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ANIMAL PRODUCTION |
protocol for synchronization of estrus in beef heifers1
* Department of Animal Sciences and Industry, Kansas State University, Northwest Research and Extension Center, Colby 67701; and
and
Department of Animal Sciences, The Ohio State University, Columbus 43210
2 Correspondence: P.O. Box 786 (phone: 785-462-6281; fax: 785-462-2315; e-mail: sandyj{at}ksu.edu).
Three experiments were conducted to evaluate methods to decrease or eliminate the detection of estrus inherent to a melengestrol acetate (MGA)-PGF2
(PGF) protocol for synchronization of estrus in heifers. In each experiment, all heifers received 0.5 mg of MGA•animal–1•d–1 for 14 d (d –32 to –19) and PGF (25 mg, i.m.; d 0, 0 h) 19 d after the last feeding of MGA (MGA-PGF protocol). In Exp. 1, heifers (n = 709) were assigned to each of the following protocols: 1) the MGA-PGF protocol with AI 6 to 12 h after detection of estrus (estrus AI; MGA-PGF); 2) MGA-PGF plus 100 µg, i.m. of GnRH on d –7 (1xGnRH) and estrus AI; or 3) MGA-PGF, GnRH on d –7, and GnRH (100 µg, i.m.) at 48 h after PGF, coincident with insemination (2xGnRH-TB48). In Exp. 2, heifers (n = 559) received the MGA-PGF protocol and were inseminated by either estrus AI or fixed-time AI (TAI) at 60 h, coincident with an injection of GnRH (GnRH-TB60). In Exp. 3, all heifers (n = 460) received the MGA-PGF protocol and were inseminated by estrus AI when detected up to 73 h. Heifers not observed in estrus by 73 h received TAI between 76 and 80 h. Half the heifers inseminated by TAI received no further treatment (TB80), and the remaining half was injected with GnRH at insemination (GnRH-TB80). Variance associated with the interval to estrus and the proportion in estrus from d 0 to 5 was similar for 1xGnRH and MGA-PGF treatments in Exp. 1. Pregnancy rate (d 0 to 5) did not differ for the MGA-PGF and 1xGnRH treatments (62.5 and 60.4%, respectively), and both were greater (P <0.05) than TAI pregnancy rate in the 2xGnRH-TB48 treatment (42.3%). In Exp. 2, the peak estrous response occurred 60 h after PGF. Pregnancy rate during the synchrony period was greater (P <0.05) for the MGA-PGF (255/401; 63.6%) than the GnRH-TB60 (74/158; 46.6%) treatment. In Exp. 3, 75.7% of heifers (348/460) were detected in estrus by 73 h and were inseminated, with a conception rate of 74.4%. Pregnancy rates after TAI did not differ between TB80 and GnRH-TB80 (14/56 = 25% and 19/56 = 33.9%, respectively). Total pregnancy rate was 63.5% for heifers inseminated after detected estrus and by TAI. Collectively, these data indicate that the exclusive use of TAI for heifers treated with the MGA-PGF protocol resulted in lower pregnancy rates than when AI was performed after detection of estrus. However, estrus AI for 3 d and TAI at the end of d 3 could result in pregnancy rates similar to those achieved after a 5-d period of detecting estrus.
Key Words: Fixed-Time Artificial Insemination • Gonadotropin-Releasing Hormone • Heifers • Melengestrol Acetate • Pregnancy Rates • Synchronization
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  J. A. Atkins, D. C. Busch, J. F. Bader, D. H. Keisler, D. J. Patterson, M. C. Lucy, and M. F. Smith Gonadotropin-releasing hormone-induced ovulation and luteinizing hormone release in beef heifers: Effect of day of the cycle J Anim Sci, January 1, 2008; 86(1): 83 - 93. [Abstract] [Full Text] [PDF]
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