Variability and interaction of some egg physical and eggshell quality attributes during the entire laying hen cycle

The aim of this study was to investigate the variability and relationships between some egg physical (egg weight, width, length, shape index and surface area) and eggshell parameters (weight and percentage, thickness, breaking strength, L*, a*, and b* values) during the entire laying hen cycle. A total of 8,000 eggs was collected every 5 weeks, from 30 to 81 weeks of hens age (10 samplings of 400 eggs/house), in 2 identical poultry houses equipped with enriched cages. For the statistical analysis, ANOVA, Bivariate Correlation, Principal Component Analysis (PCA) and hierarchical cluster analysis were used. An increase of egg weight, length and eggshell lightness (L*) associated with a reduction of eggshell percentage, breaking strength, and redness (a*) was observed as the hen aged (P<0.05). Overall, the coefficients of variation resulted <5% in width, length, shape index, and egg surface area; from 5 to 10% of egg weight, shell weight, shell percentage, shell thickness, L*, and b*; >10% of eggshell breaking strength and a*. According to the PCA, the highest changes during the laying cycle are related to egg physical parameters (32%) and to eggshell breaking strength, percentage, and thickness (26%). The egg physical parameters appeared to be strongly correlated to each other, whereas a slight correlation between eggshell breaking strength and color attributes were evidenced (-0.231 and 0.289 respectively for L* and a*; P<0.01). Hierarchical cluster analysis, based on principal components of the overall egg attributes is hereby considered, evidenced dissimilarities for eggs laid from peak production up for 39 weeks of hen age from the eggs laid afterwards. The latter group could also be divided into two subgroups, one comprising eggs laid from 44 and 53 weeks of hen age and the other from 58 weeks to the end. In conclusion, the large dataset created in this study allowed to extrapolate some robust information regarding the variability and correlations of the egg physical and eggshell quality attributes throughout the entire laying hen cycle.