BOX-COX TRANSFORMATION OF EGG-PRODUCTION TRAITS OF LAYING HENS TO IMPROVE GENETIC PARAMETER-ESTIMATION AND BREEDING EVALUATION

Egg productions between 19 and 26 (P1), 26 and 38 (P2) and 26 and 54 (P3) weeks of age were studied in two different strains of laying hens. They exhibit markedly skewed distributions. Their transformation into normally distributed traits was carried out using the Box-Cox method. The optimal Box-Cox parameter was found via an empirical approach. Values of 3.0, 6.5 and 4.8 respectively for P1, P2 and P3 were retained for the two strains studied. Transformation of these traits resulted in an increase of their heritabilities estimated by Restricted Maximum Likelihood (REML), especially for traits P2 and P3. The genetic and residual correlations between these traits were not drastically modified. Best Liner Unbiased Prediction (BLUP) applied to a reduced animal model (RAM) was performed with original and transformed data. The correlations between the two sets of estimated breeding values were 0.96, 0.90 and 0.94, respectively, for P1, P2 and P3. The percentage of hens selected with both evaluations is low when the distribution before transformation is far from the normal situation.