Light intensity and age at first egg in pullets

Layer strain pullets were reared on litter and given an 8-h photoperiod to 10 wk of age. They were then transferred to cages in which light intensity at the feed trough varied from 5 to 14 lx. Thirty birds continued on 8 h light (L):16 h dark (D) (negative controls), and another 30 birds were given 14L:10D (positive controls). Two other groups of 30 birds were given a regimen of 3 dim:8 L:3 dim:10 D with intensity during the dim phase ranging from 0.03 to 0.42 lx (very dim) or from 0.6 to 3.0 lx (marginal). Mean age at first egg (AFE) differed by 30 d between the positive and negative controls. Birds receiving very dim lighting matured 10 d earlier than the negative controls but 20 d later than positive controls. It is concluded that either the very dim light was itself nonstimulatory but had a phase shifting effect upon the biological clock that caused the 8-h normal light to fall partly in the photoinducible phase, or the first 3 h of very dim light was added to the bright phase to form an 11-h photoperiod. Birds in the top tier of the room with marginal supplementary lighting received 1.7 to 3.0 lx and matured at the same age as the positive controls, whereas those in the bottom tier received 0.6 to 0.9 lx and matured at the same age as the negative controls. Birds in the middle tier showed an intermediate AFE. It is concluded that the threshold intensity at the feed trough for white light stimulation of the photoperiodic mechanism in caged pullets lies between 0.9 and 1.7 lx. However, very dim lighting, below the threshold required for stimulation of a photoperiodic response, may shift the biological clock with unexpected consequences and, as a result, there is no known intensity of dim light that can be equated with darkness for all purposes.