Changes in eggshell conductance, water loss and hatchability of layer hens with flock age and moulting

Changes in water loss, eggshell conductance and hatchability with flock age were monitored in layer hens in a commercial hatchery. Optimal water loss for maximal hatchability of layer eggs was found to be 12 to 13% of initial egg mass at d 18 of incubation. Mass specific water vapour conductance (GH2Osp) increased linearly with flock age from 031 mg/(dgTorr) at the beginning of the first breeding season to 040 mg/(dgTorr) at its end after 77 weeks (=421 and 544 mg/(d100 gkPa), respectively). After forced moulting GH2Osp increased from 035 to 041 mg/(dgTorr) (=476 and 558 mg/(d100 gkPa), respectively). The coefficients of variation of GH2Osp increased with flock age from 14% at the beginning of the breeding season to 31% at the end of the second breeding season. In order to preserve normal incubation water loss for maximising hatchability, the humidity setting of an incubator should increase gradually, with flock age, from 53% RH to 66% RH in the first laying season and from 61% RH to 67% RH after forced moulting. A 35-fold increase (from 2 to 7%) in the difference between mean and median GH2Osp of egg batches with flock age was found, indicating increasing frequency of microscopic cracks in eggshells with flock age. This has to be taken into account when setting the humidity regime in the incubator.