Testis size, gonadotrophin secretion and pituitary responsiveness to GnRH in Romney rams during the transition from the non-breeding to the breeding season

Abstract

Romney rams, which had either an early ( n = 7) or a late ( n = 5) increase in testis size prior to the onset of the breeding season, were selected from a larger population ( n = 60). In the following year, the two groups were compared for the pattern of seasonal variation in testis size, circulating gonadotrophin concentrations and pituitary responsiveness to a gonadotrophin releasing hormone (GnRH) challenge during the transitional period from the non-breeding to the breeding season. Rams in the ‘early’ group had an earlier and greater increase in testis size than rams in the ‘late’ group, a pattern which was largely repeatable between years. Differences between the groups in the pattern of seasonal variation in testis size were associated with group differences in endocrine function. Thus, rams in the early group had a significantly ( P < 0.01) higher luteinising hormone (LH) pulse frequency in March (4.4 ± 0.4 pulses per 8 h period) than those in the late group (1.7 ± 0.3 pulses per 8 h period) and the seasonal increase in plasma follicle stimulating hormone (FSH) concentrations occurred earlier ( P < 0.10) in the early group than in the late group. There were also group differences in the pattern of seasonal variation in total LH response to the GnRH challenge. These results show that there are differences among rams of the same breed, in the pattern of seasonal variation in testis size and gonadotrophin secretion, which could potentially be used as criteria to select rams for date of onset of the breeding season in their daughters.