Genetic improvement of laying hens viability using survival analysis

The survival of about eight generations of a large strain of laying hens wa s analysed separating the rearing period (RP) from the production period (P P), after hens were housed. For RP (respectively PP), 97.8% (resp., 94.1%) of the 109 160 (resp., 100 665) female records were censored after 106 days (resp., 313 days) on the average. A Cox proportional hazards model stratif ied by flock (= season) and including a hatch-within-flock (HWF) fixed effe ct seemed to reasonably fit the RP data. For PP, this model could be furthe r simplified to a non-stratified Weibull model. The extension of these mode ls to sire-dam frailty (mixed) models permitted the estimation of the sire genetic variances at 0.261 +/- 0.026 and 0.088 +/- 0.010 for RP and PP, res pectively. Heritabilities on the log scale were equal to 0.48 and 0.19. Non -additive genetic effects could not be detected. Selection was simulated by evaluating all sires and dams, after excluding all records from the last g eneration. Thee, actual parents of this last generation were distributed in to four groups according to their own pedigree index. Raw survivor curves o f the progeny of extreme parental groups substantially differed (e.g., by 1 .7% at 300 days for PP), suggesting that selection based on solutions from the frailty models could be efficient, despite the very large proportion of censored records.