Influences of Shell Color, Genetic Background and
Hen Age on Eggshell Quality Traits of Chicken Eggs
The purpose of this study was to realize the influences of eggshell color, genetic background and hen age on eggshell quality traits. The egg traits studied were egg weight, shape index, shell weight (%), yolk weight (%), albumen weight (%), Haugh unit, shell membrane thickness, shell color measured by L a b color system and force-deformation curve measured by Instron testing machine. The eggs used in the first experiment were laid by hens of three genetic stocks, i.e., blue eggshell stock (Blue), a commercial Single Comb White Leghorn (WL) and their crossbreed offspring (Blue*WL) at 40 and 60 weeks of age. In addition, eggs laid by Blue and Blue*WL were classified by naked eye into different eggshell color phenotypes, i.e., blue, green, brown, tinted and olive. The eggs used in the second experiment were laid by the four long-term selection strains of Taiwan Country chicken at the National Chung Hsing University, i.e., L2 and D strains selected for egg production and B and S selected for growth performance. The eggs were laid at 30, 40, and 50 weeks of age. In the third experiment, eggs tested were laid by six native breeds, i.e., Hsin-Yi, Hua-Tung, Ju-Chi, Quamoy breeds in Taiwan, Shek-Ki breed from China and Nagoya breed from Japan, at 30, 40, and 50 weeks of age.
Results of Experiment 1. Eggs laid by Blue egg shell stock were much smaller with lower Haugh unit than those laid by the commercial Single Comb White Leghorns. However, their eggs had larger yolk weight (%) and higher eggshell strength at least at 40 weeks of age. Comparing eggshell quality of different color phenotypes within Blue eggshell and Blue*WL stocks revealed that brown-eggshell color and olive-eggshell color eggs in Blue eggshell stock had higher eggshell weight (%) and eggshell breaking strength than other eggshell color phenotypes, and the worst shell quality was found in blue-eggshell color phenotype. Similar result was found in Blue*WL stock, where the worst eggshell breaking strength was also found in the blue-eggshell color eggs. In addition, correlation coefficients between shell color, measured by L*, a* and b*, and eggshell strength also suggested that darker eggshell and yellow-brown eggshell eggs have a better eggshell breaking strength than the lighter ones.
Results of Experiment 2. Eggs laid by Strains L2 and D had lighter egg weight and lower eggshell breaking strength than those laid by Strains B and S. These results suggested that the genetic progress made by the selection for egg production in Taiwan Country chicken may have been on the cost of egg size and eggshell strength.
Results of Experiment 3. Comparison of egg traits among six different native chicken breeds revealed the presence of breed difference. Significant breed differences were found in shape index, egg weight, Haugh unit, albumen weight (%), yolk weight (%) and eggshell breaking strength, but not found in shell weight (%) and membrane thickness. Eggs laid by Hua-Tung had the heaviest egg weight and highest albumen weight (%) while the lowest was laid by Shek-Ki. The highest Haugh unit was found in egg laid by Nagoya and the lowest in Hua-Tung. The best eggshell breaking strength was found in eggs laid by Nagoya and Shek-ki, while the worst was in Hsin-Yi and Quemoy. Egg qualities of Nagoya were better than other breeds with higher eggshell strength breaking and Haugh unit. Eggshell breaking strength of Hsin-Yi and Quemoy was lower than other breeds.
As the hen aged, their eggs increased the size but without an increase of egg shell which resulted in a decrease of shell weight (%). In addition, the shell membrane thickness also decreased, therefore, the egg shell breaking strength decreased at older age.
Keyword: age; eggshell color; eggshell quality; genetics; Taiwan Country chicken