Genetics of Reproduction Research Scrotal circumference in yearling bulls, as a measure of testicle size, had been shown to be a highly heritable trait that is easily and accurately measured. It is an excellent predictor of age at puberty but is independent of bull libido or serving capacity. Complete breeding soundness examinations have been taken on all yearling bulls at the Center since the mid 1950s and scrotal circumference has been measured since 1969. In yearling heifers, age at puberty, reproductive tract score and pelvic measures have been taken in recent years. Age at puberty is fairly highly heritable and is lowly but favorably related to subsequent measures of reproduction and productivity. Reproductive tract score, obtained by rectal palpation, is a measure that is useful in selecting replacement heifers for breeding. Pelvic measures in both yearling bulls and heifers are highly heritable and should respond to selection. It is expected that increased pelvic area should result in less calving difficulty in two-year-old, first calf heifers. Studies on calving ease indicate that the overall measure is lowly heritable; however, component traits such as birth weight and pelvic area are highly heritable and emphasis is being placed on indicator traits of calving ease.