The relationship between testis size and stimulated plasma testosterone concentrations and its influence on mating performance in Dorper rams

Testis size, stimulated testosterone concentrations and mating performance (serving capacity, libido score and serve:mount ratio) of 14 Dorper rams were measured in the breeding season (March) in order to determine the influence of testis size and testosterone concentration on mating performance. Results indicated statistically non-significant (P>0,05) correlations between testis size and stimulated plasma testosterone concentrations. Live body mass was however significantly (P<0,05) negatively correlated (r=-o,530) to the post-copulatory mean testosterone level. In only two of three mating performance pen tests significant (P<0,05 and P<O,OI) negative correlations of -0,577 and -0,795 were obtained between scrotal circumference and serving capacity. Testis volume and serving capacity was however positively correlated (r=0,593; P<0,05) in the other test. Estimates of repeatability for serving capacity ranged from 0,102 to 0,690 and for libido score from 0,065 to 0,269. The only significant (P<0,05 and P<O,01) correlations between stimulated testosterone concentrations and mating performance were obtained between the post-copulatory testosterone levels and serving capacity (r=0,528 and 0,736 for tests 2 and 3 respectively) and for the serve:mount ratio in test 1 (r=0,557). It may therefore be concluded that serving capacity is the only criterion that offers some advantage as a selection aid to improve efficiency of reproductive performance in pen-tested rams.