the random effects of sire and AI technician. There were no differences among treatments on BCS (5.6 ± 0.63; P = 0.579), DPP (73 ± 15 d; P = 0.906), and cow age (5.4 ± 2.6 yr; P = 0.192). A total of 295 cows (54%) had activated estrus alert patches on d 0. Overall pregnancy rates differed (P < 0.001) among locations, ranging from 30.7% to 66.9%, and no treatment × location interaction was detected (P = 0.454). Average delayed between GnRH injection and delayed AI was 7 ± 1.6 h. Pregnancy rate to fixed-time AI differed (P = 0.018) among treatments. Cows in the TAI treatment had the greatest pregnancy rate (59.1 ± 0.03%) compared to cows in GnRH+TAI (43.3 ± 0.04%) group, and GnRH+8AI (50.9 ± 0.05%) was intermediate. We conclude that suckled beef cows enrolled in the 7-d CO-Synch+CIDR estrus synchronization protocol that are detected in estrus prior to the time of AI have greater pregnancy rates compared to cows that are not detected in estrus and receive an injection of GnRH and AI at 66 h after CIDR removal. Furthermore, delaying insemination after GnRH injection failed to increase pregnancy rates of cows not detected in estrus.