Timing of artificial insemination in postpartum beef cows following administration of the CO-Synch + CIDR protocol

¹ Division of Animal Science, S132 ASRC, University of Missouri, Columbia, MO 65211
² MFA Inc., 201 Ray Young Dr., Columbia, MO 65201
³ Agriculture Experiment Station, University of Missouri, Columbia 65211

^* To whom correspondence should be addressed. E-mail: pattersond{at}missouri.edu.

	Abstract

The 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 + CIDR protocol. Cows (n = 851) at two locations over two years (yr 1; n = 218, 206: yr 2; n = 199, 228) were stratified by age, BCS and days postpartum (DPP) to 1 of 2 FTAI intervals. Cows were administered GnRH (100 µg, i.m.) and equipped with a controlled internal drug release (CIDR) insert (1.38g progesterone) on d 0. Controlled internal drug release inserts were removed 7 d later at the time prostaglandin F₂ (PG; 25 mg, i.m.) was administered (d 7). Continuous estrus detection was performed at Location 2 using HeatWatch. Transmitters were fitted at the time of PG 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 PG] 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 pre-treatment 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 those that did not (123/163, 76% and 150/270, 56%, respectively). There were no treatment by location interactions within year (P > 0.10) for age, DPP, 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% and 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 pre-treatment cyclicity status (P = 0.30), and 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 FTAI at 54 h.

Key Words: artificial insemination, beef cows, controlled internal drug release insert, estrus synchronization, pregnancy rate