Critical Day Lengths for Egg Production and Photorefractoriness in the Domestic Turkey

Critical day lengths (CDL) for ovarian growth, egg production, and photorefractoriness were estimated in turkey hens during winter and summer seasons. In two experiments, one in each season, fixed day length treatments ranging from 10.5 to 16 h/d were applied to photosensitive hens at 30 wk of age. Daily egg production was used to estimate CDL for the induction of egg production as well as optimal egg production. After 24 wk on light treatments, hens used during the winter season were exposed to 20 h light: 4 h dark and subsequent changes in egg production were used to evaluate photorefractoriness. In two additional experiments, a night-interruption design was used to evaluate CDL and ovarian photoresponsiveness during winter and summer. The CDL for inducing egg production varied by season and was estimated to be 10.5 h or less during winter and about 11 h during summer. Likewise, the CDL for optimal egg production varied by season, being 11 to 11.5 h during winter and > 14 h during summer. The CDL for photorefractoriness in winter photostimulated hens was between 12 and 12.5 h. It was concluded that CDL vary by season and are not fixed, but are dynamic characteristics of photoperiodic processes. Thus, photoresponsiveness of turkey hens changes by season. In addition, CDL for initiating egg production, stimulating egg production, and inducing photorefractoriness are all different, with the CDL for photorefractoriness being longest.