Data collected on the Carnarvon Merino flock over the period 1962 to 1983 were used for this study. Variance components resulting from direct additive genetic effects, maternal additive genetic effects, maternal permanent environmental effects, as well as the relationship between direct and maternal genetic effects for several body weight and fleece traits, were estimated by DFREML procedures. Traits analysed included 120-day weaning weight (WW), body weight (W6) and greasy-fleece weight (GFW6) at six months of age, as well as body weight (W18), clean fleece weight (CFW) and mean fibre diameter (MFD) at 18 months of age. By ignoring or including maternal genetic or environmental effects, four different models of analysis were fitted to determine the most suitable model for each trait. WW and GFW6 were significantly influenced by both maternal additive genetic and permanent environmental effects. For W6, W18 and CFW, the most suitable model included a direct as well as a maternal additive genetic effect, while only the direct additive genetic effect had a sigÂ¬nificant influence on MFD. Direct and maternal heritability estimates for the various traits were as follows: 0.14 and 0.1 1 for WW; 0.18 and 0.10 for W6; 0.20 and 0.06 for GFW6; 0.43 and 0.04 for W18; and 0.26 and 0.04 for CFW. A direct heritability of 0.60 was estimated for MFD. Standard errors for these estimates ranged from 0.01 to 0.04. Positive genetic correlations between direct and maternal genetic effects were estimated for WW (0.57), W6 (0.86) and W18 (1.00).
Variance components and genetic parameters for body weight and fleece traits of Merino sheep in an arid environment
Author: M.A. Snyman, J.J. Olivier and W.J. Olivier
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