The effect of nutrition on testicular growth in mature Merino rams involves mechanisms that are independent of changes in GnRH pulse frequency.

In mature Merino rams, changes in diet to below or above the requirements for maintenance of body weight lead to changes in gonadotrophin secretion and testicular growth. However, the effects on testicular growth persist for much longer than those on LH and FSH secretion so that the gonadal and gonadotrophin responses are poorly correlated over time. This suggests that the gonadal effects may be partly independent of changes in the hypothalamic secretion of GnRH, an hypothesis tested in this study. In a short-term experiment (November, late spring, non-breeding season), we tested whether a high frequency of exogenous GnRH pulses could override the endogenous system and mimic the change in gonadotrophins seen in rams fed a high plane of nutrition. Mature Merino rams (scrotal circumference (mean +/- S.E.M.) 33.6 +/- 0.5 cm, body weight (mean +/- S.E.M.) 59.0 +/- 0.9 kg) were fed 900 g chaff + 1.6 kg lupin grain (High diet) or 360 g chaff + 60 g lupin grain (Low diet) and infused with 8 pulses of GnRH or saline daily for 5 weeks (n = 5/group). Blood was sampled every 20 min for 12 h on days-1 and 14 relative to the start of treatments. Relative to pre-treatment levels, LH pulse frequency and FSH concentrations were decreased on day 14 in saline-infused rams fed the Low diet and increased in saline-infused rams fed the High diet (P < 0.001). In GnRH-infused rams, gonadotrophin secretion was not affected by diet and the patterns of secretion of LH and FSH were similar to those in saline-infused rams fed the High diet. This model was used for a more complete endocrine analysis in a longer experiment designed to test the hypothesis that the effect of nutrition on testicular growth is partly independent of changes in the secretion of GnRH. The same treatments were imposed for 35 days on a different group of similar rams in March (autumn, mid-breeding season). Body weight and scrotal circumference were measured weekly and blood was sampled on days -1 and 14. On days -1 and 35, testosterone secretion in response to LH was tested by injecting exogenous ovine LH (NIADDK-oLH-25; 200 ng/kg body weight) to all rams. Body weight increased in rams fed the High diet and decreased in those fed the Low diet (P < 0.001) and was not affected by treatments as in experiment 1.(ABSTRACT TRUNCATED AT 400 WORDS)