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Effect of increasing day length and intermittent lighting schedules in the domestic hen on plasma concentrations of luteinizing hormone (LH) and the LH response to exogenous progesterone
GENERAL
AND
COMPARATIVE
ENDOCRINOLOGY
41, 546-553 (1980)
Effect of Increasing Day Length and Intermittent Lighting Schedules in the Domestic Hen on Plasma Concentrations of Luteinizing Hormone (LH) and the LH Response to Exogenous Progesterone S.C. WILSON Department
of Physiology
AND F.J.
CUNNINGHAM
and Biochemistry. The University Reading RG6 2AJ, England
of Reading,
Whiteknights,
Accepted February 15, 1980 Pullets were raised under a constant photoperiod of 8L:16D until 17 weeks of age at which time they were subjected either to intermittent pulses of light superimposed on a photoperiod of 8L:16D or to a photoperiod of 16L:8D. Exposure of hens, maintained under a lighting schedule of 8L:16D, to hourly pulses of light for either 10 set, 1 mitt, or 10 min during the first 8 hr of darkness or to an increased photoperiod of 16L:8D stimulated a rise in the plasma concentration of LH within 3 days and slightly advanced the onset of lay. About 6 weeks after the onset of lay the plasma concentrations of LH and the rate of lay were similar in hens maintained under a long photoperiod (16L:8D) and a short photoperiod (8L:16D) supplemented by l- or IO-min pulses of light at hourly intervals. In each of these groups the parameters measured were significantly (P < 0.02) greater than in hens exposed either to an uninterrupted short photoperiod (8L:16D) or to the photoperiod on which was superimposed IO-set pulses of light. The increased plasma concentration of LH and rate of lay observed in hens kept on a long photoperiod (16L:8D) were associated with an enhanced LH response to progesterone compared to that measured in hens exposed to 8 hr light per day.
Several studies have shown that the age at which sexual maturity is reached in the domestic hen is influenced to some extent by the number of hours of light to which the birds are exposed each day (King, 1961; McCluskey and Parker, 1963; Morris, 1967). A greater influence, however, is exerted by a change in the length of the photoperiod (Morris, 1962, 1967, 1979; Harrison et al., 1969). Thus, hens raised throughout sexual development under a long-day photoperiod of 14 hr light and 10 hr darkness (14L: 10D) lay their first egg only 7-10 days earlier than hens raised under a short-day photoperiod of 6L:18D (Morris, 1967). If, however, birds maintained under a short day length are transferred to a long day length the onset of lay can be advanced by as many as 7 weeks, depending on the age at which the transfer took place (Morris, 1962; Harrison et al., 1969). Similarly, egg production increases 546 0016~6480/80/080546-08$01.00/O Copyright All tights
@ 1980 by Academic Press, Inc. of reproduction in any form reserved.
in laying hens exposed to an increase in day length from 6 to 14 hr light per day to achieve a rate of lay as high as (Sykes, 1956) or higher than (Morris et al., 1964) that of birds raised on a constant 14-hr day length. An increase in egg production has also been achieved by the use of intermittent lighting whereby the hours of darkness are interrupted by pulses of light. In this way, hens exposed to intermittent short pulses of light achieve a higher rate of lay than do hens receiving the same amount of light in a single long pulse (Fox and Morris, 1958; Wilson and Abplanalp, 1956; Bell and Moreng, 1973). Sexual maturity is also advanced by intermittent lighting treatments in the house finch (Hamner, 1964), whitecrowned sparrow (Farner et al., 1953), and quail (Wilson et al., 1962; Follett and Sharp, 1969). In hens raised throughout sexual devel-
PHOTOPERIOD
AND LUTEINIZING
opment under a constant photoperiod of either 14 or 16 hr light per day, the plasma concentration of luteinizing hormone (LH) increases to reach a prepubertal maximum value at 3 to 4 weeks before the onset of lay. The levels of LH then fall progressively until the time of the first ovulation (Sharp, 1975; Wilson and Sharp, 1975; Wilson, 1978). With respect to LH secretion it was relevant to examine the effect on the plasma concentration of LH during sexual development of an abrupt change in photoperiod from 8 to 16 hr light per day and to ascertain to what extent the provision of pulses of light at regular intervals during the hours of darkness, to hens maintained under a short photoperiod of 8L: 16D, could substitute for a long photoperiod. The pulses of light were provided during the first 8 hr of darkness in order that the total period of uninterrupted darkness would be of equal duration to, and coincide with, that of birds maintained under 16L:8D. Also, the possibility of a relationship between photoperiod and capacity of the hypothalamus-pituitary system to respond to the injection of progesterone by a release of LH was compared in laying hens maintained under long 16L:8D and short 8L: 16D day lengths. MATERIALS
AND METHODS
Pullets of the Ross tint strain were housed under a photoschedule of 8 hr light, 16 hr darkness (8L:16D) until 17 weeks of age at which time they were transferred to individual cages and subjected to one of the following lighting treatments: Treatment 1 8L: 16D (lights on 0800 to 16:00 hr) Treatments 2, 3. 4 8L:16D with pulses of either 10 sec. I min, or IO min light at hourly intervals for the first 8 hr of darkness Treatment 5 16L:8D (lights on 08:OOto 24:00 hr)
HORMONE
RELEASE
IN HENS
547
sion. Blood (1.5 ml) was withdrawn by venepuncture into heparinized syringes during the period of the day when no preovulatory surges of LH would be expected to occur. The blood was centrifuged at 18OOgand the plasma was separated and stored at -20” until assayed for LH. LH was measured in the plasma of the 19 pullets sampled during weeks 11 to 16 and, from week 17 onward, in the plasma of 15 of the 20 pullets sampled from each lighting treatment whose age at onset of lay was closest to the age at which 50% of the total hens in each treatment group were in lay. LH and egg production. The age at onset of lay and the number of eggs laid by each hen for the duration of the experiment were noted. Times of oviposition were recorded to the nearest minute by means of an automatic recording device. The plasma concentrations of LH in blood samples taken from 40 hens in each treatment group at 27 weeks of age were compared with the number of eggs laid by each hen during the 15 days before and 10 days after the sample was taken. LH response to progesterone. At 30 weeks of age, 24 hens maintained under either 16L:8D or 8L:16D were given a single intramuscular injection of either 0.25 or 0.5 mg progesterone/kg body wt dissolved in 0.4 ml arachis oil. Injections were given at 8 hr after a midsequence ovulation. The time of ovulation was predicted to occur within 14 to 75 min of an oviposition (Warren and Scott, 1935). The concentration of LH was measured in plasma taken immediately before the injection and at intervals of 30 min thereafter for a period of 3.5 hr. Assay of LH. LH levels were measured using the radioimmunoassay system described by Follett et al., (1972). Each plasma sample was assayed in triplicate at one dilution and all plasma samples from one hen were assayed in the same assay. In the seven assays required to complete this study, the minimum detectable level of LH ranged between 0.04 and 0.09 r&ml (mean, 0.065 ? 0.006 ngiml) and the potency of a pooled sample of plasma included in each assay ranged between 5.24 and 5.77 ngiml (mean, 5.47 2 0.07 @ml)
RESULTS LH During Sexual Development
The effect of the various lighting treatments on the plasma concentrations of LH is shown in Fig. Each treatment group consisted of 96 birds and food during sexual development and water were available at all times. 1. The plasma concentration of LH rose LH during sexual de\,elopment. Between 11 and 16 gradually and was significantly (P < 0.01) weeks of age blood samples were withdrawn at weekly higher at 117 days of age than at 85 days in intervals from 19 pullets taken at random. From 17 under 8L: 16D. However, weeks to the onset of lay, blood samples were taken hens maintained within 3 days of an increase in photoperiod twice weekly from 20 hens in each of the five treatment groups. The same 20 hens were sampled on each occa- from 8 to 16 hr light per day, the plasma
548
WILSON
AND CUNNINGHAM
67-
= E s c
6-
5
: if4 I h
3-
11 77
12 64
13 91
14
15
96
105
16 112
17 119
18 126
19 weeks 133 days
Age
1. Plasma concentrations of LH in hens raised under 8L: 16D until 17 weeks of age at which time they were subjected to the following lighting treatments. 8L:16D l &; 8L:16D with eight pulses of light at hourly intervals for the first 8 hr of darkness of 10 set A---A, 1 min 0. ‘0, 10 min A-. -. -A duration; 16L:8D n 4. Up to 17 weeks values are means from 19 hens taken at random each week from the total flock of hens. From 17 weeks onward the values are the means from the same 15 of % hens subjected at that age to one of the above lighting treatments. Vertical lines renresent SEM and are not shown for three of the treatments to avoid confusion. Range of SEM: 0.21-1.21 &ml. FIG.
concentration of LH rose steeply and sig- set pulses of light whereas LH levels in nificantly (P < 0.001) from 3.43 -+ 0.63 birds with an uninterrupted period of darkrig/ml (mean + SEM, n = 19) to 8.39 + 1.21 ness, i.e., no pulses of light, did not rise rig/ml (n = 15). In contrast, the levels of LH significantly during the same period. in hens maintained under 8L:16D did not After attaining mean maximal LH conrise significantly during the same period. centrations at 124 to 127 days of age (18 The interruption of the first half of the weeks), LH levels in all treatment groups darkness period of an 8L:16D photofell until the onset of lay and at that time the schedule with eight hourly pulses of either 1 plasma concentration of LH in hens under or 10 min light also stimulated significant the long-day-length photoschedule of increases (P < 0.001) in the plasma con- 16L:8D or under a short photoperiod of centrations of LH from 3.43 ? 0.63 rig/ml (n 8L:16D supplemented by pulses of 1 or 10 = 19) to maximal values of 7.51 2 0.89 min light during the first 8 hr of darkness rig/ml (n = 15) and 8.17 5 0.69 rig/ml (n = were significantly higher (P < 0.05) than in 15), respectively, within 7 days (Fig. 1). hens given lo-set pulses or no pulses of Both the rate and magnitude of the LH in- light (Fig. 2). creases, however, were greater in hens receiving 16 hr light per day than in hens re- LH and Egg Production ceiving 8 hr light and light pulses of l- or The age at which the 96 hens maintained IO-min duration (Fig. 1). A small, though under each of the different lighting treatsignificant (P < 0.05), increase in the ments came into lay is shown in Fig. 3. It plasma concentration of LH to 5.21 + 0.57 can be seen that 50% of hens under 16L:8D &ml (n = 15) occurred in response to lo- and 8L: 16D were in lay by 146 and 151 days
PHOTOPERIOD
AND LUTEINIZING
HORMONE
RELEASE
IN HENS
549
plasma concentration of LH and the rate of lay were significantly higher (P < 0.02) in the hens exposed to 16L:8D and in hens under 8L:16D supplemented with l- or IO-min pulses of light than in hens on the uninterrupted short photoperiod or on the short photoperiod supplemented with loset pulses of light (Table l), though the differences in the plasma concentration of LH between the treatment groups were not so marked as at the onset of lay.
2
E 26 c
LH Response to Progesterone 2
Treatment of 30-week-old hens maintained under 16L:8D or 8L:16D with a I single intramuscular injection of 0.5 mg -1 0 -3 -2 Weeks before first egg progesterone/kg body wt provoked inFIG. 2. Mean plasma concentrations of LH in rela- creases of 97.0 and 96.8%, respectively, in tion to the onset of lay in groups of 15 hens raised the plasma concentration of LH within 2.0 under 8L: 16D until 17 weeks of age and then subjected to 2.5 hr of injection (Fig. 4a). The adminto the following lighting treatments: 8L: 16D 0-O; how8L: 16D with eight pulses of light at hourly intervals for istration of 0.25 mg progesterone/kg, the first 8 hr of darkness of 10 set A- - -A, 1 min ever, while provoking an increase in the 0.‘..O, IOminA-.-.Aduration; 16L:8Dm-1. plasma concentration of LH in hens mainRange of SEM: 0.14- 1.14 @ml. tained under 16L:SD failed to stimulate LH release in hens maintained under 8L:16D of age, respectively (approximately 21 (Fig. 4b), so that during 2.0 to 3.5 hr after weeks). The provision of pulses of 10 set, 1 injection the plasma LH level in the hens kept under 16L:8D was significantly (P -=c min, and 10 min light at hourly intervals during the first 8 hr of the dark period of the 0.01) higher than that in hens on the short 8L: 16D photoschedule advanced the age at photoperiod. which 50% of hens were ‘in lay by 2, 3, 4, DISCUSSION and 5 days, respectively. The mean age at onset of lay in the 15 hens of each treatment A slight advancement in the onset of lay group whose plasma concentrations of LH as a result of increasing the photoperiod were measured during sexual development from 8 to 16 hr light per day in hens at 17 was the same as, or in one group (1-min weeks of age is in agreement with previous pulses of light) 1 day later than, the date at observations by Morris and Fox (1958) and 50% lay for the total of 96 hens under each Morris (1967). treatment. The present study has shown that an inThe age at which 90% of hens were in lay crease in day length advances sexual matwas the same in hens given lo-set pulses of uration by stimulating the release of LH, light as those on an uninterrupted schedule and perhaps, as in quail (Follett, 1976) and of 8L:16D, i.e., 163 days (23 weeks). Howturkeys (Godden and Scanes, 1977), ever, 90% of hens given l- or lo-min pulses follicle-stimulating hormone (FSH). A rapid of light or a photoschedule of 16L:8D were rise in the blood concentration of LH in rein lay 7 to 9 days earlier, i.e., 154- 156 days sponse to increased photoperiod from 8 to (22 weeks) (Fig. 3). 16 hr light per day was evident within 3 At 27 weeks of age (190 days) both the days, and in this respect the hypothala1I I
550
WILSON
AND CUNNINGHAM
$ 60 c t
01 1
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134
I
I
I
140 20
I
I1
146 21
4
152
I1
158 22
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164 23
24
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170 Days Weeks
3. Age at onset of lay in hens raised under 8L:16D until 17 weeks of age and then subjected to various lighting treatments: 8L: 16D 0-O; 8L: 16D with eight pulses of light at hourly intervals for the first 8 hr of darkness of 10 set n---n, 1 min O....O, 10 min A-.-.-A duration: 16L:8D m----m. The figure is based on data from the 96 hens of each group. FIG.
mus-pituitary system of hens appears to respond rapidly to changes in photoperiod in a manner similar to that of several other avian species, notably the turkey (Godden and Scanes, 1977), canary (Nicholls, 1974), tree sparrow (Wilson and Follett, 1974), white-crowned sparrow (Follett et al., 1975), and quail (Nicholls et al., 1973; Follett and Davies, 1975). An important difference, however, between the photoperiodic responses in the domestic
hen and the quail is the fact that quail raised from hatch under a short-day photoperiod such as 6L:18D take three times as long to reach sexual maturity as those raised under a long-day photoperiod of 14L:lOD (Stein and Bacon, 1976). Hens, on the other hand, raised from hatch on the above photoperiods attain sexual maturity within 7 to 10 days of each other (Morris, 1962). Previous studies in several avian species (see introduction for references) have
TABLE 1 EFFECTOFLIGHTINGTREATMENTSON THE RATE OFLAY(NUMBEROF EGGS LAIDASA PERCENTAGE NUMBEROF HEN DAYS)DURING 173 TO 197 DAYSOF AGEANDON THEPLASMA CONCENTRATIONOF LH AT 188 DAYSOF AGE 8L: 16D
8L: 16D
8L: 16D
(8 x IO-set pulses)
(8 x I-min pulses)
(8 x IO-min pulses)
16L:8D
91.9”
92.5”
97.3h
95.8h
97.0b
2.9” -c 0.13
3.05” -+ 0.11
3.43” k 0.13
3.396 5 0.14
8L:16D
Rate of lay (%) Plasma LH (@ml)
3.41h
2 0.14
Means ? SEM, n = 40. “.OMeans with different superscripts are significantly different (P < 0.021. Note.
OF
PHOTOPERIOD
0
AND LUTEINIZING
1
I 2
HORMONE
I 3 Hours
after
I 0 inlectm
RELEASE
1
I 2
IN HENS
551
I 3
FIG. 4. Changes in the plasma concentrations of LH (means ? SEM, n = 6) after the intramuscular iniection, at 8 hr after ovulation, of (a) 0.5 and (b) 0.25 mg progesterone/kg body wt. Hens were maintained on 16L:8D (0) or 8L: 16D (0).
shown that the gonad-stimulating effects of a long photoperiod can be obtained also by supplementing a short photoperiod, i.e., 8L:16D, with intermittent pulses of light during the hours of darkness. In the present study pulses of either 1 or 10 min light at hourly intervals during the first 8 hr of darkness were sufficient to advance the age at which 90% of hens were in lay by 7 to 9 days as compared with hens maintained on an uninterrupted photoperiod of 8L: 16D. Pulses of 10 set were, however, ineffective in this respect. Furthermore, it was found that hens exposed to either a long photoperiod or a short photoperiod supplemented with pulses of 1 or 10 min light not only responded to the increases in photoperiod, at a time when prepubertal LH levels were already rising by a greater increase in the plasma concentration of LH than in hens given 10 set or no pulses of light, but also maintained higher LH levels and a higher rate of lay throughout the duration of the experiment. In fact, the similarities in age at onset of lay, rate of lay, and plasma concentration of LH at 27 weeks of age be-
tween hens given an equal number of l- or lo-min pulses of light support the observation of Wilson and Abplanalp (1956) that the frequency of the pulses of light is more important in stimulating gonadal growth than the total duration of light provided as pulses. However, differences in the extent to which pulses of 1 and 10 min light initially elevated LH levels during the prepubertal period suggest that the total duration of intermittent light is a significant factor in stimulating LH release. The transfer of hens raised under a short-day photoperiod to a long-day photoperiod or to an intermittent lighting treatment at 17 weeks of age in effect increased the magnitude of the prepubertal peak of LH since the concentration of LH in the plasma normally rises during 7 to 4 weeks before the onset of lay (Sharp, 1975; Wilson and Sharp, 1975; Wilson, 1978). Relationships between the magnitude of the prepubertal LH peak, the plasma concentration of LH before and after puberty, and the rate of lay have previously been observed in a flock of hens raised under a constant
552
WILSON
AND
CUNNINGHAM
photoperiod of 14L: 10D (Wilson, 1978). to ovarian progesterone would reduce the However, since the transfer, after the onset length of time between successive ovulaof lay, of hens raised under a short phototions and lead to the observed higher rate of period to a long one raises the rate of lay to lay in hens maintained on a long as comas high, or higher than that of birds raised pared to a short photoperiod. under a long photoperiod (McCluskey and Parker, 1963; Morris et al., 1964), it seems ACKNOWLEDGMENTS that the magnitude of the prepubertal LH The authors are grateful to the staff of Lane End peak is not important in determining the Farm, University of Reading, for the provision of potential capacity for follicular growth. facilities and to the British Egg Marketing Board ReThe present study has shown that the search and Education Trust for the Research Fellowhigher rate of lay and blood level of LH in ship awarded to S.C.W. We would also like to acknowledge the receipt of an Agricultural Research hens maintained under a long photoperiod Council grant (AG 45/157). (I 6L:8D) compared to a short one (8L: 16D) is associated with a greater sensitivity of the hypothalamus-pituitary system to reREFERENCES spond to the injection of progesterone by Bell, D. D., and Moreng, R. E. (1973). Intermittent the release of LH. Wilson and Sharp (1976) feeding and lighting of mature Leghorn hens. Poult. Sri. 52, 982-991. have shown that the stimulatory effect of Camper, P. M., and Burke, W. H. (1977). Serum esprogesterone on the release of LH is tradiol and progesterone levels of the laying turdependent on the quantity of oestradiol key hen following acute treatment with mammaand progesterone to which the hypothalalian luteinizing hormone or follicle-stimulating mus-pituitary system has been exposed at hormone. Gen. Comp. Endocrinol. 31, 224-232. Farner, D. S., Mewaldt, L. R., and Irving, S. D. the time of injection. Since LH stimulates (1953). The role of darkness and light in the steroidogenesis in the gonads of birds photoperiodic response of the testes of white(Shahabi et al., 1975; Camper and Burke, crowned sparrows. Biol. Bull. 105, 434-441. 1977; Imai and Nalbandov, 1978) the in- Follett, B. K. (1976). Plasma follicle stimulating horcreased responsiveness of the hypothalamone during photoperiodically induced sexual maturation in male Japanese quail. J. Erzdocrinof. mus-pituitary system to an injection 69, 117- 126. of progesterone may have been due to Follett, B. K., and Davies, D. T. (1975). Photosensitization of the positive feedback mechperiodicity and the neuroendocrine control of reanism by a higher circulating level of production in birds. Symp. 2001. Sot. London 35, gonadal steroids in birds exposed to long 199-224. photoperiods. In the quail (Follett, 1976) Follett, B. K., Farner, D. S., and Mattocks, P. W. (1975). Luteinizing hormone in the plasma of and turkey (Godden and Scanes, 1977) cirwhite-crowned sparrows (Zonotrichia leucophrys culating levels of follicle-stimulating horgambelii) during artificial photostimulation. Gen. mone (FSH) as well as LH are higher in Comp. Endocrinol. 26, 126- 134. birds exposed to a long than those in birds Follett, B. K., Scanes, C. G., and Cunningham, F. J. (1972). A radioimmunoassay for avian luteinizing exposed to a short daylength. If this situahormone. J. Endocrinol. 52, 359-378. tion were to prevail in the domestic hen, Follett, B. K., and Sharp, P. J. (1969). Circadian stimulatory effects of a long photoperiod on rhythmicity in photoperiodically induced gonadthe hypothalamus-pituitary-gonadal axis otropin release and gonadal growth in the quail. could conceivably hasten the maturation of Nature (London) 223, 968-971. Fox, S., and Morris, T. R. (1958). Flash lighting for egg the developing ova and reduce the time production. Nature (London) 182, 1752- 1753. after the previous ovulation at which an P. M. M., and Scanes, C. G. (1977). Effect of ovum would be sufficiently mature to be Godden, photoperiod on gonadotrophin concentrations in ovulated. This, together with an increased domestic turkeys. Brit. PO&. Sci. 18, 687-694. sensitivity of the positive feedback system Hamner, W. M. (1964). Circadian control of photo-
PHOTOPERIOD
AND LUTEINIZING
periodism in the house finch demonstrated by interrupted night experiments. Nature (London) 203, 14OC-1401. Harrison, P. C., Schumaier, G., and McGinnis, J. (1969). Reproductive development and response of White Leghorn pullets subjected to increasing day-lengths at different ages. Pou/f. Sci. 48, 1021- 1026. lmai, K., and Nalbandov, A. V. (1978). Plasma and follicular steroid levels of laying hens after the administration of gonadotropins. Biol. Reprod. 19, 779-784. King, D. F. (1961). Effects of increasing, decreasing and constant lighting treatments on growing pullets. Poult. Sci. 40, 479-484. McCluskey, W. H., and Parker, J. E. (1963). The effect of length of daily light period on reproduction in female chickens. Poult. Sri. 42, 1161-1165. Morris, T. R. (1962). The effect of changing day lengths on the reproductive responses of the pullet. Proc. 12th World’s Porrlt. Congr. Symp. Rep.. 1 IS- 123. Morris, T. R. (1967). Light requirements of the fowl. In “Environmental Control in Poultry Production” (T. C. Carter, ed.), pp. 15-39. Oliver & Boyd, Edinburgh. Morris, T. R. (1979). The influence of light on ovulation in domestic birds. In “Animal Reproduction” (H. W. Hawk, ed.), Beltsville Symposia in Agricultural Research 3, pp. 307-322. Allanheld, Osmun & Co., Montclair, N. J. Morris, T. R., and Fox, S. (1958). Artificial light and sexual maturity in the fowl. Naflrre (London) 182, 1522- 1523. Morris, T. R., Fox, S., and Jennings, R. C. (1964). The response of laying pullets to abrupt changes in daylength. &it. Poult. Sci. 5, 133- 147. Nicholls, T. J. (1974). Changes in plasma LH levels during photoperiodically controlled reproductive cycles in the canary (Serinus cnnarius). Gen. Comp.
Endocrinol.
24,
442-445.
Nicholls, T. J., Scanes, C. G., and Follett, B. K. (1973). Plasma and pituitary luteinizing hormone in Japanese quail during photoperiodically in-
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IN HENS
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duced gonadal
growth and regression. Gen. 21, 84-98. Shahabi, N. A., Bahr, J. M., and Nalbandov, A. V. (1975). Effect of LH injection on plasma and follicular steroids in the chicken. Endocrinology 96, 696-972. Sharp, P. J. (1975). A comparison of variations in plasma luteinizing hormone concentrations in male and female domestic chickens (Callus domesticus) from hatch to sexual maturity. J. Endocrinol. 67, 211-223. Stein, G. S., and Bacon, W. L. (1976). Effect of photoperiod upon age and maintenance of sexual development in female Coturnix coturnix japonica. Poult. Sci. 55, 1214-1218. Sykes, A. H. (1956). Short day-length and egg production in the fowl. J. Agr. Sci. 47, 429-434. Warren, D. C., and Scott, H. M. (1935). The time factor in egg formation. Poult. Sci. 14, 195-207. Wilson, F. E., and Follett, B. K. (1974). Plasma and pituitary luteinizing hormone in intact and castrated tree sparrows (Spizella arboreu) during a photoinduced gonadal cycle. Gen. Comp. EndoComp.
Endocrinol.
crinol.
23,
82-93.
Wilson, S. C. (1978). Relationship between plasma concentration of luteinizing hormone and intensity of lay in the domestic hen. &it. Pot&. Sci. 19, 643 -650. Wilson, S. C., and Sharp, P. J. (1975):Effects of progesterone and synthetic luteinizing hormone releasing hormone on the release of luteinizing hormone during sexual maturation in the hen (Callus domesticus).
J. Endocrinol.
67,
359-369.
Wilson, S. C., and Sharp, P. J. (1976). Induction of luteinizing hormone release by gonadal steroids in the ovariectomized domestic hen. J. Endocrinol. 71, 87-98. Wilson, W. 0.. and Abplanalp, H. (1956). Intermittent light stimuli in egg production of chickens. Poulr. SC;. 35, 532-538. Wilson, W. O., Abplanalp, H., and Arrington, L. (1962). Sexual development of Coturnix as affected by changes in photoperiods. PO&. Sci. 41, 17-22.
AND
COMPARATIVE
ENDOCRINOLOGY
41, 546-553 (1980)
Effect of Increasing Day Length and Intermittent Lighting Schedules in the Domestic Hen on Plasma Concentrations of Luteinizing Hormone (LH) and the LH Response to Exogenous Progesterone S.C. WILSON Department
of Physiology
AND F.J.
CUNNINGHAM
and Biochemistry. The University Reading RG6 2AJ, England
of Reading,
Whiteknights,
Accepted February 15, 1980 Pullets were raised under a constant photoperiod of 8L:16D until 17 weeks of age at which time they were subjected either to intermittent pulses of light superimposed on a photoperiod of 8L:16D or to a photoperiod of 16L:8D. Exposure of hens, maintained under a lighting schedule of 8L:16D, to hourly pulses of light for either 10 set, 1 mitt, or 10 min during the first 8 hr of darkness or to an increased photoperiod of 16L:8D stimulated a rise in the plasma concentration of LH within 3 days and slightly advanced the onset of lay. About 6 weeks after the onset of lay the plasma concentrations of LH and the rate of lay were similar in hens maintained under a long photoperiod (16L:8D) and a short photoperiod (8L:16D) supplemented by l- or IO-min pulses of light at hourly intervals. In each of these groups the parameters measured were significantly (P < 0.02) greater than in hens exposed either to an uninterrupted short photoperiod (8L:16D) or to the photoperiod on which was superimposed IO-set pulses of light. The increased plasma concentration of LH and rate of lay observed in hens kept on a long photoperiod (16L:8D) were associated with an enhanced LH response to progesterone compared to that measured in hens exposed to 8 hr light per day.
Several studies have shown that the age at which sexual maturity is reached in the domestic hen is influenced to some extent by the number of hours of light to which the birds are exposed each day (King, 1961; McCluskey and Parker, 1963; Morris, 1967). A greater influence, however, is exerted by a change in the length of the photoperiod (Morris, 1962, 1967, 1979; Harrison et al., 1969). Thus, hens raised throughout sexual development under a long-day photoperiod of 14 hr light and 10 hr darkness (14L: 10D) lay their first egg only 7-10 days earlier than hens raised under a short-day photoperiod of 6L:18D (Morris, 1967). If, however, birds maintained under a short day length are transferred to a long day length the onset of lay can be advanced by as many as 7 weeks, depending on the age at which the transfer took place (Morris, 1962; Harrison et al., 1969). Similarly, egg production increases 546 0016~6480/80/080546-08$01.00/O Copyright All tights
@ 1980 by Academic Press, Inc. of reproduction in any form reserved.
in laying hens exposed to an increase in day length from 6 to 14 hr light per day to achieve a rate of lay as high as (Sykes, 1956) or higher than (Morris et al., 1964) that of birds raised on a constant 14-hr day length. An increase in egg production has also been achieved by the use of intermittent lighting whereby the hours of darkness are interrupted by pulses of light. In this way, hens exposed to intermittent short pulses of light achieve a higher rate of lay than do hens receiving the same amount of light in a single long pulse (Fox and Morris, 1958; Wilson and Abplanalp, 1956; Bell and Moreng, 1973). Sexual maturity is also advanced by intermittent lighting treatments in the house finch (Hamner, 1964), whitecrowned sparrow (Farner et al., 1953), and quail (Wilson et al., 1962; Follett and Sharp, 1969). In hens raised throughout sexual devel-
PHOTOPERIOD
AND LUTEINIZING
opment under a constant photoperiod of either 14 or 16 hr light per day, the plasma concentration of luteinizing hormone (LH) increases to reach a prepubertal maximum value at 3 to 4 weeks before the onset of lay. The levels of LH then fall progressively until the time of the first ovulation (Sharp, 1975; Wilson and Sharp, 1975; Wilson, 1978). With respect to LH secretion it was relevant to examine the effect on the plasma concentration of LH during sexual development of an abrupt change in photoperiod from 8 to 16 hr light per day and to ascertain to what extent the provision of pulses of light at regular intervals during the hours of darkness, to hens maintained under a short photoperiod of 8L: 16D, could substitute for a long photoperiod. The pulses of light were provided during the first 8 hr of darkness in order that the total period of uninterrupted darkness would be of equal duration to, and coincide with, that of birds maintained under 16L:8D. Also, the possibility of a relationship between photoperiod and capacity of the hypothalamus-pituitary system to respond to the injection of progesterone by a release of LH was compared in laying hens maintained under long 16L:8D and short 8L: 16D day lengths. MATERIALS
AND METHODS
Pullets of the Ross tint strain were housed under a photoschedule of 8 hr light, 16 hr darkness (8L:16D) until 17 weeks of age at which time they were transferred to individual cages and subjected to one of the following lighting treatments: Treatment 1 8L: 16D (lights on 0800 to 16:00 hr) Treatments 2, 3. 4 8L:16D with pulses of either 10 sec. I min, or IO min light at hourly intervals for the first 8 hr of darkness Treatment 5 16L:8D (lights on 08:OOto 24:00 hr)
HORMONE
RELEASE
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547
sion. Blood (1.5 ml) was withdrawn by venepuncture into heparinized syringes during the period of the day when no preovulatory surges of LH would be expected to occur. The blood was centrifuged at 18OOgand the plasma was separated and stored at -20” until assayed for LH. LH was measured in the plasma of the 19 pullets sampled during weeks 11 to 16 and, from week 17 onward, in the plasma of 15 of the 20 pullets sampled from each lighting treatment whose age at onset of lay was closest to the age at which 50% of the total hens in each treatment group were in lay. LH and egg production. The age at onset of lay and the number of eggs laid by each hen for the duration of the experiment were noted. Times of oviposition were recorded to the nearest minute by means of an automatic recording device. The plasma concentrations of LH in blood samples taken from 40 hens in each treatment group at 27 weeks of age were compared with the number of eggs laid by each hen during the 15 days before and 10 days after the sample was taken. LH response to progesterone. At 30 weeks of age, 24 hens maintained under either 16L:8D or 8L:16D were given a single intramuscular injection of either 0.25 or 0.5 mg progesterone/kg body wt dissolved in 0.4 ml arachis oil. Injections were given at 8 hr after a midsequence ovulation. The time of ovulation was predicted to occur within 14 to 75 min of an oviposition (Warren and Scott, 1935). The concentration of LH was measured in plasma taken immediately before the injection and at intervals of 30 min thereafter for a period of 3.5 hr. Assay of LH. LH levels were measured using the radioimmunoassay system described by Follett et al., (1972). Each plasma sample was assayed in triplicate at one dilution and all plasma samples from one hen were assayed in the same assay. In the seven assays required to complete this study, the minimum detectable level of LH ranged between 0.04 and 0.09 r&ml (mean, 0.065 ? 0.006 ngiml) and the potency of a pooled sample of plasma included in each assay ranged between 5.24 and 5.77 ngiml (mean, 5.47 2 0.07 @ml)
RESULTS LH During Sexual Development
The effect of the various lighting treatments on the plasma concentrations of LH is shown in Fig. Each treatment group consisted of 96 birds and food during sexual development and water were available at all times. 1. The plasma concentration of LH rose LH during sexual de\,elopment. Between 11 and 16 gradually and was significantly (P < 0.01) weeks of age blood samples were withdrawn at weekly higher at 117 days of age than at 85 days in intervals from 19 pullets taken at random. From 17 under 8L: 16D. However, weeks to the onset of lay, blood samples were taken hens maintained within 3 days of an increase in photoperiod twice weekly from 20 hens in each of the five treatment groups. The same 20 hens were sampled on each occa- from 8 to 16 hr light per day, the plasma
548
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= E s c
6-
5
: if4 I h
3-
11 77
12 64
13 91
14
15
96
105
16 112
17 119
18 126
19 weeks 133 days
Age
1. Plasma concentrations of LH in hens raised under 8L: 16D until 17 weeks of age at which time they were subjected to the following lighting treatments. 8L:16D l &; 8L:16D with eight pulses of light at hourly intervals for the first 8 hr of darkness of 10 set A---A, 1 min 0. ‘0, 10 min A-. -. -A duration; 16L:8D n 4. Up to 17 weeks values are means from 19 hens taken at random each week from the total flock of hens. From 17 weeks onward the values are the means from the same 15 of % hens subjected at that age to one of the above lighting treatments. Vertical lines renresent SEM and are not shown for three of the treatments to avoid confusion. Range of SEM: 0.21-1.21 &ml. FIG.
concentration of LH rose steeply and sig- set pulses of light whereas LH levels in nificantly (P < 0.001) from 3.43 -+ 0.63 birds with an uninterrupted period of darkrig/ml (mean + SEM, n = 19) to 8.39 + 1.21 ness, i.e., no pulses of light, did not rise rig/ml (n = 15). In contrast, the levels of LH significantly during the same period. in hens maintained under 8L:16D did not After attaining mean maximal LH conrise significantly during the same period. centrations at 124 to 127 days of age (18 The interruption of the first half of the weeks), LH levels in all treatment groups darkness period of an 8L:16D photofell until the onset of lay and at that time the schedule with eight hourly pulses of either 1 plasma concentration of LH in hens under or 10 min light also stimulated significant the long-day-length photoschedule of increases (P < 0.001) in the plasma con- 16L:8D or under a short photoperiod of centrations of LH from 3.43 ? 0.63 rig/ml (n 8L:16D supplemented by pulses of 1 or 10 = 19) to maximal values of 7.51 2 0.89 min light during the first 8 hr of darkness rig/ml (n = 15) and 8.17 5 0.69 rig/ml (n = were significantly higher (P < 0.05) than in 15), respectively, within 7 days (Fig. 1). hens given lo-set pulses or no pulses of Both the rate and magnitude of the LH in- light (Fig. 2). creases, however, were greater in hens receiving 16 hr light per day than in hens re- LH and Egg Production ceiving 8 hr light and light pulses of l- or The age at which the 96 hens maintained IO-min duration (Fig. 1). A small, though under each of the different lighting treatsignificant (P < 0.05), increase in the ments came into lay is shown in Fig. 3. It plasma concentration of LH to 5.21 + 0.57 can be seen that 50% of hens under 16L:8D &ml (n = 15) occurred in response to lo- and 8L: 16D were in lay by 146 and 151 days
PHOTOPERIOD
AND LUTEINIZING
HORMONE
RELEASE
IN HENS
549
plasma concentration of LH and the rate of lay were significantly higher (P < 0.02) in the hens exposed to 16L:8D and in hens under 8L:16D supplemented with l- or IO-min pulses of light than in hens on the uninterrupted short photoperiod or on the short photoperiod supplemented with loset pulses of light (Table l), though the differences in the plasma concentration of LH between the treatment groups were not so marked as at the onset of lay.
2
E 26 c
LH Response to Progesterone 2
Treatment of 30-week-old hens maintained under 16L:8D or 8L:16D with a I single intramuscular injection of 0.5 mg -1 0 -3 -2 Weeks before first egg progesterone/kg body wt provoked inFIG. 2. Mean plasma concentrations of LH in rela- creases of 97.0 and 96.8%, respectively, in tion to the onset of lay in groups of 15 hens raised the plasma concentration of LH within 2.0 under 8L: 16D until 17 weeks of age and then subjected to 2.5 hr of injection (Fig. 4a). The adminto the following lighting treatments: 8L: 16D 0-O; how8L: 16D with eight pulses of light at hourly intervals for istration of 0.25 mg progesterone/kg, the first 8 hr of darkness of 10 set A- - -A, 1 min ever, while provoking an increase in the 0.‘..O, IOminA-.-.Aduration; 16L:8Dm-1. plasma concentration of LH in hens mainRange of SEM: 0.14- 1.14 @ml. tained under 16L:SD failed to stimulate LH release in hens maintained under 8L:16D of age, respectively (approximately 21 (Fig. 4b), so that during 2.0 to 3.5 hr after weeks). The provision of pulses of 10 set, 1 injection the plasma LH level in the hens kept under 16L:8D was significantly (P -=c min, and 10 min light at hourly intervals during the first 8 hr of the dark period of the 0.01) higher than that in hens on the short 8L: 16D photoschedule advanced the age at photoperiod. which 50% of hens were ‘in lay by 2, 3, 4, DISCUSSION and 5 days, respectively. The mean age at onset of lay in the 15 hens of each treatment A slight advancement in the onset of lay group whose plasma concentrations of LH as a result of increasing the photoperiod were measured during sexual development from 8 to 16 hr light per day in hens at 17 was the same as, or in one group (1-min weeks of age is in agreement with previous pulses of light) 1 day later than, the date at observations by Morris and Fox (1958) and 50% lay for the total of 96 hens under each Morris (1967). treatment. The present study has shown that an inThe age at which 90% of hens were in lay crease in day length advances sexual matwas the same in hens given lo-set pulses of uration by stimulating the release of LH, light as those on an uninterrupted schedule and perhaps, as in quail (Follett, 1976) and of 8L:16D, i.e., 163 days (23 weeks). Howturkeys (Godden and Scanes, 1977), ever, 90% of hens given l- or lo-min pulses follicle-stimulating hormone (FSH). A rapid of light or a photoschedule of 16L:8D were rise in the blood concentration of LH in rein lay 7 to 9 days earlier, i.e., 154- 156 days sponse to increased photoperiod from 8 to (22 weeks) (Fig. 3). 16 hr light per day was evident within 3 At 27 weeks of age (190 days) both the days, and in this respect the hypothala1I I
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WILSON
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134
I
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140 20
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158 22
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164 23
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170 Days Weeks
3. Age at onset of lay in hens raised under 8L:16D until 17 weeks of age and then subjected to various lighting treatments: 8L: 16D 0-O; 8L: 16D with eight pulses of light at hourly intervals for the first 8 hr of darkness of 10 set n---n, 1 min O....O, 10 min A-.-.-A duration: 16L:8D m----m. The figure is based on data from the 96 hens of each group. FIG.
mus-pituitary system of hens appears to respond rapidly to changes in photoperiod in a manner similar to that of several other avian species, notably the turkey (Godden and Scanes, 1977), canary (Nicholls, 1974), tree sparrow (Wilson and Follett, 1974), white-crowned sparrow (Follett et al., 1975), and quail (Nicholls et al., 1973; Follett and Davies, 1975). An important difference, however, between the photoperiodic responses in the domestic
hen and the quail is the fact that quail raised from hatch under a short-day photoperiod such as 6L:18D take three times as long to reach sexual maturity as those raised under a long-day photoperiod of 14L:lOD (Stein and Bacon, 1976). Hens, on the other hand, raised from hatch on the above photoperiods attain sexual maturity within 7 to 10 days of each other (Morris, 1962). Previous studies in several avian species (see introduction for references) have
TABLE 1 EFFECTOFLIGHTINGTREATMENTSON THE RATE OFLAY(NUMBEROF EGGS LAIDASA PERCENTAGE NUMBEROF HEN DAYS)DURING 173 TO 197 DAYSOF AGEANDON THEPLASMA CONCENTRATIONOF LH AT 188 DAYSOF AGE 8L: 16D
8L: 16D
8L: 16D
(8 x IO-set pulses)
(8 x I-min pulses)
(8 x IO-min pulses)
16L:8D
91.9”
92.5”
97.3h
95.8h
97.0b
2.9” -c 0.13
3.05” -+ 0.11
3.43” k 0.13
3.396 5 0.14
8L:16D
Rate of lay (%) Plasma LH (@ml)
3.41h
2 0.14
Means ? SEM, n = 40. “.OMeans with different superscripts are significantly different (P < 0.021. Note.
OF
PHOTOPERIOD
0
AND LUTEINIZING
1
I 2
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I 3 Hours
after
I 0 inlectm
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1
I 2
IN HENS
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I 3
FIG. 4. Changes in the plasma concentrations of LH (means ? SEM, n = 6) after the intramuscular iniection, at 8 hr after ovulation, of (a) 0.5 and (b) 0.25 mg progesterone/kg body wt. Hens were maintained on 16L:8D (0) or 8L: 16D (0).
shown that the gonad-stimulating effects of a long photoperiod can be obtained also by supplementing a short photoperiod, i.e., 8L:16D, with intermittent pulses of light during the hours of darkness. In the present study pulses of either 1 or 10 min light at hourly intervals during the first 8 hr of darkness were sufficient to advance the age at which 90% of hens were in lay by 7 to 9 days as compared with hens maintained on an uninterrupted photoperiod of 8L: 16D. Pulses of 10 set were, however, ineffective in this respect. Furthermore, it was found that hens exposed to either a long photoperiod or a short photoperiod supplemented with pulses of 1 or 10 min light not only responded to the increases in photoperiod, at a time when prepubertal LH levels were already rising by a greater increase in the plasma concentration of LH than in hens given 10 set or no pulses of light, but also maintained higher LH levels and a higher rate of lay throughout the duration of the experiment. In fact, the similarities in age at onset of lay, rate of lay, and plasma concentration of LH at 27 weeks of age be-
tween hens given an equal number of l- or lo-min pulses of light support the observation of Wilson and Abplanalp (1956) that the frequency of the pulses of light is more important in stimulating gonadal growth than the total duration of light provided as pulses. However, differences in the extent to which pulses of 1 and 10 min light initially elevated LH levels during the prepubertal period suggest that the total duration of intermittent light is a significant factor in stimulating LH release. The transfer of hens raised under a short-day photoperiod to a long-day photoperiod or to an intermittent lighting treatment at 17 weeks of age in effect increased the magnitude of the prepubertal peak of LH since the concentration of LH in the plasma normally rises during 7 to 4 weeks before the onset of lay (Sharp, 1975; Wilson and Sharp, 1975; Wilson, 1978). Relationships between the magnitude of the prepubertal LH peak, the plasma concentration of LH before and after puberty, and the rate of lay have previously been observed in a flock of hens raised under a constant
552
WILSON
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photoperiod of 14L: 10D (Wilson, 1978). to ovarian progesterone would reduce the However, since the transfer, after the onset length of time between successive ovulaof lay, of hens raised under a short phototions and lead to the observed higher rate of period to a long one raises the rate of lay to lay in hens maintained on a long as comas high, or higher than that of birds raised pared to a short photoperiod. under a long photoperiod (McCluskey and Parker, 1963; Morris et al., 1964), it seems ACKNOWLEDGMENTS that the magnitude of the prepubertal LH The authors are grateful to the staff of Lane End peak is not important in determining the Farm, University of Reading, for the provision of potential capacity for follicular growth. facilities and to the British Egg Marketing Board ReThe present study has shown that the search and Education Trust for the Research Fellowhigher rate of lay and blood level of LH in ship awarded to S.C.W. We would also like to acknowledge the receipt of an Agricultural Research hens maintained under a long photoperiod Council grant (AG 45/157). (I 6L:8D) compared to a short one (8L: 16D) is associated with a greater sensitivity of the hypothalamus-pituitary system to reREFERENCES spond to the injection of progesterone by Bell, D. D., and Moreng, R. E. (1973). Intermittent the release of LH. Wilson and Sharp (1976) feeding and lighting of mature Leghorn hens. Poult. Sri. 52, 982-991. have shown that the stimulatory effect of Camper, P. M., and Burke, W. H. (1977). Serum esprogesterone on the release of LH is tradiol and progesterone levels of the laying turdependent on the quantity of oestradiol key hen following acute treatment with mammaand progesterone to which the hypothalalian luteinizing hormone or follicle-stimulating mus-pituitary system has been exposed at hormone. Gen. Comp. Endocrinol. 31, 224-232. Farner, D. S., Mewaldt, L. R., and Irving, S. D. the time of injection. Since LH stimulates (1953). The role of darkness and light in the steroidogenesis in the gonads of birds photoperiodic response of the testes of white(Shahabi et al., 1975; Camper and Burke, crowned sparrows. Biol. Bull. 105, 434-441. 1977; Imai and Nalbandov, 1978) the in- Follett, B. K. (1976). Plasma follicle stimulating horcreased responsiveness of the hypothalamone during photoperiodically induced sexual maturation in male Japanese quail. J. Erzdocrinof. mus-pituitary system to an injection 69, 117- 126. of progesterone may have been due to Follett, B. K., and Davies, D. T. (1975). Photosensitization of the positive feedback mechperiodicity and the neuroendocrine control of reanism by a higher circulating level of production in birds. Symp. 2001. Sot. London 35, gonadal steroids in birds exposed to long 199-224. photoperiods. In the quail (Follett, 1976) Follett, B. K., Farner, D. S., and Mattocks, P. W. (1975). Luteinizing hormone in the plasma of and turkey (Godden and Scanes, 1977) cirwhite-crowned sparrows (Zonotrichia leucophrys culating levels of follicle-stimulating horgambelii) during artificial photostimulation. Gen. mone (FSH) as well as LH are higher in Comp. Endocrinol. 26, 126- 134. birds exposed to a long than those in birds Follett, B. K., Scanes, C. G., and Cunningham, F. J. (1972). A radioimmunoassay for avian luteinizing exposed to a short daylength. If this situahormone. J. Endocrinol. 52, 359-378. tion were to prevail in the domestic hen, Follett, B. K., and Sharp, P. J. (1969). Circadian stimulatory effects of a long photoperiod on rhythmicity in photoperiodically induced gonadthe hypothalamus-pituitary-gonadal axis otropin release and gonadal growth in the quail. could conceivably hasten the maturation of Nature (London) 223, 968-971. Fox, S., and Morris, T. R. (1958). Flash lighting for egg the developing ova and reduce the time production. Nature (London) 182, 1752- 1753. after the previous ovulation at which an P. M. M., and Scanes, C. G. (1977). Effect of ovum would be sufficiently mature to be Godden, photoperiod on gonadotrophin concentrations in ovulated. This, together with an increased domestic turkeys. Brit. PO&. Sci. 18, 687-694. sensitivity of the positive feedback system Hamner, W. M. (1964). Circadian control of photo-
PHOTOPERIOD
AND LUTEINIZING
periodism in the house finch demonstrated by interrupted night experiments. Nature (London) 203, 14OC-1401. Harrison, P. C., Schumaier, G., and McGinnis, J. (1969). Reproductive development and response of White Leghorn pullets subjected to increasing day-lengths at different ages. Pou/f. Sci. 48, 1021- 1026. lmai, K., and Nalbandov, A. V. (1978). Plasma and follicular steroid levels of laying hens after the administration of gonadotropins. Biol. Reprod. 19, 779-784. King, D. F. (1961). Effects of increasing, decreasing and constant lighting treatments on growing pullets. Poult. Sci. 40, 479-484. McCluskey, W. H., and Parker, J. E. (1963). The effect of length of daily light period on reproduction in female chickens. Poult. Sri. 42, 1161-1165. Morris, T. R. (1962). The effect of changing day lengths on the reproductive responses of the pullet. Proc. 12th World’s Porrlt. Congr. Symp. Rep.. 1 IS- 123. Morris, T. R. (1967). Light requirements of the fowl. In “Environmental Control in Poultry Production” (T. C. Carter, ed.), pp. 15-39. Oliver & Boyd, Edinburgh. Morris, T. R. (1979). The influence of light on ovulation in domestic birds. In “Animal Reproduction” (H. W. Hawk, ed.), Beltsville Symposia in Agricultural Research 3, pp. 307-322. Allanheld, Osmun & Co., Montclair, N. J. Morris, T. R., and Fox, S. (1958). Artificial light and sexual maturity in the fowl. Naflrre (London) 182, 1522- 1523. Morris, T. R., Fox, S., and Jennings, R. C. (1964). The response of laying pullets to abrupt changes in daylength. &it. Poult. Sci. 5, 133- 147. Nicholls, T. J. (1974). Changes in plasma LH levels during photoperiodically controlled reproductive cycles in the canary (Serinus cnnarius). Gen. Comp.
Endocrinol.
24,
442-445.
Nicholls, T. J., Scanes, C. G., and Follett, B. K. (1973). Plasma and pituitary luteinizing hormone in Japanese quail during photoperiodically in-
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duced gonadal
growth and regression. Gen. 21, 84-98. Shahabi, N. A., Bahr, J. M., and Nalbandov, A. V. (1975). Effect of LH injection on plasma and follicular steroids in the chicken. Endocrinology 96, 696-972. Sharp, P. J. (1975). A comparison of variations in plasma luteinizing hormone concentrations in male and female domestic chickens (Callus domesticus) from hatch to sexual maturity. J. Endocrinol. 67, 211-223. Stein, G. S., and Bacon, W. L. (1976). Effect of photoperiod upon age and maintenance of sexual development in female Coturnix coturnix japonica. Poult. Sci. 55, 1214-1218. Sykes, A. H. (1956). Short day-length and egg production in the fowl. J. Agr. Sci. 47, 429-434. Warren, D. C., and Scott, H. M. (1935). The time factor in egg formation. Poult. Sci. 14, 195-207. Wilson, F. E., and Follett, B. K. (1974). Plasma and pituitary luteinizing hormone in intact and castrated tree sparrows (Spizella arboreu) during a photoinduced gonadal cycle. Gen. Comp. EndoComp.
Endocrinol.
crinol.
23,
82-93.
Wilson, S. C. (1978). Relationship between plasma concentration of luteinizing hormone and intensity of lay in the domestic hen. &it. Pot&. Sci. 19, 643 -650. Wilson, S. C., and Sharp, P. J. (1975):Effects of progesterone and synthetic luteinizing hormone releasing hormone on the release of luteinizing hormone during sexual maturation in the hen (Callus domesticus).
J. Endocrinol.
67,
359-369.
Wilson, S. C., and Sharp, P. J. (1976). Induction of luteinizing hormone release by gonadal steroids in the ovariectomized domestic hen. J. Endocrinol. 71, 87-98. Wilson, W. 0.. and Abplanalp, H. (1956). Intermittent light stimuli in egg production of chickens. Poulr. SC;. 35, 532-538. Wilson, W. O., Abplanalp, H., and Arrington, L. (1962). Sexual development of Coturnix as affected by changes in photoperiods. PO&. Sci. 41, 17-22.