Sperm production, testicular size, serum gonadotropins and testosterone levels in Merino and Corriedale breeds

The relationships between testis size, hormone secretion and sperm producti on were studied during the spring (December) and autumn (May) in rams of tw o breeds with different breeding seasons and body weights (Corriedale and A ustralian Merino) maintained on native pastures and under natural photoperi ods in Uruguay. Blood samples were collected at 20-min intervals during a 2 60-360-min period in 13 rams (four Corriedale, nine Australian Merino) duri ng the late spring and autumn. Rams were weighed and testis size was estima ted by orchimetry at each time period. Sperm production was estimated durin g a 2-week period, 2 months before blood collection and during each week fo llowing every blood collection. There was no relationship between testicula r size and sperm production measured at the same time, nor between Live wei ght and sperm production. In contrast, testicular volume during the late sp ring was correlated with sperm production in the autumn (r = 0.65; P = 0.02 ). The autumn serum LH was higher in Corriedale than in Merino rams. LH pul satility was unaffected by season, but LH pulse frequency tended to be high er in Corriedale than in Merino rams, particularly in the late spring (2.37 versus 1.56 pulses/6 h; P = 0.08). Serum testosterone concentration was si milar in both breeds and seasons. FSH levels were higher in the late spring than in the autumn in both breeds (Corriedale: 2.83 +/- 0.48 versus 2.17 /- 0.24 ngmL(-1); Merino: 2.23 +/- 0.24 versus 1.88 +/- 0.17 ngmL(-1)). FSH and testosterone concentrations during the late spring were positively cor related with autumn sperm production (P = 0.07 and P = 0.03, respectively). In conclusion, the present experiment suggests that LH secretion is not a good parameter for the prediction of sperm production. In contrast, in our conditions (breeds and native pastures) testicular size and testosterone or FSH concentrations from the late spring may be used to predict sperm produ ction in the autumn. (C) Inra/Elsevier, Paris.