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By Peter Lewis, University of KwaZulu-Natal, South Africa
Photorefractoriness is a natural physiological condition that differentiates broiler breeders and turkey breeders from egg-type breeders and commercial layers, particularly regarding their response to lighting. It is a phenomenon that needs to be understood before lighting patterns can be correctly designed for either broiler or turkey breeders. The condition has long been recognised in turkeys but has only recently been acknowledged in broiler breeders. As a result, broiler breeder lighting recommendations have frequently been incorrect. It is worth noting that egg-laying hybrids no longer exhibit photorefractoriness and therefore have fewer constraints imposed on their lighting requirements. Photorefractoriness simply means the inability to respond to light, but more specifically the lack of a sexual response to an otherwise stimulatory day length. All seasonal breeding birds are hatched in a refractory state, termed juvenile photorefractoriness, which generally prevents them from breeding in their first year. The condition is dissipated in full-fed birds by exposure to about two months of short days, which are neutral in their ability to sexually stimulate an animal (note they are not negative) and are usually no longer than nine hours. Birds, such as broiler breeders, that have their growth controlled by the feeding programme take longer to become photoresponsive. In nature, dissipation of photorefractoriness is achieved by the short days of winter, which allows the bird to commence breeding the following spring. However, after prolonged exposure to stimulatory day lengths during the summer months, the birds again become unresponsive to light, a condition called adult photorefractoriness, and generally go out of production until they have gone through a second period of short days.Absolute and relative form
There are two forms of photorefractoriness: an absolute form, as seen in truly seasonal breeding birds like pheasants, partridges and geese, and a relative form, as exhibited by broiler and turkey breeders. In the absolute form, sexual development is severely retarded when birds are reared from hatch on long days, with some individuals never becoming sexually mature. For example, in a study in which red-legged partridge were reared from hatch on 16-hour days, the first bird did not lay its first egg until it was 68 weeks of age, and three years later more than 60% of the birds were still infertile.
In contrast, birds like broiler breeders and turkey breeders, which show the relative form, are only moderately (2-4 weeks) retarded by not being given a period of short days. Interestingly, the intense selection for egg numbers over the past 50 years has resulted in modern egg-laying hybrids no longer showing photorefractoriness. Whereas rates of lay in broiler breeders will typically be below 50% by 60 weeks of age (after about 36 weeks production) and egg laying in turkeys likely to have almost ceased after only 30 weeks, egg production in a flock of commercial egg layers may well still exceed 80% after 52 weeks in lay. Typical rates of lay for poultry species exhibiting the various forms of photorefractoriness are shown in Figure 1. Flocks of broiler breeders or breeding turkeys will contain birds in varying states of photorefractoriness, especially at the end of the breeding cycle, with some continuing to be sexually active throughout the laying season, some back in lay after having paused and spontaneously resumed egg production whilst still on long days, and others having become photorefractory and not recommencing production without experiencing a period of short days or low light intensity to dissipate the refractoriness. The effects of photorefractoriness on egg laying in females are self evident, but similar effects occur with semen production in males. Nature would not design a system in which one sex was fertile while the other was infertile.
Rearing broiler breeders
It is essential to rear broiler breeders from an early age on short days, usually 8 or 9 hours, to ensure that all birds in the flock have had their juvenile photorefractoriness dissipated by the time they are transferred to long days (≥ 11 hours) at about 20 weeks of age. When broiler breeders are reared in open-sided or inadequately light-proofed buildings, and it is not possible for them to be given short days, it is advisable to simply let them experience the naturally changing day lengths, be the photoperiods increasing or decreasing. They should not be reared on a day length equal to the expected longest natural day length (as is frequently recommended in breeder management manuals) because this will unacceptably delay maturity and reduce egg numbers. This may be the correct recommendation for egg-type stock; precocity will not be a problem even when birds are reared on increasing day lengths during the rearing period. The data in Table 1 from a study at the University of KwaZulu-Natal shows that there were no significant differences in age at 50% egg production between broiler breeders reared on increasing or decreasing day lengths and others maintained on 14 hours from day-old through to 20 weeks. However, the constant 14-hour birds laid fewer eggs, had a smaller average egg weight, and produced a lower total egg output than the birds reared under simulated naturally changing day lengths. If broiler breeders are reared on 8-hour days and photostimulated at about 20 weeks, as routinely recommended, their sexual maturity will be 3-4 weeks earlier and their egg numbers and total egg output higher than birds reared on long days (Table 2). The indisputable answer to poor light control during the rearing period is to light-proof the buildings and not to tinker with the lighting programme.
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Source: World Poultry, vol. 25, no. 12, 2010.
