Sensitivity of breeding objectives to prices and genetic parameters in Australian Corriedale and Polwarth dual-purpose sheep

Abstract

A profit equation was developed for Australian Corriedale and Polwarth dual-purpose sheep kept for both wool and lamb production. Ten traits contribute to income and costs and were included in the breeding objective. These encompass wool production, fibre diameter, and feed intake of breeding ewes and hogget ewe replacements; sale weight and carcass fat depth of lambs; and reproductive rate and mature weight of ewes. The relative economic value for each trait in the enterprise was calculated from the profit equation using income and costs for 1 year. Overall gain ($A) and gain in each trait in the breeding objective from selection using an index of hogget greasy fleece weight, hogget fibre diameter, dam's number of lambs weaned, lamb weight, and lamb fat depth are reported. The sensitivity of the genetic changes in each trait in the breeding objective and index coefficients were assessed for a range of prices of products and feed costs. Sensitivity to changes in heritabilities and genetic correlations was also assessed. The incorporation of these maternal traits into LAMBPLAN is discussed. For the standard parameters and prices used, gain in leanness accounted for one-third of overall gain ($/ewe. s.d. of selection). Other traits that contributed to overall gain were fibre diameter (28%), reproduction (18%), and growth (14%). There was a small gain through feed intake (8%), and a very small loss in wool weight. Measures of fatness, growth, and fibre diameter were the important traits in the selection index. Inclusion of hogget greasy fleece weight and dam's number of lambs weaned each added <1% to the efficiency of the selection index. Varying the sale price for lamb and the price differentials for fibre diameter and fat depth had the greatest impact on overall gain. High lamb price increased gains in both reproduction and lamb weight, whereas, high price differentials for fibre diameter and fat mainly increased gains in the particular trait. A large range in prices for wool had very little effect on the individual traits or overall. Changing feed costs had little effect on overall gain, although high feed cost reduced gains from reproduction which were compensated by reduced feed intake. Halving the heritability value for each trait reduced overall gain, largely through reduction in the trait. Varying the genetic correlations of wool production with other traits had little effect. However, when genetic correlations of reproduction with weight, fat, and fibre diameter were varied there were changes in overall gain, largely through reproduction.