The results for androstenone concentrations in belly fat samples at slaughter support the efficacy of vaccination against GnRF using the GnRF analog-diphtheria toxoid conjugate. The vaccine blocks the activity of GnRF, which is the signal for the release of luteinizing hormone and follicle stimulating hormones from the pituitary. As a consequence, testicular development is stopped and there is no further production of androstenone. At the same time, metabolism of skatole by the liver, which is blocked by steroid hormones from the mature testes, is increased