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Synchronization of estrus and artificial insemination in replacement beef heifers using gonadotropin-releasing hormone, prostaglandin F₂, and progesterone¹

G. C. Lamb^*,2, J. E. Larson^*, T. W. Geary, J. S. Stevenson, S. K. Johnson, M. L. Day, R. P. Ansotegui^#, D. J. Kesler^||, J. M. DeJarnette^¶ and D. G. Landblom^**

^* North Central Research and Outreach Center, University of Minnesota, Grand Rapids 55744; and USDA-ARS Fort Keogh Livestock and Range Research Laboratory, Miles City, MT 59301; and Department of Animal Sciences and Industry, Kansas State University, Manhattan 66506-0201; and Department of Animal Science, The Ohio State University, Columbus 43201; and ^# Department of Animal and Range Sciences, Montana State University, Bozeman 59717; and ^|| Department of Animal Sciences, University of Illinois, Urbana 61801; and ^¶ Select Sires Inc., Plain City, OH 43064; and and ^** Department of Animal and Range Sciences, North Dakota State University, Dickinson 58601

² Corresponding author: clamb{at}umn.edu

We evaluated whether a fixed-time AI (TAI) protocol could yield pregnancy rates similar to a protocol requiring detection of estrus, or detection of estrus and AI plus a clean-up TAI for heifers not detected in estrus, and whether adding an injection of GnRH at controlled internal drug release (CIDR) insertion would enhance fertility in CIDR-based protocols. Estrus in 2,075 replacement beef heifers at 12 locations was synchronized, and AI was preceded by 1 of 4 treatments arranged as a 2 x 2 factorial design: 1) Estrus detection + TAI (ETAI) (n = 516): CIDR for 7 d plus 25 mg of prostaglandin F₂ (PG) at CIDR insert removal, followed by detection of estrus for 72 h and AI for 84 h after PG (heifers not detected in estrus by 84 h received 100 µg of GnRH and TAI); 2) G+ETAI (n = 503): ETAI plus 100 µg GnRH at CIDR insertion; 3) Fixed-time AI (FTAI) (n = 525): CIDR for 7 d plus 25 mg of PG at CIDR removal, followed in 60 h by a second injection of GnRH and TAI; 4) G+FTAI (n = 531): FTAI plus 100 µg of GnRH at CIDR insertion. Blood samples were collected (d –17 and –7, relative to PG) to determine ovarian status. For heifers in ETAI and G+ETAI treatments, a minimum of twice daily observations for estrus began on d 0 and continued for at least 72 h. Inseminations were performed according to the a.m.-p.m. rule. Pregnancy was diagnosed by transrectal ultrasonography. The percentage of heifers exhibiting ovarian cyclic activity at the initiation of treatments was 89%. Pregnancy rates among locations across treatments ranged from 38 to 74%. Pregnancy rates were 54.7, 57.5, 49.3, and 53.1% for ETAI, G+ETAI, FTAI, and G+FTAI treatments, respectively. Although pregnancy rates were similar among treatments, a tendency (P = 0.065) occurred for pregnancy rates in the G+ETAI treatment to be greater than in the FTAI treatment. We concluded that the G+FTAI protocol yielded pregnancy rates similar to protocols that combine estrus detection and TAI. Further, the G+FTAI protocol produced the most consistent pregnancy rates among locations and eliminated the necessity for detection of estrus when inseminating replacement beef heifers.

Key Words: beef heifer • synchronization • timed artificial insemination

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