Plasma concentration of testosterone in young bulls in relation to age, rate of weight gain and stimulation with human chorionic gonadotrophin.

In 86 bulls ranging in age between 3 and 11 months, the maximal plasma testosterone concentration (n = 7 as well as the mean concentration (n = 7), same samples) were found to increase with age to peak values at 7-8 months with a subsequent decrease. The multiple correlation coefficient for monthly gain as a function of testosterone concentraton and age was 0.85 (P < 0.001), for monthly gain as a function of age 0.84 (P < 0.001) and for monthly gain as a function of testosterone concentration 0.44 (P < 0.001). The deviation in levels of measured plasma testosterone from the curve expressed by multiple regressiuon analyses of testosterone concentration as a function of age was correlated with the average daily weight gain during the testing period of 9 months and found to be not significant (r = 0.21). The difference between concentrations of plasma testosterone after stimulation with human chorionic gonadotrophin (HCG) and the spontaneous maximal concentration of this hormone was not signifiant. High correlation coefficients were found between maximal (r = 0.8) and mean (r = 0.6) concentratiaons of plasma testosterone before and after an injection of HCG (P < 0.001).