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Selective Breeding Case Study Chickens In The Road

SUMMARY

One of the most extraordinary results of selective breeding is the modern broiler chicken, whose phenotypic attributes reflect its genetic success. Unfortunately, leg health issues and poor walking ability are prevalent in the broiler population, with the exact aetiopathogenesis unknown. Here we present a biomechanical analysis of the gait dynamics of the modern broiler and its two pureline commercial broiler breeder lines (A and B) in order to clarify how changes in basic morphology are associated with the way these chickens walk. We collected force plate and kinematic data from 25 chickens (market age), over a range of walking speeds, to quantify the three-dimensional dynamics of the centre of mass (CoM) and determine how these birds modulate the force and mechanical work of locomotion. Common features of their gait include extremely slow walking speeds, a wide base of support and large lateral motions of the CoM, which primarily reflect changes to cope with their apparent instability and large body mass. These features allowed the chickens to keep their peak vertical forces low, but resulted in high mediolateral forces, which exceeded fore–aft forces. Gait differences directly related to morphological characteristics also exist. This was particularly evident in Pureline B birds, which have a more crouched limb posture. Mechanical costs of transport were still similar across all lines and were not exceptional when compared with more wild-type ground-running birds. Broiler chickens seem to have an awkward gait, but some aspects of their dynamics show rather surprising similarities to other avian bipeds.

INTRODUCTION

Although Darwin detailed how organisms evolve through natural selection (Darwin, 1859), he built his case partly on the knowledge that humans have used an analogous principle in the domestication of plants and animals for thousands of years. This has allowed livestock breeders to fully exploit desired phenotypic traits, resulting in dramatic and rapid changes in appearance and behaviour from their wild ancestors. A prime example of these dramatic changes can be seen in the modern broiler (a type of chicken raised specifically for meat), which has extremely rapid growth rates [18 standard deviations from its original rate across ~50 years of breeding (Whitehead et al., 2003)], a significantly larger pectoral muscle mass and increased meat yield (Barton, 1994; Lilburn, 1994; Webster, 1995; Nicholson, 1998; Corr et al., 2003a; Havenstein et al., 2003a; Havenstein et al., 2003b). However, this seeming success in the production efficiency of the modern broiler has come with unwanted consequences. In particular, musculoskeletal abnormalities and poor walking ability (commonly referred to together as ‘leg weakness’) are the most prevalent causes of culling and late mortality in the modern broiler (Pattison, 1992; Knowles et al., 2008).

Typically, leg weakness is characterised using a subjective gait scoring method, which assesses the walking ability of birds based on an abstract ideal of a ‘normal’ gait. Normal birds are considered more agile than those with an ‘abnormal’ gait, and in the worst cases, extremely abnormal birds may be incapable of sustained walking (Kestin et al., 1992). These gait scoring methods have been used extensively within the scientific community to understand the health and welfare implications in poultry (e.g. McGeown et al., 1999; Danbury et al., 2000; Weeks et al., 2000; Sandilands et al., 2011), but the actual relationship between this impaired walking ability and specific leg problems remains unclear (for a review, see Bradshaw et al., 2002). The difficulties are apparent; potential links of gait mechanics to pathology and walking ability remain merely inferential, the chance of detecting a subtle gait change correlated to pathology appears low and hence requires large sample sizes (Sandilands et al., 2011), and part of the difficulty in associating gait changes with certain pathologies is that chickens often have multiple pathologies. The way a chicken walks can therefore be a product of the underlying pathology and/or stresses, plus the bird's attempt to compensate for it.

Our first aim is therefore to quantify the locomotor dynamics of the modern broiler as an exploratory analysis of how selection has actually altered the way these birds walk and perhaps contributed to lameness. Because few studies have actually detailed objective measures of the modern broiler's gait (Reiter and Bessei, 1997; Corr et al., 1998; Corr et al., 2003b; Corr et al., 2007), here we establish the ‘normal’ gait characteristics of the modern broiler. We do this as an essential first step toward the longer-term goal of quantitatively characterising, identifying and understanding abnormal gaits in different lineages of wild and domestic poultry, including broilers. We also clarify possible misconceptions associated with what may have partly evolved to be an awkward gait for effective locomotion versus the individual perception of a ‘good’ versus ‘bad’ gait or ‘leg weakness’ in broilers. This is important to examine, because future considerations for the welfare of the modern broiler are likely to be based heavily on visual aspects of their gait.

Ironically, broiler chickens, like other galliform birds, may be considered as specialist walkers [based on their dominant locomotor mode (Tickle et al., 2007; Nudds et al., 2011)]. Yet their exaggerated lateral motions (Corr et al., 2003b) suggest that they may share more in common with other waddling, more aquatic species such as penguins, geese or ducks (Griffin and Kram, 2000; Abourachid, 2001; Usherwood et al., 2008; Nudds et al., 2011). Such waddling birds are often described as ‘awkward’ or ‘ungainly’ walkers, yet the mechanics of waddling birds still conform with the classical pendulum model of walking bipeds, associated with the conservation of mechanical energy (Cavagna, 1975; Cavagna et al., 1976; Cavagna et al., 1977). As much as 70% of the external work required to lift and accelerate the centre of mass (CoM) can be recovered as a result of this energy saving mechanism (Cavagna et al., 1977; but see Donelan et al., 2002). The second aim of our study quantifies the three-dimensional dynamics of the CoM in order to determine how broiler chickens modulate the force and mechanical energy of locomotion. Altered behavioural patterns and reduced activity levels have been reported in these birds (Weeks et al., 1994; Estevez et al., 1997; Bizeray et al., 2001; Weeks et al., 2000), which are thought to be attributable to conformation-related gait alterations causing fatigue (Abourachid, 1993; Corr et al., 2003b). We can test whether broilers require excessive work (using the metric of the mechanical cost of transport), requiring more mechanical energy from the limb muscles.

Finally, we evaluate the effects of conformation on locomotor dynamics, by investigating two pureline commercial broiler breeder lines with high performance (in terms of meat production) characteristics. These lines are typically crossbred by commercial poultry production systems in order to produce the modern broiler with desired characteristics (Anthony, 1998; Yang and Jiang, 2005). Differences in the pelvic limb musculature of these study groups have already been shown quantitatively, suggesting that differences in the gait characteristics of these lineages may exist (Paxton et al., 2010). Additionally, when the two purelines are compared at the farm level, Pureline A birds generally yield greater breast muscle mass per unit body mass and have lower average gait scores (poorer walking ability), whereas Pureline B chickens tend to have a larger body mass (~30% difference in some cases) with generally higher gait scores (H.P., unpublished data). We aim to determine whether these three lineages have adopted different locomotor strategies as a result of their altered morphology.

In addition, studies have suggested an apparent instability in broiler chickens, which – in line with the waddling gait of penguins (Kurz et al., 2008) – has been considered, somewhat speculatively, as more susceptible to falls. Specifically, broiler chickens appear to have more excessive lateral motions than more ancestrally typical ground-running birds (Cavagna et al., 1977; Gatesy and Biewener, 1991; Rubenson et al., 2004). We present the first study to investigate, albeit with an admittedly simple metric, the dynamic stability of the modern broiler. We do this by considering their gait variability and its potential role in locomotor stability (Winter, 1989; Holt et al., 1995; Dingwell and Cavanagh, 2001; Dingwell and Marin, 2006). By doing this, we aim to further highlight how morphological changes may have led to difficulties with locomotor stability in broiler chickens.

MATERIALS AND METHODS

Male commercial line birds (~42 days old) were used in this study, including two pureline commercial broiler breeder lines, referred to as Pureline A and B, and a commercial broiler strain (Table 1). The main morphological characteristics for these groups (collected from multiple cadaveric specimens) are also detailed in Table 2. These included breast muscle mass (±0.1 g; pectoralis; i.e. pectoralis major, and supracoracoideus; i.e. pectoralis minor; combined), girth (±0.1 cm), hip width (±0.1 mm), keel length (±0.1 mm) and total leg length (±0.1 mm). Hip width was taken as the distance between the trochanteric crests of the femora (birds were similarly positioned in each case), girth was measured around the circumference of the thorax of the bird (tucked under the wings), and total leg length was taken as the sum of the individual pelvic limb bones (femur, tibiotarsus and tarsometatarsus), measured from the most proximal point to the most distal point on the medial or lateral side of the bone. The bird populations were all raised under the same management conditions to ensure that any differences found were not attributed to husbandry factors, which are well known to influence the growth and leg health of broilers (Sørensen et al., 1999; Su et al., 1999; Vestergaard and Sanotra, 1999; Kestin et al., 2001; Scott, 2002; Dawkins et al., 2004; Mench, 2004; Brickett et al., 2007; Buijs et al., 2009). Those birds that were visibly lame or incapable of sustained walking were excluded from this study.

Motion capture was used to study individual birds using eight Qualisys MCH 500 cameras (Gothenberg, Sweden) that were synchronised to a Kistler 9287B force plate (Kistler Instruments, Alton, UK). The trochanteric crest of the hip and the distal phalanx of the middle toe of each limb were marked with infrared-reflective motion capture markers, thereby simplifying each limb as a linear segment. The birds were encouraged to walk over the force plate (500 Hz) parallel to the view of the cameras (167 Hz), and the marker position and the ground reaction forces in the vertical, fore–aft and mediolateral directions were recorded. All the birds had known body masses (±0.1 kg), which were taken immediately after the trials for each individual were finished.

The kinematic and force plate data were then analysed using two computer programs, Qualisys Track Manager (QTM) and MATLAB (The MathWorks, Natick, MA, USA). QTM formed a three-dimensional image of the markers' coordinates and these data were then further processed with the force plate data using custom MATLAB software. All trials were processed, but those trials where there were large gap ranges between the coordinates or where the bird was distracted were removed before further analysis. The kinematic data were filtered (Winter et al., 1974) using a low-pass, zero-lag fourth-order Butterworth digital filter with a cut-off frequency of 10 Hz. The same filter type was used for the ground reaction force data, with a cut-off frequency of 75 Hz. The kinematic data (foot markers only) were used to identify foot-down and foot-off events and these identified steps were subsequently analysed. This also allowed us to investigate any possible asymmetries (i.e. left–right limb differences) that may exist in the broiler.

Ergonomic analysis was conducted in each step to quantify mechanical energy fluctuations and to calculate the mechanical work required to move the CoM. Because the chickens walked slowly, and did not necessarily start walking ‘on cue’, there may have been some baseline drift from the force plates (an unavoidable limitation of the sensors recording over longer periods of time). To check and correct for this possibility, we assumed that the birds supported their own body mass through consecutive strides and the vertical forces were corrected accordingly. In order to reduce the error in position over time, the initial velocity conditions were calculated following methods adapted from Daley et al. (Daley et al., 2007). A path-matching technique was used where the initial velocity calculated from the kinematic data was used as an initial guess, which was then corrected to provide a base match between the CoM position calculated using the kinematics over time and the CoM position calculated through integration of the force plate data. The initial velocity selected was the value that minimised the divergence (sum of the squared differences) between the two paths and these conditions were used to calculate CoM velocity and position by the double integration of the accelerations from the force plate data.

Observation of the broilers through the length of the trial showed that they rarely walked in a straight line. The fore–aft and mediolateral forces and the CoM velocity in these two directions could therefore be under-/over-estimated depending on the direction the bird was walking in relation to the plate. The forces were thus corrected based on the angle between the CoM velocity and the force plate coordinate system. Peak forces were recorded along with step width and step length, which were defined from the lateral position of the CoM and the fore–aft position of the CoM, respectively. CoM height was defined as the average CoM position across a step. The leg length (in metres) and the leg angle (in degrees) were calculated using the CoM position and the toe marker position data. Based on basic trigonometry, if dX refers to the horizontal distance between the CoM position and the toe marker and dZ refers to the vertical distance between the CoM position and the toe marker, leg length=√(dZ2+dX2) and leg angle=180−[tan−1(dZ/dX)]. The trackway width (measured separate to step width, in order to consider the outward splay of the lower leg) was also calculated, measured as the lateral separation between the markers on the feet during the double support phase and expressed as a fraction of CoM height. The vertical and lateral displacement of the foot during swing was also considered – i.e. the peak displacement of the foot during swing relative to its position during stance – as a measure of limb circumduction and to investigate foot path variability.

The average horizontal velocity, duty factor (the fraction of the total stride cycle during which the foot is in contact with ground) (Biewener, 1983; Alexander, 1985; McMahon, 1985; Taylor, 1985) and the Froude number for each step were also recorded. The Froude number (Fr) was calculated as Fr=v2g−1l−1, where g=9.81 m s−2, l is mean hip height and v is mean velocity. The mechanical cost of transport (MCoT; J kg−1 m−1) was also considered: MCoT=Wm−1L−1, where W is absolute work performed (J), m is body mass (kg) and L is step length (m).

Only data that were considered steady state were used to draw results and conclusions in this study, as it was important to establish typical cyclical movements in these birds without halting or other perturbations. Steady state was defined based on the ground reaction force impulse and the change in CoM velocity over a step. A fore–aft impulse of 0±2 N s and a CoM velocity change of less than 35% during a step were used. The data were then sorted into eight speed categories for statistical analysis. A minimum of five data points per speed category per bird group was set in order to consider their contribution to the relationships observed as valid. Data were omitted where this condition could not be met. Whilst our statistical tests could manage a limited number of data points, because of the unsteady nature of these birds and our objective to establish normal gait characteristics, we wanted to ensure that the results were not influenced by potentially more spurious values. The computer package SPSS (IBM, Armonk, NY, USA) was used for statistical analysis to check for differences between the relationships of bird group, speed and the right or left foot with step width, step length, step frequency, leg length and angle, displacement of the foot, trackway width, peak forces, CoM energies, work performed and MCoT between bird groups. The data were analyzed using a linear mixed model, with speed, the foot used in each step (right/left) and bird group (Pureline A, Pureline B and the commercial broiler) as the fixed effects, the individual bird as the random effect and each factor previously mentioned (MCoT, step frequency, etc.) as the dependent variable. This procedure allowed the data to exhibit correlated and non-constant variability. It estimated the effects of speed, foot used and bird group on the dependent variables while adjusting for correlation due to repeated trials on each bird. P-values (≤0.05 deemed significant) were taken into consideration when analyzing the data and drawing conclusions.

Additionally, as a measure of kinematic variability, the coefficient of variation (ratio of the standard deviation to the mean) for a number of variables was used in order to have a comparable measure of dispersion among the three groups. To test for differences in morphology, a one-way ANOVA was used to test the differences among group means for significance. To validate the use of this parametric test, assumptions of normal distribution and equal variances were tested using Kolmogorov–Smirnov and Levene's tests, respectively (results displayed in Table 2). Where these assumptions were not met, an independent Kruskal–Wallis test was used. If significant differences were found (P≤0.05), a Bonferroni post hoc test was used to determine which groups were significantly different from each other. Regression analyses were also used to determine the relationship between CoM velocity and duty factor (Fig. 1), as well as any significant slope differences between step variables (Fig. 2).

Fig. 1.

Duty factor versus centre of mass (CoM) velocity for individual steps from walking chickens. Each symbol represents an individual bird, with the same symbol indicating multiple steps per bird used for this analysis. The regression lines represent a strong relationship (r2>0.4) between duty factor and CoM velocity for Pureline A and the broiler population, with a more moderate relationship (r2 between 0.2 and 0.4) for Pureline B chickens. The slopes of all three lines are statistically different from 0 (P<0.001).

We report the locomotor attributes of the modern broiler at their ‘preferred’ walking speed (mean velocity=0.25±0.02 m s−1) – i.e. the speed category used most commonly by the three groups (~40% of the data in each group) – as well as any significant relationships with speed. Although work performed is a scalar quantity, we consider the absolute values for work performed as separate components for each direction of motion in order to fully evaluate the mechanical work based on the magnitude and direction of each force vector.

All birds were examined post mortem to identify any pathological condition that may have affected the observed gait, in particular, femoral head necrosis, severe valgus deformities [greater than 45 deg is associated with lameness (Leterrier and Nys, 1992)], tibial dyschondroplasia or gross swelling of the joints.

Table 1.

Mean subject data for chicken breeds used in this study

Table 2.

General morphological characteristics of the chicken breeds used in this study

RESULTS

Gross abnormalities were not found during post-mortem examination. We therefore considered these birds to have normal limb function based on the absence of any gross pathology. It must be noted that satisfying the conditions of steady state led to differing amounts of data being excluded from this study. The largest number of steps was discounted from Pureline A (47%; 308 out of 661 ‘unsteady’ steps), with 33% (221 out of 661 ‘unsteady’ steps) and 20% (132 out of 661 ‘unsteady’ steps) being discounted from Pureline B and the broiler data sets, respectively. Collectively this accounted for 66% of the total number of steps (999) originally collected.

Very subtle morphological differences existed between the populations of our study birds (Table 2), with only significant differences in hip width (F2,27=16.5, P=<0.001) and breast muscle mass (P=<0.001). Pureline B birds had wider hips than both the broiler and Pureline A populations. Breast muscle mass varied significantly between the three groups. Pureline A had an additional 2% body mass of breast muscle mass compared with Pureline B and the broiler population, but total leg length was not statistically different between groups. Across all chicken populations, girth was ~30% larger than leg length across all bird populations. CoM height also varied between groups (F2,319.4=85.3, P=<0.001; Table 1). Pureline B chickens had the smallest CoM height, which was ~5% smaller than Pureline A birds and ~10% smaller than the broiler population.

Initial analysis of the CoM velocity and duty factor (Fig. 1) shows that the general trend among all populations was for CoM velocity to increase with a decrease in duty factor, as expected. The broiler population, which had the largest body mass (Table 1), seemed able to achieve a much broader speed range than the pureline populations, with a few broiler individuals reaching speeds between 0.6 and 1.1 m s−1. The largest fluctuations in CoM velocity can be seen in the lateral component (Table 3), with the highest fluctuations reported in Pureline A chickens. This component also differed depending on the foot used (larger for right steps; F1,311=6.49, P=0.011), and was generally smaller (~16%) in Pureline A birds (F2,311=5.59, P=0.004).

An increase in forward velocity was achieved by increasing step length and step frequency (Fig. 2), with a preference to increase step frequency at a rate slightly faster than that of step length (based on significant differences between the slope values of the two lines, P<0.001). Stance duration also decreased, whereas swing duration was kept almost constant (Fig. 3). Step width decreased with an increase in speed, with the magnitude of this effect varying significantly between bird groups. Step width decreased at a faster rate in Pureline B chickens and to a lesser degree in Pureline A chickens and the broiler population. Step width changes were more variable than changes in step length across all chicken populations. This is seen more clearly if we consider these values at the birds' preferred walking speed (Table 4). Variability in step width is shown to be approximately two times higher (~30%) than the variability in step length (~15%). Pureline B birds took longer steps than both the Pureline A and the broiler populations (F2,85.2=7.58, P=0.001), and this corresponded with a significantly lower step frequency (F2,58.0=5.89, P=0.005). Trackway width was not significantly different between groups (F2,140.0=2.66, P=0.073), remaining ~51% of mean hip height across all bird populations (Table 3).

Analysis of the chickens' general limb motions across a step cycle (Fig. 4) shows that leg angle did not change by more than ~10 deg through the stance phase of locomotion and was relatively consistent across steps (given the small standard deviations), in contrast to the swing phase of locomotion, where leg angle was much more variable. Changes in overall leg length were small between stance and swing phases, with the more noticeable differences between these events seen in the broiler population. This relates to the pathway of the feet during swing (Table 3), with the broiler population lifting their feet roughly a third higher than Pureline B birds with each step. In all bird populations, circumduction of both limbs was evident (F2,303=5.49, P=0.005), with a significantly larger lateral displacement of the right foot (F1,303=1.97, P=<0.001). This did not correlate with lateral velocity. The largest lateral displacement of both limbs was seen in Pureline B birds, which also had a significantly larger sweep angle (F2,318=11.49, P=<0.001; Table 4). Thus Pureline B birds took longer steps while drawing their feet further away from the body but at a lower elevation than broilers did.

The resulting ground reaction forces (Fig. 5) show that our study birds all tended to support forces equal to or slightly more than their body weight during a step, with peak vertical forces (Table 4) not exceeding 1.4 times body weight. These peak vertical forces were significantly larger in the broiler population (~15%) compared with the Pureline A population (F2,67.9=6.31, P=0.003). Mediolateral forces generally exceeded fore–aft forces, with both representing 10–15% of the peak vertical force. The subsequent direction of the mediolateral force corresponded to which foot was placed on the ground, with a general trend for birds to roll laterally over their supporting leg with each step. These forces were also larger in the right limb in all bird populations (F1,296.0=73.2, P=<0.001), which was more evident in the Pureline B and broiler populations, where the mediolateral forces were generally two times larger in the right limb (Table 4). Overall, the broiler population experienced significantly larger (~30%) mediolateral forces than the pureline groups (F2,84.3=10.3, P=<0.001).

The limb motions used by the birds led to a minimal change in CoM displacement (Fig. 5), which was generally less than 5% of hip height, corresponding to very small changes in gravitational potential energy. Fluctuations in kinetic energy were negligible and changes in CoM energies across all bird populations were small (fluctuating around zero) but extremely variable, with standard deviations much larger than value means (Table 4).

Fig. 2.

Step variables change with speed in walking chickens. Data are means ± s.d. The different coloured data points refer to Pureline A (red), Pureline B (blue) and the broiler population (black). Relative values were calculated by dividing step length and step width by mean hip height. Relative step frequency was calculated using the following equation, f=(hg−1)0.5, where g=9.81 m s−2 and h is mean hip height (Alexander, 1977; Alexander and Jayes, 1983). The slope values were tested for significant differences between bird populations, i.e. the coefficient for the interaction between the dependent variable and the bird group is 0. Mean slope values are displayed if no significant differences exist.

Fig. 3.

Stance (squares) and swing phase (circles) durations in walking chickens across their speed range. The different coloured data points refer to Pureline A (red), Pureline B (blue) and the broiler population (black). Individuals are not distinguished, thus data points may represent multiple steps from one bird.

The subsequent work performed in the vertical and mediolateral directions was significantly smaller in Pureline A (F2,48.4=9.32, P=<0.001 and F2,194.0=14.2, P=<0.001, respectively). Across all birds, the work performed in the mediolateral direction was of similar magnitude to the work performed in the fore–aft direction. Despite the subtle differences in locomotor dynamics observed between lines, the MCoT (work performed per kilogram body mass over a step) was not significantly different between groups (F2,68.5=1.43, P=0.247; Fig. 6). Remarkably, when compared with other ground-running birds (ostrich and guineafowl) for which adequate data exist, the MCoT appears to follow simple body-size scaling patterns (i.e. larger species toward the bottom of the plot) rather than showing a sharp divergence between specialised running birds and the more sedate, artificially selected domestic chickens.

Table 3.

Trackway width, lateral velocity and foot displacement (lateral and vertical) at preferred walking speed in chickens

DISCUSSION

Leg weakness (encompassing lameness and poor walking ability) is a topical issue concerning the health and welfare of the modern broiler chicken. Unfortunately, there are many difficulties associated with establishing the cause of leg weakness in poultry and very little is understood about their gait. Our study therefore had three major purposes: (1) to determine how selection has actually altered the way that production-line chickens walk; (2) to determine whether any changes in locomotor dynamics require excessive work, requiring more mechanical energy from the limb muscles; and (3) to establish whether a change in morphology in these chickens leads to different locomotor mechanisms. Hence, our study helps to illuminate how morphological changes may have contributed to lameness or other difficulties with locomotion in the broiler chicken.

Fig. 4.

Leg length and angle at preferred walking speed in chickens. Data are means ± s.d. (shaded areas). Stance (solid lines) and swing (dashed lines) phase are shown for the right (red) and left foot (blue).

It is well known that distinct selection pressures are applied on a line-by-line basis, so we would expect morphological differences to exist amongst our three study groups. Consequently, we found subtle differences including a large pectoral muscle mass, accounting for ~20% of total body mass (~2% larger in Pureline A chickens compared with Pureline B and the broiler population), and differences in hip width between the study populations (larger in Pureline B). This disproportionate increase in pectoral muscle mass to body mass is well documented (e.g. Havenstein et al., 1994a; Havenstein et al., 1994b; Lilburn, 1994), but under natural conditions is usually only seen in other Galliformes that require this large muscle to power a rapid take-off; e.g. grouse or partridges (Hartman, 1961; Tobalske and Dial, 2000). Broiler chickens are essentially flightless at any stage of ontogeny (authors' personal observations), and previous literature has suggested that the influence of this alone may put greater demands on the pelvic limb muscles, affecting the birds' walking ability (Abourachid, 1993; Corr et al., 2003b). The logic underlying this presumed relationship between pectoral mass and pelvic limb mechanics is that a more cranially positioned CoM requires more limb muscle effort for support. This is not uncommon among other bipeds, where the potential displacement of the CoM (greatly influenced by body size and shape) has been found to have a strong influence on aspects of locomotion [e.g. postural stability in humans (Fregly et al., 1968; Corbeil et al., 2001)], such as a wider pelvis (Pureline B birds), in combination with the large pectoral muscle mass (Hutchinson, 2004). Yet it remains unclear how other morphological changes may affect gait.

Table 4.

Dynamic gait variables for the study chickens at their preferred walking speed

Picture a six-year-old child weighing 286lb, or more than 20 stone (130kg). That, says a senior research scientist, is the reality of life for Britain's broiler chickens – and it is about to get substantially worse.

Picture a six-year-old child weighing 286lb, or more than 20 stone (130kg). That, says a senior research scientist, is the reality of life for Britain's broiler chickens – and it is about to get substantially worse.

Think of a six-year-old with a body weight of an even more monstrous 24 stone and then imagine it trying to walk. Hideous and cruel for a child – but the near future for a broiler.

The cheap poultry we have got used to in every supermarket is produced so inexpensively at a high price in suffering. Unnaturally rapid growth rates, induced by selective breeding, have led to broiler chickens becoming massively too big for their own bodies and suffering wide-spread lameness and heart failure as a result.

Yet these growth rates are about to be increased still further by British poultry breeders. Chickens that, in 1976, were reaching just over 1kg in weight at the age of six weeks are now reaching 2.6kg in the same period – and encountering severe welfare problems because of it. They will soon be reaching 3kg at the same age.

Experimental strains of these 3kg birds have already been bred at centres in Scotland and Essex and, during the next five years, will become the stock for the "crop" of 820 million broiler chickens produced in Britain every year.

Animal welfare campaigners and academics say the new growth rates will lead to an increase in the massive welfare problems that have long been associated with broiler chicken production. The breeders flatly deny it, saying the new strains have been improved and will suffer fewer welfare problems, not more. They say that, in breeding birds with even higher growth rates, they are responding to the demands of their customers – chicken producers and supermarkets.

The new birds have been bred by Britain's two main poultry breeders, Ross Breeders of Newbridge, Midlothian, and Cobb Breeding of East Hanningfield near Chelmsford. Each company is now controlled by one of the two biggest poultry breeders in the world, both US-based – Ross by Aviagen Inc and Cobb by Cobb-Vantress, Inc. Between them, they have 80 per cent of the annual breeding market of 300 million birds.

The advent of their new chicken strains was disclosed two years ago in a paper written by four Ross scientists for a meeting of the British Society of Animal Science, entitled "The challenge of genetic change in the broiler chicken". They wrote: "Live weights at 42 days have more than doubled in the past 23 years (from 1050g to 2600g) and are projected to reach 3kg by 2007."

At the weekend, executives of both Ross and Cobb confirmed to The Independent that these 3kg birds have now been bred. They did not wish to talk on the record, other than to insist that these new birds would suffer fewer of the leg and heart problems suffered by their lighter, slower-growing predecessors, although they could not give figures to indicate the size of the projected improvement.

Animal welfare campaigners rejected their assertion. Abigail Hall of the RSPCA said: "Certainly the problems will get worse, and it's totally unacceptable. They should be paying a lot more attention to the health and welfare of the current birds and the last thing they should be doing is increasing the growth rate further. We want the search for faster growth rates stopped."

Joyce d'Silva of the campaign group Compassion in World Farming said: "It's an absolute scandal. Already, millions of chickens are lame for the last week or two of their lives and here's people like Ross Breeders making that situation even worse. It is completely unacceptable in a civilised society."

Academics also expressed concern. Donald Broom, a professor of animal welfare at the University of Cambridge, said: "I think most people would find this [the new growth rate] disturbing.There has been a general ethos in the industry that what is needed is faster growth and more efficient conversion of feed to poultry meat and that's the most important thing to do. But we have reached a point where there is a very substantial amount of damage to the birds. These [new] birds are going to suffer even more pain.

"We're in a situation now where people have to turn around and go in the opposite direction, and produce birds which will grow more slowly."

There is not doubt that current growth rates have produced very serious welfare problems for broiler chickens, which are kept for their six-week lives in darkened sheds, each one containing up to 50,000 birds.

The incidence of lameness and heart failure is disputed between the chicken industry and its critics but even the industry's own figures show that lameness, for example, affects millions of birds.

Lameness is measured by a "gait score" of one to five, with three and above meaning, in the words of Peter Bradnock, chief executive of the British Poultry Council, that "the bird's welfare is compromised".

A recent survey by the industry, still unpublished, indicates that a gait score of three and above was suffered by "only" about 2 per cent of birds, Mr Bradnock said. When it was put to him that this represented about 16 million chickens, he said: "Yes of course, it's a lot of birds and it is far too many. But the industry invested millions of pounds in the past few years in improving management."

Between 3 and 4 per cent of birds die from various causes before slaughter, the survey results show – that would represent between 25 and 32 million birds annually. Non-industry estimates of lameness and mortality are significantly higher, and the Government hopes to get a more balanced picture from its own survey next year.

The physical stresses that a broiler chicken currently endures can be visualised by the aforementioned comparison with a child, – suggested to The Independent late last week by a senior research scientist with huge experience of broiler chicken welfare problems.

Begin, he said, with what is natural – the wild ancestor of our domestic chicken, the red jungle fowl Gallus gallus of south-east Asia. This handsome bird of the forest floor reaches maturity at 18 weeks of age, when it weighs less than 1kg (2.2lb).

Compare that with a human being, reaching maturity at 18 years, and weighing, let us say, 50kg, or about eight stone (which might well be an underestimate).

By 1976, the domestic chicken was reaching 1kg in a third of the time, just six weeks rather than 18. So picture, for comparison, not an eighteen-year-old but a six-year-old child weighing eight stone. Just about imaginable, perhaps.

Now, though, because of the intensive selective breeding by the broiler industry in the past 25 years, the six-week-old chicken weighs 2.6kg. That's the equivalent of a six-year-old child weighing 2.6 times 50kg – 130kg, or more than 20 stone. And in the next few years, we will see in the supermarkets – unless it is stopped – broilers attaining fully 3kg in six weeks, the equivalent of a six-year-old weighing 150kg, or nearly 24 stone.

Elliot Morley, the Government's animal welfare minister, told The Independent."We think there are justified concerns, and there are legitimate questions for the breeders to answer. The present situation with regard to broiler welfare is not acceptable and it is surely more sensible to address this now than carry on with even more increased growth."

The Government was pressing for an EU directive on broiler welfare that might be drafted next year, Mr Morley said. Asked if the directive might cover calling a halt to growth rates, he said: "I think that's a legitimate issue that has to be taken into account."

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