Brown eggshell fading with layer ageing: dynamic change in the content of protoporphyrin IX

The dynamic change in brown eggshell color as hens age has been observed, but much uncertainty still exists. We aimed to analyze the depth of eggshell color and quantity of protoporphyrin ΙΧ throughout the laying period to explore the reasons for color variation. In this study, 120 Rhode Island Red hens were used, and two eggs were collected from each individual at 26, 34, 42, 50, 60, and 70 wk of age. The eggshell color (L*, a*, b*), egg weight, eggshell dry weight, and protoporphyrin ΙΧ quantity in eggshell were measured for individual eggs. Our results showed that the intensity of brown eggshell color weaken as hens aged from 26 to 60 wk of age (L* gradually increased from 61.43 to 68.07), while eggshell lightness recovered slightly at 70 wk (L* = 64.77). The correlation analysis indicated that the content of protoporphyrin ΙΧ deposited in the eggshell was an important factor for lightness fading with the ageing process (the average r was 0.66, P < 0.01), while the egg weight had little impact on the eggshell color (the average r was 0.07, P > 0.05). The shade of the eggshell color (L* and a*) at the early laying period (26 or 34 wk) had a low correlation with the other age points (42, 50, 60, and 70 wk). However, high correlations between the shell color at 42 wk of age and subsequent ages (50, 60, and 70 wk) were found, suggesting that the intensity of eggshell color is more stable after egg-laying peaks (such as 42 wk of age). In conclusion, the intensity of brown eggshell color varies greatly among the whole laying cycle and breeders can choose the proper age for eggshell color measurements to ensure the degree of shell color in the late laying period.