Repeatability of Egg Shell Quality Measurements Within Individual Hens

Correlations of egg measurements between eggs laid by the same hen were examined using two experimental strains of White Leghorns, a control strain and a strain selected for high production and specific gravity. Eggs were collected for 56 days from 72 birds, that is, 18 pullets and 18 force-molted hens from each strain. At the beginning of the experiment, the pullets were 342 days of age and the force-molted hens 721 days of age. Egg variables measured were egg and shell weight, specific gravity, nondestructive deformation, quasi-static compression force to fracture, and shell thickness. Correlations between the same measurements on eggs laid 1 day apart were .89, .82, .67, .36, .50, and .54, respectively; these correlations decreased relatively little even for eggs laid 40 days apart. Correlations between mean measures on eggs gathered over successive 14-day periods were much higher – .94, .90, .84, .78, .85 and .81, respectively – reflecting the diminished influence of random variation within bird. Correlations within age-strain groups were smaller than the overall values, but the correlations seemed generally consistent from group to group. These results indicate that the measurements of shell strength from eggs of a bird provide a reliable indicator of the strength of future egg shells from that bird.