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Title: EFFECT OF A SINGLE SHORT-TERM REDUCTION IN PHOTOPERIOD ON PHOTOREFRACTORINESS IN TURKEY HENS

Author
item Proudman, John
item SIOPES, T - N.C. STATE UNIV

Submitted to: Poultry Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/4/2004
Publication Date: 7/2/2004
Citation: Proudman, J.A., Siopes, T.D. 2004. Effect of a single short-term reduction in photoperiod on photorefractoriness in turkey hens. Poultry Science. 83:1199-1202.

Interpretive Summary: The egg-laying season of turkey breeder hens is relatively short compared to other poultry species. One reason for this is that turkey hens more easily become photorefractory. That is, they 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 continued long day length. We have previously shown a high incidence of rPR as early as 8 weeks following photostimulation in a winter-laying flock. The present study was designed to determine whether such inhibitory factors are similarly present in a summer-laying flock, which should become photorefractory earlier than a winter-laying flock. We also tested whether a short-term reduction in day length early in the reproductive season might delay the onset of aPR and extend the length of the reproductive season. We found that a single 2-wk reduction in day length shortly after the hens reached peak egg production did not significantly reduce overall flock egg production, but it also did not improve late season egg production or retard the onset or incidence of aPR. This study confirms our previous finding that rPR occurs early in the reproductive cycle of turkey hens and shows that this response also occurs in a summer-laying flock. We interpret our results to suggest that a core percentage of hens within a flock exhibit a strong and highly variable rPR response early in the reproductive cycle regardless of season, and that these hens these rPR hens are likely to later exhibit aPR at a higher rate (and have lower associated egg production) than their flockmates. . Therefore, an indicator of the incidence of rPR early in the lay period may have a predictive value for the overall egg production of the flock. These results will be used by scientists to further study the mechanisms that cause a hen to stop laying prematurely.

Technical Abstract: In a prior study (Siopes and Proudman, 2003), we reported that a high proportion of hens in a winter-laying flock became relatively photorefractory (rPR) early in the reproductive cycle, and that successive short-term reductions in photoperiod in such hens each initially depressed egg production but then caused a rebound in rate of lay to briefly exceed that of hens that did not exhibit rPR. The present study was conducted to assess rPR in a summer-laying flock and to determine whether a single short-term reduction in day length early in the reproductive cycle might enhance egg production and delay the onset of absolute photorefractoriness (aPR). Control hens received a photoperiod of 16L:8D throughout the experiment. Experimental hens were photostimulated with 16L:8D, received a reduced (but still stimulatory) photoperiod of 11.5L:12.5D for 2 wk beginning 8 wk after photostimulation, and then were returned to 16L:8D for the remainder of the 23 wk test period. Results showed that a single 2-wk reduction in day length shortly after the hens reached peak egg production did not significantly reduce overall flock egg production, but it also did not improve late season egg production or retard the onset or incidence of aPR. The incidence of rPR was substantially less in this study than we had observed with a winter-laying flock (32.9% vs. 67.1%), but similar proportions of treated hens exhibited the most severe rPR response (a brief but complete cessation of egg production) in both studies (21.1% vs. 24.0%), and all treated hens that subsequently became aPR had shown this severe rPR response to the test photoperiod. We conclude that a core proportion of hens (approximately 1/5) exhibits a strong rPR response when presented with a reduced photoperiod early in the reproductive cycle, regardless of season of the year, and that such hens are more likely to subsequently exhibit poor egg production or become aPR than flockmates that do not exhibit rPR. .Therefore, some indication of the incidence of rPR early in the lay period may have a predictive value for the overall egg production of the flock.