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

Abstract

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.