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ARTICLE |
1 Division of Animal Science, S132 ASRC, University of Missouri, Columbia, MO 65211
2 MFA Inc., 201 Ray Young Dr., Columbia, MO 65202
3 Agriculture Experiment Station, University of Missouri, Columbia 65211
* To whom correspondence should be addressed. E-mail: pattersond{at}missouri.edu.
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Abstract |
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The objective of the experiment was to compare pregnancy rates resulting from fixed-time AI (FTAI) following administration of either one of two controlled internal drug release (CIDR)-based protocols. Heifers at three locations (Location 1, n=78; Location 2, n=61; Location 3, n=78) were assigned to one of two treatments within reproductive tract scores (RTS; 1 to 5, 1 = immature, and 5 = cycling) by age and weight. Heifers assigned to CIDR Select received a CIDR insert (1.38 g progesterone) from d 0 to 14 followed by GnRH (100 µg, i.m.) 9 d after CIDR removal (d 23) and PGF2
(PG, 25 mg, i.m.) 7 d after GnRH treatment (d 30). Heifers assigned to CO-Synch + CIDR were administered GnRH and received a CIDR insert on d 23, and PG and CIDR removed on d 30. Heifers at location 1 were fitted with a HeatWatch Estrus Detection System transmitter from the time of PG until 24 d after FTAI to allow for continuous estrus detection. Artificial insemination was performed at predetermined fixed-times for heifers in both treatments at 72 or 54 h after PG for the CIDR Select and CO-Synch + CIDR groups, respectively. All heifers were administered GnRH at the time of insemination. Blood samples were collected 10 d before and immediately before treatment initiation (d 0) to determine pre-treatment estrous cyclicity (progesterone 
0.5 ng/mL). At Location 1, estrous response during the synchronized period was greater (P = 0.06; 87 vs. 69%, respectively) and the variance for interval to estrus after PG was reduced among CIDR Select (P < 0.01) than for CO-Synch + CIDR treated heifers. Fixed-time AI pregnancy rates were significantly greater (P = 0.02) following the CIDR Select protocol (62%) compared with the CO-Synch + CIDR protocol (47%). In summary, the CIDR Select protocol resulted in a greater and more synchronous estrous response and significantly greater fixed-time AI pregnancy rates compared with the CO-Synch + CIDR protocol.
Key Words: artificial insemination, beef heifers, estrus synchronization, pregnancy rate, progestin
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