Estimation of genetic parameters for visual score traits: a simulation study

Abstract

Visual scores are assigned with no basis on an absolute reference. In this case,
evaluation references are different for each group, depending only on the structure of the group.
Thus, factors that change the structure of evaluation groups, such as group size, can affect visual
score assignment, as well as estimation of genetic parameters and prediction of breeding values,
respectively. The objective of this study to determine the consequences of assigning scores based
on a relative reference on the estimation of genetic parameters and breeding values considering
different evaluation group sizes. A simulation study was conducted considering 6 scenarios
formed by the combination of two populations (selected and unselected) and 3 group sizes (10,
40, and 100 animals) for evaluation. Genetic parameters and breeding values were estimated
using two threshold models that differed in their correction for environmental effects. Heritability
estimates for the two populations were close when the model corrected for environmental effects
(0.407 – 0.418). However, heritability levels fell over generations when the model did not consider
environmental effects (0.172 - 0.247). The size of the evaluation group mainly affected the accuracy
of breeding values prediction, since higher accuracy was related to larger evaluation group sizes
(0.576 - 0.715). We concluded that the way in which visual scores are assigned does not affect
the estimation of genetic parameters and values. However, evaluation groups of more than 40
animals should be used to obtain greater accuracy and genetic progress.