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ENVIRONMENT, WELL-BEING, AND BEHAVIOR |
* Department of Animal Science, University of California, Davis 95616; Livestock Behavior Research Unit, Agricultural Research Service, USDA, West Lafayette, IN 47907; and Department of Animal Science, Purdue University, West Lafayette, IN 47907
1 Corresponding author: jamench{at}ucdavis.edu
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ABSTRACT |
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 TOP ABSTRACT INTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Key Words: beak trimming • duck • pain • behavior • bill morphopathology
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INTRODUCTION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Trimming of the bill or beak of poultry is the most common method used to decrease the damage caused to the plumage and skin by feather pecking and cannibalistic behavior. Beak-trimming of chickens and turkeys involves removing one-third to one-half of the upper mandible, and sometimes portions of the lower mandible (Hester and Shea-Moore, 2003). This procedure is generally conducted using a hot-blade trimmer when the birds are less than 10 d of age, although some strains of poultry may require a second trim when they are older to prevent beak regrowth. Ducks are trimmed using a variety of methods, including cold-cutting with scissors (Gustafson et al., 2007), hot-blade cutting with cautery, or tip-searing, which involves holding the end of the bill against a cautery blade for 2 to 3 s. The method of trim, the amount of bill removed, and the age at which trimming is conducted vary throughout the duck industry.
Bill-trimming is controversial because of its potential to cause acute and chronic pain. In chickens a considerable body of morphological and physiological evidence can be found demonstrating the presence of neuromas in the beak following trimming (Eskeland, 1981; Gentle, 1986; Gentle et al., 1990, 1991; Cheng, 2005). These neuromas have spontaneous discharge patterns similar to those seen in human amputees who are experiencing chronic phantom limb pain (Breward and Gentle, 1985; Dubbeldam et al., 1995). In addition, trimmed pullets show significantly fewer bill-related behaviors and spend more time performing passive behaviors, such as resting and standing, than untrimmed pullets (Duncan et al., 1989; Cunningham et al., 1992). They also show more guarding behaviors, such as tucking the bill under the wing, which indicate the possibility of pain (Eskeland, 1981; Craig and Lee, 1990; Lee and Craig, 1990; Kuo et al., 1991; Cunningham et al., 1992; Gentle et al., 1997). In most cases, these behavioral effects are no longer obvious by approximately 3 wk posttrim, although they sometimes persist for months.
The severity of physiological damage caused by trimming varies according to both trimming method and age at trim (Hester and Shea-Moore, 2003). Neuromas are more likely to form when beaks are trimmed by cautery as opposed to cutting methods that do not involve cauterization (Breward and Gentle, 1985; Craig and Lee, 1990), and in chicks and poults, cold-cutting of beaks generally causes less permanent physical damage to the beak than hot cutting (Gentle et al., 1995; Grigor et al., 1995). Neuroma formation in trimmed chicken beaks tends to be minimal or absent when trimming is performed at hatch rather than at several weeks of age (Lunam et al., 1996; Gentle et al., 1997). If trimming is conducted at 10 d or younger, any neuromas that form are short-lived (Hester and Shea-Moore, 2003).
Although there have been many investigations of the physiological and behavioral effects of beak-trimming on chickens, few studies have been conducted on the effects of bill-trimming on ducks (Rauch et al., 1993). Although beak morphology is remarkably variable within and between bird species, each bill or beak is derived from similar tissues and cells during embryonic development (Noden, 1983; Le Douarin and Kalcheim, 1999; Trainor, 2003), and the bills or beaks of mature birds of different species show many similarities in anatomical and histological characteristics and physiological function (Lucas and Stettenheim, 1972). This suggests that the pathological changes and resulting chronic pain observed in beak-trimmed chickens could also potentially occur in bill-trimmed ducks. However, a recent study showed that trimming Muscovy ducks at 3 wk of age apparently caused acute but not chronic pain, because behavioral changes posttrim were short-lived, there was only a transient check in weight gain, and the bill stumps contained no neuromas (Gustafson et al., 2007). The method used to trim these ducks did not involve cautery, which may have been a contributing factor in the lack of neuroma formation (Schroedter et al., 2004), although the extensive scarring seen in the bill stumps also appeared to prevent nerve regrowth and hence neuroma formation. The purpose of the present study was to evaluate the morphopathological and behavioral effects of 2 different methods commonly used to bill-trim Pekin ducks, cutting with cautery and tip-searing, both of which are typically performed shortly after hatching.
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MATERIALS AND METHODS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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White Pekin ducklings (n = 192, 96 of each sex) were obtained from the Maple Leaf Farms Hatchery (Frankville, WI). The ducklings were moved to the Maple Leaf Farms production facility and assigned to 12 floor pens in an environmentally controlled building according to bill-trimming treatment, with 16 ducklings (8 males and 8 females) per pen. There were 4 replicate pens per treatment. The pens measured 3.66 x0.91 m and had plastic mesh flooring, with approximately 0.21 m2 of floor space per duck. Each pen contained 1 large feeder and 1 automatic trough waterer.
Ducks were whole-house brooded. The brooding temperature on d 1 was 37.8°C; this temperature was progressively decreased to reach 22.2°C when the ducks were 13 d of age, at which time brooding was discontinued. House temperatures from 14 to 22 d of age were then steadily decreased to reach 12.7°C by d 25. Over the course of the 6-wk experiment, the house temperature ranged from 12.8 to 37.8°C. The lighting regimen was 24L:0D from d 1 to 4, with a 20 lx of photophase illumination. Thereafter, the photophase illumination was reduced to 5 lx. The photoschedule from d 4 to 14 was 16L:8D, and the light period was then increased from d 15 to 36 to 22L:2D. The ducks were fed ad libitum Wisconsin starter feed (CP 24%, ME 1,386 kcal) for 2 wk and Wisconsin grower feed (CP 18%, ME 1,413 kcal) until they were processed at 6 wk of age.
Experimental Procedures
At the hatchery, 64 ducklings were trimmed using a hot-blade cut with cautery (TRIM), 64 ducklings were tip-seared (SEAR) by holding the cautery blade briefly on the tip of the bill, and the remaining 64 ducklings were held and sham-trimmed by briefly holding the cautery blade close to the bill without touching it (NOTRIM). Trimming was performed by an experienced Maple Leaf Farms employee. Ducks were weighed every 2 d for the first week, then weekly for the remainder of the study. Feather scores were taken for each duck in each pen at d 18, 21, 28, and 35 d posthatch. Three body sections (wings, back, and tail or vent regions) were scored separately on each duck. The scoring system (Table 1
) was adapted from one developed for feather-scoring chickens (Tauson et al., 1984). A total feather score was determined by summing the scores for the 3 body regions.
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The behavior of all ducks was evaluated by taking scan samples of each pen every 5 min for 30 min per pen. The behaviors recorded were resting, feeding, drinking, preening, locomotion, standing, exploratory pecking, aggressive pecking, feather pecking, and wing flapping (Gustafson et al., 2007). During wk 1, samples of each pen were taken 3 times per day: between 0600 and 1030 h (a.m. session); between 1100 and 1300 h; and between 1400 and 1700 h (p.m. session). During the following 5 wk, there were 4 observation sessions per pen per week, 2 during the a.m. and 2 during the p.m.
Bill Morphological Analysis
Six ducks per treatment were randomly selected at 3 and 6 wk of age and euthanized by cervical dislocation for bill morphological analysis. The 36 bill stumps were visually examined to assess any obvious shape abnormalities, scarring, or tumor formation. Bills were fixed in 10% buffered formalin solution for 2 mo, then decalcified in 10% formic acid in formalin solution for 10 d. Decalcification was detected by using both physical and chemical tests, bending to check flexibility (Apex Engineering Products Co., Aurora, IL) and detection by calcium oxalate (Bancroft and Gamble, 2002). Following complete decalcification, the samples were transferred into a 4% calcium thioglycolate solution for 2 wk to soften the keratin layer. After the keratin layer was removed, the upper bills were washed, dehydrated, and vacuum-embedded in paraffin wax. The embedded samples were serially sectioned longitudinally at 8-µm intervals. After deparaffinization with xylene and hydration in distilled water, alternate sections were stained with hematoxylin and eosin (Bancroft and Gamble, 2002) or Masson’s trichrome for the connective tissues (Sigma-Aldrich, St. Louis, MO), or with Bodian’s or Holm’s staining for the nerve fibers (Bancroft and Gamble, 2002). Treated sections were mounted with Permount (Sigma-Aldrich) and analyzed under a light microscope. The tissues were recognized and analyzed based on histological characteristics and the unique color reactions caused by each specific stain.
Qualitative and quantitative histological analyses of the stained sections were performed according to methods described previously (Lee et al., 1999; Ward et al., 2004; Gustafson et al., 2007). Briefly, the density of nerve fibers between the distal end of the beak stump and the frontal tip of the premaxillary bone of every 20th section was assigned a score, with 0 = no axons; 1 = some axons; 2 = moderate axons; and 3 = numerous axons. Similar scoring was used for connective tissue deposition, except that there was no 0 score. Histological observation was performed by light microscopy at 40, 200, 400, and 1,000xmagnification, and photos were taken at 40, 200, and 400xmagnification. To reduce observer bias during the analysis, the same person performed all measurements and was blind to the treatments.
Data Analysis
Scan samples were used to calculate the population behavioral time budgets for each pen at each time point. The population time budget was defined as the average proportion of time an average duck spent performing each behavior in each pen, and was calculated by dividing the total number of observations of each behavior by the total number of behaviors recorded in that session and the total number of ducks (Martin and Bateson, 1993). Because of the prediction that bill-related behaviors would be decreased and resting increased if trimming caused pain, the 10 individual behaviors recorded were reclassified into 3 broader categories for analysis: bill-related behaviors (which included drinking, preening, exploratory pecking, and feeding), non-bill-related behaviors (which included standing, locomotion, and wing flapping), and resting. However, the analysis was designed to test not only these categories, but also whether individual behaviors deviated from the mean for each category (see below). Behaviors that constituted less than 1% of the overall data set (i.e., wing flapping, feather pecking, and aggressive pecking) were excluded from the analysis to prevent floor effects by minimizing the number of zero-value data points.
All behavioral data were analyzed with SAS for Windows, version 8.0 (SAS Inst. Inc, Cary, NC). The scan sample data were analyzed with a GLM to determine the effect of treatment on the behavioral time budget of the population (pen). The model used in this analysis was
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The analysis was blocked by pen to take into account potential differences between pens, to account for repeated measures, and to avoid pseudo replication. Pen was nested within treatment. Individual behaviors were nested within category, because each behavior could occur in only 1 category. Treatment effects were crossed with category, the individual behaviors, and week. Week was treated as a continuous variable to track developmental effects and progressive changes in behaviors over time. This model tested for a difference between categories and also whether there was a difference in individual behaviors within each category. This allowed us to evaluate the importance of both individual behaviors and categories in the same analysis. The treatment xcategory interaction tested whether the shape of the category time budget differed between treatments, and the treatment xbehavior (category) interaction tested whether any individual behaviors deviated significantly from this overall pattern. Similarly, the treatment xcategory xweek interaction tested for a difference in how the category time budget shape changed between treatments over time, and the treatment xbehavior (category) xweek interaction tested whether individual behaviors deviated from this overall pattern with time.
Four types of post hoc tests were conducted to investigate a significant treatment xcategory xweek interaction. The first set estimated the slope of the lines for the categories to give the mean rate of change for each treatment over time. A significant rate of change among treatments was calculated at a Bonferroni critical alpha of 0.005 (i.e., 0.05 divided by 3 categories for 3 treatments). The second set calculated pairwise contrasts between these slopes within each behavior category at a Bonferroni critical alpha of 0.017 (i.e., 0.05 divided by 3 behavior categories) to determine whether the treatments differed in the rate of change of each behavior category. The third set compared treatment least squares means for each category during wk 1 as a single post hoc contrast for each category by using a Bonferroni-corrected critical alpha of 0.017 (i.e., 0.015 divided by 3 behavior categories). Categories identified as significant by this test were further investigated by the fourth test, in which significant differences between least squares means within a category were examined using planned comparisons with a Bonferroni critical alpha of 0.017 (i.e., 0.05 divided by 3 treatments). This procedure (i.e., set 3, followed conditionally by set 4) was repeated for each week of the experiment. The data were angular-transformed to correct for nonhomogeneity of variance. This statistical design was adapted from the method used by Chu et al. (2004) to analyze behavioral time budgets.
For each weight sample, average weight gain was calculated for each pen from the day of trimming. The data were analyzed using a GLM blocked by pen nested within treatment. The analyses tested the day xtreatment interaction, with day treated as a continuous variable. Weight gain and day were log-transformed to linearize their relationship. Average weight gain was chosen rather than absolute weight, because these data provided a far better relationship for the purpose of analysis. Post hoc estimates of each regression slope were calculated and compared by using post hoc contrasts. For each day, the least squares means of the 3 treatments were compared by Tukey’s pairwise tests.
Average feather score was calculated for each pen on each scoring day. These data were examined using a GLM blocked by pen nested within treatment. The analysis tested the day xtreatment interaction, with day treated as a continuous variable. Feather score was angular-transformed. Post hoc estimates of each regression slope were calculated and compared by using post hoc contrasts. For each day, the least squares means of the 3 treatments were compared by Tukey’s pairwise tests.
Scores for connective tissue and nerve fibers were subjected to split-plot analysis using GLM (version 9.1, SAS Inst. Inc.). The model included the main effects of treatment, time, and individual scores for each tissue, as well as the time xtreatment interaction, with individual score within treatment considered as a random variable. Connective tissue data were transformed by using an appropriate lambda value, as determined by the TRANSREG procedure (version 9.1, SAS Inst. Inc.). The Tukey-Kramer adjustment was used to compute the t-values.
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RESULTS |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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Population time budgets differed over time among the 3 treatment groups (treatment xcategory xweek interaction: F4, 873 = 4.33; P = 0.002; Figure 1). None of the estimated rates of change for any of the behavior categories differed significantly from zero. However, the rate of change in bill-related behaviors was markedly different between TRIM and NOTRIM birds (post hoc contrast: F1, 873 = 10.64; P = 0.0012), as was the rate of resting behavior between TRIM and NOTRIM birds (F1, 873 = 8.80; P = 0.0031). Accordingly, there were marked differences in behavior between the treatments during the first 2 wk posttrim. In wk 1 and 2, there was a significant difference between treatments in bill-related and resting behaviors (post hoc contrast: bill-related F2, 873 = 9.31, P <0.0001; resting F2, 873 = 9.03, P <0.0001), with TRIM and SEAR ducks resting significantly more and performing fewer bill-related behaviors than NOTRIM ducks. There were no significant differences between the SEAR and TRIM treatments during these 2 wk. By wk 3 to 5 posttrim, there were no longer significant differences between any of the treatments.
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There was a significant difference in the rate of weight gain between treatments (day xtreatment interaction: F2, 81 = 11.85; P <0.0001; Figure 2). Ducks in the TRIM group gained less weight than SEAR ducks up to d 7, and gained less than NOTRIM ducks up to d 14; SEAR and NOTRIM ducks did not differ from one another. Although the differences in weight gain were small in absolute terms (e.g., the mean difference in weight gain between TRIM and NOTRIM ducks was only 7.26 g at d 2), they were large in relative terms (e.g., at d 2 the TRIM ducks gained only 78% as much weight as the NOTRIM ducks). Nevertheless, TRIM ducks compensated for this initial check in weight gain by an overall steeper rate of gain compared with SEAR ducks (F1, 81 = 13.55; P = 0.0004) and NOTRIM ducks (F1, 81 = 21.15; P <0.0001), such that by d 21 there were no significant differences between the treatments.
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Feather scores are shown in Figure 3. Scores increased during the study, indicating deteriorating feather condition. However, this increase differed significantly among the different treatments (day xtreatment interaction: F2, 33 = 11.83; P <0.0001), with the feather condition of NOTRIM ducks deteriorating significantly faster than that of TRIM ducks (post hoc contrast of regression slopes: F1, 33 = 9.60; P = 0.0040; Bonferroni critical alpha = 0.017) and SEAR ducks (F1, 33 = 23.00; P <0.0001). The feather scores of NOTRIM ducks differed significantly from those of TRIM and SEAR ducks by 18 d of age, and these differences increased in magnitude as the study progressed. There were no significant differences in feather score between TRIM and SEAR ducks at any age, nor did the rate of change in feather condition differ between the 2 groups (F1, 33 = 2.88; P = 0.0991).
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Microscopic analysis revealed several similar morphopathological features of the TRIM and SEAR bill stumps. The surfaces of the bill stumps were well covered with the keratin sheath. The tissues under the keratin sheath were organized into 3 layers: the rhamphatheca, the epidermis, and the dermis. The regenerated dermis consisted of proliferated connective tissues, which were well supplied with blood vessels. Although both trimming methods caused connective tissue scarring, as compared with the NOTRIM method, the TRIM method caused denser scars than those induced by the SEAR method at both 3 and 6 wk posttrim (Table 2). At 3 wk posttrim, the scar tissue in the SEAR stumps had a viscous consistency, with relatively thin, sparse fibers, whereas the scar tissue in the TRIM stumps was mostly made up of thick fiber bulks arranged in variable orientations. Six weeks post-trim, the scar tissue was characteristic of dense, irregular connective tissue (Figure 4), and TRIM stumps had denser connective fiber bundles than did SEAR stumps.
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), such as Herbst corpuscles and Grandry corpuscles, were lost in both the TRIM and SEAR bill stumps. However, there was significant regrowth of nerve fibers in the SEAR stumps, whereas nerve fibers in the TRIM stumps degenerated between 3 and 6 wk of age (Table 2
and Figure 4). The nerve fibers in the SEAR stumps were similar in shape to those of untrimmed bills, that is, linear without swellings or tangled masses, whereas in TRIM stumps there were numerous nerve fibers behind, but not within, the scar tissues. This indicates that the dense scar tissues induced by the TRIM method, but not by the SEAR method, blocked nerve regrowth into the TRIM bill stumps. There were no neuromas in the bill stumps at either 3 or 6 wk after either bill-trimming method.
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DISCUSSION |
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TOPABSTRACTINTRODUCTIONMATERIALS AND METHODSRESULTSDISCUSSIONREFERENCES |
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There were no neuromas in the bill stumps of either TRIM or SEAR ducks, suggesting that these trimming procedures were not associated with chronic pain. Findings from previous studies indicate that the early spontaneous discharge from the cut end of the nerve is associated with acute pain, whereas stimulation of the later-forming neuromas leads to chronic pain. In chickens, a large amount of spontaneous activity is recorded within the distal tip of the beak stump by 5 d posttrimming in 5-wk-old chickens. By 20 to 30 d after amputation, the damaged nerve fibers form extensive neuromas within the stumps, and pain-related guarding behavior is also seen (Breward and Gentle, 1985; Gentle, 1986). However, similar to the current findings, these morphophysiological changes did not occur in turkeys that were hot-blade trimmed at 1 d of age (Gentle et al., 1995) or in chickens that were trimmed at 1 or 10 d of age (Gentle et al., 1997).
Unlike the bills of Muscovy ducks trimmed at 3 wk by cold-cutting (Gustafson et al., 2007), the bills of the ducks in this study contained blood vessels. However, both trimming treatments did result in some morphological damage to the bill, although the extent of the damage differed between the 2 treatments. Both SEAR and TRIM ducks lost mechanoreceptors in the bill. Thousands of sensory structures are located in the duck bill tip (Schroedter et al., 2004), mainly mechanoreceptors such as Herbst and Grandy corpuscles. Tactile information is collected via these sensory receptors and is used for detecting appropriate food items and excessive heat or pressure (Berkhoudt, 1977; Zweers et al., 1977; Gentle 1989).
The effects on the ducks of loss of these receptors in a commercial setting are unknown, but in this study there were certainly no long-term effects observed on feeding behavior. Despite the similarity in loss of receptors, the SEAR ducks had less scarring than the TRIM ducks, and there were nerve fibers within their bill stumps. The presence of those fibers could indicate that there was either less nerve damage or more nerve regrowth in SEAR than in TRIM ducks.
The relationship between scarring and pain depends on the type and severity of the tissue injury (Bennett, 1993; Sheen and Chung, 1993; Colburn et al., 1999). Scar tissue alters the environment of the amputated nerve endings, resulting in the blocking of nerve regrowth and in the formation of neuromas (Wu and Chiu, 1999). Neuromas are sources of substantial ectopic firing and trigger pain (Holland and Robinson, 1990; Coderre et al., 1993; Devor and Seltzer, 1999). On the other hand, scarring can actually be beneficial in preventing pain by preventing nerve regrowth and thus protecting the nerve fibers from tactile or pressure stimulation (Katz, 1992; McHugh and McHugh, 2000). However, there was no behavioral evidence that the SEAR ducks exhibited pain after the first 2 wk even though nerves were visible in the bill stump, so these nerves may serve a valuable function in maintaining sensory sensitivity in the bill without being associated with painful sensations in the bill stump.
Both the SEAR and TRIM treatments were effective in minimizing damage associated with feather pecking. The feather condition of the NOTRIM ducks was already worse than that of the trimmed ducks by 18 d of age, and by market age the plumage and skin of trimmed ducks showed little to no damage, whereas that of the untrimmed ducks showed minor to moderate damage.
In summary, trimming with both cautery and tip-searing was effective in minimizing damage caused by feather pecking. However, tip-searing may be a better trimming method than cutting with cautery from the perspective of Pekin duck welfare. Tip-searing did cause behavioral changes indicative of pain for 2 wk posttrim, but the decrease in bill-related behaviors was not severe enough to be associated with reduced weight gain during this time, as was seen in the TRIM ducks. Although neither trimming method led to neuroma formation, the SEAR method caused fewer morphological changes in the bills than the TRIM method, including less scarring and loss of nerve fibers. Whether this trimming method can be used successfully on duck species such as Muscovy, where there is a longer rearing period and hence the potential for greater bill regrowth, should be investigated.
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ACKNOWLEDGMENTS |
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Received for publication July 28, 2006. Accepted for publication May 2, 2007.
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REFERENCES |
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