Linear and threshold analysis of proportion of cracked eggs in laying hens using an animal model

The genetic background of proportion of cracked eggs was studied within the monthly periods. Records of 1,604 hens from two strains were analyzed under linear and threshold animal model. The AIREML algorithm was applied. The proportion of cracked eggs was relatively high in the beginning of egg production and decreased then to almost zero and showed a regular increase since the fifth month. Heritability was higher in the post-peak period than in the beginning of egg production. Heritability estimates of total proportion of shell cracks obtained under the threshold model (0.62 and 0.50 for M55 and V66 strains) were higher than from the linear model (0.09 and 0.07 for M55 and V44 strains). The threshold model is suggested to be more adequate than the linear model.