EFFECTS OF NUTRITION ON TESTICULAR GROWTH IN MATURE MERINO RAMS ACTIVELY IMMUNIZED AGAINST GNRH

Nutrition-induced changes in testicular size in Merino rams appear to involve both GnRH-dependent and -independent pathways. This hypothesis was tested by feeding mature Merino rams that had been actively immun ized against BSA or GnRH conjugated to BSA a diet that maintained init ial body weight or the same diet supplemented daily with 1.5 kg of lup in grain. Blood was sampled every 20 min for 24 h on days -1, 19 and 7 0 relative to the change in diet. The plasma was used to assess the ef fects of treatments on changes in LH, FSH and testosterone testosteron e were not affected by diet. In comparison, the concentrations of LH, FSH and testosterone were significantly lower in the group immunized a gainst GnRH than in rams immunized against BSA, but none of these endo crine variables was affected by nutrition. With both immunization trea tments, the testes were significantly larger in lupin-supplemented tha n in maintenance-fed rams. In the group immunized against BSA, this di fference was caused by testicular growth in lupin-supplemented rams, w hereas in the group immunized against GnRH, lupin supplementation effe ctively maintained testicular mass, rather than allowed the regression observed in maintenance-fed rams. In conclusion, differences in testi cular growth that were induced by dietary treatments in rams immunized against GnRH were not associated with changes in gonadotrophin or tes tosterone secretion. This supports the hypothesis that part of the eff ect of nutrition on testicular growth is independent of changes in GnR H secretion. The differences in testicular size observed in control ra ms were of similar magnitude to those observed in treated rams, but as sociated with large differences in plasma FSH concentrations, suggesti ng that this hormone plays an important role in this effect.