Comparison of models for genetic evaluation of scrotal circumference in crossbred bulls

The aim of this study was to evaluate the effect of including concomitant b ody weight and(or) a random dam effect in genetic evaluation models on vari ance component estimates and standard error of prediction for scrotal circu mference (SC) at 6, 8, 10, and 12 mo. Variance components and average stand ard errors of prediction were compared under models differing in either the number of related traits (M11 [SC], M12 [SC and BW]) or an uncorrelated ra ndom dam effect (M21 [SC], M22 [SC and BW]) using records on 1,547 bull cal ves. In a single-trait model (M11), estimates of direct heritabilities ((h) over cap(a)(2)) for SC were .45, .49,.57, and .66 at 6, 8, 10, and 12 mo, respectively. In a two-trait model (M12), (h) over cap(a)(2) were similar t o those in M11 model. In M21, (h) over cap(a)(2) for SC were .37, .42, .54, and .65, whereas the proportions of phenotypic variance due to dams ((d) o ver cap(2)) were .12, .11, .04, and .02 at 6, 8, 10, and 12 mo, respectivel y. Similarly, in M22, (h) over cap(a)(2) for SC were .36, .44, .56, and .65 and (d) over cap(2) were .13, .10, .02, and .02. Standard errors of predic tion for SC EBV from M22 were reduced by 2.86, 1.21, 3.02, and 1.99% relati ve to M21 and by 6.45, 2.70, 2.72, and 1.21% relative to Frill at 6, 8, 10, and 12 mo, respectively. Standard errors of prediction for SC EBV from M12 were reduced by .06, .73, 1.56, and .87% relative to M11 at 6, 8, 10, and 12 mo, respectively. The importance of the dam effect decreased with age fo r both SC and BW. These results demonstrate that a two-trait (SC and BW) an imal model would result in more accurate evaluations of yearling SC EBV in beef cattle than a single-trait model.