Worldwide, more than 7 billion hens are laying around 1.3 trillion eggs per year1. In addition, from 1960 to 2010, the worldwide egg production has shown an increase of 30%2. Thus, egg quality have gained great relevance in the egg production process. Many egg quality characteristics directly affect consumer’s acceptability3. Egg quality comprises a number of aspects related to the shell, albumen and yolk and can be divided into external and internal quality4. Both the reproductive process and consumption have a strong connection with egg quality characteristics. Altinel et al.5 reported that external and internal egg quality characteristics are relevant in poultry breeding, because of their effects on reproductive output and growth of progeny. Regarding egg conservation, It has been reported that 8% of egg production break during transport from farms to consumers6, these and the number of eggs cracked are a serious economic issue for both breeders and distributors7. Understanding correctly egg quality characteristics may diminish the rate of loss during commercial processing, considerably8. Previous studies have shown relationship between external and internal egg quality characteristics and these relations are affected by age, genotype of hen, type of rearing system and nutrition9,10. Therefore, it is important to pay attention to these characteristics in order to maintain the quality and avoid problems of preservation and marketing of eggs3.
Past investigations stated that the age of the bird has a direct effect on egg quality characteristics11 and may change these characteristics in size, weight and ratio9. External egg characteristics are strongly affected by age of hens. Average egg weight increases with the increase of the breeder age12,13. Silverside and Scott14 reported a negative correlation between age of hen and shell quality characteristics. Shell quality decreases with advancing age, which is presumably related to the increasing of egg size and egg surface area15. Other studies informed that young hens produce eggs with shell thicker than old hens8. Increasing the age of hen not only affects the external egg characteristics but also has an impact on internal quality, which decreases14,16. Egg weight elevation increases both yolk and albumen weight. However, there is a greater proportion of yolk and a smaller proportion of albumen noted with the increase of the breeder age13,17. Lapão et al.18 indicated that albumen height was affected by age of hen, from 8.80-8.30 for 32 and 59 weeks breeder age, respectively. In addition, Haugh unit presented a reduction with the increase of age of hen, from 88.6-82.1 at hens for 35 and 45 weeks of age, respectively12. Furthermore, Hy-Line19 has indicated that hen-egg rate of Hy-Line Brown laying hens diminishes until 75% or less since 80 weeks of age. The study reported herein was conducted to determine the phenotypic correlation between external and internal egg quality characteristics in 85-week-old laying hens, establishing the implications of these correlations on egg quality and production at this hen stage.
MATERIALS AND METHODS
Biological materials: A total of 288 eggs from 14485-week-old Hy-Line Brown laying hens belonging to the Poultry Experimental Unit of the Animal Science Department, Universidad Nacional Agraria La Molina were used. All hens were raised in the same conditions (temperature, humidity, feed distribution, feed time, water disposition, etc.). The eggs used for the present study were sampled within a 3-week period, one day at each week samples was taken and all egg laid during 24 hours of the day were evaluated. (90, 97 and 101 eggs at 85, 86 and 87 weeks of age, respectively). The eggs collected were evaluated using various egg quality tests for both external and internal characteristics.
Egg quality determination: All collected eggs were labeled with consecutive numbers in order to trace every egg during the whole quality evaluation process. Egg quality evaluation process started with the evaluation of egg weight (EW), for this purpose an electronic centesimal scale with a 300 g capacity was used. Then, a digital caliper was used to measure egg length (EL) and width (EG). Finally, a densimeter, 10 buckets of 20 L capacity and salt (saline solution technique) were used to determine specific gravity as described by Bennett20. Subsequently, the eggs were broken on a smooth plastic platform (previously calibrated) and the albumen and yolk weights, lengths and heights (albumen weight (AW), albumen length (AL), albumen height (AH), yolk weight (YW), yolk length (YL) and yolk height (YH), respectively) were determined using a digital caliper. Afterward, the shells were washed and stored in a plastic container (10×10 cm) for drying at room temperature (21.5°C). Finally, after 3 days, shell weight (SW) and shell thickness (ST) were measured.
Measured egg quality characteristics data were used to calculate some external and internal egg quality characteristics. These calculated characteristics were estimated using equations obtained from Kul and Seker6, Romanoff and Romanoff21, Paganelli et al.22, Singh23, Alkan et al.24 and Debnath and Ghosh25.
External egg characteristics:
where, EG and EL are the egg width and length, respectively.
Egg surface area (ESA, cm2) = 3.9782×EW0.75056
where, EW is the egg weight (g).
SW = Shell weight (mg)
ESA = Egg surface area (cm2)
Internal egg characteristics:
AH = Albumen height (mm)
DAL = Dense albumen length (mm)
DAG = Dense albumen width (mm)
where, AW and EW are the albumen (g) and egg weights (g), respectively.
Haugh unit (HU) = 100 ×log (AH-1.7EW0.37+7.6)
AH = Albumen height (mm)
EW = Egg weight (g)
YH = Yolk height (mm)
YL = Yolk length (mm)
YG = Yolk width (mm)
where, YW and EW are the yolk and egg weight, respectively.
where, YW and AW are the yolk and albumen weights (g), respectively.
Statistical analysis: Data were analyzed using a one-way ANOVA to obtain residuals. Anderson-Darling test was used to analyze the normality of the residuals of all variables evaluated using Minitab 16 Statistical Program26. Correlation analysis of egg characteristics was obtained with Pearson product-moment correlation coefficients (PCC) using SPSS 20.0 computerpackageprogram27. p<0.05 and p<0.01 were considered significant and highly significant, respectively. The coefficients obtained from the Pearson correlation analysis were interpreted as indicated in Table 1.
Descriptive statistic: Table 2 presents the descriptive statistics obtained from the data on external and internal egg characteristics. The average EW, EL, EG, SW, ST, SR, ESI, ESA, U and Specific gravity were 69.79 g, 61.36 mm, 44.81 mm, 6.28 g, 0.35 mm, 9.01%, 73.14%, 96.24 cm2, 65.24 mg cm-2 and 1.088, respectively. In addition, the average AW, AL, AG, DAL, DAG, AH, AR, AI and HU were 43.19 g, 124.7 mm, 101.9 mm, 92.46 mm, 80.75 mm, 7.10 mm, 61.81%, 8.39% and 80.48, respectively. Furthermore, the average YW, YL, YG, YH, YR, YI and Y:A were 17.17 g, 43.02 mm, 40.93 mm, 15.46 mm, 24.64%, 36.88% and 40.08%, respectively.
Phenotypic correlations among external egg characteristics: Table 3 shows the phenotypic correlations between external egg characteristics. A highly significant, weak and negative (p<0.01; -0.15) phenotypic correlation was found between EW and ESI. Likewise, a significant, weak and negative phenotypic correlation (p<0.05; -0.15) was found between EW and SR. In addition, EW and specific gravity showed a negative phenotypic correlation of -0.07. The phenotypic correlation between EW and SW was medium, positive (0.54) and highly significant (p<0.01), while that between EW and ST was not significant (p>0.05; 0.11).
We found weak, negative phenotypic correlations of EL with SR (-0.26) and specific gravity (-0.25); both correlations being highly significant (p<0.01).Correlations of EL and EG with ESI showed different results: EL and ESI showed a strong negative phenotypic correlation of -0.79, while the EG and ESI showed a medium positive phenotypic correlation of 0.38.
The phenotypic correlation between SR and ESI was weak and positive (0.17), while that between SR and ESA was weak and negative (-0.15); both correlations being highly significant (p<0.01). In addition, specific gravity showed a medium positive phenotypic correlation with SW (0.70) and ST (0.69). In addition, it showed a considerable positive phenotypic correlation with SR (0.88) and U(0.87). Both correlations were highly significant (p<0.01).