Relationship between growth rate in bulls and human chorionic gonadotropin-induced plasma testosterone concentrations.

In the selection of bulls for breeding purposes growth rate is an important characteristic. In this study 411 bulls, 5 to 11 mo of age were subjected to a human chorionic gonadotropin (hCG) stimulation test. Plasma testosterone 7 h postinjection of hCG was related to monthly weight gain in the month of blood sampling and to daily gain from 3 to 11 mo of age. The bulls were divided into two groups according to plasma testosterone concentrations; those with values above and those with values below the average for each age group. Over all ages (5 to 11 mo), the bulls in the high testosterone group had higher monthly (P less than .001) and daily (P less than .025) weight gain than those in the low testosterone group. However, when split into two groups according to age (5 to 7 and 8 to 11 mo), only the older animals showed the relationship described above. The results indicate that determination of plasma testosterone might supplement other criteria in the selection of breeding bulls for beef production. Eight to 11 mo of age, when differences between high and low levels of testosterone were greatest, would appear to be the most suitable period for sampling.