Models for Genetic Evaluation of Scrotal Circumference in Red Angus

Growth and scrotal records were obtained to estimate age of dam and age at measurement adjustment factors for yearling scrotal circumference in Red Angus bulls (n = 40,865), and to estimate genetic parameters. The linear partial regression coefficient for age at measurement was 0.0323 cm/d. The recommended factors were 0.70, 0.37, 0.08, and 0.30 cm to adjust the scrotal circumference records of yearling Red Angus bulls out of 2-, 3-, 4-, and ≥ 10-yr old cows to a mature (5 to 9 yr) age of dam equivalent. Adjusted 365-d scrotal circumference (SC365) records were fitted to animal models including direct genetic and from 0 to 3 maternal effects; however, maternal effects were negligible. Her-itability for SC365 was 0.51 ± 0.02 from a model including only direct genetic effects. Bivariate models were used to estimate parameters for SC365 with birth (BWT) and 205-d BW, and 160-d postweaning gain (PWG). Estimated genetic correlations (± 0.03) were 0.10, 0.13, and 0.13 for SC365 with direct effects on BWT, 205-d BW, and PWG, respectively. Maternal BWT had a low genetic correlation (0.05 ± 0.05) with SC365, but the genetic correlation between maternal 205-d BW and SC365 was moderate (0.35 ± 0.04). In addition to the recommendations for adjustment of scrotal circumference, these results suggest that direct genetic effects on SC365 could be used in selection programs, and that increasing selection for SC365 would be concomitant with growth up to yearling age and not antagonistic to maternal ability in Red Angus.