Variance components due to direct and maternal effects and estimation of breeding values for 12-week weight of Welsh Mountain lambs

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Abstract
The purpose of this study was to examine the importance of the genetic and maternal environmental factors influencing the 12-week weight (12WW) of Welsh Mountain lambs and to estimate genetic parameters for this trait. Records of 8880 Welsh Mountain lambs born between 1979 and 1995 were analysed. The records were from the nucleus flock of the CAMDA cooperative breeding group. In this flock selection of replacements had been based on a multi-trait index incorporating lamb growth and maternal ability as important objectives and 12WW as one of the selection criteria. Twelve models were examined, all including direct additive genetic variance and various combinations of genetic and environmental maternal effects. The most appropriate model was chosen based on log-likelihood ratio tests. It included appropriate fixed effects, and direct additive, maternal additive, maternal permanent environment and maternal common environment (litter) random effects that defined proportionally 0⋅21 (h2), 0⋅09 (m2), 0⋅06 (pe2) and 0⋅18 (ce2) of the phenotypic variance. Ignoring the additive maternal effect resulted in inflated estimates of direct heritability and ignoring the environmental effects associated with dam inflated the direct and maternal heritabilities. There was no correlation (P < 0⋅05) between the additive direct and additive maternal effects. Additive direct and maternal breeding values increased by 0⋅12 (s.e. 0⋅006) kg/year and 0Ό3 (s.e. 0⋅003) kg/year respectively. The results demonstrate the effective incorporation of selection index methodology in the context of a hill sheep flock and also the importance of several categories of maternal effects.