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Timing of artificial insemination in postpartum beef cows following administration of the CO-Synch + controlled internal drug-release protocol¹

D. C. Busch^*, D. J. Schafer, D. J. Wilson^*, D. A. Mallory^*, N. R. Leitman^*, J. K. Haden, M. R. Ellersieck, M. F. Smith^* and D. J. Patterson^*,2

^* Division of Animal Science, S132 ASRC, University of Missouri, Columbia 65211; and MFA Inc., 201 Ray Young Dr., Columbia, MO 65201; and Agriculture Experiment Station, University of Missouri, Columbia 65211

² Corresponding author: pattersond{at}missouri.edu

This experiment was designed to compare pregnancy rates in postpartum beef cows resulting from fixed-time AI (FTAI) at 54 or 66 h after administration of the CO-Synch + controlled internal drug-release (CIDR) protocol. Cows (n = 851) at 2 locations over 2 yr (yr 1, n = 218 and 206; and yr 2, n = 199 and 228 at the 2 locations, respectively) were stratified by age, BCS, and days postpartum to 1 of 2 FTAI intervals. Cows were administered GnRH (100 µg, i.m.) and were equipped with a CIDR insert (1.38 g of progesterone) on d 0. Controlled internal drug-release inserts were removed 7 d later at the time PGF₂ (25 mg, i.m.) was administered (d 7). Continuous estrus detection was performed at location 2 by using the HeatWatch Estrus Detection System; the transmitters were fitted at the time of PGF₂ and removed at the time of AI. Artificial insemination was performed at predetermined fixed times [54 h (FTAI 54; n = 424) or 66 h (FTAI 66; n = 427) after PGF₂] and all cows were administered GnRH (100 µg, i.m.) at AI. Two blood samples were collected on d –10 or –8 and immediately before treatment initiation to determine the pretreatment estrous cyclicity status of cows [progesterone 0.5 ng/mL (FTAI 54, 288/424 = 68%; FTAI 66, 312/427 = 73%; P = 0.07)]. Pregnancy rates were greater (P < 0.01) among cows that exhibited estrus than among those that did not (123/163 = 76% and 150/270 = 56%, respectively). There were no treatment x location interactions within year (P > 0.10) for age, days postpartum, or BCS; thus, the results were pooled for the respective treatments. Pregnancy rates were greater for FTAI 66 than FTAI 54 (P = 0.05; 286/426 = 67% vs. 257/424 = 61%, respectively). Pregnancy rates resulting from FTAI did not differ between year (P = 0.09), farm (P = 0.80), AI sire (P = 0.11), or technician (P = 0.64). There was no difference between pregnancy rates resulting from FTAI based on pretreatment cyclicity status (P = 0.30), and there was no difference between treatments in final pregnancy rates (P = 0.77). In summary, pregnancy rates resulting from FTAI following CO-Synch + CIDR at 66 h were greater than those resulting from FTAI at 54 h.

Key Words: artificial insemination • beef cow • controlled internal drug-release insert • estrous synchronization • pregnancy rate