Short-term endocrine and metabolic reactions before and after second immunization against GnRH in boars

Immunization of boars against GnRH inhibits synthesis of testicular steroids including androstenone (sex odour). Timing of the second vaccination (anamnestic reaction) should occur as late as possible to maintain anabolic effects of testicular hormones, but early enough to remove androstenone from body fat. Five catheterized boars received the second dose (Improvac®) at age 22 weeks. Titre, hormones and parameters reflecting protein turnover were determined in blood. An increased antibody titre and drop of LH and steroids occurred within 5 days. Metabolism adapted after 7 days. Results from this study in conjunction with previous work suggest that after two doses of Improvac given 4 weeks apart, clearance of androstenone from body fat may be achieved as early as 3 weeks after the second vaccination. Thus, it might be possible to extend the duration of anabolic effect in male pig production.