[The castration of male lambs by immunization against GnRH].

The objective of this study was to evaluate the effect of a GnRH-vaccine in the ram lamb. Experiments were performed using 20 male lambs, randomly divided into a test (GnRH-immunization) and control group (physiological NaCl-solution). At a body weight of 20 kg (age 2-3 months) and three weeks later, all animals of the test group received 2 ml of Improvac (CSL Limited, Parkville, Victoria, Australia). The body weight as well as the blood testosterone concentration were measured weekly for 16 weeks. Thereafter, blood samples for testosterone analysis were taken monthly in immunized lams only. After the booster injection testicular growth was suppressed and plasma testosterone remained at low values < 0.1 ng/ml for at least 12 weeks. The mean corresponding testosterone concentrations for the control lambs ranged between 0.1 and 0.9 ng/ml plasma. An increase of testosterone was observed in 8 of 10 immunized animals between 3 to 7 months after the booster dose. The control lambs showed a tendency for better growth rate than vaccinated animals, but the difference was not significant. Our results demonstrate that in prepubertal ram lambs two immunizations with Improvac, three weeks apart, can suppress testosterone secretion and testicular growth at least for three months after the booster injection. For a suppression of reproductive function longer than three months after the second vaccination, a third immunization is needed at this time or when testicular growth is beginning.