Spontaneous recovery of photosensitivity by turkey breeder hens given prolonged exposure to long day lengths

Department of Poultry Science, North Carolina State University, Raleigh 27695-7608, USA. tom_siopes@ncsu.edu

Three experiments were done to examine spontaneous photosensitivity (SP) and the associated renewal of egg production in Large White turkey breeder hens. In experiment 1, hens were photostimulated with 16 h of light:8 h of dark in December for 54 wk. In experiment 2, SP was examined in a different season than experiment 1. One group of hens was recycled and then photostimulated with 16 h of light:8 h of dark for 23wk. Another group of hens was not recycled and received 16 h of light:8 h of dark throughout the experiment. Egg production was used to assess SP in both experiments 1 and 2. Experiment 3 was designed to produce SP and renewed egg production during prolonged exposure (64 wk) to constant, long day lengths and to evaluate circulating thyroid hormones immediately prior to the renewal of egg production. Egg production was recorded daily to assess SP, and blood samples were taken weekly for radioimmunoassay for thyroid hormones. In both experiments 1 and 2, there was SP and renewed egg production, and this occurred in the fall season. Egg production increased gradually from less than 5% to a peak of 67% (experiment 1) and 38% (experiment 2) in November. In experiment 3, SP occurred in the fall, and egg production increased gradually and was similar to control hens from 8 wk of lay to the end of the experiment. Both groups also had similar declines in egg production following a decrease in photoperiod at wk 24 of the experiment. In the 8-wk period preceding the onset of laying, there were no significant differences between control and SP hens in plasma thyroid hormone levels. In both groups, there was a significant time effect for plasma triiodothyronine, expressed as a parabolic pattern of change. It was concluded that for turkey hens maintained in prolonged and constant long day lengths that SP and renewal of egg production occurs in the fall season. This response was independent of season of photostimulation, did not alter photoresponsiveness to shorten day lengths, and was preceded by a parabolic increase in plasma triiodothyronine.