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Poult Sci 2007. 86:2117-2125
© 2007 Poultry Science Association
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ENVIRONMENT, WELL-BEING, AND BEHAVIOR

The Impact of Nutrient Density, Feed Form, and Photoperiod on the Walking Ability and Skeletal Quality of Broiler Chickens

K. E. Brickett*, J. P. Dahiya*, H. L. Classen*,1, C. B. Annett* and S. Gomis{dagger}

* Department of Animal and Poultry Science, and {dagger} Department of Veterinary Pathology, Western College of Veterinary Medicine, University of Saskatchewan, Saskatoon, Saskatchewan, Canada S7N 5B4

1 Corresponding author: hank.classen{at}usask.ca


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The aim of this study was to examine the main and interaction effects of 3 dietary nutrient densities (low-, medium-, high-density), 2 feed forms (mash, pellet), and 2 lighting programs (20L:4D, 12L:12D) on the walking ability and skeletal quality of broiler chickens raised to a young age (35 d). Forty-eight birds per treatment (nutrient density x feed form x lighting program subclass) were randomly selected (0 d) for assessment of their gait score (GS) and BW at 11, 18, 25, and 32 d. Samples of birds from various GS classifications were selected at 32 d for radiographs and histology of the femur and tibiotarsus. Bone ash (BA) content was evaluated at 35 d from the right tibiotarsus to assess bone quality. Overall, the mean GS values were low (GS <1). An interaction existed between lighting and sex. Males provided with 20L:4D had a higher GS (0.74) than females (0.45), and this score was greater than for broilers provided with 12L:12D (0.34 and 0.26 for males and females, respectively). Feeding mash reduced the GS (0.29) compared with pellet rations (0.62). The GS increased with age, and by 32 d 2.43% of birds had a GS ≥3. Broilers fed mash had a higher BA content (50.6%) than birds fed pellet diets (49.8%), and the 12L:12D value (50.5%) was greater than for 20L:4D (48.9%). Last, males had a lower BA content (49.8%) than females (50.6%). A positive correlation existed between BW and GS based on sex, where BW at 11, 18, and 25 d affected bird mobility at 32 d (r2 = 0.39, 0.49, and 0.50 for males; r2 = 0.34, 0.37, and 0.36 for females, respectively). Radiography and histology were unaffected by GS. This study confirmed that a reduced growth rate improved GS but also demonstrated that overall bird mobility was good and the incidence of skeletal disease was low.

Key Words: broiler welfare • gait score • mobility • leg disorder • nutrition


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The continued improvement in broiler growth rate is due to a number of factors, the foremost being genetic selection (Buyse et al., 1998; Havenstein et al., 2003). Although rapid growth rate has been successful in reducing the number of days to achieve market weight (Sulistiyanto et al., 1999), there have also been associated health problems. Rapid growth rate has been associated with a higher incidence of metabolic disorders such as sudden death syndrome (SDS), ascites, and skeletal abnormalities (Buyse et al., 1998; Garner et al., 2002; Scott, 2002); consequently, reducing the growth rate has been shown to decrease the occurrence of these problems (Cook et al., 1984; Julian, 1998; Scott, 2002). With respect to skeletal abnormalities, rapid growth rate results in an increased weight load on an immature skeleton of a young broiler that likely contributes to a higher incidence of leg disorders (Julian, 1998).

Leg weakness in broiler chickens represents both an economic and an animal welfare concern and often is singled out as being of particular importance. The economic costs associated with leg weakness are due to culling on farm and to condemnations or downgrading at processing. Animal welfare aspects include the ability of affected birds to eat and drink (Garner et al., 2002; Sanotra et al., 2002) and the pain associated with the pathology of leg weakness. Lame broilers will preferentially select a ration containing an analgesic agent, more so than sound broilers (McGeown et al., 1999; Danbury et al., 2000), which suggests that they are seeking pain relief associated with their leg abnormalities (Sørensen et al., 2000). For the most part, the scientific literature describes a relatively high incidence of growth-associated leg weakness in broilers (Cook et al., 1984; Wilson et al., 1984; Skinner et al., 1991; Kestin et al., 1999; Su et al., 1999; Rath et al., 2000; Garner et al., 2002; Sanotra et al., 2002). Perhaps the most significant report from this research was by Kestin et al. (1992), who showed that approximately 90% of commercially reared birds have a detectable gait abnormality and that welfare was considered to be severely compromised for about 26% of the broilers examined. However, in more recent reports Classen et al. (2003, 2004) suggested that gait scores (GS) and the incidence of leg weakness may have improved over the last decade. In addition, McNamee et al. (1998) reported that 9.2% of flock mortality in 1 flock and 11.2% in a second flock was due to lameness, indicating that the incidence of leg abnormalities has decreased from previous reports. Because selection pressure by primary breeders continues to emphasize increased growth rate in broiler chickens, it may be anticipated that leg weakness would inadvertently evolve (Kestin et al., 1999). However, this negative attribute may be counteracted by simultaneous selection for improved bird health and skeletal strength. Kestin et al. (1999) reported that the susceptibility to leg abnormalities is altered by genotype; therefore, because of the evolving nature of commercial broilers, the periodic evaluation of leg weakness is warranted.

In turn, understanding how broiler management techniques affect the incidence of leg weakness is important to minimize its economic and animal welfare impact. One strategy to reduce leg weakness includes manipulating the rate of growth. Altering dietary energy and protein levels, implementing early feed restriction, and offering various feed forms have all been strategies previously used to manipulate the growth rate in broilers. The use of low-density rations has been shown to significantly reduce the early growth rate of broiler chickens; however, Scott (2002) found that broilers fed a low-density ration were heavier than those fed a high-density ration at 35 d of age. The reduction in BW for the high-density group was attributed to an increase in metabolic stress, because there was an increase in mortality (SDS and ascites) in broilers fed the high-density ration in contrast to those fed the low-density ration (Scott, 2002). Broilers fed mash diets have slower growth rates and a lower incidence of leg abnormalities compared with those fed pellet diets (Nir et al., 1995; Engberg et al., 2002; Scott, 2002). Adjusting broiler lighting programs is also a management factor that can be manipulated to lessen the occurrence of skeletal abnormalities. By increasing exposure to darkness, the growth rate of broiler chickens can be reduced. In conjunction with this reduced rate of growth, a corresponding decrease in the incidence of leg abnormalities and metabolic disorders has been reported (Simmons, 1982; Wilson et al., 1984; Classen and Riddell, 1989). In addition, Classen et al. (1991) suggested that metabolic changes associated with darkness may benefit broiler skeletal quality.

With respect to broiler performance, Brickett et al. (2007) showed that an interaction exists between dietary nutrient density and feed form, which affects BW, feed intake, feed conversion ratio, and absolute meat yield. In addition, these authors found that lighting programs have an effect on broiler performance and mortality independent of dietary nutrient density and feed form.

Because leg abnormalities continue to be considered a major animal welfare and production issue, a trial was performed to see how differences in broiler growth rate relate to the susceptibility of leg abnormalities. The aim of this study was to see how manipulation of the growth rate by dietary nutrient density, feed form, and photoperiod would affect the walking ability and skeletal quality of broiler chickens raised to a young age (35 d).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The experimental protocol was approved by the Animal Care Committee of the University of Saskatchewan and was performed in accordance with recommendations of the Canadian Council on Animal Care, as specified in the Guide to the Care and Use of Experimental Animals (CCAC, 1993).

Animals and Housing
Male and female Ross 308 broilers (4800) were raised under 2 different photoperiods and were fed 1 of 3 nutrient density treatments provided in either a mash or a crumble or pellet form. Chicks were randomly allocated to 1 of 8 independent rooms, consisting of 12 pens (1.68 x 1.98 m) per room, which housed 50 birds (25 males and 25 females) per pen. Each pen was provided with straw litter, a hanging feeder (0 to 21 d, 36 cm in diameter; 22 to 35 d, 44 cm in diameter), and a bell drinker (44 cm in diameter); birds had access to feed and water ad libitum. Initially, room temperatures were 35°C and were reduced by 2.8°C/wk to 22°C by 30 d.

Dietary Treatments
Three nutrient density treatments consisting of starter, grower, and finisher diets were formulated using the ideal protein concept (Boorman and Burgess, 1986; Han and Baker, 1993; Han and Baker, 1994; NRC, 1994; Mack et al., 1999) as previously described (Brickett et al., 2007). These diets were fed either as mash or in a crumble or pellet form throughout the duration of the production trial. Dietary energy was maintained in proportion to the balance of essential amino acids required by the bird for maintenance and growth. Specified amounts of starter and grower diets were based on the number of birds placed and were adjusted based on the differences in dietary energy for each treatment. This was done to ensure that all birds, regardless of the dietary treatment, consumed similar nutrients prior to changing to the grower and finisher diets.

Lighting Treatments
Broilers were initially provided with 23L:1D and 20-lx light intensity until 4 d of age, at which time the light intensity was reduced to 10 lx and the lighting programs were initiated. The 2 lighting treatments, 20L:4D and 12L:12D, were maintained until the end of the trial. The lighting programs were chosen to see whether broiler feed intake and subsequent early growth rate would be altered when the birds were offered either mash or pellet diets of various nutrient densities. Production data for this experiment were previously reported (Brickett et al., 2007).

Measurements
Birds that died during the course of the trial were necropsied and the cause of mortality was recorded. Birds demonstrating significant mobility problems and obvious skeletal abnormalities were humanely euthanized and necropsy was performed to determine the cause of lameness. All birds that died or that were euthanized were combined as a total percentage of mortality based on the birds placed at the beginning of the trial.

GS
Subjective gait scoring was used to assess bird mobility according to Garner et al. (2002). The GS consists of a 6-point scale from 0 to 5, where 0 represents no detectable gait abnormality and 5 indicates a bird incapable of standing on its feet. The GS was evaluated on individual birds (6 males and 6 females per pen), with a total of 48 (24 males and 24 females) birds per treatment (nutrient density x feed form x lighting treatment subclass). Birds were selected at random at 0 d of age and were weighed and wing-banded for identification purposes. Gait scores were assessed on d 11, 18, 25, and 32. Body weights were also collected at these times to correlate BW with GS.

Bone Quality
Based on the last GS assessment at 32 d of age, a sample of birds from various GS classifications was selected for a detailed examination of factors potentially associated with bird immobility. All birds were euthanized by cervical dislocation, and radiographs (Faxitran, Hewlett-Packard, McMinnville Division, McMinnville, OR) of the long bones were taken on 5 birds exhibiting a GS of 0, 1, and 2, and 4 birds with a GS of 3. Radiographs were taken of the right and left legs; images were then scanned into the Northern Eclipse program (Version 6.0, Empix Imaging Inc., Mississauga, Ontario, Canada), which allowed us to measure the length, width, and curvature of the femur and tibiotarsus. The length was measured from the proximal to the distal end, the width was measured at the midpoint of each bone, and the curvature was evaluated from the proximal portion of the bone to the midpoint.

A complete necropsy was then performed to check for abnormalities in the femur, tibiotarsus, metatarsus, and associated joints. For histology, samples were taken from 3 birds each with GS of 0 and 1, as well as from only 1 bird with a GS of 2.

The femoral head; a portion of the proximal tibiotarsus, sciatic plexus, and nerve; and the gastrocnemius muscle from both the right and left legs were weighed and fixed in formalin (10% buffered neutral formalin) for histological examination. Bones were then decalcified in 20% aqueous formic acid solution and trimmed. All of the tissue samples were embedded in paraffin, and 3- to 5-µm sections were stained with hematoxylin and eosin.

To assess the bone mineral content, the right tibiotarsus was obtained from 5 males and 5 females from each dietary nutrient density x feed form x lighting program subclass at 35 d of age (for a total of 120 samples). Bones were prepared and ash was measured according to the method of AOAC (1990).

Statistical Analyses
Data were analyzed using PROC GLM of SAS (SAS Inst. Inc., Cary, NC) as a 3 x 2 factorial nested within the 2 lighting programs. The ANOVA included the main effects of nutrient density (3), feed form (2), and photoperiod (2) as well as 2-, and 3-way interactions. Gait score and bone ash (BA) data were analyzed to include the main effects of nutrient density (3), feed form (2), photoperiod (2), and sex (2) in addition to 2-, 3- and 4-way interactions. Mean values were separated using Duncan’s multiple range test. The Pearson correlation procedure was used to correlate BW with GS based on age and sex. Data was considered to be statistically significant when P < 0.05.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Mortality
Overall, mortality rates were not affected by nutrient density but were affected by feed form and lighting program, with feeding mash and 12L:12D having fewer birds that died or were culled (Brickett et al., 2007). The main cause of mortality was SDS, which was affected by the lighting program (Table 1Go). Broilers provided with 20L:4D had a higher incidence of SDS (1.26%), in contrast to those provided with 12L:12D (0.77%). Although not significant (P = 0.07), broilers provided with 20L:4D had a higher incidence of death (0.56%) because of infection (peritonitis, polyserisitis, hepatitis, septicemia, pericarditis, endocarditis, meningitis) compared with those provided with 12L:12D (0.25%). There were no significant interactions between any main effects. Overall, the level of culling or mortality due to leg abnormalities was relatively low.


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  		Table 1. Causes of mortality (% of broilers placed) as affected by dietary nutrient density, feed form, and lighting program
 
GS
Increasing the nutrient density of diets resulted in a higher GS at 11 and 18 d of age, but this effect was no longer found at 25 and 32 d of age (Table 2Go). Feeding pellet rations increased the GS for all ages examined when compared with mash feeding, whereas more light (20L:4D) resulted in higher GS than 12L:12D at 25 and 32 d of age. Males had higher GS than females at 11 and 32 d of age. All mean GS values were below a score of 1 for all treatments. On an individual basis, the number of birds with a GS ≥3 did increase with age from 0.35% at 11 d of age to 2.43% at 32 d of age (Table 3Go).


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  		Table 2. Effect of nutrient density, feed form, lighting program, and sex on the walking ability1 of broiler chickens
 

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  		Table 3. Number and percentage of birds with a gait score1 of 3 or greater for each assessment period
 
Three significant 2-way interactions and 1 significant 3-way interaction were observed for the walking ability of broiler chickens (Table 2Go). A 2-way interaction was observed at 25 d of age between feed form and light (P = 0.0052); however, at 32 d of age, it was no longer significant. Another 2-way interaction was observed at 25 d of age between feed form and sex (P = 0.0053), but this interaction was also no longer significant at 32 d of age. A 2-way interaction between lighting program and sex was observed at 25 and 32 d of age (P = 0.0286 and 0.0199, respectively). Male and female broilers reared under 12L:12D had a similar mean GS at 25 and 32 d of age (0.30 and 0.34 for males; 0.36 and 0.26 for females, respectively) and were lower than broilers reared under 20L:4D (Table 4Go). As a result, broiler mobility declined when broilers were provided with 20L:4D, especially among male birds. The mean GS was similar for female birds provided with 12L:12D and 20L:4D; however, female broilers provided with 20L:4D had a lower GS compared with male broilers reared under the same program (0.58 and 0.74 for males; 0.40 and 0.45 for females, at 25 and 32 d, respectively).


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  		Table 4. Interaction between lighting program and sex on the walking ability1 of broiler chickens
 
A significant 3-way interaction among nutrient density, feed form, and sex occurred at 11 and 18 d of age (P = 0.0048 and 0.0149, respectively; Table 5Go). However, the low GS values make interpretation difficult. In addition, these interactions were not found at later ages, so they are probably of little consequence.


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  		Table 5. Interactions among nutrient density,1 feed form, and sex on the walking ability2 of broiler chickens
 
Correlations
Correlations existed between BW and GS that were influenced by age and sex (Table 6Go). The strongest correlation was observed with males, for which BW at 11, 18, and 25 d of age were correlated with the GS at 32 d of age (r2 = 0.39, 0.49, and 0.50, respectively). Although significant, BW at 32 d of age did not influence the GS as strongly at 32 d of age (r2 = 0.21). Similarly, the BW of female broilers correlated with walking ability, as shown in Table 6Go, but correlation coefficients were lower than for male broilers. Body weights at 11, 18, and 25 d of age did correlate with bird mobility at 32 d of age (r2 = 0.34, 0.37, and 0.36, respectively); similarly, BW at 32 d of age was not as strongly correlated with GS at 32 d of age (r2 = 0.20).


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  		Table 6. Correlation coefficient (r2) between BW and gait score by age and sex of broiler chickens
 
Bone Quality
BA.
There were no differences for the percentage of BA when broilers were fed diets with different nutrient densities (Table 7Go). However, feed form (P = 0.0067) and lighting program (P = 0.0186) had an effect on BA. Broilers fed mash diets had a higher percentage of BA (50.58%) compared with those fed the pellet ration (49.78%). Lighting programs had a similar effect: broilers provided with 12L:12D had a higher percentage of BA (50.47%) compared with those provided with 20L:4D (49.89%). Females had a higher percentage of BA (50.59%) compared with males (49.77%). Two significant 3-way interactions were found among sex, lighting program, and feed form (P = 0.0383) and among feed form, lighting program, and nutrient density (P = 0.0371); however, these results are difficult to interpret.


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  		Table 7. Effect of nutrient density, feed form, lighting program, and sex on bone ash (%) in broiler chickens at 35 d of age
 
Radiology.
There were minimal numerical differences among the length, width, and curvature of the femur and tibiotarsus based on the different GS classifications (GS 0, 1, 2, and 3). The angle of the femur varied from 152.9 to 164.9, whereas the angle of the tibiotarsus varied from 169.1 to 172.7.

Histology.
No detectable microscopic differences were observed on the growth plates, gastrocnemius muscle, and sciatic nerve tissues based on the GS classification (GS 0, 1, and 2). In addition, the proportions of the left and right gastrocnemius muscle were similar for birds classified with a GS of 0 to 2. This varied from 0.47% for a GS of 0 to 0.52% for a GS of 2 for both the right and left gastrocnemius muscles.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
A low incidence of leg abnormalities was observed across all treatments and ages, as indicated by birds culled and the mean GS. The main cause of culling was skeletal inflammation, primarily bacterial chondronecrosis with osteomyelitis (BCO), in agreement with the findings of Thorp and Waddington (1997) and McNamee and Smyth (2000). This is in comparison with earlier reports, in which the valgus or varus, tibial dyschondroplasia, ruptured gastrocnemius tendon, spondylolisthesis, and rotated tibia were diagnosed as the primary causes of leg disorders (Riddell, 1983; Riddell and Springer, 1985; Pattison, 1992). Riddell and Springer (1985) reported that the main causes of leg abnormalities were valgus or varus deformation (79%), tibial rotation (7%), and spondylolisthesis (14%). McNamee and Smyth (2000) indicated that the ratio of lameness due to BCO and that of noninfectious origin appears to be changing. However, it is not clear whether there is an increase in the incidence of BCO in comparison with other causes of lameness or whether there is an increase in awareness. In addition, there has been intensive selection against angular limb deformities in the modern broiler genotype and this may have contributed to the type of leg abnormalities seen in this and some other trials (Kestin et al., 1999; McNamee and Smyth, 2000).

The GS values are much lower in this study than reported by Kestin et al. (1992). Those authors examined birds to 7 wk of age (approximately 2.8 kg) and found that 90% of birds had a GS >0 and 26% had a GS ≥3. Sanotra et al. (2001) also reported a high GS in broilers reared to 1.8 kg, with 75% of birds evaluated having a GS >0 and 30% having a GS ≥3. However, Sørensen et al. (2000) found that 4-wk-old broilers had good mobility, with less than 1% of birds having a GS of 4 or 5. These GS data more closely reflect the results obtained in the current study, in which 37.5% had a GS >0 and 2.4% had a GS ≥3 at 32 d of age.

In addition, it is worth noting that the mean GS values did not exceed 1, regardless of the dietary nutrient density, feed form, or lighting program. This indicates that intensive selection against skeletal abnormalities has improved the skeletal condition, in agreement with industry perception and recent reports from our laboratory (Classen et al., 2003, 2004). Sørensen et al. (2000) reported that GS was increased to 2 at 49 d of age, compared with a mean GS of 0.9 at 28 d of age. Although overall GS values were low in this study, the birds were relatively young and were reared to a mean weight of 1.8 kg. The GS values may have increased had the birds been reared to an older age, and hence heavier BW, because the data in this study illustrate that GS increases with age. Although the management systems (feed form and lighting programs) used in this study did have an impact on growth rate and subsequent GS, the overall GS values were less than 1 and therefore did not have a negative effect on welfare, which was defined as a GS ≥3.

Differences in nutrient density neither significantly affected the growth rate or BA nor had an effect on the walking ability of broiler chickens at 32 d of age. This result indicates that leg abnormalities (inflammatory or skeletal in origin) were not affected by dietary nutrient density.

Feeding mash did not have an effect on culling for skeletal disease (inflammatory or skeletal), but there was a reduction in the GS value, in contrast to those fed pellet rations. This difference could be explained by an increase in the BA content. Broilers provided mash diets had a significantly higher bone mineral content compared with those fed pellet rations. This increase in BA could be due to an increase in activity associated with this feeding system. Skinner-Noble et al. (2005) reported that broilers fed mash rations spent less time resting (47.4%) and more time eating (18.8%) than broilers fed pellet rations (62.5 and 4.3%, respectively). In addition, broilers fed pellet rations had higher BW gains than birds fed mash (Skinner-Noble et al., 2005). Under a management system that encourages activity and slows growth, bone has the capability to adapt to changes in physical loading and use by modeling and remodeling (Rath et al., 2000; Williams et al., 2000; Tablante et al., 2003). Therefore, reducing the growth rate by feeding mash may also benefit skeletal quality by improving bone mineralization.

Lighting programs did not have an effect on skeletal disease of inflammatory or skeletal origin; however, males and females provided with 20L:4D had the highest GS. In contrast, GS values were the lowest for both males and females reared with 12L:12D. Under this management system, there was a prolonged period of rest during darkness, and this is known to have a positive effect on skeletal growth, in contrast to prolonged exposure to light, which increases the occurrence of leg abnormalities (Wilson et al., 1984; Classen and Riddell, 1989; Classen and Riddell, 1990; Classen et al., 1991; Sanotra et al., 2002). By providing longer periods of darkness, the bone mineral content was significantly greater, which could explain the improved GS when broilers were provided with 12L:12D. This observation agrees with Scott (2002), who found that toe ash content was higher in broilers exposed to 16L:8D in contrast to 23L:1D.

Although not a management technique, male broilers had a higher GS and a lower BA content than females at 32 d of age, regardless of the lighting program. Tablante et al. (2003) reported that the higher growth rate of male broilers may reduce the bone mineral content because of incomplete or inadequate bone mineralization at 42 d of age, likely because of differences in physiology (testosterone and estrogen) and how it alters bone formation and remodeling. The higher incidence in leg abnormalities in males may be the result of poor bone mineralization; therefore, the higher bone mineral content in female broilers supports the improvement in their walking ability. The significant interaction between lighting and sex demonstrated that males, which inherently are more susceptible to leg disorders (Pierson et al., 1981; Classen et al., 1991), benefit more from management programs designed to reduce the incidence of skeletal disorders.

This study demonstrated that a positive correlation existed between BW and GS based on age and sex. An increased BW at an early age was correlated with a reduction in mobility later in the production cycle. However, BW at the time of the GS assessment was not as strongly correlated with bird mobility (Table 6Go), which disagrees with Sanotra et al. (2001), who reported that BW was correlated with GS (r2 = 0.58, P < 0.001), and with Kestin et al. (1999), who also reported a correlation between BW and GS for 4 different broiler crosses at 35 d of age (cross 1: r2 = 0.53; cross 2: r2 = 0.49; cross 3: r2 = 0.35; cross 4: r2 = 0.38, P < 0.01).

No gross or histopathological differences were seen among broilers with GS values <2, but this may be due to the limited number of birds for each GS classification. This makes increasing GS from 0 to 2 difficult to attribute to an animal welfare concern based on pathological changes. It has generally been accepted that a GS ≥3 is a welfare concern, because behavioral differences are observed between lame and normal birds and pathological changes are more obvious (Sørensen et al., 2000). There were no differences in the length and width of the tibiotarsus and femur among GS classifications; these results are similar to those reported by Williams et al. (2000).

The primary cause for culling due to leg abnormalities in this study was inflammation, likely because of infectious agents (e.g., caseous arthritis and osteomyelitis). The cause of infectious leg disorders is poorly understood, making it very challenging to select against (Kestin et al., 1999). Adding to the obscurity is its apparently complex etiology. Bradshaw et al. (2002) suggested that the etiology may be the additive result of 1 or more of the following factors: genetics, growth rate, feed conversion efficiency, body conformation, exercise, circadian rhythms, nutrition, and stocking density. Cheema et al. (2003) mentioned that the intensive selection for rapid BW gains reduces immunocompetence and health and supports the "resource allocation theory". This theory states that there is a change in the allocation of resources to the different functions of the animal (Rauw et al., 1998), depending on performance standards. Cheema et al. (2003) observed that Ross 308 broilers, a strain selected for rapid growth, had a reduction in the size of primary and secondary lymphoid organs, in contrast to the Athens Canadian Randombred control. This could explain why a trend existed for an increase in infection (P = 0.07) when broilers were provided with 20L:4D, because this lighting program favored rapid BW gain. Therefore, these birds may have had a reduction in their immune function, with respect to antibody production, because antibodies are crucial for controlling bacterial challenges (Cheema et al., 2003).

Although providing longer periods of darkness or feeding mash diets or both did improve the skeletal quality of broiler chickens by allowing the skeletal system to adapt to increases in body mass, it can be argued that neither management strategy had a major effect on bird welfare, as indicated by the GS <1 and the low number of birds (2.4%) with a GS ≥3 at 32 d of age. Feeding rations of different nutrient densities had no impact on broiler mobility. Overall, based on GS, radiography, and pathology, there was little evidence to support the view that the cause of leg abnormalities in modern broilers is solely due to alterations in growth rate. In fact, despite a continued increase in growth rate from generation to generation, the incidence of leg disorders appears to be lower than found previously. Mortality is also a welfare concern and is affected by management factors such as lighting programs. Therefore, mortality should also be included as a factor in determining the welfare of a production system.


   ACKNOWLEDGMENTS
 
The authors would like to thank Lilydale Inc. and the staff at the University of Saskatchewan Poultry Centre for their contributions to this project.

Received for publication January 2, 2007. Accepted for publication April 15, 2007.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
AOAC. 1990. Official Methods of Analysis. 15th ed. AOAC, Washington, DC.

Boorman, K. N., and A. D. Burgess. 1986. Response to amino acids. Pages 99–123 in Nutrient Requirements of Poultry and Nutritional Research. Poult. Sci. Symp. 19. C. Fisher and K. N. Boorman, ed. Butterworth and Co., London, UK.

Bradshaw, R. H., R. D. Kirkden, and D. M. Broom. 2002. A review of the aetiology and pathology of leg weakness in broilers in relation to welfare. Avian Poult. Biol. Rev. 13:45–103.

Brickett, K. E., J. P. Dahiya, H. L. Classen, and S. Gomis. 2007. Influence of dietary nutrient density, feed form, and lighting on growth and meat yield of broiler chickens. Poult. Sci. 86:2172–2181.[Abstract/Free Full Text]

Buyse, J., H. Michels, J. Vloeberghs, P. Saevels, J. M. Aerts, B. Ducro, D. Berckmans, and E. Decuypere. 1998. Energy and protein metabolism between 3 and 6 weeks of age of male broiler chickens selected for growth rate or for improved food efficiency. Br. Poult. Sci. 39:264–272.[Web of Science][Medline]

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