TESTIS FUNCTION AND FEEDLOT PERFORMANCE OF BULLS ACTIVELY IMMUNIZED AGAINST GONADOTROPIN-RELEASING-HORMONE - EFFECT OF IMPLANTS CONTAINING PROGESTERONE AND ESTRADIOL BENZOATE

The effect of implants containing anabolic steroid (Synovex(R)) 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 x Red Angus bull calves were randomly assigned to a 3 x 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. The final live weight and carcass weight of immunized bulls, regardless of implant class, and implanted steers did not differ (P > .05) from the slaughter and carcass weights of unimplanted control bulls. However, the final live weight, carcass weight, and feedlot gain of unimplanted steers was lower (P < .05) than the slaughter and carcass weights and feedlot gain of control or immunized bulls or steers with Synovex implants. Immunization, like Synovex, reduced (P < .05) the masculinity of the carcasses of bulls. Taken together, these data indicate that immunization against GnRH may have practical utility as a noninvasive alternative to surgical castration in management of beef cattle. In addition, the residual levels of T secretion in immunized bulls may have anabolic effects that reduce the need for supplementation with exogenous steroid.