Photoperiodic responses of broilers. II. Ocular development.
1. Poultry are naturally long-sighted when they hatch, becoming emmetropic by 6 weeks of age. However, Cobb and Ross broilers exposed to <or=12-h photoperiods rapidly learn to eat in the dark, a behaviour which could adversely affect the establishment of emmetropia. And so, in the current study, eyes were enucleated, post mortem, at 40 and 54 d from broilers given various photoperiods between 2 and 24 h, and eye weight determined to indirectly assess the extent to which photoperiod might affect ocular development. 2. In both genotypes, there was an inverse linear relationship between absolute eye weight and photoperiod up to 21 h, but continuously illuminated birds consistently had heavier eyes. When expressed relative to body weight, eye weight for <or=21-h birds was inversely related to the logarithm of photoperiod. 3. The slope of the regression of relative eye weight on <or=21-h photoperiod log values was significantly less at 54 d than at 40 d in both genotypes, but constantly illuminated birds continued to have above average eye weights. 4. Cobb birds invariably had heavier eyes than Ross, both in absolute and relative (to body weight) terms. 5. The results support previous work suggesting that there is a minimum period of daily darkness required to maintain chicken eye growth within the normal range and show that very short photoperiods, as well as ultra-long photoperiods and continuous illumination, adversely affect ocular development, with potential welfare implications.
Lewis
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Animal and Poultry Science, University of KwaZulu-Natal, Pietermaritzburg, South Africa. pdlewis@dsl.pipex.com
Gous
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D000818:Animals; D002645:Chickens / Q000235:genetics / Q000254:growth & development / Q000502:physiology*; D005123:Eye / Q000033:anatomy & histology / Q000254:growth & development / Q000528:radiation effects*; D005838:Genotype; D007429:Intraocular Pressure / Q000528:radiation effects; D008297:Male; D017440:Photoperiod