Comparison of Different Models for Estimation of Heritability of Egg Quality Traits in Khorasan Razavi Native Fowl

Introduction Egg quality related traits are economically important in laying hens Egg quality is one of the important factors in the process of hatching. So, economical success of commercial and local flocks of hens depends on the quality of eggs produced. Increased egg quality results in higher marketability of the egg. Having eggs with higher quality is possible through improving nutrition, management and genetics. Understanding genetic structure of these traits with high accuracy will help us to design a desirable breeding program. Maternal effects can be caused by genetic or environmental differences between mothers or by the combination of the genetic and environmental differences. Advantages of Bayesian technique as a method of choice would be a promising method for providing high accurate genetic parameters estimations and having eggs with higher quality in next generations.
The purpose of current study was to estimate and compare variance components and heritability for egg quality traits in Khorasan Razavi native fowl using different animal models.
Materials and Methods The records for egg quality traits were collected from native fowl of Khorasan Razavi breeding center located in east north of Iran. In this experiment, 1000 eggs from 775 hens of 9th generation at the age of 28 to 29 weeks were collected and measured for internal and external traits. An electronic scale with an accuracy of 0.01 g was used to weigh the eggs (EW). The short and long lengths of each egg (SL and LL, respectively) were measured using Egg Form Coefficient Measuring Gauge. The eggs were broken using an Egg Shell Strength Tester to measure shell strength (SS). The height of yolk and albumen (YH and AH, respectively) were measured using a tripod micrometer (calibrated in mm) and a dial caliper to the nearest 0.01 mm was used to measure albumen and yolk diameters (AD and YD, respectively). Subsequently, yolk and albumen were carefully separated and yolk weight (YW) and albumen weight (AW) were measured. Shell weight (SW) was measured after 72 hours’ exposure to dry air. Shell thickness (ST) was measured with a Shell Thickness Meter (calibrated in mm) at the pointed end, equator and blunt end of shells and average values were used. These traits were evaluated by six different animal models through Bayesian method using Gibbs3f90 software. The most suitable model was determined by deviance information criterion (DIC) for each trait.
Results and Discussion The mean value of egg weight in this local breed was 49.66 gr. The mean value for specific gravity in present study was 1.089. Specific gravity is an important indicator to determine the quality of shell and the amount of shell to the other members. The mean values of shape index, shell strength, shell weight and shell thickness obtained in this study were 76.92, 4.24 kg/cm2, 5.19 g and 0.43 mm, respectively. The mean values of albumen weight, albumen height, yolk weight and yolk height were 28.11 g, 6.41 mm, 14.07 g, and 17.53 mm, respectively. For egg weight, specific gravity, egg length, shape index, yolk diameter and Haugh unit, a model consisted of maternal permanent environmental effects in addition to direct genetic effects was the most suitable. For egg width, shell strength, shell thickness, shell weight, yolk weight and yolk height, model including maternal genetic and permanent environmental effects in addition to direct genetic effects was the optimal model. For albumen index and albumen diameter, only direct genetic effects were affective. The estimates of direct heritability were from 0.08 (albumen height) to 0.28 (egg weight and shell weight) and maternal heritability ranged from 0.03(yolk index) to 0.13 (yolk height and albumen height). Observed differences in genetic and non-genetic parameters estimations determined by different models indicated that model choice is an important aspect for obtaining accurate estimates, which are going to be used when deciding on a breeding scheme. Generally, this study indicated that considering maternal effects in the models resulted in unbiased estimations of direct genetic variance and heritability for most of the studied traits.
Conclusion It can be concluded that all egg quality traits in Khorasan Razavi native fowl are influenced by maternal genetic and environmental effects. Therefore, including maternal effects in statistical models is essential for estimation of genetic parameters; the models included direct and maternal effects result in more accurate genetic parameters estimations for most of the studied traits.