The photoperiodic response of male broiler breeders.
The expression of photorefractoriness, a feature of seasonal breeding in birds, is important in production species, where egg production and fertility are affected by the photoperiod. Years of selection in meat-type birds have reduced the potential reproductive function, and the environmental manipulation of such genotypes is essential to maximise productivity. While it is known that egg-type pullets no longer exhibit photorefractoriness, and that female broiler breeders do, there is not much information about the response of male broiler breeders to photoperiod. Such information is important when designing lighting programmes for breeding stock, and this study aimed to gain more insight into the male response to photoperiod. A series of experiments was designed in order to achieve this. In the first the response of male broiler breeders to rearing on constant photoperiods was measured, and in the second the effects of age at photostimulation on age at sexual maturity were assessed. The response to an increase in photoperiod during the production cycle was investigated in the third trial. Evidence of photorefractoriness in males was observed, but this was not the case in all birds, possibly due to high variation in some of the fertility traits measured. It seems possible that males do not respond to photoperiod to the same extent as females. Unexpected observations of a response in some birds to early photostimulation, and the high variation observed prompted another experiment in which the heritability of the response to early photostimulation was measured as a potentially revolutionary method of selection, whilst checking that this response is not negatively linked to broiler growth rates. Left and right testis weight data, collected in the above experiments, were analysed for asymmetry. There was no consistent response in testes asymmetry to photoperiod, or evidence that asymmetry in testis weights is as a result of the imposition of genetic or environmental stress. The strong correlation found between testes dimensions and weight suggests that predictions of testis weights could be made through ultrasonics or laparoscopy.
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