ABSTRACT
This study tested the effectiveness of a conjugated GnRH vaccine for stimulating antibody production, suppressing testosterone secretion and depressing testicular development in yearling colts. Two colts were allocated to each of three groups, (1) control, (2) subcutaneous and (3) intramuscular vaccinations. Two injections of the vaccine were given 11 weeks apart. Liveweight gain was not affected by vaccination but plasma testosterone concentrations in the treated colts were suppressed and their antibody titres to GnRH were greater than 1:1000. Testicular development in the treated colts also was retarded at this time, which was approximately 28 weeks after the first injection. Semen samples, containing spermatozoa, were collected from all the colts prior to their castration at the end of the investigation. Antibody titres and testosterone concentrations in the treated colts had returned to levels similar to those of the controls by the end of the experiment. The testes of the vaccinated colts were still smaller than those of the controls on castration but were, nevertheless, increasing in size. Morphometric analysis of testicular histology and daily sperm production data indicated that the testes of the treated colts were recovering and would probably have regained normal function had they been left in situ.
