Adjusted 200-day scrotal size as a predictor of 365-day scrotal circumference.

Data collected from 264 bulls of 13 beef breeds at the Michigan Bull Test Station was evaluated to determine if scrotal circumference (SC) adjusted to 200 d of age could be used to predict scrotal circumference at 1 yr of age. Scrotal circumference of each bull was recorded on arrival at the test station and at the time of breeding soundness examination (BSE) and was adjusted to 200 and 365 d of age, respectively. Bulls with adjusted SC>34.0 cm by 365 d of age averaged a larger SC at 200 d (P < 0.0001) and faster scrotal growth (P < 0.0001) than bulls with a 365 d adjusted SC </= 34.0 cm. For all bulls evaluated, mean values differed among breeds for the 200-d SC (P < 0.0001) and the 365-d SC (P < 0.0001) but not for scrotal growth over the test period (P = 0.0773). When only bulls with an SC exceeding 34.0 cm by 365 d were considered, differences among breeds were not significant for the 200-d SC (P = 0.3067), the 365-d SC (P = 0.4083), or scrotal growth (P = 0.9248). The adjusted 200-d SC was positively correlated (partial correlation coefficient = 0.6398) with the adjusted 365-d SC. Least squares linear regression suggests that the 200-d SC accounts for nearly half the variation in scrotal size at 1 yr of age (R(2) = 0.4553). Data from this study suggests that among the bulls evaluated, 200 d old calves with SC >/= 23.0 cm had a 95% probability of achieving SC > 34.0 cm by one year of age. Calves measuring < 23 cm at 200 d had a 54% probability of achieving > 34.0 cm scrotal size by one year. This information can contribute to the selection of breeding bulls that will achieve desirable scrotal size by one year of age.