|Siopes, T - NORTH CAROLINA STATE UNIV|
Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 2, 2002
Publication Date: March 12, 2003
Citation: Siopes, T.D., Proudman, J.A. Photoresponsiveness of turkey breeder hens changes during the egg-laying season: relative and absolute photorefractoriness. Poultry Science. 82:1042-1048. 2003. 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. Wild birds first go through a latent, or "relative" photorefractory (rPR) stage, where they will cease laying only if the daylength is reduced, and an absolutely photorefractory (aPR) stage when egg-laying ceases regardless of a long daylength. We conducted an experiment to characterize the types of photorefractoriness that are found in the turkey hen, and then to determine when they appear during the reproductive cycle. We also determined whether appearance of photorefractoriness was related to plasma levels of luteinizing hormone (LH or prolactin(PRL). We found that a majority of turkey hens exhibit rPR as early as 8 weeks after photostimulation, or within 3 weeks after reaching peak egg production. Absolute PR was observed in 23% of the hens by 28 weeks after photostimulation. We found no difference in plasma levels of LH or PRL between hens that became photorefractory and those that did not. We found a marked variation among individuals in their response to an early (8 weeks after photostimulation) and brief (2 week) reduction in photoperiod that may be useful in identifying individuals that develop photorefractoriness early. These results will be used by scientists to further study the mechanisms that cause a hen to stop laying prematurely, and may provide information to help turkey breeders select individuals with the greatest reproductive potential.
Technical Abstract: Typically, photosensitive species undergo neuroendocrine changes during a reproductive season that cause them to gradually become unresponsive to a photoperiod that initially stimulated reproduction. They may first become relatively photorefractory (rPR), when they will cease egg laying only if photoperiod is reduced, and then absolutely photorefractory (aPR), when they will cease laying despite long day length. Our objective was to test the photoresponsiveness of breeder turkey hens during egg production at various times following photostimulation and to relate photoresponsiveness to rPR and aPR as well as plasma levels of prolactin (PRL) and luteinizing hormone (LH). Hens were maintained in cages in light-controlled facilities and photostimulated at 31 wk of age (September) with a photoperiod of 16L:8D. At 8, 14, and 20 wks after photostimulation, treated hens received a 2 wk exposure to an 11.5L:12.5D photoperiod and were then returned to 16L:8D. Exposure to the shortened photoperiod at 8 wks of photostimulation resulted in three distinct responses of declining egg production: non-responders (NR, 32.7% of hens), partial responders (PAR, 43.9 %), or full responders (FR, 23.4%). Egg production returned to control levels following return to a 16L:8D photoperiod. This response repeated at the 14 and 20 wk treatment periods, but with greater declines in egg production in the NR and PAR groups. The incidence of subsequent aPR in the NR, PAR, and FR groups was 5.7, 8.5 and 24%, respectively, as compared to 23.3% for the controls. Plasma LH and prolactin concentrations also declined in response to 11.5L:12.5D, and also rebounded following return to 16L:8D. The hormonal responses of NR, PAR, and FR were similar. We conclude that turkey hens exhibit varying degrees of rPR early during the egg laying season and that the incidence and severity of the rPR response increases as the laying season progresses.