Study on changes in egg quality traits and genetic parameters of white leghorn hens from 35 to 100 weeks of age

To enhance production efficiency, the egg industry increasingly extends the laying cycle of hens. However, this extension exacerbates the decline in egg quality during the late laying period. To improve egg quality throughout the extended laying period of laying hens, this study analyzed 11 egg quality traits: eggshell strength (ESS), eggshell thickness (EST), eggshell weight (ESW), eggshell ratio (ESR), egg weight (EW), yolk weight (YW), yolk ratio (YR), egg shape index (EI), Haugh unit (HU), yolk color (YC), and eggshell membrane thickness (ESMT) in 2,577 White Leghorn hens across 35, 40, 50, 60, 70, 80, 90, and 100 wk of age. Phenotype and genetic parameter variations were systematically evaluated. Results showed that ESS, YC, and ESMT had low heritability (0.05-0.25), while ESW, ESR, EST, EI, and HU exhibited moderate heritability (0.20-0.35). EW, YW, and YR had higher heritability (0.30-0.54). Heritability for EW, ESS, and HU decreased with age, while YW and YR showed increasing trends. Positive phenotypic and genetic correlations were observed among shell-related traits (ESS, ESW, ESR, EST) and between EW and YW. Conversely, YR showed negative correlations with ESW and HU. Cluster analysis grouped shell-related traits (ESR, EST, ESW, ESS) into three phases (35-50 wk, 60-70 wk, 80-100 wk) and egg-weight-related traits (EW, YW, YR) into three phases (35-40 wk, 50-60 wk, 70-90 wk). Egg quality traits were categorized into three classes: shell quality (ESS, ESW, ESR, EST), egg-weight-related traits (EW, YW, YR), and other traits (EI, HU, YC, ESMT). Weight analysis indicated higher weights for egg quality traits at 40-50 wk, suggesting this period may be more representative for selection. Among shell traits, EST had the highest weight (27.01 %), and ESS the lowest (19.85 %), prioritizing EST as a shell quality indicator. This study reveals dynamic patterns of egg quality traits during the 35-100-week laying cycle, providing insights for genetic improvement and trait simplification. These findings offer comprehensive insights into egg quality changes and genetic architecture during extended production cycles.