Abstract
Data were recorded for birth weight, birth coat score and weaning weight of between 2018 and 4235 Merino lambs born over a 17-year interval from 1986 to 2002. The resource population was divergently selected from the same base population since 1986, either for (H line) or against (L line) maternal multiple rearing ability. Direct additive genetic variances - expressed as a ratio of the total phenotypic variance (h2, direct heritability) - were estimated at 0.12 + 0.03 for birth weight, 0.70 + 0.05 for birth coat score and 0.13 + 0.04 for weaning weight. Maternal genetic variance ratios (m2) were estimated at 0.23 + 0.04 for birth weight and 0.10 + 0.04 for weaning weight. It was not significant for birth coat score. Maternal permanent environmental variance ratios (c2) for the respective traits amounted to 0.08 + 0.03 for birth weight, 0.04 + 0.02 for birth coat score and 0.08 + 0.03 for weaning weight. No consistent phenotypic or genetic differences in favour of either line were found for birth weight or birth coat score. Compared to L line lambs, phenotypic means for weaning weight were heavier in favour of H line lambs from 1995. This trend was confirmed by divergence in yearly averaged direct breeding values in the H and L lines. Expressed as percentage of the overall phenotypic mean for weaning weight, genetic changes amounted to +0.23% in the H line and -0.26% in the L line. A corresponding maternal genetic trend amounting to -0.19% of the overall phenotypic mean for weaning weight was found in the L line, while no maternal genetic change could be discerned in the H line. Divergent selection for multiple rearing ability thus did not influence birth weight and birth coat score, while the correlated response in weaning weight was favourable. Keywords: Direct heritability, Genetic trends, Maternal heritability, Parameter estimation, Smoothing splines South African Journal of Animal Science Vol.33(4) 2003: 248-256
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