Genetic analysIs of egg shell-breakage, body checking and specific gravity in pure-and cross-line White Leghorn

Two long-term selected strains of White Leghorn (PL1 and PL2), their reciprocal crosses (PL12 and PL21) and a random bred control line comprising 1626 birds were evaluated for egg shell breakage, body checking, shell quality and egg production traits at 2 stages viz., 34–38 and 68–70 weeks of age based on a total of 81, 300 eggs. Incidence of shell breakage and body checking showed a significant increase while egg specific gravity showed a decline with the advancement of age. Strain PL2 exhibiting lower incidence of shell breakage and body checking was characterized by higher ega specific gravity, egg weight, body weight and egg production as compared to the selected strain PL1. Selection for egg production and egg weight in PL2, however, did not affect shell breakage and body checking as indicated by its comparison with the unselected control line. Comparison of pures and crosses did not reveal any heterosis. Heritabilities varied between 0.08 and 0.195 for shell breakage and between 0.05 and 0.28 tor body checkinig. No consistent and large difference in heritabilities between pures and crosses was evident. The genetic correlations between the same trait at early and late stage were positive and high but much smaller than unity (0.46–0.97). Selection for the improvement of late-year shell breakage and shell quality therefore needs to be conducted at the late stage.