IMMUNIZATION OF SHEEP AGAINST GNRH EARLY IN LIFE - EFFECTS ON REPRODUCTIVE FUNCTION AND HORMONES IN RAMS

Groups of Merino ram lambs were immunized against GnRH either soon aft er birth (prepubertal) or around puberty (peripubertal) with a prototy pe commercial preparation and studied over the following 2 years to de termine the long-term effects of such immunization on reproductive dev elopment, function and hormone concentrations. The treatment was equal ly effective in suppressing reproductive function and hormones when gi ven at either time. Growth rates of immunized and controls rams were s imilar throughout the study. Prepubertal immunization delayed testicul ar growth until the rams were at least 27 weeks of age and some rams h ad subnormally sized testes even at 115 weeks of age. After peripubert al immunization, the mass of the testes declined and remained regresse d until the rams were from 51 to 90 weeks old. There was a significant correlation between total testicular mass at 115 weeks of age and dur ation (weeks) of suppression of testicular growth (prepubertal, r = -0 .7375; peripubertal, r = -0.7132, P < 0.001). Prepubertal immunization markedly delayed separation of the penis from the prepuce. The immuni zed rams did not display sexual activity until their total testicular mass reached at least 70 g. In control and immunized rams with testicu lar mass > 100 g, the semen did not differ in the percentage of sperma tozoa that were unstained (live), of normal morphology, or showed prog ressive movement. Compared with the increase in plasma LH and FSH conc entrations with age in control rams, the concentrations of these hormo nes in the immunized animals were lower and remained relatively consta nt from 46 to 90 weeks of age. Plasma FSH concentration was particular ly suppressed in immunized rams and was significantly (P < 0.05) lower than the control values at 61 and 90 weeks of age. LH release after i njection of 1 mu g GnRH at 90 weeks of age was either absent or marked ly suppressed in immunized rams compared with that for controls.