Testosterone and libido in holstein bulls of various ages.

Testosterone was measured by radio-immunoassay in the blood plasma collected from 197 and 195 Holstein bulls in the spring and fall seasons, respectively. Bulls ranged in age from 8 mo to 13.5 yr. The average value for spring was 8.0 ng/ml, higher than the value of 5.7 ng/ml in the fall. Also, there was a tendency for testosterone to increase with age of the bulls up to 6 to 7 yr. The concentration of circulating testosterone was unrelated to libido or semen quality.