Epsilon Archive for Student Projects

Abstract

Traits with low heritability estimates such as health and fertility are often assumed to have low genetic variance. However, the heritability is a ratio of additive genetic variance over all variance, and thus, the genetic variation of these traits may only be proportionally small. Health in dairy cattle is important, not only in its own right but also for human health and animal welfare. The coefficient of variation (CV) can be used to express genetic variance without units, allowing for comparison between traits regardless of their measurement units, in the same way heritability can be used. The CV is the standard deviation of the trait divided by the mean. In addition, the use of within-family variances may allow for more accurate investigation of genetic variance, because when within-family variances are large, between family variance (which is the basis for heritability estimation) is low. However, for international genetic and genomic evaluations, phenotypic measurements are not always available.
Therefore, the aim of this study was to investigate the usefulness of using CV for observations based on estimated or relative breeding values (EBV/RBV), and to investigate whether the low heritability seen for health traits, is related to low genetic variance, as defined by CV. Data for Brown Swiss cattle from 6 countries available from the Interbull evaluation of April 2014 were used and CV was calculated for within sire family variances and for the population as a whole based on international estimated breeding values (IEBV) using SAS 9.3. The breeding values were standardised differently for different countries, and were separated accordingly. However, there are reasons to believe that the interpretation of the results may be complicated because of this, and thus the validity of the CV for this evaluation may not be justified. If this is ignored for the time being, the following results were found. For all traits, population CVs were larger than sire-family CVs. Traits had either means for breeding values of 100 or 0 depending on the country, but there was no correlation between these two types of means for individual traits. Differences were found between countries for overall mean CV. Differences between different categories of traits (e.g. calving, fertility, production) were found, with production and fertility traits having the lowest overall CV across countries. Within countries, ranking of trait categories were somewhat different, but generally workability and udder health traits had high CVs. No significant correlations were found between the CV of traits and their heritability estimates, except when separating traits by country. This was only significant for Germany-Austria. Overall, the CV is useful for comparing the variance of traits with different measurement units. However, because of the nature of the breeding values in this study, it may not be appropriate to use the CV. Therefore, it is concluded that CV, although useful in itself, is not a good approach when evaluating relative breeding values across countries.