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This Article Full Text (PDF) All Versions of this Article: jas.2008-1122v1 86/10/2539    most recent Alert me when this article is cited Alert me if a correction is posted Services Similar articles in this journal Similar articles in PubMed Alert me to new issues of the journal Download to citation manager Citing Articles Citing Articles via Google Scholar Google Scholar Articles by Burns, M. G. Articles by Stevenson, J. S. Search for Related Content PubMed PubMed Citation Articles by Burns, M. G. Articles by Stevenson, J. S.

Evaluation of hCG as a replacement for GnRH in ovulation-synchronization protocols before fixed timed AI in beef cattle

M. G. Burns^*, B. S. Buttrey^*, C. A. Dobbins^*, C. A. Martel^*, K. C. Olson^*, G. C. Lamb and J. S. Stevenson^*

^* Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201 North Florida Research and Education Center, University of Florida, Marianna 32446-7906

jss{at}k-state.edu

Abstract

Two experiments were conducted during 2 yr to evaluate differences in ovulation potential and fertility in response to GnRH or hCG. In Exp. 1, 46 beef cows were given 100 µg of GnRH or 500, 1,000, 2,000, or 3,000 IU of hCG. Ovulation incidence was not different between GnRH and any of the hCG doses, indicating that ovulatory capacity of at least 500 IU of hCG was equivalent to GnRH. In Exp. 2, beef cows (n = 676) at 6 locations were assigned randomly to a 2 x 3 factorial arrangement pf treatments. Main effects were: 1) pre-timed AI (TAI) treatment GnRH or hCG) and 2) post-TAI treatment (saline, GnRH, or hCG) to initiate resynchronization of ovulation in previously inseminated cattle. Blood samples were collected (d –21 and –10) to determine progesterone concentrations and assess cyclicity. Cattle were treated with a progesterone insert on d –10 and with 100 µg of GnRH or 1,000 IU of hCG. A PGF₂ injection was given at insert removal on d –3. Cows were inseminated 62 h (d 0) after insert removal. On d 26 after first TAI, cows of unknown pregnancy status were treated with saline, GnRH, or hCG to initiate a CO-Synch protocol. Pregnancy was diagnosed 33 d after first TAI to determine pregnancies per AI (P/AI). Nonpregnant cows at 6 locations in yr 1 and 1 location in yr 2 were given PGF₂ and inseminated 56 h later, concurrent with a GnRH injection. Five weeks later, pregnancy diagnosis was conducted to determine pregnancy loss after first TAI and pregnancy outcome of the second TAI. Injection of pre-TAI hCG reduced (P < 0.001) P/AI compared with GnRH, with a greater reduction in cycling cows. Post-TAI treatments had no negative effect on P/AI resulting from the first TAI. Serum progesterone was greater (P = 0.06) 7 d after pre-TAI hCG than after GnRH and greater (P < 0.05) after post-TAI hCG on d 26 compared with saline 7 d after treatment in association with greater frequency of multiple corpora lutea. Compared with saline, injections of post-TAI GnRH and hCG did not increase second insemination P/AI and inconsistent results were detected among locations. Use of hCG in lieu of GnRH is contraindicated in a CO-Synch + progesterone insert protocol. Compared with a breeding season having only 1 TAI and longer exposure to cleanup bulls, total breeding season pregnancy rate was reduced by one third, subsequent calving distribution was altered, and 50% more AI-sired calves were obtained by applying 2 TAI during the breeding season.

Key Words: beef cattle • gonadotropin releasing hormone • human chorionic gonadotropin • ovulation synchronization • timed artificial insemination