Relative and absolute photorefractoriness in turkey hens: profiles of prolactin, thyroxine, and triiodothyronine early in the reproductive cycle.

An experiment was conducted to determine whether a commercial strain of turkey hens exhibits relative photorefractoriness (rPR) during a reproductive cycle and to ascertain whether plasma levels of certain hormones early in the reproductive cycle might be associated with subsequent expression of rPR or absolute photorefractoriness (aPR). Twenty-seven percent of hens maintained on a stimulatory photoperiod of 18L:6D for 19 wk and then given a shorter, but still stimulatory, photoperiod (13L:11D) ceased to lay and their ovaries regressed within 4 wk. These hens were considered rPR. Subsequent exposure to the 18L:6D photoperiod resulted in ovarian recrudescence in 41.7% of these PR individuals, confirming the presence of rPR at 19 wk after photostimulation. Absolute PR was observed in 15.1% of hens during a 27-wk reproductive season. Hens that became rPR or aPR exhibited significantly lower plasma prolactin levels at 8 and 14 wk after photostimulation than did hens that remained photosensitive (PS). Plasma levels of thyroxine were lower at 1 and 2 wk following photostimulation in hens that subsequently became PR than in hens that remained PS. We conclude that turkey hens may exhibit rPR and aPR during a reproductive cycle, whereas flockmates may remain PS for at least 27 wk. The presence of long daylengths, thyroid hormones, and PRL did not assure expression of PR. The expression of PR appears to be associated with reduced plasma throxine levels during a period when programming of PR is thought to occur and with reduced levels of prolactin following peak egg production.