Leg Disorders in Broiler Chickens: Prevalence, Risk Factors and Prevention
Broiler (meat) chickens have been subjected to intense genetic selection. In the past 50 years, broiler growth rates have increased by over 300% (from 25 g per day to 100 g per day). There is growing societal concern that many broiler chickens have impaired locomotion or are even unable to walk. Here we present the results of a comprehensive survey of commercial flocks which quantifies the risk factors for poor locomotion in broiler chickens. We assessed the walking ability of 51,000 birds, representing 4.8 million birds within 176 flocks. We also obtained information on approximately 150 different management factors associated with each flock. At a mean age of 40 days, over 27.6% of birds in our study showed poor locomotion and 3.3% were almost unable to walk. The high prevalence of poor locomotion occurred despite culling policies designed to remove severely lame birds from flocks. We show that the primary risk factors associated with impaired locomotion and poor leg health are those specifically associated with rate of growth. Factors significantly associated with high gait score included the age of the bird (older birds), visit (second visit to same flock), bird genotype, not feeding whole wheat, a shorter dark period during the day, higher stocking density at the time of assessment, no use of antibiotic, and the use of intact feed pellets. The welfare implications are profound. Worldwide approximately 2 x 10(10) broilers are reared within similar husbandry systems. We identify a range of management factors that could be altered to reduce leg health problems, but implementation of these changes would be likely to reduce growth rate and production. A debate on the sustainability of current practice in the production of this important food source is required.
Leg Disorders in Broiler Chickens: Prevalence, Risk
Factors and Prevention
Toby G. Knowles
*, Steve C. Kestin
, Susan M. Haslam
, Steven N. Brown
, Laura E. Green
, Andrew Butterworth
, Stuart J. Pope
, Dirk Pfeiffer
Christine J. Nicol
1 School of Veterinary Science, University of Bristol, Langford, Bristol, United Kingdom, 2 Ecology and Epidemiology Group, Biological Sciences,
University of Warwick, Coventry, United Kingdom, 3 The Royal Veterinary College, Royal College Street, London, United Kingdom
Broiler (meat) chickens have been subjected to intense genetic selection. In the past 50 years, broiler growth rates have increased
by over 300% (from 25 g per day to 100 g per day). There is growing societal concern that many broiler chickens have impaired
locomotion or are even unable to walk. Here we present the results of a comprehensive survey of commercial flocks which
quantifies the risk factors for poor locomotion in broiler chickens. We assessed the walking ability of 51,000 birds, representing 4.8
million birds within 176 flocks. We also obtained information on approximately 150 different management factors associated with
each flock. At a mean age of 40 days, over 27.6% of birds in our study showed poor locomotion and 3.3% were almost unable to
walk. The high prevalence of poor locomotion occurred despite culling policies designed to remove severely lame birds from
flocks. We show that the primary risk factors associated with impaired locomotion and poor leg health are those specifically
associated with rate of growth. Factors significantly associated with high gait score included the age of the bird (older birds), visit
(second visit to same flock), bird genotype, not feeding whole wheat, a shorter dark period during the day, higher stocking density
at the time of assessment, no use of antibiotic, and the use of intact feed pellets. The welfare implications are profound. Worldwide
broilers are reared within similar husbandry systems. We identify a range of management factors that could
be altered to reduce leg health problems, but implementation of these changes would be likely to reduce growth rate and
production. A debate on the sustainability of current practice in the production of this important food source is required.
Citation: Knowles TG, Kestin SC, Haslam SM, Brown SN, Green LE, et al (2008) Leg Disorders in Broiler Chickens: Prevalence, Risk Factors and
Prevention. PLoS ONE 3(2): e1545. doi:10.1371/journal.pone.0001545
Due to their short reproductive cycle and their worldwide popularity
as a food, poultry represent the most highly selected livestock.
Selection of broiler chickens (chickens grown for their meat) has been
primarily directed at economic traits which have reduced costs of
production [1–3]. Throughout the world the majority of broilers are
reared using very similar, modern, intensive systems of production
where birds are confined for their lifetime within high density
housing  and reared from hatch to slaughter weight within
approximately 40 days. However, there is evidence that in
optimising traits for production the resulting birds, whilst producing
meat at a low cost, have a reduced viability and reduced welfare [5–
7], with poor walking ability, or locomotion, a primary concern.
Previous research has highlighted associations between manage-
ment practices and levels of leg disorders. Most attention has focussed
on the partially effective practices of reducing feed quantity or the
nutrient density of feed [e.g. 8, 9], on providing more than 1-hour of
darkness each 24-hour period , and on attempts to increase bird
activity [11,12]. There are also known genetic effects, with genotype
influences on many traits associated with leg health [7,13–15].
Despite a large body of work investigating the effects of specific risk
factors [reviewed in 16], there has been little previous work to
examine how these practices interact on real commercial farms to
determine the overall level of leg disorders in particular flocks. There
are also many management practices, with potential influences on leg
disorders, which have not been looked at quantitatively. This study
was therefore commissioned by the UK Department for Environ-
ment, Food and Rural Affairs to investigate the extent of variation of
leg disorders within UK flocks and to identify methods by which
these disorders could possibly be controlled.
MATERIALS AND METHODS
We studied broiler flocks belonging to five major UK producers
who together accounted for over 50 per cent of UK production.
Two other relatively large companies were invited to participate
but declined. We obtained data from each producer in proportion
to their respective annual broiler production. Visits were
randomised to farm and flock and were made by veterinarians
who had completed a five-day training course to evaluate broiler
walking ability with a standardised gait scoring method .
Eighteen veterinarians with postgraduate qualifications in poultry
medicine and production, or in welfare science, acted as flock
assessors and were trained to categorise gait scores within a range
from 0 (completely normal) to 5 (unable to stand). The scoring
system primarily assesses walking ability rather than exhaustion,
with assessors trained to identify rolling gaits, limping, jerky and
unsteady movements and problems with manoeuvrability. The
scoring system is also known to correlate well with other methods
of assessing leg disorders that do not involve active movement,
such as the latency-to-lie test . Throughout the study the
uniformity of the assessors’ scoring was monitored and by the end
Academic Editor: Patrick Callaerts, Katholieke Universiteit Leuven, Belgium
Received September 19, 2007; Accepted January 14, 2008; Published February 6,
Copyright: ß 2008 Knowles et al. This is an open-access article distributed under
the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the
original author and source are credited.
Funding: This study was carried out with funding from the UK Department of
Environment, Food and Rural Affairs (Grant AW0230) with whom the design and
execution were agreed. When reviewing the paper Defra have suggested
modifications to the terminology used to describe broiler lameness within the
paper which the authors have accepted.
Competing Interests: The authors have declared that no competing interests
* To whom correspondence should be addressed. E-mail: Toby.Knowles@bristol.
PLoS ONE | www.plosone.org 1 February 2008 | Issue 2 | e1545
of the course, average scores for each category were all within half
a score. During the subsequent 18 month study, assessors were
sent at approximately six and 12 months, a tape containing new
video sequences covering a range of gait scores. The scoring of
these tapes was monitored to ensure that the assessors remained in
agreement. Reference movies of birds’ walking ability for each of
the six categories are given in the supplementary information
[Movie S1, Movie S2, Movie S3, Movie S4, Movie S5, Movie S6].
Each of 176 flocks was visited approximately three days before the
flock was depopulated for slaughter and at least 250 birds from
each flock were gait-scored from ten, pre-selected, randomised
sites within a house.
Fifty seven of the 176 flocks in our study were not ‘depopulated’
for slaughter simultaneously. Instead, one of more groups of birds
were removed sequentially over a period of days or weeks in a
process known as ‘thinning’. This process involved the removal of a
portion of the flock, usually the female birds, to allow the remaining
birds more room to grow on to a greater weight. To account for the
effects of ‘thinning’ practices, an additional 30 visits were made as
second visits approximately three days prior to a later depopulation
of one of the original 176 flocks. The flocks visited a second time
were also chosen at random from the initial set of flocks.
A primary aim of the study was to investigate possible risk factors
associated with the wide inter-flock variation in leg disorders. Of
particular interest were risk factors associated with bird husbandry
which could possibly be altered when rearing future flocks.
Information on these aspects was obtained for each flock by a
direct interview with a farm representative. The same question-
naire was used for each visit and comprised 134 questions initially
about the breeding flock that had supplied the farm, the facilities
where the eggs had been hatched, the distance and time the chicks
had been transported, and hatchery vaccination policies. Informa-
tion was then obtained about the number, weight, sex and time of
chicks placed, and their date of arrival. The largest section of the
questionnaire sought information on husbandry practices including
stocking density and thinning practices, nutritional information,
layout and construction of the house, and background information
on health, growth rates, mortality and culling policies. Finally,
information about the personnel working with the flock, the farm,
biosecurity measures and company policies was obtained. After
conducting the direct interview, each veterinary assessor collected
direct information relating to air quality, temperature, general
cleanliness and feed quality.
Statistical models were built to identify between-flock variables
that were associated with the differences in average flock walking
ability. The multilevel modelling software package MLwiN v2.01
(http://www.cmm.bristol.ac.uk/MLwiN/index.shtml ) was used
as it allowed us to create linear models within the hierarchical
structure of the data of repeated measurements on flocks and
flocks within companies.
The overall results of the survey, showing the distribution of gait
scores prior to slaughter, are given in Table 1. The figures in
Table 1 were calculated using the gait scores of a flock, weighted
by the size of the flock as given by the number of birds placed as
chicks. The minimum and maximum values in Table 1 show that
there was considerable variation in walking ability between flocks,
but overall, 27.6 per cent of birds represented by this survey had a
gait score of 3 or above.
Table 2 shows the percentage of birds within each gait score
category broken down by the five companies and by the first and
second visits. There was a large amount of variation in the
distribution of gait score in flocks between the different companies.
Company 4 notably produced only 8.5 per cent of birds with gait
scores of 3 and above at the first visit compared with 22.7 to 29.7
per cent for the other companies, and only 0.6 per cent of birds
Table 1. The estimated percentage of birds in the survey
population within each gait score category.
Mean 2.2 26.6 43.5 24.3 3.1 0.2
SD 4.8 21.1 15.9 21.3 7.0 0.5
Min 0.0 0.0 1.6 0.0 0.0 0.0
Max 34.7 82.7 74.6 83.7 45.9 3.2
Mean, SD, minimum and maximum for flocks are shown. The values are
calculated from flock averages weighted by birds placed and include first and
second visits. Total birds placed n = 4,845,962. Total birds gait scored-206
flocks6minimum of 250 birds per flock-n is approximately 51,000.
Table 2. The percentage of birds in each gait score category by producer and by first and second visit.
0 1 2 3 4 5 Mean GS Birds Placed (n) Flocks (n)
First Visit Company 1 2.9 27.0 47.4 20.4 2.1 0.2 1.92 1,484,392 71
2 1.0 21.9 49.1 25.3 2.2 0.5 2.07 191,295 10
3 1.0 21.3 48.0 28.4 1.2 0.1 2.08 486,258 20
4 3.7 44.1 43.6 7.9 0.5 0.1 1.58 773,145 26
5 1.5 29.0 41.2 24.2 3.9 0.3 2.01 1,225,925 49
0 1 2 3 4 5 Mean GS Birds Placed (n)
Second Visit Company 1 2.2 8.2 40.3 40.6 8.0 0.7 2.46 206,360 11
3 1.5 53.2 43.1 2.2 0.0 0.0 1.46 34,000 1
4 3.5 26.6 37.5 31.1 1.2 0.1 2.00 119,150 4
5 0.3 4.2 23.7 59.4 11.9 0.4 2.80 329,037 14
The table also shows the mean gait score and the number of birds represented in terms of birds placed.
Leg Disorders in Broilers
PLoS ONE | www.plosone.org 2 February 2008 | Issue 2 | e1545
with gait scores of 4 and above compared with 1.3 to 4.2 per cent
for the other companies. Table 2 also shows the deterioration of
gait over time for companies 1, 4 and 5 where a minimum of four
second visits were made, the figures for the second visit to
company 3 representing only one flock.
The average gait score for a flock was modelled in terms of the
levels of the risk factors recorded by the veterinary assessors. In
addition to the assessment of husbandry variables as linear
predictors of average flock gait score, the age at which the birds
were assessed, and a seasonal effect, were included in the model. It
was necessary to correct for both age and season at the time of
assessment, before the effects of different husbandry practices could
be properly investigated. The seasonal effect was expected to be
cyclical, so was modelled as a sinusoidal curve to identify any
cyclical variation in average gait score over a 12 month cycle. Both
Sine and Cosine terms for month (January = 1, February = 2, etc)
were used in the modelling process allowing us to look for an
annual maximum and a minimum effect of season on gait score. A
number of variables in the model were centred [Text S1] by
subtracting the mean of the variable from each of its measurements.
The most parsimonious multilevel model we obtained of the
relationship between the average gait score of a flock, and the risk
factors that were recorded within the survey, is shown in Table 3.
When company was included as a fixed effect, rather than a
random effect, in the model the parameter estimates shown in
Table 3 were not meaningfully altered and there were no
significant variable/company interactions. Similarly, when the
veterinary assessors were included in the model as fixed effects the
parameter estimates were not substantively altered. The parameter
estimates in the final model are shown and also details of which of
the variables are centred. The mean, minimum and maximum of
the predictor variables are shown in Table 4, including all those
variables which were centred.
When all other variables were held constant, there was a seasonal
pattern to the average gait score of the flocks (Table 3 and Figure 1)
with the lowest (best) gait scores occurring in March and the highest
(worst) in September. The age at which the birds were assessed was
important in determining gait score, with every extra day, across the
range of 28 to 56 days, leading to an average daily deterioration in
score of 0.048. Although each flock was visited close to slaughter
when gait is known to be poorest, within the survey as a whole we
were able to evaluate the effect of age on locomotion problems
throughout the growth period because of the wide range of age at
slaughter. A post-thinning visit was associated with an increased
average gait score of 0.25 over and above that due to the age at
which the birds were assessed, probably due to the effect of the stress
of the first thin, and/or the preponderance of larger, faster-growing
male birds remaining in the flock after thinning.
We found that a number of fundamental husbandry practices
were significantly associated with average flock gait score and these
are detailed below.
A major influence was bird genotype. Broilers worldwide are
predominantly of two types, from either one of two major
international breeder companies, labelled here A and B. Birds
from both genotypes are sometimes reared together within one
flock. For every percentage increase in Breed A birds in a flock,
from between 0 to 100 per cent, there was a 0.0024 improvement
in flock gait score.
Whole wheat is sometimes fed to broilers as part of their diet,
predominantly to improve digestive function. For every percentage
increase in dietary wheat fed, from 0 to 30 per cent, as measured
during their third week of life, there was a 0.017 per cent
improvement in flock gait score.
Table 3. The parameter estimates, their standard error and
significance for the model of average flock gait score.
Variable Variable Type
Estimate SE P
Constant 2.52 0.158 0.000
Season (Sin) Continuous 20.099 0.0408 0.016
Season (Cos) Continuous 20.035 0.0442 0.463
Age assessed (day) Continuous (Centred) 0.048 0.0049 0.000
Second visit Binary 0.25 0.089 0.005
Breed A (% in flock) Continuous 20.0024 0.00108 0.025
Dietary wheat (wk 3) % Continuous 20.017 0.0078 0.027
Average dark (hr/day) Continuous (Centred) 2 0.079 0.0283 0.005
Stocking density (kg/m
) Continuous (Centred) 0.013 0.0057 0.024
Antibiotic Binary 20.17 0.069 0.011
Binary 20.15 0.063 0.017
The parameter estimates give the amount of change in average flock gait score
for a unit change in each variable. Note, as explained in the text, factors are not
and cannot be arranged in order of importance. Positive parameter estimates
mean that an increase in the value of a variable is associated with an increase in
flock average gait score and negative param eter estimate, a decrease.
Table 4. Mean, minimum and maximum values of the
continuous predictor variables in the model of average flock
Variable Mean Min Max
Age assessed (day) 39.8 28 56
Breed A (% in flock) 85.6 0 100
Dietary wheat (wk 3) % 9.2 0 30
Average dark (hr/day) 2.9 0 8.5
Stocking density (kg/m
) 31.3 15.9 44.8
Figure 1. The modelled seasonal change in average flock gait score.
Leg Disorders in Broilers
PLoS ONE | www.plosone.org 3 February 2008 | Issue 2 | e1545
Broilers are reared under a wide variety of artificial lighting
regimes. For every 1 hour increase in the daily period of darkness,
across the range of 0 to 8.5 hours, there was a 0.079 improvement
in flock gait score.
There has been debate about the importance of stocking density
as an influence on bird welfare and locomotion . Within limits,
putting as many birds in a house as possible for each rearing cycle
will improve profitability. For every 1 kg/m
increase in stocking
density as measured at the time of the flock assessment, across a
range from 15.9 to 44.8 kg/m
, there was a 0.013 deterioration in
flock gait score.
Antibiotics are routinely used during different stages of broiler
rearing and their use can be quite difficult to quantify accurately. In
our study farmers were simply asked if a flock had received
antibiotic. A reply of ‘yes’ in this context meant that a flock had
received an extra antibiotic treatment in addition to that which
would be part of normal rearing practice. For the flocks for which the
farmer had answered ‘yes’ flock gait score was reduced by 0.17.
Broiler feed is pelleted to minimise wastage and to increase the
amount of feed that a bird can consume within a given time. In
our study, veterinary assessors made a subjective judgement of
whether the quality of the pelleting was good or poor (dusty/
broken). For flocks with poor pelleting, gait score was on average
The magnitude of a parameter estimate given in Table 3 is not an
indicator of the importance of the association of a variable with an
increase or a reduction in gait score. This is because the magnitude
of the parameter estimate is dependent upon the measurement scale
used (e.g. kg or g, hour or day). Further, it is not possible to rank
objectively the variables in terms of importance as subjective
considerations such as practicality and the perceived cost of altering
the husbandry practice have to be considered. For example, the
results show that using 100% Breed A birds compared with 100%
Breed B birds would reduce average flock gait score by 0.24. To
achieve a similar reduction in average gait score by manipulating
stocking density alone would require a reduction in stocking density of
. Despite these considerations there is value in using our
model in a predictive capacity to show the improvement in average
flock gait score that might be achieved on a hypothetical farm if each
relevant variable attained the ‘best’ value actually recorded within
the survey. Figure 2 shows the potential improvement from a
baseline flock average gait score of 2.5, the middle of the range of
possible scores. The Figure shows that no, one variable was
associated with a large shift in average flock gait towards the score
of 0, a perfect gait, but that the age at which birds are slaughtered
appears to be of primary importance, followed by the hours of
darkness that the birds are allowed.
For comparison with previous surveys we draw attention to the
prevalence of birds in our survey that had gait scores of 3 or above.
This cut-off point is important because there is evidence from studies
of the effects of analgesic drugs that birds in these categories can be in
pain [19–22], Other surveys have reported between 14.1% and
30.1% of birds with gait scores of 3 or above in different European
countries, although it is not always clear whether weighted or
unweighted estimates have been used [7,23,24]. Our survey presents
a conservative estimate of UK prevalence of leg disorders in meat
chickens because the national proportion of birds slaughtered at
second, and subsequent thinnings, when gait score tends to
deteriorate, is higher than in our sample, and because we present
results only from companies that volunteered to participate.
All companies had a policy of culling broilers and some farms
separately identified ‘‘leg culls’’ conducted because of leg disorders.
If a flock within the survey were to be heavily culled because of leg
disorders it would be expected that the overall flock gait score
would be improved. However, we found no association between
the flock average gait score and the percentage of birds culled as
‘‘leg culls’’. However, the lack of a relationship may reflect the
difficulties farmers have in recording these data in a standardised
manner across the survey as a whole. Companies themselves
report that these records are inconsistent between farms.
Despite examining a fuller range of husbandry and management
practices than previous surveys, we did not identify many novel or
previously unreported risks. An effect of season has been noted
before in the US, where a higher percentage of leg abnormalities was
reported in the summer . The strong genotype effect that we
found, confirms the important genetic component to leg disorders,
and many of the husbandry effects detected most likely alter levels of
leg disorder through direct or indirect effects on growth rate. Thus,
we consider the effect of feeding whole wheat is probably due to the
slower rate of digestion for whole wheat resulting in reduced growth
rate, whilst providing broken or dusty pellets, rather than whole
pellets, probably reduced overall consumption rates.
Birds reduce or cease their feed intake during periods of
darkness, with associated reductions in growth rate. This is the
most likely explanation for the beneficial effect of longer dark
periods on the prevalence of leg disorders. The detrimental effect
of higher stocking densities may be more complex, reflecting not
only a lack of room available for birds to move and exercise, but
also the extra environmental loading from increased biomass (e.g.
additional ammonia and litter moisture) . Finally, the
improvement in leg health observed when antibiotics were used,
was probably due to a reduction in the infectious disorders which
can cause some types of leg problem.
The study indicates that modern husbandry and genotypes,
biased towards economics of production, have been detrimental to
poultry welfare in compromising the ability of chickens to walk.
However, we demonstrate that within the current framework there
is variation in the magnitude of the problem between different
flocks, and so some scope to improve walking ability through
alterations in husbandry practice. Work needs to be carried out on
the predictability of these risks, and the economics of improved
welfare practices, for them to gain industry acceptance. An
informed balance could then be drawn between profitability and
Figure 2. A modelling exercise reveals the extent to which average
flock gait score might be improved (i.e. lowered from a notional
average score of 2.5) by adoption of the ‘best’ management practice
for each variable that we recorded within our study.
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PLoS ONE | www.plosone.org 4 February 2008 | Issue 2 | e1545
our moral obligation to maintain good standards of animal
welfare. The agreement, in May 2007, within the EU of new
regulations governing the conditions under which broilers may be
reared  is a recognition of the problems associated with
modern broiler production and is an attempt at a first step towards
remedying the situation. The new measures will include the
introduction of a maximum stocking density limit, data collection
and scientific monitoring of impacts on chicken welfare. The new
Directive will not come into force until 2010 but it will prevent
farms from stocking birds at densities over 39 kg/m
flocks where mortality levels in the past seven flocks have exceeded
a set level. The animal welfare implications of monitoring
mortality and culling rates are potentially complex. In the short-
term, the new Directive could lead to less rigorous culling of birds
with leg problems and thereby increase suffering. However, in the
longer term, the Directive could act as a stimulus to breeding
companies to produce more robust genotypes, with a reduced
susceptibility to leg disorders.
Research shows that consumers currently know little about how
broiler chickens are reared but can be shocked when presented
with information about current commercial practices . Since
the sustainability of intensive broiler production depends on
continued consumer acceptance of the farming practices involved,
the broiler industry will need to work with the scientific
community to develop more robust and healthier genotypes and
to ensure that optimal husbandry and management practices are
Text S1 Further explanation of the linear model of mean-flock-
Found at: doi:10.1371/journal.pone.0001545.s001 (0.03 MB
Movie S1 Example of a gait score of 0. Movies S1 to S6
Examples of the gait scores taken from the veterinary assessors’
Found at: doi:10.1371/journal.pone.0001545.s002 (2.19 MB
Movie S2 Example of a gait score of 1.
Found at: doi:10.1371/journal.pone.0001545.s003 (4.23 MB
Movie S3 Example of a gait score of 2.
Found at: doi:10.1371/journal.pone.0001545.s004 (2.67 MB
Movie S4 Example of a gait score of 3.
Found at: doi:10.1371/journal.pone.0001545.s005 (3.02 MB
Movie S5 Example of a gait score of 4.
Found at: doi:10.1371/journal.pone.0001545.s006 (4.25 MB
Movie S6 Example of a gait score of 5.
Found at: doi:10.1371/journal.pone.0001545.s007 (5.71 MB
We are most grateful to the broiler companies and farmers who helped
with the work and to the members of the expert steering committee, drawn
from government, the broiler industry and academia, who gave their time
to guide and support the study.
Conceived and designed the experiments: DP CN TK SK SB AB LG.
Performed the experiments: TK SK SH SB AB SP. Analyzed the data: DP
CN TK SK SH SB LG. Wrote the paper: DP CN TK SK SH LG.
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PLoS ONE | www.plosone.org 5 February 2008 | Issue 2 | e1545