INCREASE OF EGG WEIGHT WITH AGE IN NORMAL AND DWARF, PUREBRED AND CROSSBRED LAYING HENS

An exponential curve, W = P - Q exp (-Rt) was fitted to egg weights (W ) of individual hens from 8 genetic groups tested for egg production f rom 18-51 weeks of age (t). The groups were constituted of the combina tion of genotype at the sex-linked dwarfism locus (normal or dwarf) an d line (White Leghorn, Brown Egg and both reciprocal crosses). The lea st-squares mean of the residual standard deviation about the curve was between 1.27 and 1.74 g in the 8 groups and estimated values of the i nitial P - Q and mature egg P weights were between 26.9 and 33.9 g and 56.9 and 65.8 g, respectively. Effects of genotype and line as well a s heterosis were estimated for both egg weights and for rate of growth relative to the remaining expected growth (R). R was smaller for dwar f hens which thus reached mature egg weight later than normal females. Significant heterosis was found for both egg weights of dwarf hens (1 2.6 and 9.5%) and for the initial egg weight of normal ones (12.8%). N egative heterosis for R obtained for dwarf hens (-17.4%) corresponded to a flatter egg weight curve of these crossbreds.