Genetic variation of egg production traits in purebred and crossbred laying hens

Heritabilities, dominance variation and genetic correlations (rpc) among purebred and crossbred performance were estimated for egg production (number of eggs produced between 19 and 25, 26 and 38 and 26 and 54 weeks of age) and egg quality traits (average egg weight, shell strength) in four generations of two nucleus lines of egg-laying chickens and their cross, all reared in similar environments. The within-line genetic parameters were estimated using method R applied to an animal model (approach 1) and tilde-hat approximation to restricted maximum likelihood applied to a sire-dam model (approach 2). The genetic correlation between purebred and crossbred performance as well as the crossbred heritabilities were estimated based on a multivariate sire-dam model accounting for all relationships. For egg numbers and shell strength, the purebred heritabilities were low to moderate (0·12 to 0·42). They were higher when estimated under an additive model (0·25 to 0·51) but, in general, lower than the crossbred heritabilities. For egg weight, the heritabilities were always high (0·6 to 0·7). The ratio of dominance variance to total genetic variance varied between 11 and 36% with approach 1 and 5 and 56% with approach 2, indicating a large partial dominance for egg number traits and shell strength but also the difficulty of accurately estimating the dominance variance. For these traits, the estimates of the correlation between purebred and crossbred performance, rpc, were quite high (0·8 to 0·94) which contradicts the theory that traits with larger dominance and/or difference between purebred and crossbred heritabilities present lower rpc. These high rpc estimates, coupled with the absence of obvious heterosis, indicate little advantage to be gained from use of crossbred data in genetic improvement, where pure lines and crossbreds are reared in a similar non-stressful environment.