1. Changes in water loss, eggshell conductance and hatchability with flock age were monitored in layer hens in a commercial hatchery. 2. 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. 3. Mass specific water vapour conductance (GH(2)Osp) increased linearly with flock age from 0.31 mg/(d.g.Torr) at the beginning of the first breeding season to 0.40 mg/(d.g.Torr) at its end after 77 weeks (=4.21 and 5.44 mg/(d.100 g.kPa), respectively). 4. After forced moulting GH(2)Osp increased from 0.35 to 0.41 mg/(d.g.Torr) (=4.76 and 5.58 mg/(d.100 g.kPa), respectively). 5. The coefficients of variation of GH(2)Osp increased with flock age from 14% at the beginning of the breeding season to 31% at the end of the second breeding season. 6. 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. 7. A 3.5-fold increase (from 2 to 7%) in the difference between mean and median GH(2)Osp 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.
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