Effect of Delayed Step-Up Lighting on Plasma Luteinizing Hormone and Reproductive Function in Broiler Breeders F. Nøddegaard,*1 R. T. Talbot,† and P. J. Sharp† *Danish Institute of Agricultural Sciences, Research Center Foulum, DK-8830 Tjele, Denmark, and †Roslin Institute, Roslin, Midlothian EH25 9PS, Scotland, United Kingdom subsequent rate of lay in hens fed ad libitum and feedrestricted hens. Delayed photostimulation of hens did not impair the photoinduced increase in the concentration of plasma luteinizing hormone (LH) or egg production. Delayed photostimulation in cockerels failed to stimulate LH secretion. Unexpectedly, for feed-restricted hens, transfer from 3 to 8 h light/d at 264 d resulted in an increased in plasma LH and increased egg production. A similar increase in plasma LH was observed for cockerels subjected to the same lighting treatment. We concluded that, in broiler breeder hens, the reproductive response to photostimulation is not impaired if photostimulation is delayed for up to 313 d. Cockerels may not respond well to delayed photostimulation.
ABSTRACT The aim of the study was to establish, in contemporary broiler breeders, whether delayed photostimulation at 313 d results in a reproductive response similar to that after photostimulation at 134 d (standard practice). The standard lighting program was compared with a novel program in which daily hours of light were reduced to 3 h during rearing and kept at 3 h until photostimulation at 264 d (8 h) or at 313 d (16 h). This experiment was done with hens fed ad libitum or feed-restricted hens. In photostimulated and nonphotostimulated hens, feed restriction delayed the onset of egg production and enhanced the subsequent rate of laying. Standard photostimulation advanced the onset of lay and increased the
(Key words: broiler breeders, luteinizing hormone, egg production, male fertility, photostimulation) 2000 Poultry Science 79:778–783
into egg production by increasing the photoperiod to 20 h light/d and relaxing feed restriction. Egg production in these hens was inversely related to the age at photostimulation, suggesting that the capacity of the reproductive system to respond to photostimulation may decrease as a function of age per se or as a result of prolonged severe food restriction (Dunn and Sharp, 1992). The purpose of the present study was to establish how contemporary broiler breeders respond to delayed photostimulation. Measurements of changes in plasma luteinizing hormone (LH) were made as an independent correlate of the effects of photostimulation on reproductive function (Dunn and Sharp, 1990).
INTRODUCTION Maximum egg production and fertility are achieved in broiler breeders by a combination of feed restriction and lighting patterns (Katanbaf et al., 1989; Yuan et al., 1994; Hocking, 1996) with the aim of stimulating hens as early as possible into laying fertile, settable eggs. Delayed photostimulation of broiler breeders may be necessary because of management or market requirements, but limited information is available on the interaction between feed restriction and lighting patterns after broiler breeders have been allowed to come into breeding condition while maintained on short days. A low-level of egg production observed in second-year dwarf broiler breeders maintained on 3 h light/d after an induced molt was significantly stimulated after an increase in day length to 20 h light/d (Sharp et al., 1992). In an associated study, dwarf broiler breeders were kept out of lay by severe feed restriction while exposed to 8 h light/d for 22, 36, and 52 wk (Dunn and Sharp, 1992) and then were stimulated
MATERIALS AND METHODS Birds and Lighting Treatments Cobb broiler breeder2 hens (240) were reared from 1d-old in floor pens (2 × 2 m) in groups of 20, and 60 Cobb broiler breeder cockerels were in groups of 30 (2 × 4 m). Both sexes were exposed to one of two lighting programs
1 To whom correspondence should be addressed: Flemming. [email protected]
2 Cobb 500, Cobb Denmark A/S, Frederiksha˚b DK-7183 Randbøl, Denmark. Received for publication August 2, 1999. Accepted for publication December 21, 1999.
Abbreviation Key: LH = luteinizing hormone; L:D = photoperiod of hours light:h darkness.
PHOTOPERIODIC RESPONSES OF BROILER BREEDERS
FIGURE 1. Photoperiodic lighting patterns (A, B, B1, and B2) used in the study.
(Figure 1). There were six pens of hens and one pen of males in each program. Lighting program A was as recommended by the Cobb Breeding Company (Breeder Management Guide - Cobb 500, 1997). At 134 d the photoperiod of Program A was increased from 8 to 11 h and thereafter was increased by increments of 1 h at 14-d intervals until it reached 16 h. In lighting pattern B, designed to delay the onset of breeding, the photoperiod was decreased from 23 to 3 h between 1–134 d in steps of 1 h at 7-d intervals. At 264 d, one-half of the B groups (B1) were kept in the same pen while the photoperiod was increased to 8 h; thereafter, at 313 d, the photoperiod was increased to 16 h. The rest of the B groups were moved to identical pens at 261 d to maintain exposure to a 3-h photoperiod (B2); thereafter, at 313 d, the photoperiod was increased to 16 h. At the onset of lay, each pen of hens was provided with three mature cockerels selected at random from the male pens. The first egg was recorded for each pen, so an individual pen received the males on the day of its first egg. Egg-laying records were kept for each pen, and eggs were incubated for 10 d and then candled to measure male fertility.
Feeding Treatments The six pens of 20 hens in each lighting program were divided into three feeding regimens. Two pens were fed ad libitum, two were fed to attain 75% of the ad libitum fed body weight, and two pens of hens were provided a restricted regimen as recommended by the Cobb Breeding Company (Breeder Management Guide - Cobb 500, 1997) In each lighting program, cockerels were fed to attain an average body weight 10% above that of hens fed ad libitum. Birds were weighed individually each week to calculate the amount of feed to be given to the restricted groups in order to maintain their target growth curves.
Plasma LH and Phospholipoprotein Measurements Eight cockerels, eight hens fed the recommended regimen, and eight hens fed ad libitum were selected for blood
sampling from groups exposed to lighting programs A, B1, and B2. No blood samples were taken from hens fed to maintain 75% of the body weight of the ad libitum group. Blood samples (1 mL) were taken toward the end of the light period, by direct venipuncture from a wing vein at 130, 163, 196, 227, 261, 269, 311, 318, and 358 d of age. After centrifugation, plasma was removed and stored at –20°C. Plasma LH was measured in a single radioimmunoassay (Sharp et al., 1987). The intraassay coefficient of variation was 4.6%. Plasma phospholipoprotein was detected as described by Nøddegaard (1989). Hens in lay have high plasma levels of a distinct phospholipoprotein complex, and this complex forms a nontransparent precipitate immediately upon dilution with water (Nøddegaard, 1989).
Statistical Procedures Changes in concentrations of plasma LH in response to changes in photoperiod were tested for significance by a two-sided, paired t-test that compared values before and after an increase in photoperiod. The same test was used to compare LH levels between treatment groups at fixed points in time.
RESULTS Body Growth Curves The growth curves of hens exposed to the two lighting patterns were almost the same within each of the three feeding treatments with the exception that the hens fed ad libitum grew to a heavier adult weight in Group B than in Group A (Figure 2). The body weights of birds in the three feed-restricted groups were markedly different; the birds fed the recommended restricted regimen were the lightest. After about 190 d, differences in body weights between the three feeding treatments became progressively less pronounced (Figure 2). In hens in Group B1, there was no change in body weight associated with increased photoperiod at Days 264 and 313 or in group B2 with transfer to another pen on Day 261 (Figure 2 insert). The growth curves of the cockerels were maintained 10% above that of the hens fed ad libitum. There were no changes in male body weight at Days 264 and 313 associated with increased photoperiod (data not shown).
Plasma LH In hens in Group A, plasma LH increased after photostimulation at 134 d (ad libitum, P = 0.013; recommended restricted regimen, P = 0.068). Thereafter, plasma LH decreased progressively until Day 358. In Group A, there were no significant differences in plasma LH concentration between the hens fed ad libitum or feed-restricted hens between Days 130 and 358 (Figure 3).
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In cockerels in Group A, plasma LH did not increase after photostimulation at 134 d and decreased progressively until Day 358 (Figure 4). From 163 to 227 d, plasma LH was lower in hens in Group B1 and B2 than in the stimulated group (A), whereas there was no significant difference in plasma LH in cockerels in these lighting treatments (cf Figures 3 and 4). In hens in Group B1 fed the recommended restricted regimen, plasma LH increased significantly after Day 264 when the photoperiod was increased from 3 to 8 h and again after Day 313 when the photoperiod was increased to 16 h (Figure 3). In hens in Group B2 fed the recommended restricted regimen, plasma LH did not change immediately after they were moved to another pen while maintained on 3 h light/d. Plasma LH increased significantly in these hens after 313 d when the photoperiod was increased from 3 to 16 h (Figure 3). In hens in Group B1 fed ad libitum, plasma LH did not change significantly after the photoperiod was increased from 3 to 8 h at Day 264 but increased after the photoperiod was increased from 8 to 16 h at Day 313 (Figure 3). In hens in group B2 fed ad libitum, maintained on 3 h
light/d, plasma LH decreased (P = 0.03) after transfer to another pen on Day 261 (Figure 3). Plasma LH increased significantly in these hens after the photoperiod was increased to 16 h on Day 313 (Figure 3). In cockerels in Group B1, the increase in photoperiod from 3 to 8 h on Day 264 was associated with an increase in plasma LH, whereas no associated change in plasma LH was observed in control Group B2 cockerels retained on 3 h light/d. There were no changes in plasma LH in cockerels in Groups B1 and B2 after the photoperiod was increased to 16 h light/d on Day 313 (Figure 4).
Egg Production and Fertility Age at the onset of egg production in photostimulated (Group A) and nonphotostimulated (Group B) hens was related to the level of feed restriction (Figure 5); birds fed the recommended restricted regimen came into lay much later. The rate of egg production in photostimulated and nonphotostimulated hens was also related to the level of feed restriction; the hens fed ad libitum laid at a lower rate than the feed-restricted hens (Figure 5). Irrespective of the level of feed restriction, in Group A, egg production
FIGURE 2. Body weight growth curves for broiler breeder hens fed ad libitum (ad lib), or to attain 75% of the ad libitum body weight (75%), or on the Cobb-recommended feed restriction regimen (recom). The hens were exposed to lighting Regimen A or B shown in Figure 1. Inserted: Body weight growth curves for hens in Regimen B1 or B2 (Figure 1) during photostimulation.
PHOTOPERIODIC RESPONSES OF BROILER BREEDERS
FIGURE 5. Egg production in broiler breeders photostimulated from Day 134 by using lighting pattern A (see Figure 1) or retained on short days until Day 263 by using lighting pattern B (Figure 1). There were three groups of hens in each lighting treatment: fed ad libitum (ad lib), 75% of the ad libitum body weight (75%), or the Cobb recommended feed restriction regimen (recom). HDP = Hen-day egg production.
FIGURE 3. Changes in plasma luteinizing hormone (LH) in broiler breeder hens after increases in photoperiod. a) Gradual increase during Days 134 to 203 (lighting pattern A shown in Figure 1). b) Increase at Days 264 and 313 (lighting pattern B1) or increase only at Day 313 (lighting pattern B2). The hens were fed either ad libitum (ad lib) or the Cobb recommended feed restriction regimen (recom). *P < 0.05, **P < 0.01, or ***P < 0.001, compared with the preceding value.
decreased with advancing age. In Group B hens maintained on 3 h light/d, the low rate of lay was due, in part, to the presence of 25% nonlayers in the group fed the recommended restricted regimen and 65% nonlayers in the group fed ad libitum. This finding was concluded from observations on the immediate reaction of the plasma upon dilution with 9 volumes ice-cold distilled water (data not shown). Development of a nontransparent precipitate indicated that the hens were laying. In Group B1 fed the recommended restricted or 75% of the regimen ad libitum, egg production increased after transfer from 3 to 8 h light/d (Figure 6). Egg production in Group B2, fed the recommended restricted regimen and transferred to another pen while held on 3 h light/ d, decreased (Figure 6). Egg production increased steeply in all groups, irrespective of the level of feed restriction after the photoperiod was increased to 16 h at 313 d (Figure 6). This production level was similar to peak levels of egg production (Days 200 to 232) observed in three groups of birds on the same feed restriction regimen and photostimulated on 134 d (Figures 5 and 6). The fertility of eggs from the hens fed as recommended and exposed to the standard step-up lighting pattern was greater than 90% from 3 wk after onset of lay until the end of the experiment (data not shown). The fertility of eggs from hens in Groups B1 and B2 held on 3 h light/d was very poor (14 to 32%) but increased to 61% when the photoperiod was increased to 8 h/d and to 80% (B1) and 87% (B2) when increased to 16 h/d (data not shown).
DISCUSSION FIGURE 4. Changes in plasma luteinizing hormone (LH) in broiler breeder males after a gradual increase in photoperiod during Days 134 to 203 (lighting pattern A, shown in Figure 1), or increases at Days 264 and 313 (lighting pattern B1 shown in Figure 1), or an increase only at Day 313 (lighting pattern B2). The cockerels were fed to be 10% heavier than hens fed ad libitum. *P < 0.05 compared with the preceding value.
These results demonstrate the interaction between feed restriction and photoperiod in the control of egg production in contemporary broiler breeders. Feed restriction delayed the onset of lay and increased the rate of lay in photostimulated and nonphotostimulated hens. Photostimulation (increasing daily hours of light) advanced the
NØDDEGAARD ET AL.
FIGURE 6. Egg production in broiler breeders marginally photostimulated at Day 264 and then fully stimulated at Day 313 by using lighting pattern B1 or only fully stimulated at Day 313 using pattern B2 (see Figure 1). The hens were fed ad libitum (a) or fed to attain 75% of the ad libitum body weight (b), or fed according to the Cobb-recommended feed restriction regimen (c). HDP = Hen-day egg production.
onset of lay, and increased the rate of lay, irrespective of the degree of feed restriction. Photostimulation at 134 and 313 d resulted in similar peaks of egg production within each feeding treatment. This observation shows that, in contemporary broiler breeder hens held on short days, there is no decrease in the capacity of the reproductive system to respond to photostimulation in the first year, provided the level of feed restriction is not sufficiently severe to prevent egg laying. It appears from an earlier study (Dunn and Sharp, 1992), that in broiler hens held on short days, feed restriction sufficient to prevent egg laying progressively decreases the capacity of the reproductive system to respond to photostimulation in the first year. The progressive decrease in plasma LH in female broiler breeders after initial photostimulation at 134 d was correlated with decreased egg production. Similar observation has been made of hens from other lines of broiler breeders (Williams and Sharp, 1978, Sharp et al., 1992) and on table egg-laying lines (Tanabe et al., 1981). This age-related decrease in
plasma LH is likely to be a direct cause of reduced female reproductive function. In males, a similar age-related LH decrease (Figure 6) did not seem to impair reproductive function, because more than 90% of the eggs from the hens in the standard lighting program (A), and fed according to recommendations, were fertile throughout the experiment. However, it cannot be excluded that sperm count decreased during the study but not sufficiently so to affect fertility. Improved fertility after increases in photoperiod of Program B was not related to LH scretion because, although plasma LH in the males increased when the photoperiod was increased from 3 to 8 h, it did not do so when the photoperiod was increased from 8 to 16 h. Improved fertility after an increase in photoperiod may have been due to altered social interactions between the sexes. Males were probably fully sexually developed in the short-day regimes (Parker and McCluskey, 1965), but 3 and 8 h of light may not have allowed sufficient time for mating activity to ensure complete female fertility. Sreekumar and Sharp (1998) have previously reported failure of short-day cockerels to show an increase in plasma LH after photostimulation around the age of somatic maturation. This apparent lack of photo-responsiveness can be explained as a result of the photoperiodicindependent increase in plasma LH that occurs at this age. The failure of cockerels to show an increase in plasma LH after photostimulation at 313 d, at a time when plasma LH levels were low, was unexpected. This observation suggests that in cockerels unlike hens, there is a decrease in the capacity of the reproductive system to respond to photostimulation with advancing age during the first year. It remains to be established whether this is due to age per se or to the severity of the level of feed restriction. A further unpredicted observation in this study was the increase in plasma LH in males and females fed the recommended restricted regimen and transferred from 3to 8-h photoperiods at 264 d. This increase in plasma LH appeared to be physiologically significant because it was associated with an increase in egg production and probably also fertility in males. Because amounts of food were unchanged and there was no unexpected upward deviation in the body weight curves for cockerels or hens transferred from 3 to 8 h (Figure 2, insert), the increase in plasma LH and reproductive function cannot be explained as being due to an unintentional increase in feed intake. The possibility that an increase in photoperiod from 3 to 8 h may be photostimulatory should be considered. The birds of Group B experienced a gradual decrease of photoperiod to 3 h during the first 19 wk of life, whereas the birds of Program A experienced a fast decrease to 8 h. The gradual and more sustained decrease probably made the birds of Program B more responsive to the late light stimulation—more responsive than if these birds had experienced the standard rearing part of Program A. It is well established for chickens held on 6- or 8-h photoperiods that the critical day length for photoinduced LH release is about 10 to 11 h (Dunn and Sharp, 1992; Sharp, 1993). It is possible that the day length thresh-
PHOTOPERIODIC RESPONSES OF BROILER BREEDERS
old for stimulation was lowered during the unusually long period of exposure to 3 h. It is theoretically possible that for hens held on very short photoperiods (i.e., 3 h), the circadian rhythm of photoinducibility is phasedadvanced sufficiently for the photoinducible phase (Φi) to be illuminated for 1 d after transfer to 8 h light:16 h darkness (8L:16D) (Follett and Pearce-Kelly, 1991). Further exposure to 8L:16D lighting cycles would be expected to result in a rapid re-entrainment of the circadian rhythm of photosensitivity such that the lighting cycle would not be photoinductive. Because exposure to 1 long d for quail is known to trigger increased LH secretion up to 10 d (Follett and Pearce-Kelly, 1991), a similar mechanism may account for the increase in LH secretion observed in broiler hens and cockerels after transfer from 3 to 8 h (Figures 3 and 4). However, this explanation is not supported by experiments on quail entrained to 2L:22D or 10L:14D. In these birds, there was no evidence that Φi, as is measured by increased plasma LH, after exposure to 1 long d was advanced in birds entrained to 2L:22D relative to those entrained to 10L:14D (Follett and PearceKelly, 1991). In conclusion, these results show that delayed photostimulation in broiler breeder hens fed the recommended restricted diet does not impair the initial photoperiodic response of the reproductive system; however, this may not apply to male broiler breeders in which delayed photostimulation failed to stimulate LH release.
ACKNOWLEDGMENTS The day-old chickens were kindly provided by Cobb Danmark, Frederiksha˚ b, Randbøl, Denmark. F. Nøddegaard was supported by the Danish Ministry of Food, Agriculture and Fisheries and the Danish Poultry Council. R. T. Talbot and P. J. Sharp were supported by a BBSRC core strategic grant.
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