Absence of a seasonal effect on the critical day length for photorefractoriness in turkey breeder hens

Abstract

Two experiments were conducted to determine whether the critical day length (CDL) for photorefractoriness remained stable or varied by season of the year. The first experiment was done in the fall with photosensitive hens that were exposed to fixed day length treatments ranging from 12 to 16 h/d for 24 wk. At 24 wk of treatment, all hens were given 20 h light (L):4 h dark (D) and subsequent changes in egg production were used to evaluate photorefractoriness. This experimental approach was the same as that used by Siopes (1994) to estimate the CDL for photorefractoriness in winter-photostimulated hens. It was determined that the CDL for photorefractoriness in fall-photostimulated hens was 12 to 12.5 h, the same as that obtained in the earlier report for winter-photostimulated hens. A second experiment was done to further test the possibility that the CDL for photorefractoriness was static during the year. A gradually increasing photoperiod was started in winter at 12L:12D, just under the CDL for photorefractoriness but just over the CDL for egg production. Subsequent egg production and expression of photorefractoriness were compared to controls to estimate the dynamics of the CDL for photorefractoriness. Results supported a static CDL for photorefractoriness and suggest that any daily photoperiod exceeding 12 h will activate processes leading to photorefractoriness, independent of season.