Effect of nutrition on seasonal patterns of LH, FSH and testosterone concentration, testicular mass, sebaceous gland volume and odour in Australian cashmere goats

The effects of season and diet on LH, FSH and testosterone concentrations, testicular weight, sebaceous gland volume and male odour were examined in mature bucks fed ad libitum diets of Low or High quality for 16 months under natural photoperiod at 29°S 153°E (n=6 per treatment). Each week plasma was sampled, the bucks were weighed, scored for male odour and assessed for testicular weight based on scrotal circumference. Each month a skin sample was taken from the occipital region for histological assessment of sebaceous gland volume. For each variable there was a clear circannual cycle that was significantly influenced by dietary treatment. The timing of seasonal changes in LH and testosterone concentration, sebaceous gland volume and odour score was similar with a mid-autumn peak. In each case the High diet advanced, extended the duration and increased the magnitude of the seasonal elevation. Change in testicular weight was not associated with these variables, instead being strongly correlated with voluntary feed intake and change in liveweight, themselves subject to seasonal variation with a winter/spring peak. The High diet induced large increases in liveweight and testicular weight during the first months of the experiment without influencing the seasonally low concentrations of FSH, LH and testosterone present at the time. These data demonstrate that the male Australian cashmere goat, like the female, exhibits marked reproductive seasonality, and that nutrition is a powerful modulator of the seasonal cycles. They suggest that testosterone concentration, sebaceous gland volume and odour score are ultimately dependent upon LH secretion which appears to be under strong seasonal (photoperiodic) control, with the effects of enhanced nutrition limited to periods when photoperiodic inhibition is waning. On the other hand testicular size, and therefore sperm production, appear to be primarily dependent on changes in voluntary feed intake and growth, with the seasonal cycle of testicular size more a consequence of the seasonal appetite/growth cycle than changing gonadotrophin concentrations.