Genetic inference from comparison of Pure-and Cross-line white leghorns for egg shell quality and economic traits

A total of 2675 pure and cross- line pullets were assessed for egg shell quality, shell breakage and egg body checking between 36 and 40 weeks of age comprising 82,283 eggs. Eggs of crosses had higher specific gravity (1.089 vs 1.087) and lower incidence of shell breakage (2.61 vs 3.06%) and body checking (1.28 vs 1.42%) with heterosis being 1.4, 14.7 and 9.9% respectively. Cross- line pullets also had significantly better performance than pures for egg production traits but heterosis for these traits was lower than for egg shell quality traits. Crosses had higher heritability than the pures for specific gravity (0.63±0.15 vs 0.31±0.07) and body checking (0.31± 0.11 vs 0.09±0.03). However reverse trend was evident for shell breakage (0.11±0.04 for pures vs. - 0.01±0.07 for crosses). Specific gravity had high negative genetic correlation with shell breakage (− 0.85±0.06 and - 0.68±0.10) and positive genetic correlation with body checking (0.49±0.15 and 0.13±0.15) in pures and crosses. Shell breakage and body checking had low positive phenotypic, genetic and environmental correlations. Egg production had negative genetic correlation with specific gravity in both pures and crosses. Of the various possible genetic correlations of shell traits with economic traits, the obvious and significant ones were between egg weight and body checking (positive), and between 20-week body weight and shell breakage (positive). Results indicated that selection for egg production is expected to reduce shell quality which may be improved through mass selection for egg specific gravity.