Poult. Sci.
HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS
QUICK SEARCH:   [advanced]


     

Poult Sci 2009. 88:191-198. doi:10.3382/ps.2008-00264
© 2009 Poultry Science Association
This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Siopes, T. D.
Right arrow Articles by Proudman, J. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Siopes, T. D.
Right arrow Articles by Proudman, J. A.

PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION

Exposure duration to long day lengths associated with the expression of photorefractoriness in turkey breeder hens

T. D. Siopes*,1 and J. A. Proudman{dagger}

* Department of Poultry Science, North Carolina State University, Raleigh 27695-7608; and {dagger} Biotechnology and Germplasm Laboratory Building 200, USDA-Agricultural Research Service, Beltsville Agricultural Research Center-East, Beltsville, MD 20705

1 Corresponding author: tom_siopes{at}ncsu.edu


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
In addition to inducing egg production, exposure to long days concomitantly activates processes that eventually result in photorefractoriness (PR) and cessation of egg production. Experiments were conducted to evaluate the duration of exposure to long days that result in these processes. In each of 3 experiments, we subjected Large White turkey breeder hens to long days (16 or 18 h per day) for differing lengths of time from initial photostimulation and then returned them to a photoperiod (12L:12D) that provided sufficient, but decreased, photoperiodic drive to support egg production but not induce PR. Photoresponsiveness was then evaluated by egg production after a return to a longer day length (20 h per day) late in the lay period and beyond the mean onset of PR typical for these turkey hens. Hens that have undergone any reduction in photoresponsiveness should not increase egg production in response to the increased photoperiod. From experiments 1 and 2, exposure to long days for as little as 1 d and as much as 9 wk from initial photostimulation did not result in an alteration in subsequent photoresponsiveness. This was based on an increased egg production response to a change in photoperiod from 12L:12D to 20L:4D after 20 wk of photostimulation that was similar to controls held continuously on 12L:12D and opposite to the response of controls held continuously on 18L:6D. It was clear that PR had been fully programmed by 20 wk of exposure to long days. Exposure to long days for 12 wk (experiment 3) resulted in a partial alteration of subsequent photoresponsiveness. It was concluded that programming of PR during late spring-summer season occurs after 9 wk of long day exposure, is not fully expressed by 12 wk of long days, and can be fully expressed by 20 wk of photostimulation.

Key Words: photoperiod • photorefractoriness • turkey • egg production • season


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Photostimulation of reproductive function with long day lengths is followed at some point during the reproductive season in birds by full or partial unresponsiveness to long days and subsequent gonadal regression and molt. This is referred to as photorefractoriness (PR), and PR in turkey breeder hens has been described by Siopes (2001, 2002) and Siopes and Proudman (2003).

Photorefractoriness for turkeys consists of 2 phases, absolute PR (molt and no eggs on long day lengths) and relative PR that precedes and is a lesser form of absolute PR. Hens will lay eggs during relative PR, and relative PR can only be observed by a decrease in long day lengths to a lesser, but still long, day length. Photorefractoriness is a natural process that leads to molt, allows replenishment of feathers, and assures that reproductive activity will occur at times of the year that maximize chances for survival of the young in nature.

Seasonal breeding in turkey hens, like most temperate zone birds, is a balance between phases of photosensitivity and PR. Although photosensitive, the turkey hen responds to day lengths greater than the critical day length (CDL) for photosexual stimulation by laying eggs and many, but not all, of these hens subsequently spontaneously cease lay and become PR. The minimal day length that subsequently results in PR is called the CDL for PR and has been reported to be between 12 and 12.5 h of light per day for turkey hens (Siopes, 1994, 1998). This CDL is static, remaining the same throughout the year. The CDL for photoperiodic drive (PD), which is the minimal duration of light resulting in normal egg production, varies during the year from as little as about 11.5 h to more than 16 h of light per day for normal egg production (Siopes, 1994, 2007). This means that turkey hens will respond to light differently at different times of the year for egg production but the same for the development of PR and provides an important basis for understanding the reproductive response of turkey hens to long days.

In a practical sense, PR is a problem in that there is an inverse relationship between PR and egg production that is reflected in a lack of persistence of egg laying. The presence of PR can then have considerable adverse effects on reproductive performance and breeder economics. The goal in lighting management of turkeys is to minimize PR and maximize PD.

Nicholls et al. (1988) and Wilson (1997) suggested that both the stimulation of reproduction and subsequent programming of PR occur by the interaction of thyroid hormones and long day length early in photostimulation. This model, developed using starlings and tree sparrows, suggests that PR is programmed by the interaction of thyroxine and long days (Nicholls et al., 1988; Wilson and Reinert, 1999; Dawson, 2001; Wilson, 2001; Mishra et al., 2004) and that programming occurs as early as 1d (Dawson, 2001) or 7d (Dawson et. al, 1985) but within a few weeks of long day exposure. The effects of long days on activating PR are delayed from those activating PD. Once the mechanism(s) are set (programmed), PR progresses inextricably toward overt expression some weeks later.

Little is known of similar mechanisms for programming of PR in turkeys, but long day lengths and thyroxine are required for ovarian development and PR (Lien and Siopes, 1989; Siopes, 1997). We have reported that circulating thyroid hormones immediately before and after photostimulation (–6 d to +14 d) are not indicative of a programming event for PR in turkeys (Proudman and Siopes, 2002, 2006). Also, Siopes (2001, 2002) indicated that the spontaneous onset and expression of PR in turkey hens is highly variable (including no PR expression), with an average onset of 18 to 24 wk of photostimulation depending on season. The associated range for the onset of PR was 12 to 32 wk after photostimulation.

From this sparse information for turkeys, one reasonable interpretation is that long day effects on establishing (programming) PR occur within 18 wk of photostimulation and that the expression of PR after programming is highly variable, including an absence of PR in some hens. If programming occurs as in wild birds, it would be expected to occur soon after initial photostimulation and well before overt expression noted above.

The purpose of the experiments herein was to further clarify the exposure time to long days from initial photostimulation that subsequently resulted in a decreased photoresponse to long days. This is the point when long days had affected or programmed PR such that the hens had become partially or fully unresponsive to increased long day exposure. This information is useful with regard to better understanding photoresponsiveness of turkey hens. In addition, if the time gap between initial photostimulation of reproduction (PD) and programming of PR is sufficiently large and we know when programming of PR starts after PS, we then have useful information for improving lighting management to minimize PR and improve egg production.


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
Three experiments were conducted to estimate how many long days are needed to establish the PR response in groups of turkey hens. Hens were subjected to long days for differing lengths of time and then returned to a photoperiod (12L:12D) that decreases PD but is known to support egg production and not induce absolute PR (Siopes, 1994). Each experiment included a long day control and a control group that would remain photosensitive (constant 12L:12D). All groups of hens were tested for photoresponsiveness by an increase in photoperiod (increased PD) late in the lay cycle. Hens that had been programmed for PR or have otherwise undergone any long day-induced reduction in photoresponsiveness should not increase egg production in response to the increased photoperiod.

It is notable that PR can be largely defined by a spontaneous stop in egg laying while exposed to long day lengths. In the negative control long day hens designed to maximize PR, some egg laying occurs at and during the time of testing for photoresponsiveness. Most, but not all, of these hens would eventually become PR, but onset of PR in turkey hens is highly variable (Siopes, 2001). Thus, present in any group of hens late in a typical lay cycle will be photosensitive (laying eggs), relative PR (laying eggs), and absolute PR (not-laying eggs) hens. The reference to this group as PR, even though low level egg production is present, is indicative of their lack of response to an increased day length and contrasts the treatment group responses.

Each experiment was done in light-controlled buildings that were not temperature-controlled but were insulated and mechanically ventilated. Feed and fresh water were provided for intake ad libitum throughout the study.

Experiment 1

This experiment tested the effect of long days during the first 4 wk of photostimulation on subsequent expression of PR. The hens were in floor pens with wood-shavings litter, and each pen had 4 nest boxes and 1 feeder and waterer. The lighting was by incandescent lamps at a mean light intensity of about 54 lx at turkey head height. Light was controlled by mechanical clocks and electronically recorded to validate control. Yearling, Large White Nicholas breeder hens were light-restricted with short days (8L:16D) for 10 wk to ensure photosensitivity. Groups of hens were then photostimulated in January with 16L:8D (lights on at 0500 h) and after 1, 14, or 28 d were given 12L:12D (lights on at 0500 h), which continued until 24 wk from initial photostimulation. This photoperiod (12L:12D) is considered to be gonadostimulatory and is known to induce and support egg production but not induce PR. Thus, there were 3 treatment groups based on days of exposure to 16L:8D, including 1 negative control group maintained on 16L:8D and 1 positive control group maintained on 12L:12D for 24 wk of photostimulation. Each group consisted of 2 to 3 hens in each of 8 replicate pens.

Photoresponsiveness was then assessed by providing 20L:4D for 8 wk to all groups starting in July and 24 wk from initial photostimulation. At this time of year, the CDL for normal egg production exceeds 12 h per day, and enhancement of PD requires very long day lengths. At the start of exposure to 20L:4D, egg production ranged from 30 to 40% hen-day egg production among the treatment groups. During the 6-wk test period, data were collected for daily egg production by pen to evaluate changes in photoresponsiveness. Hens that had been programmed for PR by prior long day exposure should not respond (increase egg production) to the increased PD provided by increasing the photoperiod from 12 to 20 h of light per day.

Experiment 2

This experiment tested the effects of long days during the first 1, 5, and 9 wk of photostimulation on subsequent expression of PR and served to extend the observations of experiment 1. Female parent line BUTA (British United Turkeys of America, Lewisburg, WV) Large White breeder hens were raised from 1 d of age following the guidelines of the primary breeder. Birds were raised on a 14L:10D photoperiod until 18 wk of age and then on a 6L:18D photoperiod until 30 wk of age to ensure photosensitivity. At 29 wk of age, hens were moved to laying pens in 2 rooms with independent light control of sodium vapor lamps set to provide 50 lx at bird height. Electronic time clocks with battery backup provided precise control of the light-dark cycle. Birds were photostimulated at 30 wk of age (April 29) with a photoperiod of 18L:6D (lights on at 0400 h). Individual egg production and nesting activity were monitored by trap-nesting, with the nests checked and hens expelled 5 times per day. Hens were trained to use the trap nests from the start of egg production (about 2 to 3 wk after photostimulation) for about 3 wk. Any hens that did not lay consistently in the trap nests thereafter or hens that became broody were removed from the experiment.

Treatment hens received 1, 5, or 9 wk of a 18L:6D daily photoperiod followed by 12L:12D until 20 wk of photostimulation. Control hens received either 18L:6D or 12L:12D as daily photoperiods continuously for 20 wk. We then tested for photosensitivity and programming of PR in all 5 treatment groups of hens by restricting light to 8L:16D for 2 wk starting September 8 and then stimulating with 20L:4D for 6 wk. There were 18 hens in each of the groups. Hens not recovering egg production when returned to long days were deemed absolutely PR. Classification of a hen as PR was confirmed by examination of the ovary at necropsy.

Experiment 3

From results of experiment 2, it was clear that PR had been programmed by 20 wk of photostimulation but not by 9 wk with a daily photoperiod of 18L:6D. Experiment 3 was done to get a better estimate of where between 9 and 20 wk of photostimulation that 18L:6D influences the programming of PR and to see if one can deter PR without adversely affecting egg production. The idea was to give 18L:6D as long as we could to maximize PD then switch to 12L:12D (<CDL for PR) before 18L:6D altered photoresponsiveness. The 12L:12D photoperiod was theorized to sustain egg production without inducing PR and thus increases persistence of lay (i.e., late cycle production). In short, we would be maximizing PD to initiate egg production and minimizing PR, both essential components to maximizing egg production.

One hundred eighty female parent line BUTA (British United Turkeys of America) Large White breeder hens were raised from 1 d of age following the guidelines of the primary breeder and in the same facility and management as per experiment 2. Birds were photostimulated at 30 wk of age (April 12) with a photoperiod of 18L:6D (lights on at 0400 h). The experiment consisted of 2 groups of hens. There were 69 hens in the treatment group with 7 pens (replicates) of 9 to 10 hens per pen and 78 control hens in 8 pens of 10 to 11 each. Treated hens received 12 wk of an 18L:6D daily photoperiod from April 12 to July 5 followed by 12L:12D photoperiods to 18 wk (August 10) of photostimulation. We then tested for photosensitivity by returning to a photoperiod of 20L:4D for 5 wk. Hens not increasing egg production when returned to long days were deemed PR. Classification of a hen as PR was confirmed by examination of the regressed ovary at necropsy. A control group was maintained on a 18L:6D photoperiod treatment throughout.

Statistical Analysis

An ANOVA was used to evaluate specific time points during egg production in each experiment using the GLM procedures of SAS Institute (1998). The least squares mean option was used to estimate significant differences among treatment means. Statements of statistical significance were based on P ≤ 0.05 unless specified otherwise. Repeated measures ANOVA was used to evaluate treatment effects over time in each experiment.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Experiment 1

Exposing hens to long days (16L:8D) for 1 d, 2 wk, or 4 wk at the start of photostimulation did not prevent an increase in egg production when day length was increased 24 wk from initial photostimulation, but continuous exposure to 16L:8D did (Figure 1Go). During the 6-wk test period on 20L:4D, the range in egg production increases among each of the 3 treatment groups was similar and ranged from 75 to 100%.


Figure 1
View larger version (19K):
[in this window]
[in a new window]

  		Figure 1. Photoresponsiveness of turkey hens to a photoperiod of 20 h of light per day after 24 wk of exposure to a photoperiod of 12L:12D except for initial exposure to 16L:8D for either 1, 14, or 28 d. Photoresponsiveness was evaluated by daily egg production for 6 wk after 20L:4D. Data are expressed as mean percentage change in egg production from that at the start of 20L:4D. Horizontal line represents the baseline mean hen-day egg production for a 2-wk period immediately before the start of 20L:4D. Control groups were maintained throughout the test on either 12L:12D or 16L:8D.

 
At the start of the testing for the presence of PR, only 2/16 of the positive control (12L:12D) hens were nonlayers as compared with 9/15 negative controls (16L:8D). It was expected that none of the hens in 12L:12D would be nonlayers, but 2 occurred. This was likely because 12 h of light per day is very close to the threshold for inducing PR and indicates minor individual hen variation in that threshold level. That 14/16 in this group remained photosensitive (produced eggs) supports the view that 12 h of light per day has only little or no effect on inducing PR and supports use of this light treatment to allow egg production without induction of PR and thus allows assessment of the effect of various exposures to longer day lengths on programming of PR.

Experiment 2

Full-term egg production response of all treatment groups is given in Figure 2Go. There was no significant difference in onset, peak, or overall egg production among the treatment and control hens up to the time of testing photoresponsiveness at 20 wk of photostimulation. The response to a 2-wk period of 8L:16D was also similar. However, after a change from 8 to 20 h of light per day, all treatment groups sharply increased egg production, whereas the long day control group did not. This may be more clearly seen in Figure 3Go as a percentage change in production level from that occurring at the start of the test for photoresponsiveness (20 wk from initial photostimulation).


Figure 2
View larger version (29K):
[in this window]
[in a new window]

  		Figure 2. Mean hen-day egg production during the 28-wk experimental period. Hens were given either 1, 5, or 9 wk of initial exposure to a photoperiod of 18L:6D followed by 12L:12D to 20 wk of the test. At 20 wk, all hens were tested for photoresponsiveness by acute exposure to short day lengths (8L:16D) for 2 wk and then 6 wk of exposure to 20L:4D. Control hens received either 18L:6D or 12L:12D throughout the test.

 

Figure 3
View larger version (20K):
[in this window]
[in a new window]

  		Figure 3. Relative changes in hen-day egg production after a 6-wk exposure to a photoperiod of 20L:4D late in the lay period. Photoresponsiveness of hens after 20 wk of exposure to a photoperiod of 12L:12D except for initial exposure to 18L:6D for either 1, 5, or 9 wk. After 20 wk from initial photostimulation, hens were tested for photoresponsiveness by 2 wk of 8L:16D to depress egg production, followed by 20L:4D as indicated by the bar at the top of the figure. Data are expressed as mean percentage change in egg production from that at the start of 8L:16D. Horizontal line represents the baseline mean hen-day egg production for a 2-wk period immediately before the start of 8L:16D. Control groups were maintained throughout the test on a daily photoperiod of either 12L:12D or 18L:6D. Cumulative eggs per hen during wk 20 to 28 are given on the right margin. a,bMeans with no common letters differ significantly (P ≤ 0.05).

 
Only 6 of 18 control (18L:6D) hens remained photosensitive at the end of this experiment, whereas all hens in the 12L:12D control group and all treated hens remained photosensitive. We believe that the 6 control hens expressing some photoresponsiveness to 20 h of light per day in experiment 2 are not the consequences of a lack of development of PR but rather a spontaneous recovery from diminished photoresponsiveness or PR to prolonged exposure to long days that is typical of turkey breeder hens during this time of the year (Siopes, 2005).

Experiment 3

An evaluation of exposure to long days for the initial 12 wk of photostimulation on subsequent photoresponsiveness is illustrated in Figure 4Go. After 12 wk of long days (18L:6D), a change in photoperiod to 12L:12D resulted in a decline in egg production. Of the 69 hens in this group, 40% ceased laying eggs within 6 wk. Subsequent exposure to 20L:4D resulted in a resumption of egg production to the level of the controls in 4 wk.


Figure 4
View larger version (20K):
[in this window]
[in a new window]

  		Figure 4. Mean hen-day egg production during the 24-wk experimental period. Treatment hens received a photoperiod of 18L:6D for the first 12 wk of photostimulation followed by a test of photoresponsiveness initiated with 6 wk of 12L:12D followed by 6 wk of 20L:4D. Arrows indicate the start and end of the 12L:12D photoperiod treatment. Control hens received 18L:6D throughout the experiment.

 

   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
From the results of these 3 experiments, it was clear that the effects of long days in promoting PR in turkey breeder hens start only after a prolonged exposure time. Certainly, the long day effects are not occurring with exposure to 1 long day, as has been reported for a wild bird by Dawson (2001). From experiments 1 and 2, it can be interpreted that exposure to long days for as little as 1 d and as much as 9 wk did not result in an alteration in subsequent photoresponsiveness or apparent programming of PR. It was clear, though, that PR had been programmed by 20 wk of exposure to long days.

Hens treated with long days for as long as 9 wk at the start of photostimulation followed by a photoperiod of 12L:12D (experiment 2) that is below the CDL for PR programming responded to a switch from 12L:12D to 20L:4D late in lay differently than that seen in the 18L:6D (PR programmed) controls and similar to the 12L:12D (PR not programmed) controls. This was the basis for the statement that 9 wk of treatment was insufficient time to program the mechanism that subsequently results in decreased photoresponsiveness (expression of PR). Therefore, programming of PR by long days occurred after 9 wk of photostimulation.

An alternative but less plausible explanation of the results of experiment 1 and 2 is possible if the 12L:12D photoperiod was not perceived as a long day but rather a short day. If so, any effects of the initial 16L:8D or 18L:6D treatments on photoresponsiveness would be obscured by the subsequent long-term effect of 12L:12D to have some degree of resensitizing the hens to light; that is, some degree of recycling could have occurred. Thus, the initial long day exposure would have been nullified or at least masked to some degree, and the positive response of egg production to a late season increase in photoperiod would be caused by a short day renewed photosensitivity (i.e., recycling).

The precise maximum hour of light per day to allow recycling of turkey hens to occur has not been reported. However, recycling of turkey hens can occur with 11.5 h but not 13 h of light per day when given for at least 8 wk (unpublished data). There are no reports of the effect of 12L:12D photoperiods on recycling turkey hens. Giving 12L:12D for at least 8 wk could theoretically allow an eventual recycling of turkey hens, because 12 h of light is below the CDL for PR programming (≥12 h, ≤12.5 h). However, 12 h of light is also greater than the daily minimum required for PD and egg production by turkey hens and so provides photostimulation (PD) sufficient to induce egg production (Siopes, 1994).

Dawson and McNaughton (1993) have noted that a 12L:12D photoperiod may be interpreted by male starlings as a long or a short day, depending on prior photoperiod treatment. According to their report, changing from a 16L:8D to a 12L:12D photoperiod would result in 12 h of light being interpreted as a short day and gradual resensitization of the male starlings within 3 mo. Because the 12L:12D treatment in experiments 1 and 2 did not decrease the rate of egg production from that in the long day controls or induce molt, the hens were likely interpreting 12L:12D as a long day rather than a short day. In support of this view is that it has never been reported that recycling (short day reinitiation of photosensitivity) can occur coincident with sustained egg production, and characteristically, recycling is associated with molting. It is concluded that 12L:12D was interpreted as a long day by the turkey hens, and because 12 h of light per day is below the CDL for PR, the results support that the effects of long days on programming PR occurred after 9 wk of exposure to 16 or more hours of light per day.

Egg production declined, albeit only a partial depression in eggs amounting to a decrease of about 15%, when the 18L:6D photoperiod was changed to 12L:12D after 12 wk in experiment 3. This suggests that a change in photoresponsiveness had occurred that was different from when 12L:12D was given at 9 wk of photostimulation with 18L:6D in experiment 2 (no decrease in egg production). Because experiment 2 and 3 were done at very similar times of the year, a seasonal variation in CDL cannot account for the different responses. However, the difference in initial exposure time to 18L:6D (9 wk vs. 12 wk) could account for the different egg production responses between experiments.

We have previously shown that photoresponsiveness to decreases in a long photoperiod to a lesser, but still long, photoperiod gets progressively larger as the lay cycle progresses and the balance between PD and PR favors PR and a diminishing PD. This appears to be a transition from relative PR to absolute PR, and relative PR may start as early as 8 wk after initial photostimulation (Siopes and Proudman, 2003). Therefore, the hens in experiment 3 had more exposure time (12 wk) to 18 h of light per day than those in experiment 2 (9 wk) and thus had progressed further, but not completely, toward PR. That is, they had decreased PD and were relatively PR. This is supported by the increased egg production after switching from 12L:12D to 20L:4D (Figure 4Go, experiment 3) and is consistent with our previous report (Siopes and Proudman, 2003). Egg production declined (albeit partially) when 18L:6D was changed to 12L:12D after 12 wk of photostimulation (experiment 3). This diminishes the practical utilization of such a program and suggests that such a program could be useful for deferring PR and therefore extending the lay period only if the change to 12L:12D occurred before 12 wk of photostimulation.

Siopes (2001) previously reported that the effects of long days on programming PR became evident between 16 and 19 wk of photostimulation when evaluated in the winter season with the associated short CDL for PD. There is not a discrepancy between these results and the earlier onset of PR (9 to 12 wk) observed in the present experiments. The variance is consistent with a seasonal variation in the CDL as reported by Siopes (1994); that is, the CDL for PD is much shorter in the winter than summer season. Experiments 2 and 3 were done in summer and used 12L:12D to test for PR compared with a winter experiment using a test photoperiod of 13L:11D in the prior report (Siopes, 2001). As a consequence, there was a much larger relative decrease in photoperiod from the existing CDL during summer evaluations of PR than during winter. This should more readily expose relative PR in summer than winter seasons and further illustrates the seasonal variations in photoresponsiveness in turkey hens.

From the 3 experiments, it cannot be definitively concluded how many long days (wk) are required to activate (program) PR. Part of this is because photoresponsiveness to long days progressively dissipates during the lay period as a transition from relative (partial) PR to absolute PR. Further, photoresponsiveness varies with season of the year (Siopes, 1994). The expression of absolute PR by individual turkey hens (stop lay and molt while in long days) is highly variable but unequivocal, with initial onsets generally at about 12 to 13 wk of photostimulation and mean onsets at 17 to 25 wk of photostimulation depending on season (Siopes, 2001, 2002). Programming of PR has clearly occurred before these times, but relative PR must be considered, because it is part of, and precedes, absolute PR.

The time that relative PR starts after photostimulation is more equivocal than for absolute PR. A prior study with results similar to those in experiment 3 (partial decline of egg production on decreased day lengths for a short period followed by resumption of eggs on return to the prior long day) showed this to occur by 8 wk of photostimulation in the winter season (Siopes and Proudman, 2003). From experiments 2 and 3, we can say that the first indication of an altered photoresponsiveness (relative PR) has occurred at 12 wk of photostimulation but not at 9 wk of photostimulation. This applies to hens tested in the summer season, and seasonal variations are likely considering our prior reports noted above (Siopes, 1994; Siopes and Proudman, 2003).

The results of the current study are consistent with our prior report that relative PR is an earlier and lesser form of absolute PR. How light programs PR remains unknown but is certainly neural, and it seems the number of long days required to achieve this programming may involve one or more factors including high variability among hens and high variability in the expression of PR. Programming of PR during late spring-summer occurs after 9 wk of long day exposure, is not fully expressed by 12 wk of long days, and is fully in place by 20 wk of photostimulation.


   ACKNOWLEDGMENTS
 
This work is part of the USDA Multi-State Research Project S-1020, Enhancing Reproductive Efficiency of Poultry. We gratefully acknowledge the excellent technical assistance of Vickie Hedgpeth (Department of Poultry Science, North Carolina State University).

Received for publication June 27, 2008. Accepted for publication September 22, 2008.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
Dawson, A. 2001. The effects of a single long photoperiod on induction and dissipation of reproductive photorefractoriness in European starlings. Gen. Comp. Endocrinol. 121:316–324.[CrossRef][Web of Science][Medline]

Dawson, A., A. R. Goldsmith, and T. J. Nicholl. 1985. Development of photorefractoriness in intact and castrated male starlings (Sturnis vulgaris) exposed to different periods of long-day lengths. Physiol. Zool. 58:253–261.

Dawson, A., and F. J. McNaughton. 1993. Circannual rhythms in starlings: Response of castrated birds to 12-hr days depends on photoperiodic history. J. Biol. Rhythms 8:141–150.[Abstract/Free Full Text]

Lien, R. J., and T. D. Siopes. 1989. Effects of thyroidectomy on egg production, molt, and plasma thyroid hormone concentrations of turkey hens. Poult. Sci. 68:1126–1132.[Web of Science][Medline]

Mishra, M. K., F. E. Wilson, T. S. Scanlan, and G. Chiellini. 2004. Thyroid hormone-dependent seasonality in American tree sparrows (Spizella arborea): Effects of GC-1, a thyroid receptor β-selective agonist, and of iopanoic acid, a deiodinase inhibitor. J. Comp. Physiol. [B] 174:471–479.[Medline]

Nicholls, T. J., A. R. Goldsmith, and A. Dawson. 1988. Photorefractoriness in birds and comparison with mammals. Physiol. Rev. 68 :133–176.[Free Full Text]

Proudman, J. A., and T. D. Siopes. 2002. Relative and absolute photorefractoriness in turkey hens: Profiles of prolactin, thyroxine, and triiodothyronine early in the reproductive cycle. Poult. Sci. 81:1218–1223.[Abstract/Free Full Text]

Proudman, J. A., and T. D. Siopes. 2006. Potential role of thyroid hormones and prolactin in the programming of photorefractoriness in turkey hens. Poult. Sci. 85:1457–1461.[Abstract/Free Full Text]

SAS Institute. 1998. SAS/STAT Guide for Personal Computers. Version 6 ed. SAS Inst. Inc., Cary, NC.

Siopes, T. D. 1994. Critical day lengths for egg production and photorefractoriness in the domestic turkey. Poult. Sci. 73:1906–1913.[Web of Science][Medline]

Siopes, T. D. 1997. Transient hypothyroidism reinitiates egg laying in turkey breeder hens: Termination of photorefractoriness by propylthiouracil. Poult. Sci. 76:1776–1782.[Abstract/Free Full Text]

Siopes, T. D. 1998. Absence of a seasonal effect on the critical day length for photorefractoriness in turkey breeder hens. Poult. Sci. 77:145–149.[Abstract/Free Full Text]

Siopes, T. D. 2001. Temporal characteristics and incidence of photorefractoriness in turkey hens. Poult. Sci. 80:95–100.[Abstract/Free Full Text]

Siopes, T. D. 2002. Photorefractoriness in turkey breeder hens is affected by age and season. Poult. Sci. 81:689–694.[Abstract/Free Full Text]

Siopes, T. D. 2005. Spontaneous recovery of photosensitivity by turkey breeder hens given prolonged exposure to long day lengths. Poult. Sci. 85:1457–1467.[Web of Science]

Siopes, T. D. 2007. Lighting for summer egg production by turkeys: Day length and light intensity. Poult. Sci. 86:2413–2419.[Abstract/Free Full Text]

Siopes, T. D., and J. A. Proudman. 2003. Photoresponsiveness of turkey breeder hens changes during the egg-laying season: Relative and absolute photorefractoriness. Poult. Sci. 82:1042–1048.[Abstract/Free Full Text]

Wilson, F. E. 1997. Photoperiodism in American tree sparrows: Role of the thyroid gland. Pages 159–169 in Perspectives in Avian Endocrinology. S. Harvey and R. Etches, ed. Soc. Endocrinol., Bristol, UK.

Wilson, F. E. 2001. A test of the hypothesis that T3 is the "seasonality" hormone in American tree sparrows (Spizella arborea): Intracerebroventricular infusion of iopanoic acid, an inhibitor of T3 synthesis and degradation. J. Comp. Physiol. [B] 171:113–119.[CrossRef][Medline]

Wilson, F. E., and B. D. Reinert. 1999. Long days and thyroxine program American Tree sparrows for seasonality: Evidence for temporal flexibility of the breeding season of euthyroid females. Gen. Comp. Endocrinol. 113:136–145.[CrossRef][Web of Science][Medline]





This Article
Right arrow Abstract Freely available
Right arrow Full Text (PDF)
Right arrow Alert me when this article is cited
Right arrow Alert me if a correction is posted
Services
Right arrow Similar articles in this journal
Right arrow Alert me to new issues of the journal
Right arrow Download to citation manager
Right arrow reprints & permissions
Citing Articles
Right arrow Citing Articles via Google Scholar
Google Scholar
Right arrow Articles by Siopes, T. D.
Right arrow Articles by Proudman, J. A.
Right arrow Search for Related Content
PubMed
Right arrow Articles by Siopes, T. D.
Right arrow Articles by Proudman, J. A.

HOME HELP FEEDBACK SUBSCRIPTIONS ARCHIVE SEARCH TABLE OF CONTENTS