Comparison of a 16- versus a 19-day interval between controlled internal drug release removal and prostaglandin F2α following a 14-day controlled internal drug releasetreatment and fixed-time artificial insemination in postpartum beef cows1

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This experiment compared 2 long-term controlled internal drug release (CIDR)-based protocols to synchronize estrus before fixed-time artificial insemination (FTAI) in postpartum beef cows. Cows were assigned to treatments by age, BCS, and days postpartum. Cows assigned to the 14- to 19-d CIDR-PGF2α protocol (n = 196) received CIDR inserts (1.38 g progesterone [P4]) from d 0 to 14 and PGF2α (25 mg, i.m.) 19 d after CIDR removal on d 33. Cows assigned to the 14-to-16-d CIDR-PGF2α protocol (n = 195) received CIDR inserts from d 3 to 17 and PGF2α 16 d after CIDR removal on d 33. Cows were artificially inseminated on d 36, 72 h after PGF2α, with GnRH (100 μg, i.m.) at FTAI. Cows were exposed for natural service 14 d after FTAI for 75 d. Blood samples for P4 were collected at d -10 and 0 to determine pretreatment estrous cyclicity status and again at PGF2α. Blood samples for estradiol (E2) were collected at PGF2α and FTAI. HeatWatch estrus detection transmitters were used from CIDR removal until FTAI to determine onset of estrus after CIDR removal and PGF2α. Dominant follicle diameter was determined at PGF2α and FTAI. Pregnancy diagnosis was performed 70 d after FTAI and confirmed at d 140 of gestation. Estrous response after CIDR removal was similar between treatments. Cows in both treatments had similar size dominant follicles on d 33 at PGF2α and d 36 at FTAI. Progesterone at PGF2α was greater (P = 0.03) for 14-to-16-d compared to 14-to-19-d treated cows. Mean concentrations of E2 at PGF2α were similar between treatments but were greater (P = 0.01) at FTAI for 14-to-16-d compared to 14-to-19-d treated cows. Estrous response after PGF2α was greater (P < 0.01) for 14-to-19-d compared to 14-to-16-d treated cows (47.4 vs. 29.7%, respectively). Pregnancy rate resulting from FTAI was affected by the treatment × age group interaction (P = 0.08). Pregnancy rate after FTAI among cows ≥ 4 yr tended to be greater (P = 0.06) for 14-to-19-d compared to the 14-to-16-d treated cows, suggesting that the 14-to-19-d schedule works better for older age cows compared with the 14-to-16-d schedule. Final pregnancy rates were similar between the 2 treatments. In summary, these data indicate that a range in intervals from CIDR removal to PGF2α may be feasible when using long-term CIDR-based protocols in cows and raise questions that warrant further study regarding the benefits of extending this interval based on cow age.