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Performance of Layers Given Two-Hour Midnight Lighting as Growing Pullets
2003 Poultry Science Association, Inc.
Performance of Layers Given TwoHour Midnight Lighting as Growing Pullets S. Leeson,1 L. J. Caston, and J. D. Summers Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario N1G 2W1 Canada
Primary Audience: Flock supervisors, Nutritionists SUMMARY Two experiments were conducted to record layer performance of growing pullets that had received various midnight lighting treatments. In these studies, pullets were grown on 12 h of light (6:00 a.m. to 6:00 p.m.) in experiment 1 and on 8 h of light (7:30 a.m. to 3:30 p.m.) in experiment 2. Some pullets received 2 h of supplemental light at midnight (12:00 a.m. to 2:00 p.m.) from either 0 to 18, 4 to 18, 8 to 18, 12 to 18, 0 to 12, 0 to 8, or 0 to 4 wk. Each rearing treatment was tested with eight replicate groups of four adjacently and individually caged birds. From 18 to 70 wk of age, the greatest number of eggs (317 or 315) were produced by pullets given midnight lighting from 0 to 18 or 4 to 18 wk, respectively. Introducing the extra light late in the growing period (12 wk) induced early maturity and had a negative effect on late cycle egg production. Most other production parameters were little influenced by the midnight lighting treatments. The only detrimental effect of midnight lighting occurred when it was used only during the late rearing period, likely because it induced sexual maturity too early. Key words: egg production, midnight lighting, pullet, sexual maturity 2003 J. Appl. Poult. Res. 12:313–320
DESCRIPTION OF PROBLEM Attaining mature weight-for-age with Leghorn pullets is often difficult [1], especially in hot environments [2]. Leeson et al. [3] have recently tried 2 h of midnight lighting as a potential means of stimulating feed intake and growth of immature pullets. The additional lighting had some effect on early growth, especially when pullets were grown on very short days and fed mash diets (diet composition is shown in Table 1). However, the midnight lighting advanced sexual maturity, which suggests the pullets perceived midnight as dawn, 1
effectively increasing their daylength to 18 h in some of the treatments. Early maturity induced by an increase in photoperiod is usually thought to result in a very small egg size [4]. The aim of these studies was to investigate the adult layer performance of pullets subjected to midnight lighting at various times during their adolescent growing period. The growing performance of these pullets to 18 wk of age is detailed in a previous publication [3].
MATERIALS AND METHODS Experiment 1 Two hundred fifty-six Shaver White pullets were moved to laying cages at 18 wk of age.
To whom correspondence should be addressed: e-mail: [email protected].
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314 TABLE 1. Percentage diet composition Corn Soybean meal (48%) Dical PO4 Limestone Vit-Min PMXA Salt DL-methionine
Light was provided by incandescent bulbs, with 23 lx from 0 to 5 d, followed by 8 lx thereafter. At time of transfer to the laying cages, lighting was increased to 14L:10D and from 20 wk onwards was at 16L:8D. Environmental temperature was maintained at 22°C. The experiment was conducted for 13 28-d periods, during which time the egg production was continuously monitored. Eggs collected on the last 2 d of periods, 1, 3, 5, and 13 were weighed and shell deformation assessed as a measure of shell quality. Albumen height was also measured at the end of periods 1 and 13. Daily feed intake was monitored for the last 2 d of periods 1, 3, 5, and 13 in order to obtain information relevant to the online production cycle. Eggs were assigned a commercial weight category according to the following criteria: small (42 to 48 g), medium (49 to 56 g), large (57 to 63 g), and extra large (>63 g). Birds were weighed at the end of periods 1 (20 wk) and 13 (70 wk) of age.
58.84 30.00 1.67 8.07 1.00 0.31 0.11 100.00
A Provided per kilogram of diet: retinyl palmitate, 4.4 mg; cholecalciferol, 40 µg; dl-a-tocopheryl acetate, 11 mg; riboflavin, 8 mg; biotin, 0.22 mg; pantothenic acid, 15 mg; vitamin B12, 12 µg; niacin, 50 mg; choline, 600 mg; vitamin K3, 3.3 mg; folic acid, 1 mg; ethoxyquin, 120 mg; manganese, 70 mg; zinc, 70 mg; copper, 10 mg; iron, 60 mg; and selenium, 0.3 mg.
The cages were a two-tiered, stacked arrangement of two back-to-back rows. Pullets were selected at random from within eight rearing treatments involving use of a 2-h period of midnight supplemental lighting from 0 to 18 wk of age. Pullets in treatment 1 were reared on constant light for 12 h (6:00 a.m. to 6:00 p.m.), whereas in Treatment 5, birds also always received 2 h of supplemental light (12 midnight to 2:00 a.m.). In treatments 2, 3, and 4, the midnight lighting was provided from 0 to 4, 0 to 8, or 0 to 12 wk, respectively. For treatments 6, 7, and 8, the midnight lighting was from 12 to 18, 8 to 18, or 4 to 18 wk, respectively. Each rearing treatment was tested with eight replicate groups of four adjacently caged birds in the light-tight layer facilities.
Experiment 2 The design of the experiment was very similar to that described for experiment 1. In this study immature pullets were grown on 8 h of light (7:30 a.m. to 3:30 p.m.) with or without midnight lighting (midnight to 2:00 a.m.). Light source and intensity were as described in experiment 1. Eight pullet treatments with midnight lighting were used in a manner com-
TABLE 2. Egg production from 18 to 70 wk of age (experiment 1) Pullet light treatmentA 1
3
5
7
9
11
13
Total egg number (18 to 70 wk)
66.5ab 59.5ab 55.0b 33.5c 58.7b 76.0a 59.4ab 55.9b 10.8 **
95.5 97.7 97.0 97.9 96.1 95.8 94.9 98.2 5.93 NS
94.6 96.7 94.3 96.1 95.0 91.6 94.4 96.7 6.87 NS
91.0 94.8 88.8 96.8 90.7 86.2 90.9 91.7 9.54 NS
89.2 85.9 87.7 91.3 89.6 84.1 85.2 89.6 11.1 NS
84.2 82.3 85.3 88.2 87.8 82.0 83.7 87.7 12.5 NS
65.3ab 68.8ab 78.0ab 81.9a 77.0ab 60.8b 68.1ab 75.6ab 13.2 *
308 310 312 312 315 302 307 317 24.0 NS
Percentage egg production (hen-day per 28 d period) Treatment 1 2 3 4 5 6 7 8 Significance a–c
0 to 4 wk 12 12 + 2 12 + 2 12 + 2 12 + 2 12 12 12 SEM
4 to 8 wk 12 12 12 + 12 + 12 + 12 12 12 +
2 2 2
2
8 to 12 wk
12 to 18 wk
12 12 12 12 + 12 + 12 12 + 12 +
12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01. A
12 12 12 + 12 + 12 + 12 12 12 +
2
2 2 2
4 to 8 wk
12 12 12 12 + 12 + 12 12 + 12 +
2 2
2 2
8 to 12 wk 12 12 12 12 12 + 12 + 12 + 12 + 2 2 2 2
12 to 18 wk 52.4 51.2 52.7 49.4 51.4 52.4 51.2 50.4 2.9 NS
Egg weight (g) 21.4 21.4 20.1 19.8 20.8 21.2 21.3 21.1 1.55 NS
Eggshell def.B (µm)
Period 1
8.77 8.73 9.03 8.69 8.28 8.81 8.62 8.68 0.47 NS
Albumen height (mm)
A
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. B Eggshell deformation. **P < 0.01.
a–c
12 12 + 12 + 12 + 12 + 12 12 12
1 2 3 4 5 6 7 8 SEM Significance
2 2 2 2
0 to 4 wk
Treatment
Pullet light treatmentA
TABLE 3. Egg characteristics (experiment 1)
58.9 59.0 59.1 59.9 59.2 58.2 58.0 59.3 2.2 NS
Egg weight (g) 21.8 22.8 22.7 22.5 21.7 21.7 21.6 22.4 1.67 NS
Eggshell def. (µm)
Period 3
62.2 61.8 62.5 62.7 60.5 61.1 60.9 61.5 2.36 NS
Egg weight (g) 22.8 23.6 23.7 22.8 22.7 23.5 22.9 24.2 1.68 NS
Eggshell def (µm)
Period 5
65.9 66.0 65.1 65.1 65.2 66.7 64.5 66.2 3.92 NS
Egg weight (g)
25.2 25.5 26.1 24.6 24.6 25.1 23.7 27.2 3.33 NS
Eggshell def. (µm)
Period 13
7.35a 6.77a 6.64ab 6.95a 6.54ab 5.84b 6.74ab 6.57ab 0.64 **
Albumen height (mm)
LEESON ET AL.: MIDNIGHT LIGHTING 315
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316 TABLE 4. Percentage egg grading (experiment 1) Pullet light treatmentA Period 1 Treatment 1 2 3 4 5 6 7 8
0 to 4 wk 12 12 + 12 + 12 + 12 + 12 12 12
4 to 8 wk
12 2 12 2 12 + 2 2 12 + 2 2 12 + 2 12 12 12 + 2
8 to 12 wk
12 to 18 wk
12 12 12 12 + 12 + 12 12 + 12 +
12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
SB
M
Period 3
L
XL
12.5 75.0 12.5 0 100.0 0 0 87.5 12.5 25.0 75.0 0 12.5 75.0 12.5 0 87.5 12.5 0 87.5 12.5 25.0 75.0 0
0 0 0 0 0 0 0 0
M
L
XL
Period 5
Period 13
L
L
XL
XL
12.5 75.0 0 62.5 37.5 25.0 75.0 12.5 87.5 0 62.5 37.5 12.5 87.5 12.5 87.5 0 75.0 25.0 37.5 62.5 0 87.5 12.5 62.5 37.5 0 100.0 0 100.0 0 75.0 25.0 37.5 62.5 12.5 87.5 0 62.5 37.5 0 100.0 12.5 87.5 0 87.5 12.5 0 100.0 0 100 0 87.5 12.5 12.5 87.5
12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. S (small) = 42 to 48 g; M (medium) = 49 to 56 g; L (large) = 57 to 63 g; XL (extra large) = > 63 g.
A B
TABLE 5. Body weight and feed intake (experiment 1) Pullet light treatmentA
Feed intake (g/bird per d)
BW (g) Treatment 1 2 3 4 5 6 7 8 SEM Significance
0 to 4 wk 12 12 + 12 + 12 + 12 + 12 12 12
4 to 8 wk
2 2 2 2
12 12 12 + 12 + 12 + 12 12 12 +
8 to 12 wk 12 12 12 12 + 12 + 12 12 + 12 +
2 2 2
2
12 to 18 wk 12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
Period 1 1,579 1,553 1,589 1,555 1,569 1,539 1,583 1,588 100.0 NS
Period 2 1,848 1,827 1,938 1,851 1,812 1,841 1,878 1,872 178.3 NS
Period 1 ab
92.9 90.8ab 91.4ab 83.4b 94.2a 95.2a 93.4a 88.4ab 6.14 **
Period 3
Period 5
Period 13
102.9 101.0 101.4 100.6 103.3 100.2 99.3 102.0 6.03 NS
104.5 102.9 103.8 104.9 104.6 104.7 105.1 108.0 6.56 NS
109.8 106.3 112.8 107.9 109.8 110.0 106.8 111.5 9.52 NS
a,b
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. **P < 0.01. A
TABLE 6. Egg production (experiment 2) Percentage egg production (Hen-day per 28-d period)
Pullet light treatmentA Treatment 1 2 3 4 5 6 7 8 SEM Significance a–d
0 to 4 wk
4 to 8 wk
8 to 12 wk
12 to 16 wk
1
2
3
4
5
6
8 + + + + 8 8 8
8 8 + + + 8 8 +
8 8 8 + + 8 + +
8 8 8 8 + + + +
26.2b 19.2bc 13.3cd 10.0d 9.6d 56.7a 52.6a 12.3cd 4.86 **
93.6a 88.6abc 82.1cd 84.0bcd 77.9d 90.9ab 87.9abc 83.6bcd 5.53 **
97.0a 96.3ab 95.9ab 95.4ab 93.0ab 94.6ab 90.2b 97.0a 4.06 *
94.5 94.9 92.6 91.5 90.2 92.9 88.4 93.8 4.34 NS
94.1 94.9 91.7 93.8 90.5 91.2 88.8 92.7 5.32 NS
91.0ab 91.8a 91.8a 93.4a 86.7abc 79.9bc 79.0c 92.1a 7.06 **
8 8 8 8
2 2 2 2
8 8 8
8
2 2 2
8 8
2
8 8
2 2 2 2
8 8 8 8
2 2 2 2
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01. A
8
8 8 8
8 8 + + + 8 8 +
2
2 2 2
4 to 8 wk
8 8
8 8
8 8 8 + + 8 + + 2 2
2 2
8 to 12 wk
8 8 8 8
8 8 8 8 + + + + 2 2 2 2
12 to 18 wk
A
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01.
a–c
2 2 2 2
8 + + + + 8 8 8
1 2 3 4 5 6 7 8 SEM Significance
8 8 8 8
0 to 4 wk
Treatment
Pullet light treatmentA
TABLE 7. Egg characteristics (experiment 2)
53.5 52.5 52.9 52.9 53.7 52.0 53.0 53.7 0.95 NS
Egg weight (g) 20.2 21.4 20.4 20.4 19.6 20.8 19.8 20.7 0.89 NS
Eggshell def. (µm)
Period 2 Egg weight (g) 56.4ab 56.5ab 56.5ab 56.7ab 58.1a 55.6b 55.3b 57.6a 1.29 **
Albumen height (mm) 8.96ab 9.01a 8.57ab 8.81ab 8.80ab 8.47ab 8.16b 8.76ab 0.35 *
Period 4
57.8abc 57.8abc 59.4a 58.7ab 59.4a 56.4c 57.2bc 59.3a 1.56 **
Egg weight (g)
28.9 27.9 26.7 27.6 26.2 28.9 27.6 28.4 2.47 NS
Eggshell def. (µm)
Period 5
62.5 58.3 59.0 59.1 61.0 56.7 57.6 60.1 3.95 NS
Egg weight (g)
24.1ab 25.4a 24.5ab 24.6ab 22.6b 24.5ab 23.6ab 23.8ab 1.50 *
Eggshell def. (µm)
Period 6
LEESON ET AL.: MIDNIGHT LIGHTING 317
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318 TABLE 8. Egg grading (experiment 2) Pullet light treatment Period 2 Treatment 1 2 3 4 5 6 7 8
Period 4
Period 5
Period 6
0 to 4 wk
4 to 8 wk
8 to 12 wk
12 to 18 wk
M
MB
L
M
L
XL
M
L
XL
8 + + + + 8 8 8
8 8 + + + 8 8 +
8 8 8 + + 8 + +
8 8 8 8 + + + +
100 100 100 100 100 100 100 100
50.0 50.0 25.0 37.5 12.5 50.0 75.0 0
50.0 50.0 75.0 62.5 87.5 50.0 25.0 100.0
0 0 0 0 0 37.5 0 0
100.0 100.0 100.0 100.0 100.0 62.5 75.0 100.0
0 0 0 0 0 0 25.0 0
12.5 0 12.5 0 0 37.5 12.5 0
75 100 87.5 100 87.5 62.5 87.5 100
12.5 0 0 0 12.5 0 0 0
8 8 8 8
2 2 2 2
8 8 8
8
2 2 2
8 8
2
8 8
2 2 2 2
8 8 8 8
2 2 2 2
8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. M (medium) = 49 to 56 g; L (large) = 57 to 63 g; XL (extra large) = > 63 g.
A B
parable to the schedule described for experiment 1 (Table 6). We intended to continue this layer study for 13 28-d periods, just as with experiment 1. However, during Period 7, layers inadvertently received a 1% calcium grower diet, and the trial was terminated. Production parameters only to the end of period 6 are described here, and measurements were as detailed for experiment 1. However, feed intake was measured only at the end of Period 2. Statistical Analysis Statistical analysis was the same for experiments 1 and 2. The objective in each experiment was to observe how the lighting regimen applied to pullets in the rearing period affected subsequent layer performance in terms of egg production and how egg quality parameters and treatments were defined by these regimens. Egg production and egg quality parameters as well as BW and feed intake were subjected to analysis of variance, and those response variables resulting in a significant F-test were further analyzed using Tukey’s test. Significance was accepted at P < 0.05.
RESULTS AND DISCUSSION In experiment 1, the highest egg production in Period 1 (18 to 22 wk) was for pullets introduced to midnight lighting at 12 wk (treatment 6, Table 2), whereas the lowest egg production was seen for treatment 4 birds, in which the midnight lighting was stopped at 12 wk. Throughout most of the experiment, egg production was unaffected by rearing treatment,
although significant differences were seen again in Period 13 (P < 0.05, Table 2), in which the converse to Period 1 occurred, with the early-maturing treatment 6 birds laying the fewest eggs and vice versa for the more slowly maturing treatment 4 birds. Overall, there was a major decline in egg production during Period 13, being greater than anticipated, and although this coincided with a period of hot weather, birds did not seem unduly stressed and pen temperature did not exceed 28°C. The early-maturing treatment 6 birds produced the least number of eggs to 70 wk. Rearing treatments had few effects on egg weight or shell quality measurements; however the early-maturing treatment 6 pullets exhibited an exceptionally low albumen height in Period 13, being significantly different (P < 0.05) to the latematuring treatment 4 birds, which had the best albumen height at this older age (Table 3). Egg grading is shown in Table 4. Rearing treatment had little effect on BW or feed intake (Table 5). The same trends were seen in Experiment 2 regarding earlier egg production for pullets receiving midnight lighting initially at 8 or 12 wk (treatments 6 and 7, Table 6). However, these earlier-maturing birds exhibited a dramatic decline in egg production during Period 6 compared to all other treatment groups. The earlier-maturing treatment 6 and 7 birds also showed the lowest albumen height during Period 2 of the study and the smallest egg size during Periods 4 and 5 (Table 7). After Period 4, the earliest-maturing treatment 6 birds were laying the smallest eggs, according to egg grad-
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319
TABLE 9. Body weight and feed intake (experiment 2) Feed intake (g/bird per d)
Pullet light treatmentA BW (g) 0 to 4 wk 8 8+2 8+2 8+2 8+2 8 8 8 SEM Significance
4 to 8 wk
8 8 8
8
8 8 + + + 8 8 +
8 to 12 wk
2 2 2
8 8
2
8 8
8 8 8 + + 8 + +
2 2 2 2
12 to 18 wk
8 8 8 8
8 8 8 8 + + + +
Period 1 d
2 2 2 2
1,202 1,206d 1,223c 1,238b 1,238b 1,268a 1,238b 1,221c 8.04 **
Period 2 bcd
1,541 1,532cd 1,571abc 1,595ab 1,620a 1,485d 1,485d 1,600a 36.7 **
Period 2 89.8 89.3 90.7 90.0 88.3 88.2 89.9 92.8 5.12 NS
a–d
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. **P < 0.01. A
ing classification (Table 8). Shell quality was unaffected until Period 6, when treatment 2 eggs showed a general decrease in shell quality. While being the heaviest birds at 20 wk of age (Period 1, Table 9), the early-maturing treatment 6 layers were the smallest birds by the end of Period 4. Only one observation of feed intake was undertaken, at the end of Period 2, when intake was unaffected by rearing treatment (Table 9). Midnight lighting of growing immature pullets seems to influence sexual maturation. While the effects of daylength and intensity per se are well known [5, 6, 7]) we are unaware of any information on the effect of midnight lighting of immature pullets on adult performance. P. D. Lewis [8] suggests that pullets will perceive the midnight lighting as an “effective” increase in photoperiod and will likely respond in the classical manner, as though this is a photostimulation. Normally one expects earliermaturing birds to produce smaller eggs throughout the life of the flock [1]. However, BW is little affected, and this is the reason that manipulation of maturity, by this means, has little influence on egg size. The effects of midnight lighting on sexual development are likely confounded by the light treatment induced at
time of move to the laying cages. In experiment 1, for example, the layers received 14 h of light per day when moved to the laying cages. For some pullets, this apparent stimulation (from an 8 h/d base in experiment 2) may, in fact, represent a decline in daylength, assuming midnight lighting is the cue for dawn in treatments 4, 5, 7, and 8, which were subjected to midnight lighting immediately prior to time of transfer. A step down in effective daylength (from 18 to 14 h) will likely delay maturity and so temper the effect of the stimulation occurring in the pullet house. Another trial would have to be conducted to separate out these stimulatory vs. inhibitory effects of change in daylength. The duration of midnight lighting of pullets seems critical. The technique should not be used in older pullets (>12 wk) as a means of stimulating late-cycle growth rate because the early induced maturity seems to have long term negative effects on egg production. If midnight lighting is used throughout the growing period, the pullet is effectively subjected to a constant long daylength, and so this has no effect on sexual maturation. Most eggs (315 to 317 in 52 wk) were produced by pullets given midnight lighting from 4 to 18 or 0 to 18 wk (treatments 8 and 5, respectively; Table 2).
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CONCLUSIONS AND APPLICATIONS 1. Providing growing Leghorn pullets with 2 h of supplemental light starting at midnight was studied as a potential technique for manipulating growth and development. 2. Midnight lighting has the potential to influence sexual maturation, because the pullet seems to perceive midnight as the start of the day, and the time from 2 a.m. until “lights-on” at around 6 a.m. is perceived as “light.” 3. If midnight lighting is first introduced at 12 wk of age, it has the effect of stimulating maturity. Conversely, if midnight lighting is given only from 0 to 12 wk, it has the effect of delaying maturity. 4. The safest system would be to use midnight lighting throughout rearing, where the perceived “long-day” is nonstimulatory to maturity because photoperiod is constant. Pullets grown on this program produced the most eggs from 18 to 70 wk (315 to 317).
REFERENCES AND NOTES 1. Leeson, S., and J. D. Summers. 1997. Feeding programs for growing egg-strain pullets. Pages 121–140 in Commercial Poultry Nutrition. University Books, Guelph, ON, Canada. 2. Leeson, S., and J. D. Summers. 1991. Growth and development of Leghorn pullets subjected to abrupt changes in environmental temperature and dietary energy level. Poult. Sci. 70:1732–1738. 3. Leeson, S., L. J. Caston, and J. D. Summers. 2002. Potential for midnight lighting to influence development of Leghorn pullets. J. Appl. Poult. Res. Submitted. 4. Leeson, S., L. J. Caston, and J. D. Summers. 1991. Significance of physiological age of Leghorn pullets in terms of subsequent reproductive characteristics and economic analysis. Poult. Sci. 70:37–43. 5. Lewis, P. D., and T. R. Morris. 1998. A comparison of the effects of age at photostimulation on sexual maturity and egg
production in domestic fowl, turkeys, partridges and quail. World’s Poult. Sci. 54:119–128. 6. Lewis, P. D., T. R. Morris, and G. C. Perry. 1999. Light intensity and age at first egg in pullets. Poult. Sci. 78:1227–1231. 7. Lewis, P. D., and T. R. Morris. 1999. Light intensity and performance of domestic pullets. World’s Poult. Sci. 55:241–250. 8. Lewis, P. D. 2003. Personal communication.
Acknowledgments This work was supported by the Poultry Industry Center, Guelph, Ontario, Canada and the Ontario Ministry of Agriculture and Food, Guelph, Ontario, Canada. We would like to thank Peter Lewis for his ideas in interpretation of the photostimulatory effects of midnight lighting.
Performance of Layers Given TwoHour Midnight Lighting as Growing Pullets S. Leeson,1 L. J. Caston, and J. D. Summers Department of Animal and Poultry Science, University of Guelph, Guelph, Ontario N1G 2W1 Canada
Primary Audience: Flock supervisors, Nutritionists SUMMARY Two experiments were conducted to record layer performance of growing pullets that had received various midnight lighting treatments. In these studies, pullets were grown on 12 h of light (6:00 a.m. to 6:00 p.m.) in experiment 1 and on 8 h of light (7:30 a.m. to 3:30 p.m.) in experiment 2. Some pullets received 2 h of supplemental light at midnight (12:00 a.m. to 2:00 p.m.) from either 0 to 18, 4 to 18, 8 to 18, 12 to 18, 0 to 12, 0 to 8, or 0 to 4 wk. Each rearing treatment was tested with eight replicate groups of four adjacently and individually caged birds. From 18 to 70 wk of age, the greatest number of eggs (317 or 315) were produced by pullets given midnight lighting from 0 to 18 or 4 to 18 wk, respectively. Introducing the extra light late in the growing period (12 wk) induced early maturity and had a negative effect on late cycle egg production. Most other production parameters were little influenced by the midnight lighting treatments. The only detrimental effect of midnight lighting occurred when it was used only during the late rearing period, likely because it induced sexual maturity too early. Key words: egg production, midnight lighting, pullet, sexual maturity 2003 J. Appl. Poult. Res. 12:313–320
DESCRIPTION OF PROBLEM Attaining mature weight-for-age with Leghorn pullets is often difficult [1], especially in hot environments [2]. Leeson et al. [3] have recently tried 2 h of midnight lighting as a potential means of stimulating feed intake and growth of immature pullets. The additional lighting had some effect on early growth, especially when pullets were grown on very short days and fed mash diets (diet composition is shown in Table 1). However, the midnight lighting advanced sexual maturity, which suggests the pullets perceived midnight as dawn, 1
effectively increasing their daylength to 18 h in some of the treatments. Early maturity induced by an increase in photoperiod is usually thought to result in a very small egg size [4]. The aim of these studies was to investigate the adult layer performance of pullets subjected to midnight lighting at various times during their adolescent growing period. The growing performance of these pullets to 18 wk of age is detailed in a previous publication [3].
MATERIALS AND METHODS Experiment 1 Two hundred fifty-six Shaver White pullets were moved to laying cages at 18 wk of age.
To whom correspondence should be addressed: e-mail: [email protected].
JAPR: Research Report
314 TABLE 1. Percentage diet composition Corn Soybean meal (48%) Dical PO4 Limestone Vit-Min PMXA Salt DL-methionine
Light was provided by incandescent bulbs, with 23 lx from 0 to 5 d, followed by 8 lx thereafter. At time of transfer to the laying cages, lighting was increased to 14L:10D and from 20 wk onwards was at 16L:8D. Environmental temperature was maintained at 22°C. The experiment was conducted for 13 28-d periods, during which time the egg production was continuously monitored. Eggs collected on the last 2 d of periods, 1, 3, 5, and 13 were weighed and shell deformation assessed as a measure of shell quality. Albumen height was also measured at the end of periods 1 and 13. Daily feed intake was monitored for the last 2 d of periods 1, 3, 5, and 13 in order to obtain information relevant to the online production cycle. Eggs were assigned a commercial weight category according to the following criteria: small (42 to 48 g), medium (49 to 56 g), large (57 to 63 g), and extra large (>63 g). Birds were weighed at the end of periods 1 (20 wk) and 13 (70 wk) of age.
58.84 30.00 1.67 8.07 1.00 0.31 0.11 100.00
A Provided per kilogram of diet: retinyl palmitate, 4.4 mg; cholecalciferol, 40 µg; dl-a-tocopheryl acetate, 11 mg; riboflavin, 8 mg; biotin, 0.22 mg; pantothenic acid, 15 mg; vitamin B12, 12 µg; niacin, 50 mg; choline, 600 mg; vitamin K3, 3.3 mg; folic acid, 1 mg; ethoxyquin, 120 mg; manganese, 70 mg; zinc, 70 mg; copper, 10 mg; iron, 60 mg; and selenium, 0.3 mg.
The cages were a two-tiered, stacked arrangement of two back-to-back rows. Pullets were selected at random from within eight rearing treatments involving use of a 2-h period of midnight supplemental lighting from 0 to 18 wk of age. Pullets in treatment 1 were reared on constant light for 12 h (6:00 a.m. to 6:00 p.m.), whereas in Treatment 5, birds also always received 2 h of supplemental light (12 midnight to 2:00 a.m.). In treatments 2, 3, and 4, the midnight lighting was provided from 0 to 4, 0 to 8, or 0 to 12 wk, respectively. For treatments 6, 7, and 8, the midnight lighting was from 12 to 18, 8 to 18, or 4 to 18 wk, respectively. Each rearing treatment was tested with eight replicate groups of four adjacently caged birds in the light-tight layer facilities.
Experiment 2 The design of the experiment was very similar to that described for experiment 1. In this study immature pullets were grown on 8 h of light (7:30 a.m. to 3:30 p.m.) with or without midnight lighting (midnight to 2:00 a.m.). Light source and intensity were as described in experiment 1. Eight pullet treatments with midnight lighting were used in a manner com-
TABLE 2. Egg production from 18 to 70 wk of age (experiment 1) Pullet light treatmentA 1
3
5
7
9
11
13
Total egg number (18 to 70 wk)
66.5ab 59.5ab 55.0b 33.5c 58.7b 76.0a 59.4ab 55.9b 10.8 **
95.5 97.7 97.0 97.9 96.1 95.8 94.9 98.2 5.93 NS
94.6 96.7 94.3 96.1 95.0 91.6 94.4 96.7 6.87 NS
91.0 94.8 88.8 96.8 90.7 86.2 90.9 91.7 9.54 NS
89.2 85.9 87.7 91.3 89.6 84.1 85.2 89.6 11.1 NS
84.2 82.3 85.3 88.2 87.8 82.0 83.7 87.7 12.5 NS
65.3ab 68.8ab 78.0ab 81.9a 77.0ab 60.8b 68.1ab 75.6ab 13.2 *
308 310 312 312 315 302 307 317 24.0 NS
Percentage egg production (hen-day per 28 d period) Treatment 1 2 3 4 5 6 7 8 Significance a–c
0 to 4 wk 12 12 + 2 12 + 2 12 + 2 12 + 2 12 12 12 SEM
4 to 8 wk 12 12 12 + 12 + 12 + 12 12 12 +
2 2 2
2
8 to 12 wk
12 to 18 wk
12 12 12 12 + 12 + 12 12 + 12 +
12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01. A
12 12 12 + 12 + 12 + 12 12 12 +
2
2 2 2
4 to 8 wk
12 12 12 12 + 12 + 12 12 + 12 +
2 2
2 2
8 to 12 wk 12 12 12 12 12 + 12 + 12 + 12 + 2 2 2 2
12 to 18 wk 52.4 51.2 52.7 49.4 51.4 52.4 51.2 50.4 2.9 NS
Egg weight (g) 21.4 21.4 20.1 19.8 20.8 21.2 21.3 21.1 1.55 NS
Eggshell def.B (µm)
Period 1
8.77 8.73 9.03 8.69 8.28 8.81 8.62 8.68 0.47 NS
Albumen height (mm)
A
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. B Eggshell deformation. **P < 0.01.
a–c
12 12 + 12 + 12 + 12 + 12 12 12
1 2 3 4 5 6 7 8 SEM Significance
2 2 2 2
0 to 4 wk
Treatment
Pullet light treatmentA
TABLE 3. Egg characteristics (experiment 1)
58.9 59.0 59.1 59.9 59.2 58.2 58.0 59.3 2.2 NS
Egg weight (g) 21.8 22.8 22.7 22.5 21.7 21.7 21.6 22.4 1.67 NS
Eggshell def. (µm)
Period 3
62.2 61.8 62.5 62.7 60.5 61.1 60.9 61.5 2.36 NS
Egg weight (g) 22.8 23.6 23.7 22.8 22.7 23.5 22.9 24.2 1.68 NS
Eggshell def (µm)
Period 5
65.9 66.0 65.1 65.1 65.2 66.7 64.5 66.2 3.92 NS
Egg weight (g)
25.2 25.5 26.1 24.6 24.6 25.1 23.7 27.2 3.33 NS
Eggshell def. (µm)
Period 13
7.35a 6.77a 6.64ab 6.95a 6.54ab 5.84b 6.74ab 6.57ab 0.64 **
Albumen height (mm)
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316 TABLE 4. Percentage egg grading (experiment 1) Pullet light treatmentA Period 1 Treatment 1 2 3 4 5 6 7 8
0 to 4 wk 12 12 + 12 + 12 + 12 + 12 12 12
4 to 8 wk
12 2 12 2 12 + 2 2 12 + 2 2 12 + 2 12 12 12 + 2
8 to 12 wk
12 to 18 wk
12 12 12 12 + 12 + 12 12 + 12 +
12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
SB
M
Period 3
L
XL
12.5 75.0 12.5 0 100.0 0 0 87.5 12.5 25.0 75.0 0 12.5 75.0 12.5 0 87.5 12.5 0 87.5 12.5 25.0 75.0 0
0 0 0 0 0 0 0 0
M
L
XL
Period 5
Period 13
L
L
XL
XL
12.5 75.0 0 62.5 37.5 25.0 75.0 12.5 87.5 0 62.5 37.5 12.5 87.5 12.5 87.5 0 75.0 25.0 37.5 62.5 0 87.5 12.5 62.5 37.5 0 100.0 0 100.0 0 75.0 25.0 37.5 62.5 12.5 87.5 0 62.5 37.5 0 100.0 12.5 87.5 0 87.5 12.5 0 100.0 0 100 0 87.5 12.5 12.5 87.5
12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. S (small) = 42 to 48 g; M (medium) = 49 to 56 g; L (large) = 57 to 63 g; XL (extra large) = > 63 g.
A B
TABLE 5. Body weight and feed intake (experiment 1) Pullet light treatmentA
Feed intake (g/bird per d)
BW (g) Treatment 1 2 3 4 5 6 7 8 SEM Significance
0 to 4 wk 12 12 + 12 + 12 + 12 + 12 12 12
4 to 8 wk
2 2 2 2
12 12 12 + 12 + 12 + 12 12 12 +
8 to 12 wk 12 12 12 12 + 12 + 12 12 + 12 +
2 2 2
2
12 to 18 wk 12 12 12 12 12 + 12 + 12 + 12 +
2 2 2 2
2 2 2 2
Period 1 1,579 1,553 1,589 1,555 1,569 1,539 1,583 1,588 100.0 NS
Period 2 1,848 1,827 1,938 1,851 1,812 1,841 1,878 1,872 178.3 NS
Period 1 ab
92.9 90.8ab 91.4ab 83.4b 94.2a 95.2a 93.4a 88.4ab 6.14 **
Period 3
Period 5
Period 13
102.9 101.0 101.4 100.6 103.3 100.2 99.3 102.0 6.03 NS
104.5 102.9 103.8 104.9 104.6 104.7 105.1 108.0 6.56 NS
109.8 106.3 112.8 107.9 109.8 110.0 106.8 111.5 9.52 NS
a,b
Means within columns with no common superscripts are significantly different. 12 = 12 h of light (6:00 a.m. to 6:00 p.m.); 12 + 2 = 12 h of light plus 2 h, from midnight to 2:00 a.m. **P < 0.01. A
TABLE 6. Egg production (experiment 2) Percentage egg production (Hen-day per 28-d period)
Pullet light treatmentA Treatment 1 2 3 4 5 6 7 8 SEM Significance a–d
0 to 4 wk
4 to 8 wk
8 to 12 wk
12 to 16 wk
1
2
3
4
5
6
8 + + + + 8 8 8
8 8 + + + 8 8 +
8 8 8 + + 8 + +
8 8 8 8 + + + +
26.2b 19.2bc 13.3cd 10.0d 9.6d 56.7a 52.6a 12.3cd 4.86 **
93.6a 88.6abc 82.1cd 84.0bcd 77.9d 90.9ab 87.9abc 83.6bcd 5.53 **
97.0a 96.3ab 95.9ab 95.4ab 93.0ab 94.6ab 90.2b 97.0a 4.06 *
94.5 94.9 92.6 91.5 90.2 92.9 88.4 93.8 4.34 NS
94.1 94.9 91.7 93.8 90.5 91.2 88.8 92.7 5.32 NS
91.0ab 91.8a 91.8a 93.4a 86.7abc 79.9bc 79.0c 92.1a 7.06 **
8 8 8 8
2 2 2 2
8 8 8
8
2 2 2
8 8
2
8 8
2 2 2 2
8 8 8 8
2 2 2 2
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01. A
8
8 8 8
8 8 + + + 8 8 +
2
2 2 2
4 to 8 wk
8 8
8 8
8 8 8 + + 8 + + 2 2
2 2
8 to 12 wk
8 8 8 8
8 8 8 8 + + + + 2 2 2 2
12 to 18 wk
A
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. *P < 0.05. **P < 0.01.
a–c
2 2 2 2
8 + + + + 8 8 8
1 2 3 4 5 6 7 8 SEM Significance
8 8 8 8
0 to 4 wk
Treatment
Pullet light treatmentA
TABLE 7. Egg characteristics (experiment 2)
53.5 52.5 52.9 52.9 53.7 52.0 53.0 53.7 0.95 NS
Egg weight (g) 20.2 21.4 20.4 20.4 19.6 20.8 19.8 20.7 0.89 NS
Eggshell def. (µm)
Period 2 Egg weight (g) 56.4ab 56.5ab 56.5ab 56.7ab 58.1a 55.6b 55.3b 57.6a 1.29 **
Albumen height (mm) 8.96ab 9.01a 8.57ab 8.81ab 8.80ab 8.47ab 8.16b 8.76ab 0.35 *
Period 4
57.8abc 57.8abc 59.4a 58.7ab 59.4a 56.4c 57.2bc 59.3a 1.56 **
Egg weight (g)
28.9 27.9 26.7 27.6 26.2 28.9 27.6 28.4 2.47 NS
Eggshell def. (µm)
Period 5
62.5 58.3 59.0 59.1 61.0 56.7 57.6 60.1 3.95 NS
Egg weight (g)
24.1ab 25.4a 24.5ab 24.6ab 22.6b 24.5ab 23.6ab 23.8ab 1.50 *
Eggshell def. (µm)
Period 6
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318 TABLE 8. Egg grading (experiment 2) Pullet light treatment Period 2 Treatment 1 2 3 4 5 6 7 8
Period 4
Period 5
Period 6
0 to 4 wk
4 to 8 wk
8 to 12 wk
12 to 18 wk
M
MB
L
M
L
XL
M
L
XL
8 + + + + 8 8 8
8 8 + + + 8 8 +
8 8 8 + + 8 + +
8 8 8 8 + + + +
100 100 100 100 100 100 100 100
50.0 50.0 25.0 37.5 12.5 50.0 75.0 0
50.0 50.0 75.0 62.5 87.5 50.0 25.0 100.0
0 0 0 0 0 37.5 0 0
100.0 100.0 100.0 100.0 100.0 62.5 75.0 100.0
0 0 0 0 0 0 25.0 0
12.5 0 12.5 0 0 37.5 12.5 0
75 100 87.5 100 87.5 62.5 87.5 100
12.5 0 0 0 12.5 0 0 0
8 8 8 8
2 2 2 2
8 8 8
8
2 2 2
8 8
2
8 8
2 2 2 2
8 8 8 8
2 2 2 2
8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. M (medium) = 49 to 56 g; L (large) = 57 to 63 g; XL (extra large) = > 63 g.
A B
parable to the schedule described for experiment 1 (Table 6). We intended to continue this layer study for 13 28-d periods, just as with experiment 1. However, during Period 7, layers inadvertently received a 1% calcium grower diet, and the trial was terminated. Production parameters only to the end of period 6 are described here, and measurements were as detailed for experiment 1. However, feed intake was measured only at the end of Period 2. Statistical Analysis Statistical analysis was the same for experiments 1 and 2. The objective in each experiment was to observe how the lighting regimen applied to pullets in the rearing period affected subsequent layer performance in terms of egg production and how egg quality parameters and treatments were defined by these regimens. Egg production and egg quality parameters as well as BW and feed intake were subjected to analysis of variance, and those response variables resulting in a significant F-test were further analyzed using Tukey’s test. Significance was accepted at P < 0.05.
RESULTS AND DISCUSSION In experiment 1, the highest egg production in Period 1 (18 to 22 wk) was for pullets introduced to midnight lighting at 12 wk (treatment 6, Table 2), whereas the lowest egg production was seen for treatment 4 birds, in which the midnight lighting was stopped at 12 wk. Throughout most of the experiment, egg production was unaffected by rearing treatment,
although significant differences were seen again in Period 13 (P < 0.05, Table 2), in which the converse to Period 1 occurred, with the early-maturing treatment 6 birds laying the fewest eggs and vice versa for the more slowly maturing treatment 4 birds. Overall, there was a major decline in egg production during Period 13, being greater than anticipated, and although this coincided with a period of hot weather, birds did not seem unduly stressed and pen temperature did not exceed 28°C. The early-maturing treatment 6 birds produced the least number of eggs to 70 wk. Rearing treatments had few effects on egg weight or shell quality measurements; however the early-maturing treatment 6 pullets exhibited an exceptionally low albumen height in Period 13, being significantly different (P < 0.05) to the latematuring treatment 4 birds, which had the best albumen height at this older age (Table 3). Egg grading is shown in Table 4. Rearing treatment had little effect on BW or feed intake (Table 5). The same trends were seen in Experiment 2 regarding earlier egg production for pullets receiving midnight lighting initially at 8 or 12 wk (treatments 6 and 7, Table 6). However, these earlier-maturing birds exhibited a dramatic decline in egg production during Period 6 compared to all other treatment groups. The earlier-maturing treatment 6 and 7 birds also showed the lowest albumen height during Period 2 of the study and the smallest egg size during Periods 4 and 5 (Table 7). After Period 4, the earliest-maturing treatment 6 birds were laying the smallest eggs, according to egg grad-
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319
TABLE 9. Body weight and feed intake (experiment 2) Feed intake (g/bird per d)
Pullet light treatmentA BW (g) 0 to 4 wk 8 8+2 8+2 8+2 8+2 8 8 8 SEM Significance
4 to 8 wk
8 8 8
8
8 8 + + + 8 8 +
8 to 12 wk
2 2 2
8 8
2
8 8
8 8 8 + + 8 + +
2 2 2 2
12 to 18 wk
8 8 8 8
8 8 8 8 + + + +
Period 1 d
2 2 2 2
1,202 1,206d 1,223c 1,238b 1,238b 1,268a 1,238b 1,221c 8.04 **
Period 2 bcd
1,541 1,532cd 1,571abc 1,595ab 1,620a 1,485d 1,485d 1,600a 36.7 **
Period 2 89.8 89.3 90.7 90.0 88.3 88.2 89.9 92.8 5.12 NS
a–d
Means within columns with no common superscripts are significantly different. 8 = 8 h of light; 8 + 2 = 8 h of light plus 2 h, from midnight to 2:00 a.m. **P < 0.01. A
ing classification (Table 8). Shell quality was unaffected until Period 6, when treatment 2 eggs showed a general decrease in shell quality. While being the heaviest birds at 20 wk of age (Period 1, Table 9), the early-maturing treatment 6 layers were the smallest birds by the end of Period 4. Only one observation of feed intake was undertaken, at the end of Period 2, when intake was unaffected by rearing treatment (Table 9). Midnight lighting of growing immature pullets seems to influence sexual maturation. While the effects of daylength and intensity per se are well known [5, 6, 7]) we are unaware of any information on the effect of midnight lighting of immature pullets on adult performance. P. D. Lewis [8] suggests that pullets will perceive the midnight lighting as an “effective” increase in photoperiod and will likely respond in the classical manner, as though this is a photostimulation. Normally one expects earliermaturing birds to produce smaller eggs throughout the life of the flock [1]. However, BW is little affected, and this is the reason that manipulation of maturity, by this means, has little influence on egg size. The effects of midnight lighting on sexual development are likely confounded by the light treatment induced at
time of move to the laying cages. In experiment 1, for example, the layers received 14 h of light per day when moved to the laying cages. For some pullets, this apparent stimulation (from an 8 h/d base in experiment 2) may, in fact, represent a decline in daylength, assuming midnight lighting is the cue for dawn in treatments 4, 5, 7, and 8, which were subjected to midnight lighting immediately prior to time of transfer. A step down in effective daylength (from 18 to 14 h) will likely delay maturity and so temper the effect of the stimulation occurring in the pullet house. Another trial would have to be conducted to separate out these stimulatory vs. inhibitory effects of change in daylength. The duration of midnight lighting of pullets seems critical. The technique should not be used in older pullets (>12 wk) as a means of stimulating late-cycle growth rate because the early induced maturity seems to have long term negative effects on egg production. If midnight lighting is used throughout the growing period, the pullet is effectively subjected to a constant long daylength, and so this has no effect on sexual maturation. Most eggs (315 to 317 in 52 wk) were produced by pullets given midnight lighting from 4 to 18 or 0 to 18 wk (treatments 8 and 5, respectively; Table 2).
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CONCLUSIONS AND APPLICATIONS 1. Providing growing Leghorn pullets with 2 h of supplemental light starting at midnight was studied as a potential technique for manipulating growth and development. 2. Midnight lighting has the potential to influence sexual maturation, because the pullet seems to perceive midnight as the start of the day, and the time from 2 a.m. until “lights-on” at around 6 a.m. is perceived as “light.” 3. If midnight lighting is first introduced at 12 wk of age, it has the effect of stimulating maturity. Conversely, if midnight lighting is given only from 0 to 12 wk, it has the effect of delaying maturity. 4. The safest system would be to use midnight lighting throughout rearing, where the perceived “long-day” is nonstimulatory to maturity because photoperiod is constant. Pullets grown on this program produced the most eggs from 18 to 70 wk (315 to 317).
REFERENCES AND NOTES 1. Leeson, S., and J. D. Summers. 1997. Feeding programs for growing egg-strain pullets. Pages 121–140 in Commercial Poultry Nutrition. University Books, Guelph, ON, Canada. 2. Leeson, S., and J. D. Summers. 1991. Growth and development of Leghorn pullets subjected to abrupt changes in environmental temperature and dietary energy level. Poult. Sci. 70:1732–1738. 3. Leeson, S., L. J. Caston, and J. D. Summers. 2002. Potential for midnight lighting to influence development of Leghorn pullets. J. Appl. Poult. Res. Submitted. 4. Leeson, S., L. J. Caston, and J. D. Summers. 1991. Significance of physiological age of Leghorn pullets in terms of subsequent reproductive characteristics and economic analysis. Poult. Sci. 70:37–43. 5. Lewis, P. D., and T. R. Morris. 1998. A comparison of the effects of age at photostimulation on sexual maturity and egg
production in domestic fowl, turkeys, partridges and quail. World’s Poult. Sci. 54:119–128. 6. Lewis, P. D., T. R. Morris, and G. C. Perry. 1999. Light intensity and age at first egg in pullets. Poult. Sci. 78:1227–1231. 7. Lewis, P. D., and T. R. Morris. 1999. Light intensity and performance of domestic pullets. World’s Poult. Sci. 55:241–250. 8. Lewis, P. D. 2003. Personal communication.
Acknowledgments This work was supported by the Poultry Industry Center, Guelph, Ontario, Canada and the Ontario Ministry of Agriculture and Food, Guelph, Ontario, Canada. We would like to thank Peter Lewis for his ideas in interpretation of the photostimulatory effects of midnight lighting.