Two N.Z. sets of phenotypic and genetic parameters were used to compare expected genetic responses from index selection for an economic breeding objective favouring the lean and penalising the fat content of lamb carcasses. The effect of different economic values for lean relative to fat were examined over the range corresponding to the 'biological' objectives, weight-adjusted fat and lean growth rate. Particular attention was given to the magnitude and direction of responses in lean and fat. Increasing the economic emphasis on lean relative to fat progressively increased the lean response but was associated with an increasing fat response in an undesirable direction. Different measures of carcass fatness varied in their ability to discriminate among genes affecting fat and lean differentially when included in a selection index along with growth rate, and were superior to eye muscle depth. Good estimates of the relative economic values are required for derivation of optimum selection strategies, especially if indexes of low discriminating power are to be used for selection.
Proceedings of the New Zealand Society of Animal Production, Volume 51, , 353-358, 1991
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