Comparison of long-term controlled internal drug release-based protocols to synchronize estrus and ovulation in...

Two experiments were conducted to examine the necessity of adding a GnRH injection to a 14-d controlled internal drug release (CIDR)-based protocol for synchronization of estrus and ovulation in postpartum beef cows. The experiments were designed to characterize long-term CIDR-based protocols in cyclic and noncyclic postpartum beef cows on the basis of estrous response, follicular dynamics, and serum steroid hormone concentrations. In Exp. 1 and 2, crossbred lactating beef cows (n = 40 and 38, respectively) were randomly assigned to 1 of 2 treatments by age, days postpartum (DPP), BCS, and estrous cyclicity status: 1) cows received a CIDR from d 0 to 14 followed by GnRH 9 d after CIDR removal (d 23) and PGF2α on d 30 (CIDR Select) or 2) CIDR administration from d 0 to 14 followed by PGF2α 16 d later (d 30; Show-Me-Synch). Estrus detection was performed using HeatWatch transmitters applied from CIDR removal to AI. Cows in Exp. 1 were artificially inseminated based on detected estrus whereas cows in Exp. 2 were inseminated at a fixed time. In both experiments, follicle turnover on d 25 of treatment was greater among CIDR Select-treated cows (P < 0.001) compared with Show-Me-Synch-treated cows. In Exp. 1, CIDR Select-treated cows tended to have a reduced (P = 0.06) variance for the interval to estrus after PGF2α than Show-Me-Synch-treated cows. Also, cows assigned to the CIDR Select protocol had greater concentrations of progesterone (P < 0.05) on the day before PGF2α administration as well as greater concentrations of estradiol-17β (P < 0.01) 48 h after PGF2α administration. In Exp. 2, mean dominant follicle diameter on d 23 and at fixed-time AI (FTAI) did not differ between treatments (P > 0.10), but Show-Me-Synch-treated cows had larger follicles at d 28 (P < 0.001) and tended to have larger follicles at PGF2α (d 30; P = 0.06) compared with cows assigned to CIDR Select. In summary, the administration of GnRH on d 23 of a long-term CIDR-based estrus synchronization protocol increased follicle turnover; however, both long-term CIDR-based protocols yielded similar physiological outcomes among estrous-cycling and anestrous postpartum beef cows.