Poult Sci 2007. 86:917-920
© 2007 Poultry Science Association
PHYSIOLOGY, ENDOCRINOLOGY, AND REPRODUCTION |
Effects of Photoperiod on Ovarian Morphology and Carcass Traits at Sexual Maturity in Pullets
H. Chen*,
R. L. Huang*,1,
H. X. Zhang
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K. Q. Di*,
D. Pan* and
Y. G. Hou*
* College of Animal Science and Technology, Agricultural University of Hebei; Hebei University, Baoding, Hebei 071001, China
1 Corresponding author: dkhrl{at}mail.hebau.edu.cn
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ABSTRACT
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This paper is concerned with the effects of photoperiod on ovarian morphology and carcass traits at sexual maturity in egg-type hens. Two hundred fifty-six commercial egg-type pullets were initially subjected to a photoperiod of 23L:1D, which was reduced to 22L:2D at 1 wk, to 18L:6D at 2 wk, and to 16L:8D at 3 wk. From 4 to 20 wk, the photoperiod was 8L:16D. At 20 wk, 32 pullets were individually caged in individually lit cages, with 8 cages per unit. Two cage units were placed into 4 photoperiods of 17L:7D, 15L:9D, 13L:11D, and 11L:13D, respectively. Each bird was processed when it reached sexual maturity (SM), and carcass and ovarian morphology were assessed. The results showed that photoperiod had an effect on the timing of SM, and the age at first egg was 5.7 d earlier for hens exposed to the 17L:7D photoperiod than the 11L:13D photoperiod. However, photoperiod had no effect on BW at SM. A photoperiod of 11L:13D limited ovarian follicle formation and increased carcass protein and lipid compared with birds on longer photoperiods, whereas the 17L:7D photoperiod restricted ovary and oviduct full development. These results indicated that excessively long and short photoperiods can restrict reproductive development in egg-type hens.
Key Words: photoperiod • sexual maturation • carcass trait • ovarian morphology • pullet
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INTRODUCTION
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Reproductive characteristic in egg-type hens can be assessed by examining egg-laying characteristics such as age at first egg, sequence lengths, and intersequence pause lengths. Alternatively, some indicators of reproductive characteristics can be identified by studying ovary morphology during sexual maturation at the first oviposition and at the end of lay (Renema et al., 2001; Robinson et al., 2001). Light may be the most critical of all environmental factors affecting reproduction in birds (Olanrewaju et al., 2006). It is usual to rear egg-type pullets on 8-h day length until they reach 18 to 20 wk, followed by initial weekly increments of 30 min or 1 h thereafter to a peak of 14- to 16-h day length. Pullets exhibit juvenile photorefractoriness (Morris, 1966). Light intensity had no effect on the timing of sexual maturity (SM) or BW at SM, but ovary weight and number of large yellow follicles (LYF) responded positively to increasing light intensity (Renema et al., 2001). The reported responses of abruptly different size increments in photoperiod tend to provide insufficient treatments to determine the optimal day length to use during SM and the laying period (Robinson et al., 1998; Ciacciariello and Gous, 2005). It is possible that egg-type hens may also react differently to prepubertal increases in day length. The objective of this research was to compare the effects of different photoperiods on carcass and reproductive morphology, egg production, and laying patterns at SM. The relationship of production profiles with ovarian morphology and carcass characteristics at SM were also examined.
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MATERIALS AND METHODS
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Stocks and Pullet Management
Two hundred fifty-six Hyline (Brown) commercial egg-type pullets were reared to 20 wk of age. The calculated nutrient analyses of the diets are reported in Table 1
. Feed and water were provided ad libitum throughout the study. Pullets were reared in 16 cages of 16 birds each in light-proof facilities. Each group had 4 replicates. Chicks were initially subjected to a photoperiod of 23L:1D, which was reduced to 22L:2D at 1 wk, to 18L:6D at 2 wk, and to 16L:8D at 3 wk. From 4 to 20 wk the photoperiod was 8L:16D. Illumination was provided by two 15-W compact fluorescent lamps producing a mean illuminance of 15 ± 2.4 lx at the top of cages. Pullets’ beaks were trimmed at 7 d of age, and all pullets were wing-banded at 6 wk. All experimental procedures were approved by the Animal Research Committee of Hebei.
Experiment Design
At the age of 20 wk 2 pullets from each replicate were selected, weighed, and randomly placed in individual, illuminated, standard laying cages. The portable cage units consisted of 8 cages. The cages units were exposed to 4 different photoperiods of 17L:7D, 15L:9D, 13L:11D, and 11L:13D. Feed and water were provided ad libitum throughout the study. The age at first egg was recorded.
Carcass and Reproductive Traits at Sexual Maturity
The weight of the first egg for each hen was recorded, and feed was withdrawn overnight (12 to 20 h) to facilitate gut clearance. The following morning, the pullets were killed by cervical dislocation, and the intact carcass was weighed. The weights of the abdominal fat pad (including the fat surrounding the gizzard), breast muscle (pectoralis major and minor), liver, oviduct, ovary, and stroma were recorded. The stroma weight was composed of the ovarian tissue remaining after the LYF were counted and removed. The number of small yellow follicles and follicle cell in the oviduct were recorded. Birds were inspected for incidence of internal ovulation, internal oviposition, ovarian regression, and follicular atresia.
All carcass components, except the liver and oviduct, were returned to the carcass and stored at –20°C until analysis was performed. Each carcass was pressure cooked for 4 h and homogenized using an industrial blender. Duplicate 150-g samples of each homogenized carcass were frozen, processed, and analyzed as described by Renema et al. (1999). Carcass samples were analyzed in duplicate for dry matter, ash, CP, and petroleum ether-extractable lipid content. The liver and oviduct were individually frozen, stored, freeze-dried, and ground, and the total lipid content was determined by petroleum ether extraction.
Statistical Analysis
The experiment was analyzed as a single factorial design. Sources of variation were different photoperiod. All data were analyzed by 1-way analyses of variance using the GLM procedures of SPSS (SPSS Inc., Chicago, IL). When significant differences were determined for the main effects, comparisons among means were made using the least significant difference procedure. Unless otherwise stated, all statements of significance were assessed using P < 0.05.
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RESULTS AND DISCUSSION
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Age at Sexual Maturity
Exposure to photoperiods of 17L:7D, 15L:9D, 13L:11D, and 11L:13D significantly affected age at SM (Table 2). The average age at first egg was 144.8 d for the 17L:7D and 150.5 d for the 11L:13D. The results showing that the largest advance in age at first egg for egg-type hens reared on 8-h photoperiods was achieved by transferring them to a 17L:7D photoperiod did not agree well with the predicted most stimulatory photoperiod for egg-type pullets (Lewis et al., 2002, 2003). Although earlier SM may represent a faster rate of follicular recruitment after lighting, it is also likely to be influenced by variation in age of hypothalamic maturation.
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Table 2. Age, BW, BW gain, breast muscle, and abdominal fat pad at sexual maturity (SM) in egg-type laying hens subjected to various photoperiods
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It has been suggested that there is a BW or body composition threshold for the onset of sexual maturation (Brody et al., 1980, 1984). All lighting programs were effectively able to stimulate the sexual maturation process. However, photoperiod had no effect on BW or absolute abdominal fat pad or breast muscle weight at SM (Table 2
). Relative abdominal fat pad weight was 2.93% of BW in pullets exposed to 17L:7D, compared with 3.81, 3.98, and 3.98% for pullets exposed to 15L:9D, 13L:11D, and 11L:13D, respectively (Table 2).We conclude that earlier SM caused less nutrient allocation to the abdominal fat pad.
Carcass and Reproductive Morphology
Liver weight and lipid content of birds exposed to 17L:7D were not significantly higher than those of birds exposed to 15L:9D, 13L:11D, or 11L:13D. Liver protein content of pullets exposed to 17L:7D was not significantly lower than that of the other groups (Table 3). Relative values for carcass protein, lipid, ash, and moisture did not differ among the groups (Table 3). Values for the oviduct and ovary are shown in Table 4. Significant differences were detected only for the weight of the stroma, which weighed significantly less in pullets exposed to the 17L:7D photoperiod than in those exposed to the 15L:9D, 13L:11D, and 11L:13D photoperiods (P < 0.05; Table 4). The different photoperiods had no significant effects on first egg weight, LYF total or mean weight, LYF lipid content, or small yellow follicle number or weight (Table 5). The LYF protein content was highest in pullets exposed to the 13L:11D photoperiod than in those exposed to the 11L:13D photoperiod (Table 5).
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Table 3. Liver weight, liver lipid and liver protein content, carcass protein, carcass lipid, carcass ash, and carcass water at sexual maturity in egg-type laying hens subjected to various photoperiods
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Table 4. Oviduct length, oviduct weight and protein content, lipid content, ovary weight and percentage, and stroma weight at sexual maturity in egg-type laying hens subjected to various photoperiods
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Table 5. First egg weight, LYF1 total weight, LYF mean weight, number of LYF and SYF2 at sexual maturity in egg-type laying hens subjected to various photoperiods
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Even though accelerating photoperiods resulted in a 5.7-d advance in SM, it was unlikely that it would be a more profitable alternative to rearing birds. Brody et al. (1980, 1984) and Soller et al. (1984) had reported the importance of threshold BW for the onset of SM. Because the reduced age at first oviposition in the 17L:7D photoperiod also was associated with a numerically lower BW, BW must not have been a limiting factor in achievement of SM or selection had effectively altered the threshold limit. However, abdominal fat pad weight and total carcass lipid content were not significantly different between photoperiod at SM, which would appear to support a minimum fat requirement for SM, as previously reported by Bornstein et al. (1984). Robinson et al. (1999) found no significant differences in carcass composition of broiler breeder pullets photostimulated to reach SM at different ages; and although there were differences in Single Comb White Leghorn pullets (Robinson et al., 1999), the greatest trend toward increases in carcass at composition of this experiment were found in pullets exposed to the 11L:13D photoperiod. Soller et al. (1984) found that fat content alone was not sufficient to initiate SM but felt that there may be a lean body mass requirement instead. Lean body mass has also been found to be more closely related to LYF number at SM than is carcass lipid (Hocking, 1993). Because of observed differences in sexual maturation rate and fat content in broiler breeders, Renema et al. (1999) stated that the apparent link between BW, lipid, or protein content in maturing pullets may better relate to more specific metabolic changes. Birds in the current study may simply have similar fat content at SM because of similar feed management and growth curves. Whereas current data suggested that there is a minimum fat requirement for the onset of lay, care must be taken to understand the actual mechanisms.
Although the 11L:13D treatment stimulated sexual maturation, it was not a full response. Ovary weights tended to be reduced, and lipid stores increased, relative to the longer photoperiod groups. In the 17L:7D photoperiod treatment, numerically decreased contents of all lipid measures and numerically lower values for most ovarian morphology suggested that overlong photoperiod limited sexual development. These findings may indicate that the optimal photoperiod for egg-type hens in SM is about 13L:11D.
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ACKNOWLEDGMENTS
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This project was funded by the Science Institute of Hebei province and Poultry Breeding Farms of Agricultural University in Hebei. Infrastructure support was provided by the College of Animal Science and Technology, Agricultural University of Hebei.
Received for publication October 10, 2006.
Accepted for publication January 28, 2007.
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REFERENCES
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ABSTRACT
INTRODUCTION
