Physiology and Endocrinology

The efficacy of active immunization against GnRH as a non-invasive alternative to physical castration was assessed in Hereford × Angus bull calves. Calves were randomly assigned to 1 of 4 treatment groups; unimmunized calves (n = 8), bull calves actively immunized against GnRH (n = 30), or calves castrated at 3 months of age and either non-implanted (n = 9) or implanted with Synovex (n = 9). Immunized calves received primary immunization prior to weaning and secondary immunization at feedlot entry. Anti-GnRH titer was evident in all immunized animals (final titer = 31.3 ± 2.4% of total 125I-GnRH bound by 1:1000 serum dilution). Anti-GnRH antibodies were not evident in steers or unimmunized bulls. Final serum concentration of testosterone (.7 ± .2 and 3.0 ± .4 ng/ml, respectively) and testes weight at slaughter (164 ± 23 and 549 ± 35 g, respectively) were reduced (p < .01) in immunized, relative to unimmunized, bulls. The final live weight of non-implanted steers (472 ± 13 kg) was significantly lower that the final weight of unimmunized bulls (530 ± 20 kg). In contrast, the final live weight of immunized bulls (493 ± 8 kg) did not differ (p > .05) from the value noted in unimmunized bulls. Aggressive characteristics were comparable (p > .05) in immunized bulls and non-implanted steers and significantly lower than the level of aggressive behavior noted in unimmunized bulls. Level of intramuscular fat and quality grade were comparable (p > .05) in non-implanted steers and immunized bulls and significantly higher than values noted in unimmunized bulls. These data indicate that active immunization against GnRH suppresses testicular function and reduces the aggressive behavioral traits of male cattle. Feedlot performance and carcass traits are comparable in immunized bulls and steers. Taken together these data indicate that immunization against GnRH is an effective alternative to physical castration in the management of beef cattle.