Sustained testicular atrophy in bulls actively immunized against GnRH: potential to control carcase characteristics

The objectives were to determine whether active immunization against gonadotrophin releasing hormone (GnRH) induced a long-term suppression of testicular function in bulls, and to ascertain the effects of immunization against GnRH on carcase and meat quality characteristics. In experiment 1, 6-month-old Zebu bulls were assigned to: control (n=25), no treatment; immunized (n=31), immunized against GnRH at 0 and 4 months (anti-GnRH(2)), with a sub-set of bulls (n=17) immunized again at 10 months (anti-GnRH(3)). After the second immunization, testicular growth ceased for 2 months in 14/31 (45%) bulls and for at least 6 months in 17/31 (55%) bulls. Among the latter bulls (anti-GnRH(3)) the testes did not grow for >1 year after the third immunization in 5/17 (30%) bulls. In experiment 2, 22-month-old Zebu bulls were assigned to: control (n=14), no treatment; immunized (n=17), immunized against GnRH at 0, 2 and 4 weeks. The testes decreased (P<0.05) in size for 2 months after immunization in 11/17 (65%) bulls and then re-initiated growth, whilst in 6/17 (35%) bulls the testes continued to decrease in size for 4 months and did not re-initiate growth for 1 year. At slaughter, the latter immunocastrated bulls had carcase and meat quality characteristics the same as contemporary bulls that had been castrated before puberty. The findings demonstrated that active immunization against GnRH can induce a long-term suppression of testicular function in a proportion of bulls. Also, when bulls are immunocastrated after puberty, carcase and meat quality traits change from those typical of entire bulls to traits that are characteristic of long-term castrated bulls.