The effect of the laying hen’s age on some egg quality traits and their correlation and coefficient of determination were investigated. Three hundred, 22 - 38 weeks old ISA-Brown laying hens reared in battery cages under confined system were used. Sixty eggs were randomly collected nine times at different ages, 30 eggs in morning hours and a similar number in late afternoon. Egg shape index, egg weight, yolk weight, albumen weight, yolk to albumen ratio, shell weight and shell thickness were examined. Correlation and coefficient of determination were calculated using linear equation. The effect of laying hen’s age on all egg quality traits showed a significant (p<0.01) treatment influence. Moreover, correlation and coefficient of determination between age and egg quality traits showed a significant (p<0.01) treatment effect. Laying hen’s age determines egg quality traits as indicated by the positive correlation and hence it is highly recommended in selection programs for genetic improvement.
The quality of the hen’s egg is of primary concern to the producers as it contributes for better economy price of fertile and table eggs. The success of a production flock is dependent on the total quantity of qualitative eggs produced (Monria et al., 2003). Hence, in breeding industry, high egg quality is of enormous importance (Bain, 2005). This is because the embryonic development depends on some egg quality traits (Onagbeson et al., 2007). Evidence shows that consumers usually prefer table eggs of better internal and external quality for eggs acceptability (Stadelman, 1977). Egg quality could provide a convenient means of assessing stress in laying hens (Reynard and Savory, 1999). However, a number of factors including age of the laying hen were reported to influence the quality of the hen’s egg (Anyaegbu et al., 2016). Egg shape index is an important egg quality trait to poultry breeders due to its significance in estimating air cell volume (Tserveni-Gousi and Yannakopoulus, 1995). It declines as the laying hen’s age progresses (Van Den Brand et al., 2004, Suk and Park, 2001). Egg weight was reported as a function of the laying hen’s age (Gilbert et al., 1978) and generally it increased with the hen’s age (Rizz and Chiericato, 2005, Van Den Brand et al., 2004, Ledur et al., 2002, Sliversides and Scott, 2001). Egg weight could be predicted as the sum of yolk, albumen and shell. The weights of yolk and albumen increased with age (Johnson and Gous, 2007a), but yolk increased at a faster rate than albumen (Johnson and Gous, 2007b). Consequently, yolk to albumen ratio increased with the progress of the laying flock’s age (Rizz and Chericato, 2005, Van Den Brand et al., 2004, Suk and Park, 2001).Yolk to albumen ratio became of primary importance to food processors as the demand for liquid eggs increased (Hussein et al., 1993).
Shell weight associated positively with egg weight and increased with age during the first cycle of egg production while, it is deteriorated during the second cycle of egg production (Suk and Park, 2001). However, shell weight generally increased with age (Eva and Levinka, 2009) while, shell thickness decreased with age (Sliversides and Scott, 2001, AL-Batshan et al., 1994, Roland, 1978). Eggshell quality seemed to be a major concern to commercial egg producers based on the magnitude of broken and weak shell (Shawk et al., 1990). The knowledge of correlation among productive traits was imperative for construction of selection indices to maximize the rate of genetic improvement (Gehan and Ensaf, 2012). It was helpful in determining the direction of selection since the selection for one trait might affect positively or negatively the performance of other traits (Mondal et al., 2006). The magnitude of correlation is an indication of how closely linear the variables under study. Another value of correlation is the coefficient of determination that has provided relationship between variables (Fajemilehin, 2017). However, genotype and the laying hen’s age seemed to be the most important factors influencing egg quality traits (Hassan, 2009). Hence, the aim of this study was to investigate the effect of laying hen’s age on some egg quality traits, and their correlation as well as coefficient of determination.
Materials and Methods
Three hundred ISA-Brown laying hens, 22-38 weeks old, reared in layer battery cages under confined system in a 16- lighting and 8- darkness hours were used. Temperatures of 20-23 were maintained and relative humidity stood at 60%. Feed and water were provided ad-libitum. A typical layer diet was fed to the experimental birds. Sixty eggs were randomly collected nine times at one-week interval. At each time, 30 eggs collected in early morning hours and a similar number of eggs in late afternoon. Egg shape index was calculated (Amer, 1959) as follows-
Egg shape index (Egg width Egg length) 100
Using an electronic balance, each individual egg was weighted. By separating the yolk from the albumen, yolk weight was then determined. The shell weight was determined after washing the shell to remove any residual albumen (Shawket, 1990) and dried at room temperature for three days (Amer, 1959). Albumen weight was calculated (Ahn et al., 1997) as follows-
Albumen weight= (Total egg weight) (Yolk weight Shell weight)
Shell thickness was measured using a micrometer, after carefully removing the dried shell’s inner membranes. The measurements were made at the blunt end, the middle and the pointed end of the egg. Shell thickness was then calculated as an average of three readings (Shawket, 1990).
Data collected were managed and analyzed using analysis of variance (ANOVA) that included in SPSS version 16.0 (2007). Means between the variables were compared using Fisher’s Least Significant Difference (LSD) test at 1% level. All results were presented as mean standard error. The values of correlation (r) and coefficient of determination (r²) between age and egg quality traits were evaluated using Pearson’s Correlation Analysis at 1% level.
Results and Discussion
The results of the effect of the laying hen’s age on the egg quality traits are shown in Table 1 while the correlation and the coefficient of determination values between the age and the traits are shown in Table 2.
Table 1: Effect of the Laying hen’s age on egg quality traits
|Age (weeks)||Shape Index* (%)||Egg Weight*(g)||Yolk Weight*(g)||Albumen Weight*(g)||Yolk to Albumen*(ratio)||Shell Weight*(g)||Shell Thickness*(mm)|
|22||78.69ª±1.14||51.82ⁱ ±0.55||12.29ⁱ ±0.10||33.85ⁱ±0.61||0.363ʱᵍ ±0.02||5.62ⁱ ±0.15||0.352ª±0.02|
|24||78.55ᵇ±1.16||53.96ʱ ±0.93||12.84ʱ±0.14||35.24ʱ±1.01||0.364ᵍ ±0.02||5.82ʱ ±0.12||0.349ᵇ ±0.02|
|26||78.39ᶜ ± 1.18||55.52ᵍ ±1.04||13.26ᵍ±0.17||36.24ᵍ±1.01||0.366ᵉᶠ±0.02||5.95ᵍ ±0.14||0.347ᵇᶜ ±0.02|
|28||78.20ᵈ ±1.19||56.55ᶠ ±1.61||13.56ᶠ ±0.28||36.89ᶠ ±1.44||0.367ᵉ ±0.02||6.03ᶠ ±0.11||0.346ᶜᵈ ±0.01|
|30||77.99ᵉ±1.21||58.03ᵉ ±1.48||13.95ᵉ±0.24||37.85ᵉ±1.38||0.369ᶜᵈ±0.02||6.16ᵉ±0.14||0.345ᶜᵈᵉ ±0.01|
|32||77.78ᶠ ±1.23||59.71ᶜᵈ ± 1.84||14.39ᶜᵈ±0.30||38.94ᵈ±1.65||0.370ᶜᵇ±0.02||6.30ᵈ±0.11||0.344ᵈᵉᶠ±0.01|
|34||77.58ᵍ ±1.26||60.32ᶜ ±2.08||14.57ᶜ±0.34||39.33ᶜ ±1.83||0.371ªᵇ ±0.02||6.35ᶜ ±0.09||0.344ᵈᵉᶠ±0.02|
|36||77.39ʱ ±1.27||61.18ᵇ±2.07||14.81ᵇ ±0.33||39.88ᵇ±1.85||0.372ª±0.02||6.42ᵇ ± 0.11||0.343ᵉᶠᵍ ±0.01|
|38||77.23ⁱ ±1.29||61.94ª±1.98||15.01ª ±0.30||40.38ª ±1.81||0.372ª ±0.02||6.49ª±0.13||0.342ᶠᵍʱ±0.01|
*Significant difference (p<0.01); a-i: Means within a column not sharing common superscripts letters are significantly different at 1% level.
Egg shape index was significantly (p 0.01) influenced and showed a decline (78.69 with the advancing age. This result supports the findings of Van Den Brand et al. (2004), Suk, and Park (2001). The decrease in egg shape index with age indicates that eggs become elongated in shape as the hen ages. The possible explanation for this result could be based on the early findings of Asmundson and Baker (1940) who had documented that with advancing hen age, egg weight increased in volume and when forced through the narrow isthmus of the oviduct it became elongated.
Table 2: The correlation (r) and coefficient of determination (r²) values between the Laying Hen’s Age and the Egg quality Traits
|Traits||Correlation (r)||Coefficient of Determination (r²)||Significance|
|Egg shape index||0.4||0.15||*|
|Yolk: albumen ratio||0.39||0.15||*|
*Significant difference (p 0.01)
The correlation analysis relating egg shape index to age was positively significant (p 0.01), but weak (0.40), suggesting a weak association between them. Furthermore, based on coefficient of determination value (0.15), the contribution of the age on the relationship between age and egg shape index was only about 15%. Effect of laying hen’s age on egg weight showed a significant (p 0.01) treatment. Egg weight revealed an increase with increasing age of the laying hen (51.82 0.55 61.94 g). Numerous investigators including Rizz and Chiericato (2005), Van Den Brand et al. (2004), Ledur et al. (2002), Sliversides and Scott (2001) have observed an increase in egg weight as the age advances. Hence, an egg weight increased proportionally to the increase in age of the laying hen. Moreover, the egg weight seemed to be a function of laying hen age (Gilbert et al., 1978). The correlation analysis between the hen’s age and egg weight revealed a significant (p 0.01) positive relationship as egg weight was highly (0.90) correlated with age. The coefficient of determination value (0.80) shows thats the contribution of age on the relationship between age and egg weight is almost 80%, suggesting that egg weight is more or less closely associated with laying hen’s age.
The influence of the laying hen’s age on yolk weight revealed a significant (p 0.01) treatment increase. Yolk weight was increased (12.29 – 15.01 0.30 g) as the hens aged. Apparently, this result is similar to the findings of Johnson and Gous (2007a), Hussien et al. (1993), Zhirong and Jeong (1991). However, the increase in yolk weight could be based on that yolk weight is equally dependent upon increasing flock age or increasing egg weight (Fletcher et al., 1983). The correlation analysis between age of laying hens and the yolk weight showed a significant (p 0.01) positive association. The correlation value (0.95) showed a strong association between age and yolk weight, whereas, the coefficient of determination value (0.91) emphasized high age contribution on its relationship with yolk weight.
Albumen weight was significantly (p 0.01) influenced by the age. As the age of flock advanced, albumen weight showed an increase (33.85 0.61 – 40.38 1.81 g) and this observation is in line with results obtained in similar studies (Johnson and Gous, 2007a, Hussien et al., 1993, Fletcher et al., 1983). Previous result of age and egg weight might justify the increased albumen weight with advancing flock age based on the findings of Hussien et al. (1993) who pointed out that albumen weight is directly proportional to egg weight. The correlation analysis indicated that albumen weight was significantly (p 0.01) positively associated with age. As observed, the correlation value between age and albumen weight (0.83) was lower than that between age and yolk (0.95). Johnson and Gous (2007b) concluded that yolk weight increases at a faster rate than the albumen weight. This conclusion had provided a support to such an observation. The coefficient of determination value (0.68) explains a fair contribution of age in its relationship with albumen weight of 68 % compared to the coefficient determination between age and yolk (0.91).This provides additional support to the above observation.
Yolk to albumen ratio was influenced by the laying hen’s age and revealed a significant (p 0.01) result. Yolk to albumen ratio increased progressively as the age of the laying hens increased (0.363 0.02 – 0.372 0.02). It is in line with results of similar studies reported by Rizz and Chericato (2005), Van Den Brand et al. (2004), Suk and Park (2001), Ahn et al. (1997), Hussien et al. (1993). The increase in the ratio is likely due to the faster yolk increase than the albumen as the age of the laying advances (Johnson and Gous, 2007b). The correlation analysis between the age and the ratio indicated a positive significant (p 0.01) association. The correlation between the two variables (0.39) seemed to be weak. The coefficient of determination value (0.15) implies that the contribution of age on its relationship with the ratio is only 15%.Such result was expected since yolk to albumen ratio is dependent on the volumes of yolk and albumen as the former increases at a faster rate than the latter (Johnson and Gous, 2007b).
The influence of the hen’s age on shell weight indicated a significant ( p 0.01) treatment effect. Shell weight was found to increase with increased age (5.62 0.15 – 6.49 0.13 g). This study confirms the previous findings of Eva and Levinka (2009), Suk and Park (2001). In this study, young birds in their first cycle of egg production were used to justify the increased shell weight with age (Suk and Park, 2001). The correlation analysis between age and shell weight showed a positive significant (p 0.01) relationship. Shell weight strongly (0.91) correlated to age and the contribution of age on its relationship with shell weight was almost 83% as the coefficient of determination value (0.83) implies. Considering that, egg weight is a function of laying hen’s age (Gilbert et al., 1979), the relatively high positive correlation between shell weight and egg weight during the first cycle of egg production was documented by Roland (1979), Suk and Park (2001). This could justify the high correlation values between age and shell weight obtained in this study.
Shell thickness revealed a significant (p 0.01) treatment difference with age. Shell thickness was found to decrease with advancing age (0.352 0.02 – 0.342 0.01 mm).This result is in line with the findings of Sliversides and Scott (2001), AL-Batshan et al. (1994), Roland (1979).The decline in shell thickness with advancing age could be explained by a limited shell amount to spread over large surface area. The correlation analysis explained the relationship between age and shell thickness was shown to be positively significant (p 0.01) and fair (0.69), indicating that the two variables were not well associated. Nevertheless, the coefficient of determination value (0.47) in one way or another has highlighted such a weak relationship between age and shell thickness.
This study has concluded that laying hen’s age determines egg quality traits as indicated by positive correlation values. Hence it is highly recommended in selection programs for genetic improvement of layers.
Conflict of Interest
The authors declare no conflict of interest.