Immunization of heifers against gonadotropin-releasing hormone: antibody titers, ovarian function, body growth, and carcass characteristics1

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Abstract
To investigate the effects of active immunization of cyclic beef heifers with different doses of a human serum albumin-Cys-Gly-GnRH (HSA-GnRH) conjugate on antibody titers, ovarian function, body growth, and carcass characteristics, 32 heifers (BW = 477 ± 7.1 kg; mean ± SE) were assigned to one of four immunization treatments: .1 mg of HSA or .01, .1, or 1.0 mg of HSA-GnRH, respectively. All heifers received a primary (d 0) and booster (d 28) immunization using DEAE-dextran as adjuvant. The duration of the experiment was 158 d. Overall antibody titers against GnRH were greater (P < .05) for heifers immunized against GnRH (13 ± 3.3, 22 ± 3.8, and 19 ± 2.8% binding at a plasma dilution of 1: 640 for Treatments 2 to 4, respectively) than for controls (1 ± .1%). The numbers of heifers that became anestrous (plasma progesterone < .5 ng/mL for > 21 d) were 1/8, 8/8, 7/8, and 8/8, respectively. The interval from primary immunization to anestrus (40.7 ± 6 d) and the duration of anestrus (78 ± 7 d) were not affected by dose of HSA-GnRH conjugate. The number of ovulations detected was reduced (P < .05) in GnRH-immunized (4.6 ± .64, 4.0 ± .70, and 3.6 ± .60 for Treatments 2 to 4, respectively) compared with control heifers (9.4 ± .20). During induced anestrus, follicular growth was generally arrested ( < 5 mm in diameter) and plasma estradiol decreased.(ABSTRACT TRUNCATED AT 250 WORDS)

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