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Title: RELATIVE AND ABSOLUTE PHOTOREFRACTORINESS IN TURKEY HENS: PROFILES OF PROLACTIN, THYROXINE, AND TRIIODOTHYRONINE EARLY IN THE REPRODUCTIVE CYCLE

Author
item Proudman, John
item SIOPES, T - NORTH CAROLINA UNIVERSITY

Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 4/3/2002
Publication Date: 12/1/2002
Citation: Proudman, J.A., Siopes, T.D. Relative and absolute photorefractoriness in turkey hens: profiles of prolactin, thyroxine, and triiodothyronine early in the reproductive cycle. Poultry Science. 81:1218-1223. 2002.

Interpretive Summary: Turkey hens have the poorest reproduction of any commercial poultry species. One reason for this is that many turkeys become photorefractory like wild birds. That is, they cease to lay despite still having a stimulatory daylength. In wild birds, this end to the reproductive season is thought to be programmed at the time they begin laying by the thyroid hormone, thyroxine. We conducted an experiment to characterize the types of photorefractoriness that are found in the turkey hen, and then to determine whether levels of certain hormones in the blood early in the reproductive cycle may program the hen to stop laying prematurely. Our results showed that the turkey hen expresses both absolute photorefractoriness and relative photorefractoriness. Unlike wild birds, blood levels of thyroxine were not related to whether the hen continued to lay. Low levels of prolactin in the blood following peak egg production appeared to be indicative of whether the hen would later become photorefractory. These results suggest that selection of turkeys for growth and egg production have altered the control of seasonal breeding. Programming of refractoriness by thyroxine does not seem important, allowing birds to lay longer. These results will be used by scientists to further study the mechanisms that cause a hen to stop laying prematurely.

Technical Abstract: An experiment was conducted to determine whether a commercial strain of turkey hens exhibits relative photorefractoriness 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 relative photorefractoriness or absolute photorefractoriness. Twenty-seven npercent of hens maintained on a stimulatory photoperiod 18L:6D for 19 wk and then given a shorter (but still stimulatory) photoperiod ceased to lay and their ovaries regressed within 4 wk. These hens were considered relatively photorefractory. Subsequent exposure to the 18L:6D photoperiod resulted in ovarian recrudescence in 41.7% of these photorefractory individuals, confirming the presence of relative photorefractoriness at 19 wk after photostimulation. Absolute photorefractoriness was observed in 15.1% of hens during a 27-wk reproductive season. Hens which became either rrelatively or absolutely photorefractory exhibited significantly lower plasma prolactin levels at 8 and 14 wk after photostimulation than did hens which remained photosensitive. Plasma levels of thyroxine were lower at 1 and 2 wk following photostimulation in hens that subsequently became PR than in hens that remained photosensitive. We conclude that turkey hens may exhibit both relative and absolute photorefractoriness during a reproductive cycle while flockmates may remain photosensitive for at least 27 wk. Plasma thyroxine levels soon after photostimulation, during a period when programming of photorefractoriness is thought to occur, do not appear to be related to whether turkey hens remain photosensitive. Therefore, thyroid hormones may not be as important in regulating the photorefractory response in turkeys as in wild birds.