Comparison of long- versus short-term CIDR-based protocols to synchronize estrus prior to fixed-time AI in postpartum beef cows

This experiment was conducted to compare pregnancy rates in postpartum beef cows resulting from fixed-time AI (FTAI) after treatment with controlled internal drug release (CIDR)-based protocols to synchronize estrus. Cows assigned to the Show-Me-Synch (n = 167) protocol received a CIDR from d 0 to 14, and prostaglandin F2α (PGF2α) on d 30. Cows assigned to 7-d CO-Synch + CIDR (n = 177) received a CIDR and gonadotropin releasing hormone (GnRH) on d 23. On d 30, CIDRs were removed and PGF2α was administered. Blood sampling occurred on d −10 and 0 of treatment to determine estrous cyclicity status (progesterone ≥0.5 ng/mL estrous cycling). Treatments were balanced on age, DPP and BCS. Estrous detection was performed using HeatWatch from PGF2α to FTAI. Artificial insemination was performed at predetermined fixed times (72 h, Show-Me-Synch; 66 h, 7-d CO-Synch + CIDR) and all cows were administered GnRH at FTAI. This experiment was conducted over a two year period; no differences were found between years so the data were pooled for further analysis. Pregnancy rate resulting from FTAI did not differ (P > 0.10) between technicians or AI sires. Pregnancy rate resulting from FTAI was similar between treatments (P = 0.20); however, cows that exhibited estrus prior to FTAI had a higher pregnancy rate (P < 0.01) than for those that did not. Pregnancy rate at the end of the breeding period was similar between treatments (P = 0.28). In summary, FTAI pregnancy rates were similar among postpartum beef cows following treatment with either a short- or long-term CIDR-based estrous synchronization protocol.