doi:10.5455/vetworld.2013.263-266 Genetic analysis of egg quality traits in White Leghorn chicken

Aim: The present study was undertaken to estimate the genetic parameters and to assess the inheritance pattern of egg quality traits in White Leghorns. Materials and Methods: Data on 480 eggs produced by 40 week old pullets of 4 genetic groups were used for studying egg quality traits. Heritabilities and correlations were estimated by full sib correlation method using Mixed Model Least Squares and Maximum Likelihood (LSMLMW) computer program. Results: The egg weight, haugh unit, yolk index, albumen index, yolk weight, albumen weight, shell weight and shell thickness ranged from 50.01 ± 0.48 to 53.89 ± 0.43 g, 65.38 ± 0.92 to 80.98 ± 1.01, 0.341 ± 0.003 to 0.353 ± 0.003, 0.056 ± 0.002 to 0.087 ± 0.002, 14.16 ± 0.13 to 15.58 ± 0.12 g, 30.92 ± 0.39 to 33.18 ± 0.39 g, 4.32 ± 0.05 to 5.12 ± 0.05 g and 0.336 ± 0.003 to 0.376 ± 0.003 mm, respectively. Heritability estimates for egg weight, yolk index, albumen index and albumen weight ranged from low to medium while those of haugh unit, yolk weight, shell weight and shell thickness ranged from low to high. The genetic and phenotypic correlations of egg weight with other egg quality traits except shell quality traits were mostly positive and moderate to high. High positive genetic and phenotypic correlations between haugh unit and other traits were observed. Genetic correlation of yolk weight with albumen weight was positive while that with shell quality traits was mostly negative. Shell weight was positively correlated with shell thickness. Conclusion: Significant genetic group differences were observed for various egg quality traits studied. The heritability estimates for different egg quality traits were low to moderate. The association among egg quality traits was positive in general.


Introduction
collected and used for studying external and internal egg quality traits.Each sire mated to two dams Poultry farming occupies a pivotal position in contributed to 6 progeny at the rate of three progeny per bringing about rapid economic growth.In India, during dam. the last five decades, poultry has evolved from subsis-Eggs were weighed individually to the accuracy tence farming to an integrated and highly commercial of 0.01 g.Thereafter, the eggs were broken and business oriented enterprise, the development of which albumen height, haugh unit (HU) etc. were measured was not only in size but also in productivity, using Egg Quality tester (EMT 5200, Japan).Length sophistication and quality [1].India is the third largest and width of thick albumen were measured at the producer of eggs and fifth largest producer of poultry longest point by Vernier Callipers with an accuracy of meat in the world, producing 55.64 billion eggs and 0.05 mm.Width of the yolk was measured twice at the about 648,900 tons of poultry meat in 2008 [2].The right angles to each other and averaged.Height of the overall quality of egg has been considered as the most yolk was measured at the highest part by Ames important trait by the producers and consumers as well micrometer with an accuracy of 0.1 mm.Yolk was for which continuous genetic evaluation of different separated from albumen and weighed nearest to 0.1 g. egg quality traits has become essential in the present The shells of broken eggs were dried at room market scenario.Therefore, this investigation was temperature before shell weight was recorded to 0.1 g taken up to estimate the genetic parameters and to accuracy.Shell thickness was measured nearest to 0.01 assess the inheritance pattern of various egg quality mm using a dial thickness gauge at two places on the traits in three strains and control population of White shell (at broader and at narrower ends) and averaged.Leghorns.
Albumen and yolk indices were determined as the ratio

Materials and Methods
of height to width of albumen and yolk as per standard procedures suggested [3,4].Albumen weight was A total of 480 eggs produced by 40 week old calculated by subtracting the total weight of the two pullets (120 from each of the three pure lines and one components (yolk + shell) from the egg weight.control line of White Leghorns) mated to 20 sires were Heritabilities and correlations were estimated by also reported in different breeds [14][15][16].full sib correlation method using Mixed Model Least Yolk index and albumen index were highest in Squares and Maximum Likelihood (LSMLMW) computer IWI strain (0.353±0.003 and 0.087±0.002,respectiprogram [5]. vely) and lowest in IWK strain (0.341±0.003 and 0.056 ±0.002, respectively).There were reports of higher

Results and Discussion
yolk index value (17) and higher albumen index value Phenotypic means: Significant differences (P<0.01)(11) and very lower albumen index values [7, 8 and 10] were observed among the three strains and control in comparison to the present findings.The weights of population with respect to various egg quality traits yolk, albumen and shell and their percent weights over (Table -1).The IWK strain had significantly higher egg the egg weight were measured and analyzed for weight (53.89 ± 0.43 g) when measured from other two comparison.The IWK strain had higher yolk, albumen strains and control population.The three strains in the and shell weights as expected on the ground that it had present study were obtained from different sources and higher egg weight, while percent weights varied in the with different selection criteria.Similar differences trend.Similar observations [10,15] were reported in were reported by earlier authors in White Leghorns White Leghorns, while higher percent shell weights [6][7][8][9][10][11]. Egg weight ranged from 50.01 ± 0.48 to 53.89 ± [12,[18][19][20] were observed in native / backyard poultry.0.43 g, which is in agreement with the published This suggests that the shells of White Leghorns reports [8,[11][12][13].Haugh unit, a measure of albumen contribute lesser proportion to the egg weight when quality was in the range of 65.38 ± 0.92 in IWK to compared to native / backyard poultry.In the present 80.98±1.01 in IWI strain, which is in accordance with study, the shell thickness ranged from 0.336 ± 0.003 in the published reports [6,[8][9][10][11].Higher haugh unit IWH to 0.376 ± 0.003 mm in IWI strain, which is values than those observed in the present study were comparable to many reports in literature [7,8,10,14].Heritability estimates are on diagonals; Genetic correlations are in above diagonals and phenotypic correlations are in below diagonals The reports of higher shell thickness values [13, 15, 16, percent yolk weight and percent albumen weight were 21] in backyard / other poultry could be due to their also reported [23].The genetic correlation of albumen higher percentage contribution to egg weight.
weight with shell weight was positive in all strains while it was negative with shell thickness except in Heritability Estimates: Egg weight was found to be control population.The phenotypic correlation between moderately heritable as the heritability ranged from these traits was varied.Shell weight was positively 0.180 ± 0.122 to 0.293 ± 0.022, similar to the earlier correlated with shell thickness in all the populations at reports [6,7,10] in White Leghorns.The heritability genetic, phenotypic and environmental levels.This estimate of haugh unit ranged from 0.217 ± 0.177 to indicates that genetic or environmental factors 0.322 ± 0.224 across the populations studied, which is affecting shell weight would bring about simultaneous in agreement with the observations of different earlier change in shell thickness, since both of them constitute authors [7,8,10,11].The heritability estimates of the shell strength.High positive genetic and phenotypic albumen and yolk indices in some populations could correlations (0.792 and 0.610, respectively) between not be obtained probably because of small population shell percent and shell thickness were also reported size.Yolk index and albumen index were found to be [23].low to moderately heritable.Reviewed literature had Conclusion varied results ranging from 0.01 [17] to 0.70 [7].The heritability estimate of yolk weight and albumen Significant genetic group differences were weight were in the range of 0.097 to 0.721 and 0.124 to observed for various egg quality traits studied.The 0.250, respectively.Available literature on these estimates heritability estimates for different egg quality traits is very scanty except one report of similar heritability were low to moderate.The association among egg estimates for these traits [10].The heritability quality traits was positive in general.Further studies estimates of shell weight and shell thickness were are required in finding out the association of external found to be low to highly heritable which ranged from and internal egg quality parameters so as to take 0.161 to 0.419 across different strains, indicating that appropriate breeding plans for producing customer selection could be applied for the improvement of shell friendly quality eggs.quality.These findings corroborate with the previous reports [7,10,17].

Authors' contribution
SD, GPM and RC designed and implemented the study.Correlations: The genetic and phenotypic correlations SD, GPM recorded and analysed the data.SD drafted of egg weight with egg quality traits were mostly the manuscript.SD, GPM, MM and RC revised the positive and moderate to high in magnitude while they manuscript.All authors read and approved the final varied with shell quality parameters.Similar positive manuscript.correlations of egg weight with other egg quality traits in White Leghorns were reported [17,23].As haugh unit is a measure of albumen height corrected for egg The first author is thankful to the Sri Venkateswara weight, the high genetic (0.375 to 0.912) and phenotypic Veterinary University for permitting the study and to correlations (0.726 to 0.914) with albumen index are the officials and staff of the Project Directorate on not surprising.The correlations of haugh unit with Poultry, Hyderabad for providing facilities during the other traits were also positive and moderate to high in study.magnitude.These findings are in agreement with the report of very high (0.895) genetic correlation with In contrast, highly negative correlations between 6. Chatterjee, R. N., Sharma, R. P., Niranjan, M., Reddy, B. L.
Competing interests albumen index and varied correlations with other traitsAuthors declare that they have no competing interest.[23].The genetic correlations of Yolk index with albumen index, yolk weight and albumen weight were rable to earlier reports [17,23].The genetic correlation natural position to the yolk index as determined after of yolk weight with albumen weight was positive in all separating the yolk from albumen.Poultry Science 27: 376-380.strains while that with shell weight and shell thickness 5. Harvey, W. R. (1990) User's guide.LSMLMW and MIXMDL was negative.Phenotypic correlation with albumen PC-2 version.Mixed model least-squares and maximum weight, shell weight and shell thickness was positive.likelihood computer programme.Ohio, U.S.A.

Table - 1
. Least squares means for egg quality traits in White Leghorns

Table - 2
. Heritability and correlation coefficients for egg quality traits in different strains of White Leghorns