Effects of nutrition on seasonal changes in scrotal circumference, testosterone and pituitary responsiveness to exogenous GnRH in Corriedale rams

Abstract

In this study we investigated the effects of nutrition on seasonal changes in live weight (LW), scrotal circumference (SC), plasma testosterone and LH secretion of Corriedale rams in Uruguay under rearing conditions representative of the country's extensive grazing systems. In Experiment 1, where animals were feeding on improved ( n=13) or native pastures ( n=12), LW and SC were significantly influenced by month and plane of nutrition ( p<0.0001). There was also a significant interaction between month and plane of nutrition ( p<0.0001). Both groups showed similar seasonal patterns of change in LW and SC, with the highest values recorded in late summer (February) and the lowest in winter (June). Plane of nutrition had no effect on plasma concentrations of testosterone, but testosterone was significantly influenced by month ( p<0.0001). In both treatments, levels of testosterone were lowest in winter and spring (June–November) and highest in early autumn (March). The LH and testosterone responses following an injection of GnRH were significantly affected by month ( p<0.001) but were not related to dietary treatment. The LH response was lowest in summer (December), and the lowest testosterone response was observed in winter (June). In Experiment 2 a group of six rams was transferred from native to improved pasture in winter (July). LW increased significantly within the first month following transfer to the higher plane of nutrition ( p<0.05), but no changes were observed in SC until three months later. From summer (December) to autumn (April) rams were supplemented with concentrate to maintain LW, but SC decreased significantly from February to April ( p<0.05). The findings indicate that changes in SC induced by nutritional status might be governed by pathways independent of GnRH, but that the overall yearly testicular cycle of the Corriedale ram seems to be controlled by mechanisms operating at the hypothalamo–pituitary level.