Peripheral plasma testosterone concentrations in bulls around puberty

It is now well established that dairy bulls attain their mature rate of spermato-genesis by 1 year of age (Attal & Courot, 1963; Macmillan & Hafs, 1968) and that gonadotrophins and steroid hormones are involved in the process of initiation and maintenance of spermatogenesis (Steinberger & Steinberger, 1969). Already by this age, testosterone is the most important androgen secreted by the interstitial tissue (Lindner, 1969). Rawlings, Hafs & Swanson (1972) observed an increase in the plasma levels of testosterone in five bulls up to 11 months of age and then a drop the following month. Since the levels found were highly variable, it was apparent that data from quite a large number of animals would be required to establish whether there is a significant increase or decrease in the plasma levels. Blood samples were collected from the jugular veins of fifty-one French Friesian bulls at 9, 12 and 15 months of age. The sampling period occurred over 1 year. The bulls were maintained under similar daily conditions since they were brought to the performance test station when they were 8 days old. Fro