The productive response of broiler breeder hens to lighting and growth manipulation during rearing.
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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 houses.