Juvenile body weight (JBWT) and the reproductive traits, hen housed egg production (HHP), rate of lay expressed as hen day percent (HD%), age at first egg (AFE) and egg weight (EWT) on approximately 8 overlapping generations of a broiler-type female line were available to study (1) their genetic (co)variances and dispersion parameters and (2) the effect of transformation of the egg production data on the magnitude of heritability and the genetic correlation structure. Bi- and multivariate animal model REML estimations were feasible for the present sizeable data sets (including 573127 animals) by reducing the models to univariate forms using scaling and transformation. Heritabilities were low for HHP (0.14) and HD% (0.10), intermediate for AFE (0.34) and high for EWT (0.55). Genetic correlations between JBWT and HHP (-0.18), HD% (-0.20) and AFE (0.15) were low, whereas EWT was highly correlated to JBWT (0.63) and only the bivariate heritability estimate of EWT was increased by including all the JBWT data (0.55 increased to 0.64). Genetic correlations of HHP with HD% and AFE were large (0.92 and -0.76, respectively), while HD% and AFE were less correlated (-0.45). EWT was little correlated to the other reproductive characters. Box-Cox transformation of HHP and HD% and logarithmic transformation of AFE increased the heritability estimates (0.21, 0.18 and 0.38 for HHP, HD% and AFE, respectively), but had generally little effect on the genetic correlation estimates. Comparison to earlier literature estimates revealed substantial differences in the genetic correlation structure. Possible reasons were discussed. A better understanding of the genetic and physiological relationships between these traits can optimise selection strategy and management within the selection programme.
