Two strains of White Leghorns (PL1 and PL2) were studied at three stages of laying viz; 36–40; 52–56 and 68–72 weeks of age for % shell weight; shell weight per unit surface area (SWUSA) and egg weight. The means for % shell and SWUSA showed a consistent decline over stages with decline being more sharper from stage II to III than from I to II. The coefficients of variation showed an increase over stages. The heritabilities for both the measures of shell-quality showed a decline over stages which was attributable to both the decrease in genetic variance and the increase in phenotypic variances. The genetic correlations among different stages for % shell and SWUSA were close to unity in PL 2 but PL 1 had a very low correlation with stage III. It is concluded that selection in strain PL 2 may be based on early shell-quality but in PL 1 selection at middle stage (52–56 weeks) would be more effective.
Chicken, egg shell, inheritance
