Case Study: Scrotal Circumference in Beef Bulls-Prediction of Measures at 365 Days of Age from Measures at 240 Days of Age with Data from the Tucumcari Bull Test

Measurement of scrotal circumference (SC) is included in a breeding soundness exam for for bovine bulls. A minimum of 30 cm of SC is recommended for yearling bulls (Kennedy et al., 2002). Objectives were to evaluate the association of SC adjusted to 240 d of age with SC adjusted to 365 d of age using data from the Tucumcari Bull Test (n = 2,514). Scrotal circumference adjusted to 240 d of age was also used to assess its effectiveness to predict 365-d SC of at least 30 cm. Bulls with 365-d SC greater than or equal to 30 cm (n = 2,418) had 240-d SC of 26.9 ± 0.1 cm, and bulls with 365-d SC less than 30 cm (n = 96) had 240-d SC of 22.1 ± 0.5 cm. Correlation of 0.7 (P < 0.01) was detected between adjusted 240- and 365-d SC, and phenotypic trend slightly increased (linear; P < 0.01) from 1982 to 2006 for these traits. Initial SC was a significant (P < 0.01) effect in prediction of 365-d SC. Plotting of the lower 95% confidence limit of the predicted 365-d SC regression line suggested a 240-d SC of 22.5 cm as a culling level to achieve SC of at least 30 cm. Probability estimated (P < 0.01) by standard-normal distribution revealed that a minimum of 22 cm was needed at 240 d of age to achieve 365-d SC of at least 30 cm. Nonetheless, yearling bull breeding soundness exam failure rate due to insufficient SC was less than 4% in these data.