Abstract:
Body weight is considered an important trait for the selection of replacement animals in both wool
and mutton sheep. Knowledge of the genetic variance of each trait and covariances among traits is
essential for effective genetic evaluation and improvement programs. It is important that estimated
breeding values for performance traits should be estimated as accurately as possible. This could be
achieved by fitting the most appropriate statistical model, which accounts for all known non-genetic
effects, as well as correctly partitioning the genetic variance into its various sources. The aim of this
study was to identify the most appropriate models for estimation of breeding values for body weights
recorded at different ages in Merino sheep. Various statistical procedures, including uni- and
multivariate linear models employing restricted maximum likelihood methods, random regression and
repeatability models were evaluated. The dataset used in this study comprises body weight data
recorded at different ages in the Grootfontein Merino stud from 1968 to 2012. The total number of
males and females for which birth weight was recorded, were 7794 and 8317 respectively. The
univariate direct heritability of body weight increased with an increase in age. Direct heritability
estimates were 0.20 ± 0.03 for birth weight, 0.16 ± 0.02 for weaning weight, 0.51 ± 0.04 for 15-month
body weight and 0.40 ± 0.05 for 3-year adult body weight. Maternal heritability estimates were 0.11 ±
0.02 for birth weight, 0.04 ± 0.01 for weaning weight and 0.08 ± 0.02 for 15-month body weight. The
genetic correlation between direct and maternal effects was negative for all weights where it was
included and ranged from -0.95 ± 0.14 for 6-month body weight to -0.28 ± 0.09 for birth weight. The
repeatability model including direct and maternal genetic effects, without splines, was the most
appropriate repeatability model for estimation of genetic parameters for body weight. The accuracy of
the estimated breeding values were determined using Spearman rank correlations and number and
proportion of common animals in the Top 10% and Top 1% lists. The comparison of estimated
breeding values for body weights obtained with univariate, multivariate and repeatability models
revealed that the multivariate model was the most efficient method due to the high accuracies
obtained with this procedure. These results will be implemented when estimating breeding values for
body weights for the animals in the Merino reference population during the development phase of a
suitable SNP key to be used in genomic selection for body weight in South African Merino sheep.
