Seasonal changes in plasma testosterone concentration and mating activity in Border Leicester, Poll Dorset, Romney and Suffolk rams

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Abstract
Seasonal changes in the plasma testosterone profile and mating activity were compared for Border Leicester, Poll Dorset, Romney and Suffolk rams (three of each breed) maintained under field conditions at latitude 35¦S. Border Leicester, Romney and Suffolk showed maximum testosterone levels from February to May. Peak mating activity occurred from March to August, but the precise time and duration of the peak differed between the three breeds. A correlation (r = 0.65, P<0 .05) existed between seasonal changes in plasma testosterone and mating activity in Border Leicester but not in Romney or Suffolk. However, in all three breeds, peak testosterone levels preceded peak mating activity by 1-2 months. Poll Dorset rams showed an extended period of high testosterone levels from August to March but no seasonal peak in mating activity. There was no real relationship, in any breed, between plasma testosterone concentration and mating activity for individual rams within any month.

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