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Absence of a seasonal effect on the critical day length for photorefractoriness in turkey breeder hens
PHYSIOLOGY AND REPRODUCTION Absence of a Seasonal Effect on the Critical Day Length for Photorefractoriness in Turkey Breeder Hens1 T. D. SIOPES2 Department of Poultry Science, North Carolina State University, Raleigh, North Carolina 27695-7608
(Key words: turkey, light, photorefractoriness, critical day length, photoperiod) 1998 Poultry Science 77:145–149
sion and subsequent molt and remains a major mechanism limiting the continuous production of semen and eggs in birds. Both photoperiodic drive and photorefractoriness are activated at the start of exposure of photosensitive birds to photoperiods long enough to induce gonadal development (Nicholls et al., 1988; Wilson and Reinert, 1993, 1996; Reinert and Wilson, 1996). However, in turkey hens the full expression of photorefractoriness is delayed and starts at about 18 wk of continuous photostimulation with 14 h of light/d (Lien and Siopes, 1989). That the CDL for photoperiodic drive may be less than that for photorefractoriness has important practical implications in the development of lighting schedules to promote one without the other. If a photoperiod is utilized that stimulates egg production but does not activate photorefractoriness, then a greater persistence of lay should occur, resulting in more eggs per hen. This difference was addressed in the report by Siopes (1994) but with marginal success, presumably because the fixed photoperiod that was used (11.5 to 12 h) provided insufficient photoperiodic drive in the face of an increasing CDL for photoperiodic drive as the laying period progressed. This result will be re-evaluated in the present study in Experiment 2.
INTRODUCTION With respect to photoperiodic responses of birds, the minimum duration of a daily photoperiod that induces an effect is referred to as the critical day length (CDL). Sharp (1993) has reported minimum and maximum CDL for the domestic hen without consideration of season. In a prior report, it was noted that the CDL for egg production by turkeys was not fixed but rather varied by time of year (Siopes, 1994). In addition, the CDL for egg production was different (less) than that for photorefractoriness for hens photostimulated in January (11 to 11.5 vs 12 to 12.5 h, respectively). In general, photoperiodic drive refers to the degree of switching-on of reproductive function (egg production) by light, whereas photorefractoriness refers to the degree of switching-off of reproductive responses to light. These have been addressed for the domestic hen (Sharp, 1993) and other avian species (Sharp, 1996). Photorefractoriness is associated with gonadal regres-
Received for publication January 30, 1997. Accepted for publication August 14, 1997. 1The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products mentioned, nor criticism of similar products not mentioned. 2To whom correspondence should be addressed: [email protected]
Abbreviation Key: CDL = critical day length; D = dark; L = light.
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obtained in the earlier report for winter-photostimulated hens. A second experiment was done to further test the possibility that the CDL for photorefractoriness was static during the year. A gradually increasing photoperiod was started in winter at 12L:12D, just under the CDL for photorefractoriness but just over the CDL for egg production. Subsequent egg production and expression of photorefractoriness were compared to controls to estimate the dynamics of the CDL for photorefractoriness. Results supported a static CDL for photorefractoriness and suggest that any daily photoperiod exceeding 12 h will activate processes leading to photorefractoriness, independent of season.
ABSTRACT Two experiments were conducted to determine whether the critical day length (CDL) for photorefractoriness remained stable or varied by season of the year. The first experiment was done in the fall with photosensitive hens that were exposed to fixed day length treatments ranging from 12 to 16 h/d for 24 wk. At 24 wk of treatment, all hens were given 20 h light (L): 4 h dark (D) and subsequent changes in egg production were used to evaluate photorefractoriness. This experimental approach was the same as that used by Siopes (1994) to estimate the CDL for photorefractoriness in winter-photostimulated hens. It was determined that the CDL for photorefractoriness in fallphotostimulated hens was 12 to 12.5 h, the same as that
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MATERIALS AND METHODS
Experiment 1 Yearling Large White turkey breeder hens (Nicholas) at the end of their first season of lay were light-restricted with 8 h of light/d [8 h light (L):16 h dark (D)] for 8 wk to ensure photosensitivity. Hens were then exposed to one of the following light treatments: 12, 12.5, 13, 13.5, 14, or 16 h of light/d. There were 24 hens per treatment group in four replicate floor pens of 6 hens each. All light treatments were started in September (fall) and continued for 24 wk. Egg production was recorded daily, by pen, for calculations of cumulative weekly egg production (eggs per hen). After 24 wk of light treatments, all hens were abruptly exposed to 20 h of light/d (20L:4D), and photoresponsiveness was evaluated, by original light treatment group, for the subsequent 6-wk period based on the relative change in egg production. The switch to 20L:4D occurred in the month of March when the mean percentage hen-day egg production among the light treatment groups had declined to about 40 to 50%. As noted earlier, photorefractoriness is programmed at the start of photostimulation, therefore this is a test of photoresponsiveness as a consequence of events occurring previously, in September.
Experiment 2 Results of Experiment 1 indicated that the CDL for photorefractoriness did not appear to change with season. Experiment 2 was undertaken to support this contention by evaluating the effect on egg production of a gradually
increasing photoperiod designed to sustain full photoperiodic drive with the least duration of daily light. If the CDL for photorefractoriness is static throughout the year and remains at 12 to 12.5 h, then a gradually increasing photoperiod starting near the CDL for photoperiodic drive in January (11 to 11.5 h), but below the CDL for photorefractoriness, should provide only a brief period following initial photostimulation in which photorefractoriness is not activated. Egg production should be normal but this delay in activation of photorefractoriness should be reflected in 1) some (slight) persistence in lay; and, 2) some difference in photoresponsiveness at the end of the typical laying period. If the CDL for photorefractoriness is not static but increases in a like manner as the CDL of photoperiodic drive, then photorefractoriness should not be activated and a clear (strong) persistence of lay should occur. In addition, photorefractoriness should be minimal or absent at the end of a typical laying period and the hens thus strongly photoresponsive. There were two treatment groups: 1) Controls, which received 16L:8D, and 2) Experimental, which received 12L:12D, gradually increasing by 6 min/wk, based on results from a previous study (Siopes, 1994). This procedure was designed to keep the daily photoperiod just above a gradually but steadily increasing CDL for photoperiodic drive as the laying season progresses. All light treatments were controlled by computer and verified by electronic recording of all on and off times. The treatments were started in January and continued for 27 wk. There were 32 hens per treatment group in four replicate pens of 8 hens each for the control group, and 56 hens in eight replicate pens of 7 hens each for the experimental group. Egg laying was recorded daily, by pen. After 27 wk of light treatments, all hens were abruptly exposed to 20L:4D to test for photorefractoriness. Photoresponsiveness was evaluated, by original light treatment group, for the subsequent 6-wk period based on the relative change in egg production. The switch of light treatments to 20L:4D occurred in the month of July.
Statistical Analysis Statistical analyses were done by one-way ANOVA and repeated measures ANOVA using the General Linear Models (GLM) procedures of the SAS Institute (1990). Statement of statistical significance are based on P < 0.05 unless specified otherwise.
RESULTS All treatments in Experiment 1 resulted in similar egg laying. However, as expected, the cumulative eggs per hen to 24 wk of lighting was slightly lower (P = 0.15) in the treatment groups receiving 12, 12.5, and 13 h light/d than in those hens receiving 16 h light/d (77, 75, and 85 eggs per hen vs 89 eggs per hen, respectively). This result was quite similar to the response reported earlier (Siopes, 1994) for hens photostimulated in August under otherwise the same conditions.
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It seems that all that would be required to sustain photoperiodic drive is to regularly increase the photoperiod from about 11.5 h during the laying period. However, results would be dependent on what happens to the CDL for photorefractoriness, that is, does it change (is it dynamic) or does it remain unchanging (is it static) during the laying period? This information is not known for any avian species. If photorefractoriness increases in a like manner as the CDL for photoperiodic drive, it should be possible to sustain photoperiodic drive without activating photorefractoriness by careful changes in photoperiod during the laying period. Lighting programs to minimize photorefractoriness in chickens by successive increment in light were without benefit (Morris et al., 1995). The primary purpose of this study was to determine whether the CDL for photorefractoriness in turkey hens is static or changing during the year. Because an earlier report (Siopes, 1994) established the CDL for photorefractoriness in turkey hens to be 12 to 12.5 h for hens photostimulated in January, the first experiment of this study was designed to obtain the same information for hens photostimulated at a different time of year, the fall. The hen strain, physical facilities, diet, and management practices were the same as in the previous study.
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After 24 wk of treatment, egg production had slowly declined to about 40 to 50% among treatments. The photoresponsiveness of the hens to an abrupt increase in light to 20L:4D at this time is shown in Figure 1. An increase in egg production within 4 wk occurred only in those hens previously exposed to 12 h of light/d. Those hens previously exposed to greater than 12L had unchanged or decreased egg production. The egg production response of hens from Experiment 2 is shown in Figure 2. The hens subjected to gradually increasing photoperiods (Experimental group) had a response that was generally the same as the controls, as cumulative eggs per hen to 24 and 27 wk of treatment was similar between treatment groups (Table 1). In addition, weekly egg production was the same or slightly less from 6 to 22 wk of treatment as determined by repeated measure ANOVA. Thereafter, a cross-over of egg production occurred, and although the experimental group remained consistently above the controls, the difference was not significant.
TABLE 1. Cumulative egg production for turkey hens exposed to gradually increasing photoperiods of 6 min/wk from 12 h light (L):12 h dark (D) for 24 and 27 wk. Controls were maintained on a fixed photoperiod of 16L:8D Eggs per hen Treatment
0 to 24 wk
0 to 27 wk
Control Increasing light RMSE P
87.9 82.5 9.6 0.38
94.3 91.3 11.2 0.67
The photoresponsiveness of the hens in Experiment 2 to an abrupt increase in light to 20L:4D at 27 wk of treatment is also shown in Figure 2. Clearly, the Controls were completely unresponsive (photorefractory), whereas the Experimental group housed under a gradually increasing photoperiod had a distinct, but weak, increase in egg production.
DISCUSSION From the results of Experiment 1, it was clear that the CDL for photorefractoriness in hens photostimulated in the fall was between 12 and 12.5 h of light/d. This response was identical to that reported earlier (Siopes, 1994) for turkey hens photostimulated in January, and suggests, but certainly does not prove, that the CDL for photorefractoriness is static during the year. If true, this result, combined with the fact that the CDL for photoperiodic drive increases during the year from about 11 h in winter to greater than 14 h in summer (Siopes, 1994), provides considerable insight into the response of turkey hens to different durations of photoperiod. Thus, any daily photoperiod greater than 12 h will activate processes leading to photorefractoriness and, based on other reports (see the review of Nicholls et al., 1988), the greater the duration of light the sooner the onset and effects of photorefractoriness on depressing egg production. This conclusion would argue against the practical use of unnecessarily long photoperiods. Only for a transient period during winter could CDL for photoperiodic drive be optimal without activating photorefractoriness because normal cumulative egg production can be attained with 11.5 to 12 h of light/d
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FIGURE 1. Photoresponsiveness of hens to a photoperiod of 20 h light (L):4 h dark (D)/d following 24 wk exposure to photoperiods ranging from 12 to 16 h of light/d. Photoresponsiveness was evaluated by egg production for 4 wk following 20L:4D. Data are expressed as mean percentage change in egg production from that at the start of 20L:4D. The mean hen-day egg production ranged from 40 to 50% among treatments at the start. Original day length treatments (hours of light per day): 16 (◊), 14 (◊), 13.5 (»), 13 (o), 12.5 (∫), and 12 (ÿ).
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for hens photostimulated in winter (Siopes, 1994). This result is consistent with those of Experiment 2 in the present study (Figure 2). That the CDL for photorefractoriness would be unchanging during the year seems to make sound biological sense. Photorefractoriness is a major mechanism regulating termination of reproductive function and is generally regarded to exist to assure that birds in nature produce offspring only at a time of year with optimum chances of success, generally late winter and spring. Therefore, a static low level CDL for photorefractoriness, generally lower than the CDL for photoperiodic drive, could be of value to assure that the mechanism can be activated over a wide range of photoperiodic drive conditions that induce egg laying. Interestingly, this baseline threshold (12 to 12.5 h) is similar to natural day lengths occurring at the equator, where avian breeding periods are generally much longer than at other latitudes. Unfortunately, the presence of photorefractoriness presents a problem for year-around production desired in commercial bird industries. Results of the present study, plus those of Siopes (1994) allow a reasonable speculation on why photoperiods ranging from 12 to 16 h (or more) of light/d can produce the same cumulative eggs per hen during a full lay period (24 wk) started in winter. Cumulative eggs per hen are the same but the temporal pattern of how this was accomplished is not. The patterns are different because of different effects on photorefractoriness and photoperiodic drive as a consequence of a static CDL for photorefractoriness and an increasing CDL for photoperiodic drive. For example: 11.5 to 12 h of light/d are barely above the CDL for photoperiodic drive but below
the CDL for photorefractoriness. Thus, these photoperiods provide reduced photoperiodic drive but also decreased photorefractoriness, the combined effect of which results in slowed onset of lay and reduced peak of lay but persistence in lay. Egg production on 16 h (or more) of light/d has fast onset, a high sharp peak, but a strong spontaneous decline due to strong photoperiodic drive and increased photorefractoriness, respectively. An increasing CDL for photoperiodic drive and a static CDL for photorefractoriness are also consistent with reduced egg production during summer. Longer photoperiods are required in summer than winter to get normal egg production and these longer photoperiods accelerate the effects of photorefractoriness. In the present study, the gradually increasing photoperiod treatment group (Experiment 2) started with 12 h of light in winter and increased 6 min/wk. This photoperiod was therefore above the CDL for photoperiodic drive and initially, but temporarily, just below the CDL for photorefractoriness. Cumulative egg production was similar to fixed-phase control hens on 16L:8D, whereas weekly egg production was generally lower and a clear but slight persistence in lay occurred (Figure 2). Furthermore these hens, in contrast to controls, had at least some degree of photoresponsiveness remaining at the end of the lay period (27 wk of photostimulation). In turkey hens on 14 h of light/d, the mean onset of photorefractoriness has been reported to be 23 wk of photostimulation (Lien and Siopes, 1989). A reasonable explanation of the response of these hens is that the gradually increasing light treatment did not prevent photorefractoriness, but for a short time early in the year it did not activate, or only marginally
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FIGURE 2. Mean weekly percentage hen-day egg production of turkey hens exposed for 27 wk to either a fixed daily photoperiod of 16 h light (L):8 h dark (D) or a photoperiod gradually increasing from 12L:12D at 6 min/wk. At 27 wk of treatment all hens were given a photoperiod of 20L: 4D to evaluate photoresponsiveness. Inserted are the statistical summaries from repeated measures ANOVA during the 27-wk treatment period.
CRITICAL DAY LENGTH FOR PHOTOREFRACTORINESS
ACKNOWLEDGMENT The author gratefully acknowledges the excellent technical assistance of Robert Neely.
REFERENCES Bacon, W. L., and K. E. Nestor, 1977. The effect of various lighting treatments or the presence of toms on reproductive performance of hen turkeys. Poultry Sci. 56:415–420. Lien, R. J., and T. D. Siopes, 1989. Turkey plasma thyroid hormone and prolactin concentrations throughout an egg
laying cycle and in relation to photorefractoriness. Poultry Sci. 68:1409–1417. Marsden, S. J., W. S. Cowan, and L. M. Lucas, 1962. Effect of gradual and abrupt lengthening of photoperiod on reproduction responses of turkeys. Poultry Sci. 41: 1864–1868. McCartney, M. G., V. L. Sanzer, K. I. Brown, and V. D. Chamberlin, 1961. Photoperiodism as a factor in the reproduction of the turkey. Poultry Sci. 40:368–376. Morris, T. R., P. J. Sharp, and E. A. Butler, 1995. A test for photorefractoriness in high-producing stocks of laying pullets. Br. Poult. Sci. 36:763–769. Nicholls, T. J., A. R. Goldsmith, and A. Dawson, 1988. Photorefractoriness in birds and in comparison with mammals. Physiol. Rev. 68:133–176. Reinert, B. D., and F. E. Wilson, 1996. Thyroid dysfunction and thyroxine dependent programming of photoinduced ovarian growth in American tree sparrows (Spizella arborea). Gen. Comp. Endocrinol. 103:71–81. SAS Institute, 1990. SAS/STAT User’s Guide. Version 6, Fourth Edition. Vol. 2. SAS Institute Inc., Cary, NC. Sharp, P. J., 1993. Photoperiodic control of reproduction in the domestic hen. Poultry Sci. 72:897–905. Sharp, P. J., 1996. Strategies in avian breeding cycles. Anim. Reprod. Sci. 42:505–513. Siopes, T. D., 1994. Critical day lengths for egg production and photorefractoriness in the domestic turkey. Poultry Sci. 73: 1906–1913. Wilson, F. E., and B. D. Reinert, 1993. The thyroid and photoperiodic control of seasonal reproduction in American tree sparrows (Spizella arborea) J. Comp. Physiol. B 63: 563–573. Wilson, F. E., and B. D. Reinert, 1996. The timing of thyroid dependent programming in seasonally breeding male American tree sparrows (Spizella arborea). Gen. Comp. Endocrinol. 103:82–92.
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activated, photorefractoriness. This light treatment delayed the onset sufficiently that the full effects of photorefractoriness had not occurred by 27 wk of treatment. Thus, there were slightly different degrees of photorefractoriness between the two light treatment groups, which is consistent with the idea that CDL for photorefractoriness remains static at about 12 to 12.5 h during the year. Had the CDL for photorefractoriness become elevated, especially early after lighting, the degree of photoresponsiveness would be expected to be much greater than what was observed. The overall egg laying performance by turkey hens in gradually increasing photoperiods is consistent with existing literature (McCartney et al., 1961; Marsden et al., 1962; Bacon and Nestor, 1977). That is, there were no distinct advantages over fixed photoperiods even when used at an optimum time of year to sustain photoperiodic drive and minimize photorefractoriness. Morris et al. (1995) have also recently reported no advantage to using increasing photoperiods designed to minimize photorefractoriness in chickens.
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(Key words: turkey, light, photorefractoriness, critical day length, photoperiod) 1998 Poultry Science 77:145–149
sion and subsequent molt and remains a major mechanism limiting the continuous production of semen and eggs in birds. Both photoperiodic drive and photorefractoriness are activated at the start of exposure of photosensitive birds to photoperiods long enough to induce gonadal development (Nicholls et al., 1988; Wilson and Reinert, 1993, 1996; Reinert and Wilson, 1996). However, in turkey hens the full expression of photorefractoriness is delayed and starts at about 18 wk of continuous photostimulation with 14 h of light/d (Lien and Siopes, 1989). That the CDL for photoperiodic drive may be less than that for photorefractoriness has important practical implications in the development of lighting schedules to promote one without the other. If a photoperiod is utilized that stimulates egg production but does not activate photorefractoriness, then a greater persistence of lay should occur, resulting in more eggs per hen. This difference was addressed in the report by Siopes (1994) but with marginal success, presumably because the fixed photoperiod that was used (11.5 to 12 h) provided insufficient photoperiodic drive in the face of an increasing CDL for photoperiodic drive as the laying period progressed. This result will be re-evaluated in the present study in Experiment 2.
INTRODUCTION With respect to photoperiodic responses of birds, the minimum duration of a daily photoperiod that induces an effect is referred to as the critical day length (CDL). Sharp (1993) has reported minimum and maximum CDL for the domestic hen without consideration of season. In a prior report, it was noted that the CDL for egg production by turkeys was not fixed but rather varied by time of year (Siopes, 1994). In addition, the CDL for egg production was different (less) than that for photorefractoriness for hens photostimulated in January (11 to 11.5 vs 12 to 12.5 h, respectively). In general, photoperiodic drive refers to the degree of switching-on of reproductive function (egg production) by light, whereas photorefractoriness refers to the degree of switching-off of reproductive responses to light. These have been addressed for the domestic hen (Sharp, 1993) and other avian species (Sharp, 1996). Photorefractoriness is associated with gonadal regres-
Received for publication January 30, 1997. Accepted for publication August 14, 1997. 1The use of trade names in this publication does not imply endorsement by the North Carolina Agricultural Research Service of the products mentioned, nor criticism of similar products not mentioned. 2To whom correspondence should be addressed: [email protected]
Abbreviation Key: CDL = critical day length; D = dark; L = light.
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obtained in the earlier report for winter-photostimulated hens. A second experiment was done to further test the possibility that the CDL for photorefractoriness was static during the year. A gradually increasing photoperiod was started in winter at 12L:12D, just under the CDL for photorefractoriness but just over the CDL for egg production. Subsequent egg production and expression of photorefractoriness were compared to controls to estimate the dynamics of the CDL for photorefractoriness. Results supported a static CDL for photorefractoriness and suggest that any daily photoperiod exceeding 12 h will activate processes leading to photorefractoriness, independent of season.
ABSTRACT Two experiments were conducted to determine whether the critical day length (CDL) for photorefractoriness remained stable or varied by season of the year. The first experiment was done in the fall with photosensitive hens that were exposed to fixed day length treatments ranging from 12 to 16 h/d for 24 wk. At 24 wk of treatment, all hens were given 20 h light (L): 4 h dark (D) and subsequent changes in egg production were used to evaluate photorefractoriness. This experimental approach was the same as that used by Siopes (1994) to estimate the CDL for photorefractoriness in winter-photostimulated hens. It was determined that the CDL for photorefractoriness in fallphotostimulated hens was 12 to 12.5 h, the same as that
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MATERIALS AND METHODS
Experiment 1 Yearling Large White turkey breeder hens (Nicholas) at the end of their first season of lay were light-restricted with 8 h of light/d [8 h light (L):16 h dark (D)] for 8 wk to ensure photosensitivity. Hens were then exposed to one of the following light treatments: 12, 12.5, 13, 13.5, 14, or 16 h of light/d. There were 24 hens per treatment group in four replicate floor pens of 6 hens each. All light treatments were started in September (fall) and continued for 24 wk. Egg production was recorded daily, by pen, for calculations of cumulative weekly egg production (eggs per hen). After 24 wk of light treatments, all hens were abruptly exposed to 20 h of light/d (20L:4D), and photoresponsiveness was evaluated, by original light treatment group, for the subsequent 6-wk period based on the relative change in egg production. The switch to 20L:4D occurred in the month of March when the mean percentage hen-day egg production among the light treatment groups had declined to about 40 to 50%. As noted earlier, photorefractoriness is programmed at the start of photostimulation, therefore this is a test of photoresponsiveness as a consequence of events occurring previously, in September.
Experiment 2 Results of Experiment 1 indicated that the CDL for photorefractoriness did not appear to change with season. Experiment 2 was undertaken to support this contention by evaluating the effect on egg production of a gradually
increasing photoperiod designed to sustain full photoperiodic drive with the least duration of daily light. If the CDL for photorefractoriness is static throughout the year and remains at 12 to 12.5 h, then a gradually increasing photoperiod starting near the CDL for photoperiodic drive in January (11 to 11.5 h), but below the CDL for photorefractoriness, should provide only a brief period following initial photostimulation in which photorefractoriness is not activated. Egg production should be normal but this delay in activation of photorefractoriness should be reflected in 1) some (slight) persistence in lay; and, 2) some difference in photoresponsiveness at the end of the typical laying period. If the CDL for photorefractoriness is not static but increases in a like manner as the CDL of photoperiodic drive, then photorefractoriness should not be activated and a clear (strong) persistence of lay should occur. In addition, photorefractoriness should be minimal or absent at the end of a typical laying period and the hens thus strongly photoresponsive. There were two treatment groups: 1) Controls, which received 16L:8D, and 2) Experimental, which received 12L:12D, gradually increasing by 6 min/wk, based on results from a previous study (Siopes, 1994). This procedure was designed to keep the daily photoperiod just above a gradually but steadily increasing CDL for photoperiodic drive as the laying season progresses. All light treatments were controlled by computer and verified by electronic recording of all on and off times. The treatments were started in January and continued for 27 wk. There were 32 hens per treatment group in four replicate pens of 8 hens each for the control group, and 56 hens in eight replicate pens of 7 hens each for the experimental group. Egg laying was recorded daily, by pen. After 27 wk of light treatments, all hens were abruptly exposed to 20L:4D to test for photorefractoriness. Photoresponsiveness was evaluated, by original light treatment group, for the subsequent 6-wk period based on the relative change in egg production. The switch of light treatments to 20L:4D occurred in the month of July.
Statistical Analysis Statistical analyses were done by one-way ANOVA and repeated measures ANOVA using the General Linear Models (GLM) procedures of the SAS Institute (1990). Statement of statistical significance are based on P < 0.05 unless specified otherwise.
RESULTS All treatments in Experiment 1 resulted in similar egg laying. However, as expected, the cumulative eggs per hen to 24 wk of lighting was slightly lower (P = 0.15) in the treatment groups receiving 12, 12.5, and 13 h light/d than in those hens receiving 16 h light/d (77, 75, and 85 eggs per hen vs 89 eggs per hen, respectively). This result was quite similar to the response reported earlier (Siopes, 1994) for hens photostimulated in August under otherwise the same conditions.
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It seems that all that would be required to sustain photoperiodic drive is to regularly increase the photoperiod from about 11.5 h during the laying period. However, results would be dependent on what happens to the CDL for photorefractoriness, that is, does it change (is it dynamic) or does it remain unchanging (is it static) during the laying period? This information is not known for any avian species. If photorefractoriness increases in a like manner as the CDL for photoperiodic drive, it should be possible to sustain photoperiodic drive without activating photorefractoriness by careful changes in photoperiod during the laying period. Lighting programs to minimize photorefractoriness in chickens by successive increment in light were without benefit (Morris et al., 1995). The primary purpose of this study was to determine whether the CDL for photorefractoriness in turkey hens is static or changing during the year. Because an earlier report (Siopes, 1994) established the CDL for photorefractoriness in turkey hens to be 12 to 12.5 h for hens photostimulated in January, the first experiment of this study was designed to obtain the same information for hens photostimulated at a different time of year, the fall. The hen strain, physical facilities, diet, and management practices were the same as in the previous study.
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147
After 24 wk of treatment, egg production had slowly declined to about 40 to 50% among treatments. The photoresponsiveness of the hens to an abrupt increase in light to 20L:4D at this time is shown in Figure 1. An increase in egg production within 4 wk occurred only in those hens previously exposed to 12 h of light/d. Those hens previously exposed to greater than 12L had unchanged or decreased egg production. The egg production response of hens from Experiment 2 is shown in Figure 2. The hens subjected to gradually increasing photoperiods (Experimental group) had a response that was generally the same as the controls, as cumulative eggs per hen to 24 and 27 wk of treatment was similar between treatment groups (Table 1). In addition, weekly egg production was the same or slightly less from 6 to 22 wk of treatment as determined by repeated measure ANOVA. Thereafter, a cross-over of egg production occurred, and although the experimental group remained consistently above the controls, the difference was not significant.
TABLE 1. Cumulative egg production for turkey hens exposed to gradually increasing photoperiods of 6 min/wk from 12 h light (L):12 h dark (D) for 24 and 27 wk. Controls were maintained on a fixed photoperiod of 16L:8D Eggs per hen Treatment
0 to 24 wk
0 to 27 wk
Control Increasing light RMSE P
87.9 82.5 9.6 0.38
94.3 91.3 11.2 0.67
The photoresponsiveness of the hens in Experiment 2 to an abrupt increase in light to 20L:4D at 27 wk of treatment is also shown in Figure 2. Clearly, the Controls were completely unresponsive (photorefractory), whereas the Experimental group housed under a gradually increasing photoperiod had a distinct, but weak, increase in egg production.
DISCUSSION From the results of Experiment 1, it was clear that the CDL for photorefractoriness in hens photostimulated in the fall was between 12 and 12.5 h of light/d. This response was identical to that reported earlier (Siopes, 1994) for turkey hens photostimulated in January, and suggests, but certainly does not prove, that the CDL for photorefractoriness is static during the year. If true, this result, combined with the fact that the CDL for photoperiodic drive increases during the year from about 11 h in winter to greater than 14 h in summer (Siopes, 1994), provides considerable insight into the response of turkey hens to different durations of photoperiod. Thus, any daily photoperiod greater than 12 h will activate processes leading to photorefractoriness and, based on other reports (see the review of Nicholls et al., 1988), the greater the duration of light the sooner the onset and effects of photorefractoriness on depressing egg production. This conclusion would argue against the practical use of unnecessarily long photoperiods. Only for a transient period during winter could CDL for photoperiodic drive be optimal without activating photorefractoriness because normal cumulative egg production can be attained with 11.5 to 12 h of light/d
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FIGURE 1. Photoresponsiveness of hens to a photoperiod of 20 h light (L):4 h dark (D)/d following 24 wk exposure to photoperiods ranging from 12 to 16 h of light/d. Photoresponsiveness was evaluated by egg production for 4 wk following 20L:4D. Data are expressed as mean percentage change in egg production from that at the start of 20L:4D. The mean hen-day egg production ranged from 40 to 50% among treatments at the start. Original day length treatments (hours of light per day): 16 (◊), 14 (◊), 13.5 (»), 13 (o), 12.5 (∫), and 12 (ÿ).
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for hens photostimulated in winter (Siopes, 1994). This result is consistent with those of Experiment 2 in the present study (Figure 2). That the CDL for photorefractoriness would be unchanging during the year seems to make sound biological sense. Photorefractoriness is a major mechanism regulating termination of reproductive function and is generally regarded to exist to assure that birds in nature produce offspring only at a time of year with optimum chances of success, generally late winter and spring. Therefore, a static low level CDL for photorefractoriness, generally lower than the CDL for photoperiodic drive, could be of value to assure that the mechanism can be activated over a wide range of photoperiodic drive conditions that induce egg laying. Interestingly, this baseline threshold (12 to 12.5 h) is similar to natural day lengths occurring at the equator, where avian breeding periods are generally much longer than at other latitudes. Unfortunately, the presence of photorefractoriness presents a problem for year-around production desired in commercial bird industries. Results of the present study, plus those of Siopes (1994) allow a reasonable speculation on why photoperiods ranging from 12 to 16 h (or more) of light/d can produce the same cumulative eggs per hen during a full lay period (24 wk) started in winter. Cumulative eggs per hen are the same but the temporal pattern of how this was accomplished is not. The patterns are different because of different effects on photorefractoriness and photoperiodic drive as a consequence of a static CDL for photorefractoriness and an increasing CDL for photoperiodic drive. For example: 11.5 to 12 h of light/d are barely above the CDL for photoperiodic drive but below
the CDL for photorefractoriness. Thus, these photoperiods provide reduced photoperiodic drive but also decreased photorefractoriness, the combined effect of which results in slowed onset of lay and reduced peak of lay but persistence in lay. Egg production on 16 h (or more) of light/d has fast onset, a high sharp peak, but a strong spontaneous decline due to strong photoperiodic drive and increased photorefractoriness, respectively. An increasing CDL for photoperiodic drive and a static CDL for photorefractoriness are also consistent with reduced egg production during summer. Longer photoperiods are required in summer than winter to get normal egg production and these longer photoperiods accelerate the effects of photorefractoriness. In the present study, the gradually increasing photoperiod treatment group (Experiment 2) started with 12 h of light in winter and increased 6 min/wk. This photoperiod was therefore above the CDL for photoperiodic drive and initially, but temporarily, just below the CDL for photorefractoriness. Cumulative egg production was similar to fixed-phase control hens on 16L:8D, whereas weekly egg production was generally lower and a clear but slight persistence in lay occurred (Figure 2). Furthermore these hens, in contrast to controls, had at least some degree of photoresponsiveness remaining at the end of the lay period (27 wk of photostimulation). In turkey hens on 14 h of light/d, the mean onset of photorefractoriness has been reported to be 23 wk of photostimulation (Lien and Siopes, 1989). A reasonable explanation of the response of these hens is that the gradually increasing light treatment did not prevent photorefractoriness, but for a short time early in the year it did not activate, or only marginally
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FIGURE 2. Mean weekly percentage hen-day egg production of turkey hens exposed for 27 wk to either a fixed daily photoperiod of 16 h light (L):8 h dark (D) or a photoperiod gradually increasing from 12L:12D at 6 min/wk. At 27 wk of treatment all hens were given a photoperiod of 20L: 4D to evaluate photoresponsiveness. Inserted are the statistical summaries from repeated measures ANOVA during the 27-wk treatment period.
CRITICAL DAY LENGTH FOR PHOTOREFRACTORINESS
ACKNOWLEDGMENT The author gratefully acknowledges the excellent technical assistance of Robert Neely.
REFERENCES Bacon, W. L., and K. E. Nestor, 1977. The effect of various lighting treatments or the presence of toms on reproductive performance of hen turkeys. Poultry Sci. 56:415–420. Lien, R. J., and T. D. Siopes, 1989. Turkey plasma thyroid hormone and prolactin concentrations throughout an egg
laying cycle and in relation to photorefractoriness. Poultry Sci. 68:1409–1417. Marsden, S. J., W. S. Cowan, and L. M. Lucas, 1962. Effect of gradual and abrupt lengthening of photoperiod on reproduction responses of turkeys. Poultry Sci. 41: 1864–1868. McCartney, M. G., V. L. Sanzer, K. I. Brown, and V. D. Chamberlin, 1961. Photoperiodism as a factor in the reproduction of the turkey. Poultry Sci. 40:368–376. Morris, T. R., P. J. Sharp, and E. A. Butler, 1995. A test for photorefractoriness in high-producing stocks of laying pullets. Br. Poult. Sci. 36:763–769. Nicholls, T. J., A. R. Goldsmith, and A. Dawson, 1988. Photorefractoriness in birds and in comparison with mammals. Physiol. Rev. 68:133–176. Reinert, B. D., and F. E. Wilson, 1996. Thyroid dysfunction and thyroxine dependent programming of photoinduced ovarian growth in American tree sparrows (Spizella arborea). Gen. Comp. Endocrinol. 103:71–81. SAS Institute, 1990. SAS/STAT User’s Guide. Version 6, Fourth Edition. Vol. 2. SAS Institute Inc., Cary, NC. Sharp, P. J., 1993. Photoperiodic control of reproduction in the domestic hen. Poultry Sci. 72:897–905. Sharp, P. J., 1996. Strategies in avian breeding cycles. Anim. Reprod. Sci. 42:505–513. Siopes, T. D., 1994. Critical day lengths for egg production and photorefractoriness in the domestic turkey. Poultry Sci. 73: 1906–1913. Wilson, F. E., and B. D. Reinert, 1993. The thyroid and photoperiodic control of seasonal reproduction in American tree sparrows (Spizella arborea) J. Comp. Physiol. B 63: 563–573. Wilson, F. E., and B. D. Reinert, 1996. The timing of thyroid dependent programming in seasonally breeding male American tree sparrows (Spizella arborea). Gen. Comp. Endocrinol. 103:82–92.
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activated, photorefractoriness. This light treatment delayed the onset sufficiently that the full effects of photorefractoriness had not occurred by 27 wk of treatment. Thus, there were slightly different degrees of photorefractoriness between the two light treatment groups, which is consistent with the idea that CDL for photorefractoriness remains static at about 12 to 12.5 h during the year. Had the CDL for photorefractoriness become elevated, especially early after lighting, the degree of photoresponsiveness would be expected to be much greater than what was observed. The overall egg laying performance by turkey hens in gradually increasing photoperiods is consistent with existing literature (McCartney et al., 1961; Marsden et al., 1962; Bacon and Nestor, 1977). That is, there were no distinct advantages over fixed photoperiods even when used at an optimum time of year to sustain photoperiodic drive and minimize photorefractoriness. Morris et al. (1995) have also recently reported no advantage to using increasing photoperiods designed to minimize photorefractoriness in chickens.
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