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Uncharacteristically large spreading area on a flat surface of broken egg negatively affects egg quality assessment and reduces eggs' economic value. In this study, we investigated the heredity of the egg content spreading area as well as the relationships between the egg content spreading area and egg quality traits and properties. We measured the total egg content spreading area (TECA), outer thin albumen area (OTAA), inner thick albumen area (ITAA), yolk area (YA), and egg quality traits for 1414 newly laid eggs from 487 27-wk-old White Leghorn pure line pullets. The genetic parameters of egg content spreading areas were estimated. The phenotypic and genetic correlations between egg content spreading area and egg quality traits were analyzed. The differences in the properties of eggs with similar egg weight but markedly different TECA were also analyzed. The heritability estimates for TECA, OTAA, ITAA, and YA were low to moderate, with values of 0.214, 0.176, 0.340, and 0.280, respectively. Egg weight was related to TECA with a phenotypic correlation of 0.450 (P 0.01) and a genetic correlation of 0.349. A high genetic correlation (-0.731) was found between TECA and Haugh unit. In eggs with larger TECA, the weight and total solid content of outer thin albumen (OTA) and moisture content of inner thick albumen (ITA) were significantly higher, whereas the weight and total solid content of ITA was markedly lower, but no differences (P 0.05) were found in the pH of OTA and ITA, moisture content of OTA, as well as the eggshell strength, thickness, and non-destruction and fracture deformation between eggs with similar egg weight but markedly different TECA. These results suggest that the egg content spreading area can be regulated via the direct selection strategy or indirect selection of the ratio of OTA to ITA in the breeding program.

Eggshell quality is associated with hatchability, and ultrastructural organization is an important eggshell quality parameter. To the determine the relationship of shell ultrastructural properties with hatchability, we measured the effect of eggshell thickness, mammillary layer thickness, and the average size of mammillary cones on hatchability. Eggs with thick shells and thick mammillary layers had the highest incidence of hatching. As such, hatchability correlated positively with both eggshell thickness (r = 0.30; P < 0.05) and mammillary layer thickness (r = 0.28; P < 0.05). In addition, the correlation between the average size of mammillary cones and hatchability was not statistically significant. However, a significant positive correlation was observed between the average size of mammillary cones and mammillary layer thickness. It can be concluded that eggshell thickness and mammillary layer thickness affect hatchability.