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ARTICLE |
1 Division of Animal Sciences, University of Missouri, Columbia 65211
2 MFA, Inc., Columbia 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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This experiment was designed to compare pregnancy rates in postpartum beef cows resulting from fixed-time AI (FTAI) after treatment with one of two protocols to synchronize estrus and ovulation. Crossbred, suckled beef cows (n = 650) at four locations (n = 210; n = 158; n = 88; n = 194) were assigned within a location to one of two protocols within age group by days postpartum and BCS. Cows assigned to the MGA Select treatment (MGA Select; n = 327) were fed melengestrol acetate (MGA; 0.5 mg·hd-1·d-1) for 14 d, GnRH (100 µg i.m. Cystorelin) was injected on d 26, and PGF2
(PG; 25 mg i.m. Lutalyse) was injected on d 33. Cows assigned to the CO-Synch + CIDR protocol (CO-Synch + CIDR; n = 323) were fed carrier for 14 d, were injected with GnRH and equipped with an EAZI-BREED Controlled Internal Drug Release insert (CIDR; 1.38 g progesterone) 12 d after carrier removal, and PG (25 mg i.m. Lutalyse) was injected and CIDR were removed on d 33. Fixed-time AI was performed at 72 or 66 h after PG for MGA Select and CO-Synch + CIDR groups, respectively. All cows were injected with GnRH (100 µg i.m. Cystorelin) at the time of insemination. Blood samples were collected 8 and 1 d before the start of MGA or carrier to determine estrous cyclicity status of cows (estrous cycling versus anestrus) before treatment [progesterone 
0.5 ng/mL; (MGA Select, 185/327, 57%; CO-Synch + CIDR, 177/323, 55%); P = 0.65]. There was no difference (P = 0.20) between treatments associated with pregnancy rate to FTAI (MGA Select, 201/327, 61%; CO-Synch + CIDR, 214/323, 66%). There was also no difference (P = 0.25) between treatments in final pregnancy rate at the end of the breeding period (MGA Select, 305/327, 93%; CO-Synch + CIDR, 308/323, 95%). These data indicate that pregnancy rates were comparable after FTAI, following administration of the MGA Select or CO-Synch + CIDR protocols. Both protocols provide opportunities for beef producers to utilize AI and potentially eliminate the need to detect estrus.
Key Words: Beef cow, controlled internal drug release insert, estrus synchronization, fixed-time artificial insemination, progestin, melengestrol acetate
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