Testis function and feedlot performance of bulls actively immunized against gonadotropin-releasing hormone: effect of implants containing progesterone and estradiol benzoate

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Abstract
The effect of implants containing anabolic steroid (Synovex®) on testis function, feedlot performance, and carcass traits was evaluated in bulls, steers, and bulls actively immunized against gonadotropin-releasing hormone (GnRH). Ninety-one Hereford or Hereford × Red Angus bull calves were randomly assigned to a 3 × 2 factorial experiment. The treatments were 1) unimmunized control (n =30), 2) actively immunized against GnRH (n = 31), or 3) castrate (n =30). The classes were 1) unimplanted (n = 45) or 2) implanted (n =46) with Synovex. Animals in treatment Groups 2 and 3 were either immunized with a GnRH-keyhole limpet hemocyanin (KLH) conjugate or castrated at 3.6 ± .1 mo of age. Animals assigned to Class 2 received Synovex-C at the time of initial treatment and Synovex-S at weaning and feedlot entry. Animals in treatment Group 2 received a booster immunization at feedlot entry. Anti-GnRH titer was evident at weaning and remained elevated to slaughter in all (31 of 31) immunized animals. Anti-GnRH titer was not evident in steers or unimmunized bulls. Immunization and(or) Synovex decreased (P < .05) serum concentrations of testosterone (T) and scrotal circumference (SC) at weaning. Similarly, SC, testis weight, and tissue concentrations of spermatozoa at slaughter were significantly reduced by immunization, Synovex, or the immunization/Synovex combination. Immunization, but not Synovex, reduced (P < .05) serum levels of T at slaughter. The masculinity of immunized bulls was less (P < .05) than that of control bulls.(ABSTRACT TRUNCATED AT 250 WORDS)

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