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PRODUCTION, MODELING, AND EDUCATION |
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* Department of Agricultural, Food and Nutritional Science, University of Alberta, Edmonton, Alberta, Canada, T6G 2P5; and Agriculture Research Division, Alberta Agriculture and Food, Edmonton, Alberta, Canada, T6G 2P5
2 Corresponding author: martin.zuidhof{at}gov.ab.ca
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Key Words: broiler breeder • genetic strain • body weight profile • egg production • chick production
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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Photostimulating breeders early (18 wk of age) may result in earlier sexual maturation (Yuan et al., 1994; Robinson et al., 1996) but with additional production of very small (<52 g) eggs, which in some markets are too small to be set. Flocks that are photostimulated early are also more likely to have poor uniformity in the onset of sexual maturation, because birds vary in time of attainment of thresholds for BW, carcass composition, and ability to release gonadotropin-releasing hormone to come into production. In contrast, late photostimulation (PS) ensures that most birds are physically mature enough to respond to a photostimulatory cue, resulting in flocks that come into production more uniformly (Hocking, 1996; Robinson et al., 1996). Consequently, age at PS in modern broiler breeders has been delayed from 18 wk of age (1980s) to 22 or 24 wk of age.
Providing excess feed during sexual maturation can cause an over-stimulation in ovarian follicle development (Van Middelkoop, 1971; Hocking et al., 1987; Yu et al., 1992) in the form of additional large yellow follicles (LYF). With moderate over-feeding, LYF numbers have been shown to increase by as little as 1 LYF (Robinson et al., 1998a), which ultimately can result in a 10-egg reduction in overall productivity (Robinson et al., 1998b). Over-feeding for as little as 2 wk between PS and peak production can permanently hinder fertility and hatchability (Ingram and Wilson, 1987).
This trial was carried out to determine if there is a production effect on strains being reared on alternate BW profiles through the measurement of various indicators of reproductive and metabolic function. A second objective was to determine if some strains of breeders could be photostimulated early (18 wk) if they were reared on a heavier BW profile from 4 wk of age. To meet these objectives, 3 strains of hens were reared on 4 different growth curves and were photostimulated at 18 or 22 wk of age. This study will provide a better understanding of the physiological mechanisms driving the interaction among nutrient allocation, selection for growth rate, and age at PS as they affect reproductive traits.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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This project was carried out according to the Guide to the Care and Use of Experimental Animals (CCAC, 1993). The experimental protocol was approved by the University of Alberta’s Faculty of Agriculture, Forestry and Home Economics Animal Policy and Welfare Committee.
Stocks and Management
The details of the genetic stocks used and the rearing phase management have been described in a companion paper (Robinson et al., 2007). The 3 strains used were the Hubbard Hi-Y (Hubbard-ISA, Walpole, NH), Ross 508, and Ross 708 (Aviagen Inc., Huntsville, AL), which were obtained commercially and reared in floor pens in groups of 70 chicks per pen (n = 560 per strain). The chicks were reared following a photoperiod of 24L:0D for the first 3 d and 8L:16D to 18 wk of age. Chicks were individually identified with neck tags. All pullets were fed ad libitum for the first 4 wk of age.
The control BW profile (standard) was the mean of the 3 commercial strains used. A low profile based on an early reduction in feed allocation was followed by a period of large feed allocations during sexual maturation. Birds of this low profile were 25% lower in BW than the standard pullets at 12 wk of age. A moderate profile was based on birds being 150% of the standard birds at 12 wk followed by lower rate of gain to 32 wk. A high profile was based on birds being 200% of 12-wk BW of the standard birds. This profile had minimal BW gains during sexual maturation. The details of the diets fed have been published in a companion paper (Robinson et al., 2007). Briefly, a starter was fed from wk 0 to 5, a grower was fed from 5 to 22 wk, and a breeder diet was fed after 22 wk. The breeder diet was formulated to provide 16.0% CP, 2,830 kcal/kg, and 3.5% Ca. Quantitative feed restriction was applied with daily feeding. Feed was allocated on a pen basis based on BW, BW gain, and predicted BW gain in the immediate week. Actual feed allocation values for the 4 BW profiles are presented in Figure 1
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Data Collection.
The egg production period ended at 58 wk of age. Every egg was classified as normal, soft-shelled, shell-less, having other shell abnormalities, or double-yolked. Egg weights were recorded on every egg to 32 wk and 4 d per week thereafter. Early production characteristics included average first egg weight, settable egg age (first 2 consecutive days where egg weight exceeded 52 g), the interval from sexual maturity to settable egg age, and the number of small (<52 g) eggs. Production parameters included all eggs (total eggs), normal eggs, defective eggs, settable eggs, prime sequence length, mean sequence length, and number of sequences longer than 10 d. Beginning at 28 wk of age, hens were inseminated with 50 µL of pooled semen from identical age Ross males. Inseminations were carried out at 7-d intervals. Hatchability was determined weekly, and breakouts of hatch residue were conducted biweekly to determine fertility and hatchability of fertile eggs. Embryonic mortality (1 to 7 d, 8 to 14 d, and 15 to 21 d) and the number of chicks dead in shell plus cull chicks were determined. Chick production was calculated based on the number of settable eggs and overall hatchability.
Statistical Analysis.
Production and carcass trait data were analyzed as a 3-way ANOVA using the MIXED procedure of SAS (SAS 9.1, SAS Institute Inc., Cary, NC), with BW curve, PS age, and strain as fixed effects. Differences between least squares means were determined using pairwise differences and were reported as significant at the P< 0.05 level.
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RESULTS AND DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTS AND DISCUSSIONREFERENCES |
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). The high birds that had the heaviest growth rates early needed to have their growth held back post-PS to reach the 32-wk target, whereas the low and standard birds, which were held back early, were fed more feed post-PS to reach the 32-wk target BW.
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). Overall, the 18WK birds were reported to enter lay approximately 16 d sooner than the 22WK birds (Renema et al., 2007), although the maturation interval between PS and first oviposition was 12 d faster in birds photostimulated at 22 wk of age. Any advantage of the 18WK PS age was lost in the time it took to reach the point where 52-g eggs were produced (Table 1). The interval from sexual maturity to settable egg time was 22.8 d compared with 9.3 d in 18WK and 22WK hens, respectively (Table 1). The 22WK hens entered lay with a mean egg weight of 47 g, which was 3.6 g heavier than that of the 18WK hens. This concurs with Joseph et al. (2002), who reported a 2.4-g increase in egg weight following the delay of PS to 23 rather than 20 wk of age. The 18WK hens produced 15.6 more small eggs during the egg production period (Table 1). Reducing the number of small eggs produced could affect the number of settable eggs in sites where a 52-g minimum hatching egg weight is required. The increase in BW associated with delaying PS would be expected to increase egg weight, because age and BW are positively correlated with egg weight (McDaniel et al., 1981).
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). The 18WK hens came into production earlier and peaked earlier than did 22WK hens (Figure 3, panel A). Previous studies have demonstrated that birds entering lay following delayed PS compensate for this later start by laying more frequently (Yuan et al., 1994; Robinson et al., 1996; Renema et al., 2001). The fact that this did not occur in the current study is attributed to several factors. With several of the BW profile treatments, the larger pullets may have the physical and hypothalamic maturity to respond quickly to PS at 18 wk. Heavier birds with a higher fat content typically enter lay sooner (Summers and Leeson, 1983). In the current study, birds on the heavier BW profiles commenced production sooner and with a higher abdominal fat pad size than the lower BW profiles (Renema et al., 2007).
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), final settable egg values were 142.9 (18WK) and 149.4 (22WK; Table 2). Robinson et al. (1996) also noted no difference in settable egg age in birds photostimulated across a wide range of ages.
There was an interaction of BW profile with PS age. Within the 22WK birds, small egg production was similar among BW profiles. However, when birds were photostimulated at 18 wk, 18WK-high birds produced 44 small eggs compared with 33 in moderate birds and a mean of 22 in low and standard birds (Table 1). The higher BW profile treatments photostimulated at 18 wk started producing eggs earlier (Figure 4, panel A). Although the more uniform response of birds to the later PS age was expected (Robinson et al., 1996; Joseph et al., 2002), the distribution of small eggs among BW profiles in 18WK birds was unexpected. Furthermore, the interval between sexual maturity and settable egg age was nearly double in moderate and high compared with low and standard hens (Table 1). Clearly, the lower feed allocation required to keep the moderate and high birds on their BW target during sexual maturation affected some early production traits. With little effect of growth history on initial egg weight, age may be more important than BW in influencing early egg weight. Feeding levels may also have interacted with growth history and BW to affect egg size (Table 1). Fit statistics using mixed model analysis (data not shown) support these hypotheses. Prime laying sequence length was affected in a similar manner, with the prime sequence of low and standard hens averaging 7 d longer than that of moderate and high hens (Table 2). These results do not agree with the general understanding that small hens produce smaller eggs and large hens produce larger eggs (McDaniel et al., 1981). However, Hudson et al. (2001) observed that from 20 to 30 wk of age, low BW birds in a nonuniform group gained more weight than heavy birds. Because feed allocation in the early lay period may have been excessive for the smaller birds, they suggest that this might have contributed to increased egg weight.
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). With the exception of the poor-producing Ross 708-high hens, all strain x BW profile combinations were remarkably similar in egg production (Table 2). In the current study, very tight control on rate of gain in combination with frequent small changes in feed allocation may have circumvented metabolic disruptions, which may be the underlying cause of some of these noted hormonal disruptions.
The effect of strain on egg production was uniform across total, normal, and settable egg production, with lower average egg production in Ross 708 hens (Table 2; Figure 3). Variability in productivity was apparent among strain grown on particular profiles (Table 2). Within the strain x BW profile interaction, the Ross 708-low, Ross 708-standard, and Ross 708-moderate hens had a similar egg production rate to most other strain x BW profile combinations, whereas the Ross 708-high hens produced approximately 23 fewer eggs than other treatment combinations with this strain. In contrast, Ross 508-high birds laid the same number of eggs as Ross 508-standard birds (mean = 178.5). When compared at the main effect level, neither total nor settable egg production differed among BW profiles (Table 2, Figures 3 and 4). However, settable egg production to 58 wk ranged from 153.9 eggs in low birds to 133 eggs in high birds, with high birds having fewer eggs than birds on any other BW profile (Table 2)—a difference based on increased small egg production in the high BW profile group (Table 1).
Laying Sequence Analysis
Unlike egg production, there was no effect of PS age on laying sequence traits (Table 2). This observation was surprising, considering that one of the ways birds photo-stimulated later can make up for low early production is to lay more frequently and in longer laying sequences (Robinson et al., 1990).
Excess feed can limit both prime sequence length and mean laying sequence length compared with that of normally feed-restricted hens (Robinson et al., 1991). Although prime sequence length is an early lay parameter that can be affected by prepeak feeding, mean sequence length characterizes the effect of BW profile over the productive life of the bird. Values of both mean sequence length and number of laying sequences greater than 10 decreased sequentially between low and high birds (Table 2). Both measurements provided a similar distribution of significant differences among the means. It is a very interesting finding that feed allocation in the prepeak period can affect traits like laying sequence length in the long term. Mean sequence length was higher in Hubbard Hi-Y (4.0 d) and Ross 508 hens (3.7 d) than in Ross 708 hens (3.1 d; Table 2). All strains differed when sequences > 10 were compared. The difference between the 2 measures indicates that the Hubbard Hi-Y hens lay more sequences greater than 10 d in length. Although the mean sequence length for Ross 708-standard and Ross 708-moderate hens were similar, values for number of sequences greater than 10 were reduced in Ross 708-moderate birds (0.69) than in Ross 708-standard birds (1.67; Table 2). The Ross 708-high birds had almost no sequences greater than 10 d, although mean sequence length was still 2.4 d, on average. A 3.16-d mean sequence length in Ross 508-moderate birds was associated with 1.27 sequence greater than 10 d. In Ross 708-moderate birds, a 3.14-d mean sequence length was found with only 0.69 sequences greater than 10 d. Calculating sequences greater than 10 d may artificially mask how sequence data is interpreted. For example, a single 24-d sequence would make a bird appear less valuable than one with two 12-d laying sequences. A more representative way of weighting this data is with mean weekly sequence length, in which a running average of current sequence length is assessed (Joseph et al., 2002).
Defective Egg Production
Defective egg production did not differ much due to PS, strain, or BW profile treatments (Table 2). The Ross 708 hens laid 0.21 double-yolked eggs per bird compared with an average of 0.55 for Hubbard Hi-Y and Ross 508 hens. This made up a very small portion of their overall egg production. Most of these double-yolked eggs were produced during the peak feeding period, early in egg production. As the BW profiles merged at 32 wk, the moderate and high hens were receiving much less feed to hold back their rate of BW gain (Figure 2). This period of reduced feed allocations began very early in production and encompassed the period in which many pullets were commencing lay. As a result, production of both shell-less eggs and double-yolked eggs was approximately half that of the low and standard hens (Table 2). Ultimately, the standard hens produced significantly more shell-less eggs than moderate or high hens, and the low and standard hens produced significantly more double-yolked eggs than the moderate hens. This demonstrates that defective egg production may be more affected by feeding level than by previous feed-restricted feeding levels or growth history.
Fertility and Hatchability
The average hatchability after 30 wk of age was 82% in 22WK hens compared with 79% in 18WK hens (Table 3). Fertility followed a similar pattern (87 and 84%, respectively, P = 0.053). Embryonic mortality was unaffected by PS age, strain, or BW profile (Table 3). It averaged 3.5, 1.6, and 1.8% during wk 1, 2, and 3 of incubation, respectively.
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). However, the Ross 708-low and Ross 708-high hens consumed less feed than the other strains in these groups to meet their target BW objectives (data not shown). This may limit the availability of trace nutrients to the embryo. The developing embryos of modern high-breast yield strains are particularly susceptible to vitamin deficiencies, which will result in death, malformation, or some other atypical response (Leeson and Summers, 2001).
Chick production did not vary among any of the main effects (Table 3). Total chick production averaged 135 chicks per hen to 58 wk of age. There was a significant strain x BW profile interaction; the Hubbard Hi-Y-standard and Ross 708-high groups had the lowest chick production within BW profile for their respective strains. For the Ross 708 hens, this was simply a result of poor egg production, because fertility and hatchability traits for the Ross 708-high hens were good (Table 3). The Hubbard Hi-Y-standard hens suffered from a combination of poor egg production and hatchability, resulting in the production of fewer chicks than either the Hubbard Hi-Y-low or Hubbard Hi-Y-moderate groups.
The current study demonstrated that feed intake was found to have more effect on egg size and early production traits than BW profile. Early egg size and prime sequence length were reduced in the larger (moderate and high) hens early in lay, corresponding with lower feed allocation between PS and 32 wk of age. The BW profiles affected egg production traits differently among strains. Ross 708-high hens were unable to mobilize nutrients from storage, because they were needed under conditions of dietary deficiency to support sexual maturation. The Hubbard Hi-Y-high hens were not hindered by the very low feed allocations during sexual maturation. Although 18-wk PS resulted in increased egg production to 58 wk compared with a 22-wk PS age, this was primarily due to the production of additional small eggs. Genetic strains differ in their ability to respond to alternate rearing BW profiles due to differences in their ability to manage nutrient allocation.
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ACKNOWLEDGMENTS |
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FOOTNOTES |
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Received for publication January 24, 2007. Accepted for publication June 15, 2007.
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| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
52 g) for main effects of PS age (D), strain (E), and rearing BW profile (F). Body weight profiles included standard (mean of breeder BW targets for strains used), low (12-wk BW target = 25% lower than standard followed by rapid gain to 32 wk), moderate (12-wk BW target = 150% of standard followed by lower rate of gain to 32 wk), and high (12-wk BW target = 200% of standard followed by minimal growth to 32 wk).