The impact of heterogeneity of variances on the genetic evaluation of performance traits in Nellore cattle

ABSTRACT The objective was to evaluate the existence of heterogeneity of variances and its impact on the genetic evaluation of ponderal performance in sires of the Nellore breed. We used records of adjusted body weights at 210 (W210), 365 (W365), and 450 (W450) days of age. Both W365 and W450 were combined by principal component analyses using the first component (PC). Average daily gain (ADG) was obtained by difference between W450 and W210. The classes of standard deviations (SD) for W210, PC, and ADG were obtained by the standardization of means of herd-year means subclasses, with positive values composing the high SD and values equal and less than zero composing the SD. The model included the fixed effects of contemporary group and age at calving as a covariate, random genetic additive, and maternal genetic (except for PC) effects, and the permanent maternal environment. Variance components were obtained by Gibbs sampling. Posterior means of heritability in analyses without considering heterogeneity of variances ranged from 0.15±0.01 to 0.31±0.01. Posterior means of genetic correlations between the two classes of SD for W210, PC, and ADG were equal to 0.85±0.04, 0.83±0.03, and 0.71±0.08, respectively. Spearman correlation to breeding values of sires for ADG as the selection intensity increased in them, and the correlations between breeding values in general analyses were more correlated with those predicted in the high DP. Therefore, when there is a higher selection intensity on the sires only for the ADG criterion, there is a significant presence of the heterogeneity of variances and impact on the genetic evaluation of the sires. Thus, for ADG, the predictions of breeding values obtained by the genetic evaluation model in which the heterogeneity of variances are not considered are more weighted by the class of greater heterogeneity.

摘要 本研究旨在评估内洛尔（Nellore）品种公牛体重性能遗传评估中的方差异质性存在情况及其影响。本研究使用了210日龄（W210）、365日龄（W365）及450日龄（W450）的校正体重记录。通过主成分分析对W365与W450进行合并，提取第一主成分（PC）。日平均增重（ADG）通过W450与W210的差值计算得到。针对W210、PC及ADG的标准差（SD）等级，通过对群-年子类的均值进行标准化获得：正值划归高SD等级，零及负值划归低SD等级。模型包含如下固定效应：同期组、作为协变量的产犊年龄，随机遗传加性效应、母体遗传效应（PC除外）以及永久性母体环境效应。方差组分通过吉布斯抽样（Gibbs sampling）获得。未考虑方差异质性的分析中，遗传力的后验均值范围为0.15±0.01至0.31±0.01。W210、PC及ADG的两个SD等级间的遗传相关后验均值分别为0.85±0.04、0.83±0.03与0.71±0.08。随着对公牛ADG的选择强度提升，其育种值的斯皮尔曼相关系数，以及常规分析中育种值间的相关系数，均与高SD组的预测结果相关性更强。因此，当仅针对ADG指标对公牛施加较高选择强度时，方差异质性会显著存在，并对公牛的遗传评估产生影响。据此，针对ADG指标，未考虑方差异质性的遗传评估模型所得到的育种值预测结果，会更多地受到异质性更高的等级的加权影响。