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An experiment is reported, in which shell weight per unit surface area was measured for eggs laid by pedigreed pullets at 33, 54 and 65 weeks of age, using a Rhode Island Red strain and a White Leghorn strain. Specific gravity and albumen quality were also measured and records of individual egg production were available.The heritability of "shell thickness " (i.e. shell weight per unit surface area) in the Rhode Island Reds declined from 0.57 in the first period to 0.33 in the last period. The genetic correlation between thickness in the first period and thickness in the last period was 0.90. The relative selection efficiencies expected by basing selection on early shell thickness with the object of improving late shell thickness were greater than 1.0, indicating that early selection would be the best method of improving shell quality in this strain. In the Leghorn flock, estimates of the heritability of shell thickness were low (0.07 to 0.20) and variable. The genetic correlations between thickness measured in different periods were generally lower than for the Rhode Island Reds, and relative selection efficiencies varied widely. The Leghorns had much thicker shells than the Rhode Island Reds at all times of the year but showed little prospect of further improvement in shell thickness whichever time of measurement was used as a basis for selection.Specific gravity was found to be highly genetically correlated with shell thickness and so would be a good estimator of shell thickness, although more eggs per pullet must be measured to give the same accuracy as direct measurement of shell weight per unit area. There were no adverse genetic correlations between thickness and the other traits measured, except perhaps in the case of albumen quality.It is concluded that the lack of any general improvement in end-of-year shell strength is probably not due to low heritability, to adverse genetic correlations or to inaccurate methods of measuring shell thickness, but to a lack of selection pressure.

Approximate standard errors of genetic parameter estimates were obtained using a simulation technique and approximation formulae for a simple statistical model. The similarity of the corresponding estimates of standard errors from the two methods indicated that the simulation technique may be useful for estimating the precision of genetic parameter estimates for complex models or unbalanced population structures where approxi mation formulae do not apply. The method of generating simulation populations in the computer is outlined, and a technique of setting approximate confidence limits to heritability estimates is described.

The refractive index (Zeiss Abbe Model A) was compared to percentage-protein.(Technicon autoanalyzer) and percentage-solids (oven dried, 108° C. for one hour) in 505 samples of egg white. The linear regression and correlation parameter estimates indicated that the refractive index is a better predictor of percentage-protein than solids, although percentage-protein and percentage solids are highly correlated.

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