Effect of Sampling Status and Adjustment for Heterogeneous Variance on Bias in Bull Evaluations

Abstract

Holstein bulls were assigned to sampling categories (AI stud, AI nonstud, or non-AI) based on bull code, controller number, and age at semen distribution. The AI stud bulls were sampled through traditional progeny-testing programs of 13 AI organizations; AI nonstud bulls had AI semen collection reported by another organization or by multiple organizations. The non-AI bulls had no reported AI semen collection. Actual daughter yield deviations for these three groups of bulls were compared with expected performance (parent average) to provide an indication of whether evaluations were free from bias for daughter yield deviations. Mean difference of daughter yield deviation from parent average was close to 0 kg for animal model evaluations of all 22,930 bulls but was positively biased by 46 kg of milk for AI nonstud bulls. Mean ETA and reliabilities for parents were highest for AI stud bulls and lowest for non-AI bulls. The AI stud bulls varied least and were intermediate for mean management, approximated as mean daughter yield minus bull ETA. Management was highest for AI nonstud bulls, which suggested that adjustment for heterogeneous variance might reduce bias. However, the effect of this adjustment on mean difference of daughter yield deviation from parent average was small.

Sire×contemporary group interactions for birth weight and preweaning growth traits in the Asturiana de los Valles beef cattle breed

“…The cause of this interaction effect does not seem to be the heterogeneity of variances (Canavesi, 1993), which leads to the consideration that this is a result of the existence of an environmental covariance between the daughters of a given sire within a given contemporary group because of preferential treatment. This could be more important when some sires are used and evaluated in a small number of herds (Powell et al, 1994). This scenario may be compatible with a common situation in beef cattle improvement programs, thus leading to a more intense effect of the environmental covariances existing among sires and herds.…”

“…Meyer (1987) and Calus et al (2002) reported estimations of this interaction of between 2.5% and 3% of the phenotypic variance, thus leading to this effect being included in some dairy cattle evaluation programs (Powell et al, 1994). The cause of this interaction effect does not seem to be the heterogeneity of variances (Canavesi, 1993), which leads to the consideration that this is a result of the existence of an environmental covariance between the daughters of a given sire within a given contemporary group because of preferential treatment.…”

Within-Herd Heritability Estimated with Daughter–Parent Regression for Yield and Somatic Cell Score

“…Efforts to standardize records to constant genetic or phenotypic variance have resulted in improvements in the accuracy of sire PTA (Wiggans and VanRaden, 1991;Powell et al, 1994;van der Werf et al, 1994;Weigel and Lawlor, 1994). The use of direct estimates of within-herd heritability (h 2 WH ) may be more precise than heritability inferred from herd variance.…”

Accuracy and Stability of National and International Somatic Cell Score Evaluations

“…Bulls reported by 3 yr of age as having semen distributed to at least 40 herds are assigned sampling status "S", and other bulls are assigned sampling status "O". Powell et al (2004b) reported that O-code bulls tended to be overevaluated relative to S-code bulls for yield, but to a lesser extent than found in previous years (Powell et al, 1994). Conclusions from yield traits should not be extrapolated to SCS.…”

“…Powell et al (2004b) reported declines in yield PTA over time for Holstein bulls that were, or had been, in active AI service. A number of studies have shown that bulls sampled outside the programs of the major AI organizations were initially overevaluated for yield (Cassell et al, 1992;Powell et al, 1994;Powell et al, 2004b). To provide information on how bulls were sampled, the National Association of Animal Breeders (NAAB) added sampling status to their cross-reference program (Sattler, 1990).…”