Neuroendocrine control of decreased persistency of egg-laying in domestic hens : evidence for the development of photorefractoriness

Egg-laying in hens exposed for more than 11 months to photostimulatory daylengths was intermittent and associated with a reduction in numbers of yellow-yolky ovarian follicles. Old laying hens (105 weeks) had lower concentrations of luteinizing hormone (LH in the pituitary gland and plasma and reduced pituitary gland responsiveness to chicken LH-releasing-hormones (LHRH-I and 11) in vivo, when compared with young laying hens (28 weeks). Four weeks after transfer from 14 to 8h light/day, egg production almost stopped in old, but not in young hens, although plasma LH concentrations decreased in all birds. After transfer from 14 to 20h light/day, plasma LH increased in young, but not in old, hens, without a change in the rate of egg production. Reproductive function was enhanced in old hens returned to long days after induction of a moult and ovarian regression by reducing daylength and dietary restriction. Moulted hens had a greater rate of egg production, higher concentrations of plasma LH and a greater pituitary-gland responsiveness to LHRH-II in vivo than unmoulted control hens. After transfer from 14 to 8h light/day, egg-laying decreased more rapidly in unmoulted than in moulted hens; transfer to 17h light/day increased egg production in moulted, but not in unmoulted, birds. Induction of ovarian regression in old hens by dietary restriction alone also enhanced reproductive function after the dietary restriction was relaxed. Egg-laying was more persistent in hens brought into lay for a second year by transferring them from 3 to 11h light/day than in hens transferred from 3 to 20h light/day. Egg production was stimulated in hens maintained on 3 or 11h light/day for 42 weeks, after transfer to 20h light/day. Egg production ceased in hens maintained on 20h light/day for 46 weeks, after transfer to 3h light/day. These observations are consistent with the view that poor persistence of laying in hens <2 years old and exposed continuously to long days is caused, in part, by a reduction in hypothalamic gonadotroph function. This reduction in neuroendocrine function may be due, in part, to the development of relative photorefractoriness.