Proceedings of the World Congress on Genecs Applied to Livestock Producon, 11.215 1 Estimation of variance components and heritabilities for body weight from birth to six years of age in Merino sheep using random regression models K.R. Nemutandani 1 , M.A. Snyman 1 , W.J. Olivier 1 & C. Visser 2 1 Grootfontein Agricultural Development Institute, Private Bag X529, Middelburg (EC), 5900, South Africa 2 Department of Animal and Wildlife Sciences, University of Pretoria, Pretoria, 0084 carina.visser@up.ac.za (Corresponding author) Summary Genetic parameters and (co)variance components for body weight in sheep estimated with random regression models are relatively scarce. The aim of this study was to identify the most appropriate random regression model for estimation of variance components and genetic parameters for body weights recorded from birth until six years of age in Merino sheep. The dataset used in this study comprised body weight data recorded from birth until six years of age in the Grootfontein Merino stud from 1968 to 2012. Random regression models fitted included direct genetic, maternal genetic and animal and maternal permanent environmental effects as random effects in various combinations. The model fitting second order polynomials for the direct and maternal genetic effects and 11 splines separating ages 1, 2, 4, 8, 12, 15, 20, 32, 44, 56 and 68 months was the most suitable model. Three datasets were created. The first included data on all ewes from birth until six years of age and rams from birth until 15 months of age; the second included data from only ewes from birth until six years of age; the third included data from ewes and rams from birth until 15 months of age. The additive genetic variance and consequently the phenotypic variance, as well as the direct heritabilities obtained in this study are much higher than those obtained elsewhere with random regression models in sheep. However, in all but one of the other studies, the datasets included body weights up to much younger ages (16 months) than the current study. The model fitting one polynomial for direct and maternal genetic effects and including 11 splines was compared for the three datasets. It was evident that direct genetic variances and heritabilities were similar until 15 months of age between the dataset including all animals and the one including rams and ewes up till the age of 15 months. These values were, however, higher than published literature values. The direct variances obtained with the dataset including only data from ewes, yielded much lower values, which were comparable to literature values up until 15 months of age. Including records for ram lambs up until 15 months of age together with ewes from birth until adult ages yielded direct genetic variances that fall outside the normal values reported for sheep. This raises the question if random regression is the most suitable procedure for the analysis of body weight in sheep when including data on all available animals from birth until adult age. Keywords: direct genetic variance, phenotypic variance Introduction Recently, random regression models have become a more common method to use for the analyses of growth traits. The random regression model (RR) allows environmental effects specific to the time of recording to be accounted for and can also accommodate genetic differences in the shape of each animal's growth curve (Meyer, 2004; Schaeffer, 2004).