|dc.description.abstract||This study was designed to provide information that would enable the development of a theory to
predict age at sexual maturation and settable egg production in broiler breeder hens submitted to a
variety of constant or increasing photoperiods and with diverse growth curves. Six trials were
conducted using three strains of broiler breeder females housed in floor pens or individual cages.
The treatments covered a wide range of growth profiles during the rearing period, from slow
growth to achieve 2100g at 24 weeks, to fast growth achieving 2100g at 15 weeks of age. The
lighting treatments included 8, 11 and 16-h constant photoperiods, photostimulation at various
ages between 10 and 24 weeks, abrupt or gradual increases in daylength, and transfers to a 10, 11,
12 or 16-h final photoperiod in lay.
The results show that broiler breeders exhibit photorefractoriness, and that the adult form starts
developing from about 56 weeks of age. They also suggest that photorefractoriness contributes
towards the accelerated decline in egg production observed at the end of the laying period.
Relaxations of feed restriction during the rearing period and earlier transfers to a stimulatory
photoperiod were successfully used to advance sexual maturity by up to 3 weeks compared with
conventionally managed controls. Furthermore, birds subjected to constant photoperiods reached
sexual maturity later than birds that had been photostimulated at 20 weeks of age. Settable egg
production progressively improved when birds were transferred to stimulatory daylengths at older
ages, until about 20 weeks, but subsequent delays in photostimulation did not result in any further
increase in egg numbers. Delaying photostimulation of conventionally grown birds beyond 28
weeks and maintaining them on constant 8 or 16-h photoperiods negatively affected egg
production. Maintaining birds on constant 11-h photoperiods had a less deleterious effect on egg
production. Increasing the photoperiod from 8 to 12 h resulted in a significant improvement in
settable egg production compared with birds transferred to 16 h.
Prediction equations were produced to estimate mean age at sexual maturity for control birds
subjected to constant photoperiods, and for birds reared on a control or fast growth curve and
photostimulated at between 10 and 24 weeks of age. Data presented in this thesis suggest that, to
minimise the accentuated decline in egg production typically seen late in the laying period, birds
kept in light-tight houses should be transferred to photoperiods shorter than the currently
recommended 16 h. Finally, photorefractoriness provides an improved understanding of the
causes of erratic performance frequently observed in out-of-season flocks kept in open sided