©2008 Poultry Science Association, Inc.
Validation of a Three-Point Gait-Scoring System for
Field Assessment of Walking Ability
of Commercial Broilers
A. B. Webster,*1 B. D. Fairchild,* T. S. Cummings,† and P. A. Stayer‡
*Department of Poultry Science, University of Georgia, Athens 30602;
†College of Veterinary Medicine, PO Box 6100, Mississippi State, MS 39762;
and ‡Sanderson Farms, PO Box 988, Laurel, MS 39441
Primary Audience: Researchers, Broiler Producers, Animal Welfare Auditors
SUMMARY
A 3-point gait-scoring system used to evaluate broiler walking ability in welfare audits of
commercial flocks in the United States was compared with the 6-point Kestin system. In 2 university trials, market-age broilers of 2 commercial varieties were gait-scored by 2 observers for
each scoring system. Subsamples of birds were rescored and evaluated in latency-to-lie (LTL)
tests. Too few birds had significant walking difficulties in these trials to allow for a good comparison of the 2 scoring systems across all the gait score categories, but the data were encouraging despite sampling limitations. There was a significant association between the 2 systems,
and both had substantial between-observer agreement. Both scoring systems had significant
correlations with LTL, but the variation of LTL was too high to give gait score or LTL much predictive value for each other. In the field observations, 2 teams of observers scored broilers 47 to
61 d of age on 5 commercial farms each. Two pairs of observers on each team scored the same
birds, each pair using the 3-point system or the Kestin system. Broilers with walking problems
were oversampled to obtain an adequate number in each gait score category. Weighted κ statistics showed substantial between-observer agreement in each system but more so in the 3-point
system, suggesting that the application of the 3-point system was more consistent between observers. Spearman correlations between 3-point and Kestin scores for individual birds indicated
good correspondence between the 2 systems. The simplicity of the 3-point gait-scoring system
appears to facilitate between-observer agreement, making it preferable to more complex systems for use in commercial animal welfare audits. The correspondence between the gait-scoring
systems validates the 3-point system in light of the 6-point Kestin system.
Key words: broiler, gait score, walking ability, animal welfare audit
2008 J. Appl. Poult. Res. 17:529–539
doi:10.3382/japr.2008-00013
DESCRIPTION OF PROBLEM
Modern strains of broilers are susceptible to
leg problems compared with unselected strains
of chickens. Under some circumstances, inci1
Corresponding author: [email protected]
dence of leg disorder can be high, associated
with elevated rates of mortality [1–3]. Impaired
gait has also been found to be linked to pain or
discomfort [4, 5]. Observations such as these
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530
have led many people to expect that commercial broiler flocks routinely have large percentages of birds with walking difficulty and have
fueled concern that the broiler industry is living
with a level of bird welfare that is unacceptably
poor. Fortunately, there is growing evidence that
environmental factors can have a considerable
influence on incidence of leg problems in broilers and that leg abnormalities can be kept to
low levels under good management [6, 7]. This
evidence has come in part by means of evaluations of walking ability of birds in commercial
flocks.
Because animal welfare assurance has become an important aspect of the marketing of
poultry products to food retailers, the need has
arisen for an appropriate method to measure the
incidence of leg disorder in commercial broiler
flocks. Visual inspection of walking ability (i.e.,
gait scoring) offers the advantage of allowing
noninvasive evaluation of large numbers of
birds in a short period of time. A valid and feasible method for gait scoring broilers on commercial farms is needed so that auditors can apply a consistent scoring method and minimize
inequities in the assessment of different farms.
The method should allow a broiler house to be
evaluated in a reasonable length of time. Gaitscoring systems have been developed to assess
leg condition in breeder stocks and have been
adapted for evaluation of broilers [8, 9]. The
systems used by these authors define 6 categories of walking abnormality on an ordinal scale
of severity. The differences between categories
are subtle enough to make gait scoring perhaps more difficult and slower than necessary
in a commercial production environment. Other
studies used fewer categories to score walking ability but did not evaluate the systems in
a context comparable to the real-time observations necessary for an animal welfare audit or
for comprehensive sampling of farms as part of
a company animal welfare program [6, 10]. We
have developed a real-time, 3-point (3PT) gaitscoring system that is currently being used on
commercial farms in the United States. The 3PT
system seeks to identify broilers as having no
impairment of walking ability (score 0), having
obvious impairment but still ambulatory (score
1), and having severe impairment and not able to
walk without great difficulty (score 2). The 3PT
system needs to be validated for reliability and
comparability with other scoring systems.
Motivation to walk can influence evident
walking ability, and even pain-related behavior
such as limping can be overridden by attentional mechanisms [11]. Understanding this, some
researchers have sought alternate methods of
assessing leg weakness. Weeks et al. [12] described the latency-to-lie (LTL) test, wherein
broilers are made to stand in shallow water. Being a novel and aversive stimulus, the water is
thought to produce avoidance motivation strong
enough to induce broilers to stand as long as possible. In LTL tests, birds with good walking ability are able to stand a long time, whereas those
with poorer gait scores sit down sooner. Berg
and Sanotra [13] modified the LTL test with the
intent of making it easier to use in commercial
broiler houses.
In the present study, the 3PT gait-scoring
system was compared with adaptations of the
6-point Kestin gait-scoring system [8, 9] in 2
university trials and in 10 commercial broiler
flocks. Subsamples of birds in the university trials were rescored to assess retest reliability and
were evaluated in LTL tests to compare the different scoring systems against a common standard.
MATERIALS AND METHODS
University Trials
In each of 2 trials, straight run broiler chicks
of 2 varieties were obtained from commercial
hatcheries and housed in an environmentally
controlled room on the University of Georgia
(UGA) Poultry Research Farm in separate floor
pens bedded with pine shavings. One hundred
fifty Cobb 500 [14] and 200 Ross 708 [15]
chicks were placed in trial 1, and 200 chicks of
each variety were placed in trial 2. Space allocation was 0.7 ft2 per bird. The photoperiod was
24 h of light from d 1 to 7 and 20L:4D from d
8 to 42 (or d 45, trial 2), when the trials ended.
Standard UGA practices were followed for flock
management, and UGA-formulated broiler feed
and water were provided ad libitum. Body
weights were obtained at the beginning and at
the end of the trial.
Gait-scoring observations were conducted
at the end of each trial. A total of 681 broilers
Webster et al.: THREE-POINT GAIT-SCORING SYSTEM
were observed in the 2 trials. This number is
less than the number housed minus mortality,
because some unscored broilers escaped past
the divider separating unscored and scored birds
and were lost among the scored birds. Two observers scored each bird according to the 3PT
gait-scoring system, and 2 other observers simultaneously scored the same birds according
to a 6-point gait-scoring system. The sex of each
broiler was noted. The observers consisted of
poultry science professors, technicians, graduate
students, and undergraduate students. Although
all were familiar with broiler chickens, most had
no prior experience with gait scoring and had to
be trained. During training, an experienced individual explained the categories of the different gait-scoring systems, which were provided
to trainees in written form, and then worked
with the trainees to score live birds according
to the scoring system they were assigned until
they became proficient. In trial 1, all observers
used stopwatches to record the time it took to
determine the gait score of each bird. Although
the original purpose was to compare the length
of time it took to score birds using the different systems, the time data prompted us to switch
from the 6-point modified Kestin system [9]
used in trial 1 to the 6-point Kestin system [8]
in trial 2. The observers in trial 1 gravitated toward the Kestin system by overlooking the time
criteria built into the modified Kestin system.
It was thought that animal welfare auditors and
other gait scorers working in commercial situations would have the same tendency, making
the modified system impractical for field use.
The gait-scoring systems thus used in this study
are outlined in Table 1. During gait scoring,
the observers stood outside the pen of the birds
where they could avoid viewing the broilers
from above. A handler in the pen used folding
wire catch pens to clear space for birds to walk
freely without being impeded by other birds.
The handler separated broilers from the flock
and encouraged them to walk in the open area
in the direction of other birds. This encouragement consisted, as necessary, of approaching the
bird from behind, tapping the catch pen near the
bird, or gently prodding the bird with a length
of polyvinyl chloride tubing or a foot. Although
the effort made to get birds to walk was not rigorously standardized, there was no advantage
531
in trying to excessively motivate them, because
fear or novelty can cause a chicken to ignore a
painful leg condition and run or walk normally
[11], which would decrease the numbers of birds
recorded in the poorer gait score categories and
compromise comparison of the gait-scoring systems. When a broiler without evident leg abnormality refused to walk, the handler left it where
it was and went on to other birds. The reluctant
bird usually could be induced to walk after other
birds had traveled past it.
In each trial, up to 10 broilers of each variety
representing each category of the 6-point system were identified for subsequent rescoring of
their walking ability and for participation in LTL
tests. Only those birds were selected for which
both observers using each system agreed on the
gait score (total, 90 birds). This was done to provide a common basis for subsequent analyses
comparing the gait-scoring systems. Broilers
with gait abnormalities were oversampled from
the population to obtain a balanced distribution
of birds across the score categories as much as
possible. The gait rescoring and LTL tests were
carried out separately. The second gait-scoring
test was conducted in the same manner as the
first, except that it involved only the identified
birds. The LTL tests were done in the same room
that housed the broilers, using a procedure similar to that of Berg and Sanotra [13]. For these
tests, the birds were removed from their home
pen and placed in tubs containing an inch (2.5
cm) of lukewarm water. The time it took for the
broiler to first attempt to sit down was recorded,
at which time the bird was removed and replaced with another bird. A test was terminated
after 600 s if the bird stood that length of time.
Three tubs were monitored at 1 time by a single
observer. The tubs were surrounded by cardboard walls high enough to prevent test birds
from seeing other broilers. In trial 1, gait scoring
and LTL tests for Ross 708 broilers took place
on d 39 and gait rescoring on d 40. For Cobb 500
broilers, all observations were done on d 41. In
trial 2, gait scoring of both broiler varieties was
carried out on d 45 and gait rescoring and LTL
tests on d 46.
Although 2 broiler varieties were used to
reinforce the generality of the study, the intent
was not to make a direct comparison of the 2
varieties. The age of the parent flocks was not
JAPR: Field Report
controlled, and the 2 stocks were grown on an
identical lighting and nutritional program, which
may not have been the optimum for one or the
other variety. The walking ability observed may
not have reflected the optimum leg condition
that could be achieved with either stock. Gait
score data will not be presented separately for
the broiler varieties.
Complete lameness
5
Field Observations
Severe impairment of function but still capable of walking
4
No detectable abnormality.
Slight defect in walking ability that is difficult to define. An uneven gait.
Definite and identifiable defect in gait but with little hindrance of movement.
Obvious gait defect, which affects ability to move about (e.g., limp, jerky or unsteady stride,
or splaying of a leg).
Severe gait defect. Capable of walking with difficulty when driven or strongly motivated.
Squats at first opportunity.
Bird cannot walk. May shuffle on shanks or hocks with assistance of wings.
Bird can walk at least 5 ft with a balanced gait. Bird may appear ungainly but with little effect on function.
Bird can walk at least 5 ft but with a clear limp or decidedly awkward gait.
Bird will not walk 5 ft. May shuffle on shanks or hocks with assistance of wings.
3-point gait-scoring system
0
None
1
Obvious impairment
2
Severe impairment
Kestin gait-scoring system (adapted from Kestin et al. [8])
0
None
1
Detectable but unidentifiable abnormality
2
Identifiable abnormality that has little effect on overall function
3
Identifiable abnormality that impairs function
Criterion
Degree of impairment
Gait score
Table 1. Description of gait-scoring systems used to evaluate walking ability of market-age broilers
532
Observations were made on 10 commercial
farms contracted to a broiler complex in Mississippi over 2 d in the month of September. Only
1 house was used on each farm. A total of 1,381
birds was observed. All houses were solid-wall
construction and designed for tunnel ventilation.
They each held 20,000 to 25,000 broilers on litter. Two teams of observers, comprised of university professors, veterinary medical students
from Mississippi State University, and broiler
company employees, scored broilers on 5 farms
each. The flocks ranged in age from 47 to 61 d
of age, and all birds were the Ross 308 variety
[15]. Although familiar with broiler chickens,
most of the observers had not done gait scoring
before the study, so training was conducted first
for these individuals. The training consisted of
a classroom session to explain the categories of
the 2 gait-scoring systems being compared and
discuss important aspects of bird catching and
handling procedures during gait scoring. This
was followed by a session with a commercial
flock in which catching and observation procedures were demonstrated and trainees scored
birds representing a full range of walking ability
according to the system assigned to them (3- or
6-point) until they demonstrated proficiency.
In the field observations, 2 observers scored
birds using the 3PT system, and 2 others simultaneously scored the same birds using the
Kestin system in each commercial flock. The
sex of each bird was recorded. The intent was
to observe about 20 birds representing each of
the 6 Kestin gait score categories for a total of
120 broilers per flock. Catch pens were used to
enclose and observe 3 groups of 20 to 30 birds in
succession at different locations along the side
wall of the house. After each group was gathered, a space of approximately 10 ft was cleared
of birds along the wall and out to the first line
of drinkers (about 5 ft from the wall). The catch
Webster et al.: THREE-POINT GAIT-SCORING SYSTEM
pen was opened toward the open space and the
birds were encouraged as gently as possible
by similar methods as for the university trials
to walk out of the pen. Typically, the broilers
would leave the enclosure singly or in small
groups. The observers stood several feet away
from and slightly behind the pen opening so that
they could see the birds from the side but not be
perceived as an obstacle to broilers attempting
to leave the pen. The broilers were gait scored as
they left the catch pen, much as would be done
in an animal welfare audit. Most of these birds
were found to be in the Kestin 0 or 1 category,
so each flock was searched in different zones to
come up with up to 3 additional pens of 25 to 30
broilers having evident walking abnormalities.
These were scored in the same manner as the
other birds to make up the numbers of birds in
the Kestin 2 to 5 categories as much as possible.
Thus, the greater Kestin categories, reflecting
greater leg problems, were oversampled in regard to their true incidence. To avoid overly disturbing the flocks, no further effort was made to
find lame broilers even if some Kestin categories still lacked the desired number of birds.
Data Analysis
Various procedures of SAS [16] were employed to carry out data analyses enabling comparison of the 2 gait-scoring systems. The data
from the 2 university trials and the 10 commercial flocks were combined, respectively, and
analyzed without attempting to partition out
trial or farm effects. To evaluate between-observer agreement within each system (i.e., how
similarly observers using the same gait-scoring
system scored birds), the FREQ procedure of
SAS [16] was used to construct frequency tables
matching the scores of the 2 observers for every bird and calculate weighted kappa (κ) values
and Spearman rank correlations between the 2
sets of scores. The κ value is a measure of the
strength of agreement between ratings within a
scoring system. Values ranging from 0.4 to 0.6,
0.6 to 0.8, and 0.8 to 1.0 are considered to reflect
moderate agreement, substantial agreement, and
almost perfect agreement, respectively. The κ
value was weighted to compensate for the different number of categories in each gait-scoring
system, because it is to be expected, for instance,
that a 1-category difference in score between
533
observers would reflect a larger disagreement
in the 3PT system than the 6-point system. The
same type of analysis was done with the data
from the university trials to evaluate withinobserver (i.e., test-retest) agreement using the
broilers that were rescored for walking ability.
The CORR procedure of SAS [16] was used to
calculate Spearman correlations between the
LTL times and first gait scores of the selected
broilers in the university flocks and to calculate
Spearman partial correlations to assess the association remaining between LTL and 1 gait-scoring system after removing the association that
could be attributed to the other gait-scoring system. Finally, the 3PT and Kestin systems were
compared directly in the university trials and
the field observations. The FREQ procedure of
SAS [16] was used to produce frequency tables
for each of the 4 combinations of observers (i.e.,
3PT1-K1, 3PT1-K2, 3PT2-K1, and 3PT2-K2).
The output data sets were combined and averaged to produce a composite frequency for each
3PT-Kestin score combination. After rounding
the frequencies to the nearest integer, the FREQ
procedure was used again to produce the Spearman rank correlation between the ratings of the
2 scoring systems. Kappa values could not be
calculated, because the 2 scoring systems had
different numbers of categories. Spearman partial correlations adjusting for the effect of sex
were calculated in addition to the simple Spearman correlations but are not reported, because
the sex effect was found to be negligible.
RESULTS AND DISCUSSION
University Trials
Even though growth rates were high in the
university trials, averaging 57 and 63 g/bird per
day for Ross 708 and 63 and 67 g/bird per day
for Cobb 500 birds in trials 1 and 2, respectively, relatively few broilers evidenced significant
walking disability. As a result, almost 90% of
the birds were assigned a score of 0 in the 3PT
system, and, correspondingly, fewer than 5% of
the birds were given Kestin scores of 3 or greater (Table 2). For gait rescoring, the deliberate
oversampling of birds with gait abnormalities
increased the percentages of birds in the greater
score categories somewhat (Table 2), but these
categories were still underrepresented for a bal-
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534
Table 2. Percentage of birds (mean of observer averages; range in parentheses) in each gait score category for the
3-point and Kestin gait-scoring systems in each test1
Score
Study
Test
Scoring
system
University trials
First gait score
3-point
Kestin
Gait rescore2
Latency-to-lie
3-point
Kestin
3-point
Kestin
Field observations
First gait score
3-point
Kestin
0
89.2
(0.6)
73.7
(5.0)
73.9
44.4
299
(28)
341
(38)
63.2
(1.2)
37.0
(0.7)
1
2
9.5
1.2
(1.0)
(0.5)
19.6
3.4
(5.7)
(1.5)
19.4
6.7
31.7
8.3
176
43
(52)
(43)
221
182
(35)
(90)
23.5
13.3
(2.2)
(0.9)
21.0
11.6
(3.6)
(2.2)
3
4
5
—
—
—
0.3
(0)
—
2.2
—
0.7
(0)
—
3.9
—
0
(0)
—
0
(0)
—
9.7
(1.1)
11.5
(1.1)
2.4
(0.7)
—
9.4
—
177
(80)
—
9.1
(5.0)
1
Latency-to-lie is in seconds to indicate average latencies (SE in parentheses).
No measure of variation is indicated, because selection of broilers for rescoring was dependent on agreement of gait scores
between observers. The greater-numbered gait score categories were oversampled to maximize their representation in the
rescore test.
2
anced comparison of the 2 gait-scoring systems
across all levels of walking ability. The numbers
of broilers available in the commercial flocks
allowed oversampling to achieve better balance
of birds across the gait score categories (Table
2), but even so, the relative rarity of birds with
serious walking disabilities in the commercial
flocks made it not possible to equalize numbers
in each category without overly disturbing the
flocks.
Kappa values indicated substantial betweenobserver agreement within both the 3PT and
Kestin gait-scoring systems in the university trials but somewhat more so for the Kestin system
(Table 3). Similarly, Spearman rank correlations
indicated highly significant positive associations between observers in each scoring system,
but the association was stronger between the observers using the Kestin system. The deficient
numbers of birds in the greater score categories
and the possible effect of specific pairs of observers mitigate against giving too much importance at this point to the apparent difference in
between-observer agreement between the 2 gaitscoring systems.
Within-observer agreement based on rescoring the gaits of selected birds is a measure of
how well individual observers agree with their
original gait assessment. Because some time
elapsed between the first and second gait-scoring events, the walking performance of the birds
itself may have changed, affecting assessment of
within-observer reliability. Nonetheless, weighted κ values indicate substantial within-observer
agreement, and the strength of agreement was
virtually the same for both gait-scoring systems.
Likewise, Spearman rank correlations provide
evidence of highly significant, positive associations between first and second gait scores, which
were equivalent for the 2 scoring systems (Table
3). Within-observer agreement for the Kestin
system was close to that reported originally by
Kestin et al. [8], even though the second test
took place the following day as opposed to later
the same day as in Kestin et al. [8]. Garner et
al. [9] reported much stronger within-observer
reliability with the Kestin system using a shorter
test-retest interval and with birds being scored
from video clips recorded under standardized
circumstances.
The results for the LTL tests in the university trials are presented in Table 2. Birds having no gait abnormalities had the longest LTL,
as has been shown in other studies [12, 13]. The
variability of the measure appeared to increase
among birds in the greater gait score categories
except for Kestin categories 4 and 5, in which
the few birds involved sat immediately. Al-
Webster et al.: THREE-POINT GAIT-SCORING SYSTEM
535
Table 3. Between-observer and within-observer agreement and association between latency-to-lie and gait scores
for the 3-point and Kestin gait-scoring systems1
Study
Measure
Agreement
3-point system
Kestin system
University trials
Between-observer agreement2
Weighted κ
0.63
(0.19)
0.62
(P < 0.0001)
0.67
(0.22)
0.68
(P < 0.0001)
−0.41
(P < 0.0001)
−0.14
(P > 0.05)
0.78
(0.06)
0.84
(P < 0.0001)
0.72
(0.11)
0.71
(P < 0.0001)
0.68
(0.18)
0.69
(P < 0.0001)
−0.43
(P < 0.0001)
−0.18
(P > 0.05)
0.65
(0.05)
0.74
(P < 0.0001)
Spearman correlation
Within-observer agreement
Weighted κ
Spearman correlation
Latency-to-lie
Spearman correlation
Spearman correlation (partial)3
Field observations
Between-observer agreement
Weighted κ
Spearman correlation
In parentheses: 95% confidence interval for weighted κ values and probability level for statistical significance for Spearman
correlations.
2
Between-observer and within-observer agreement based, respectively, on comparison between observers of first gait scores
for all broilers observed and between first and second gait score within observers for rescored broilers.
3
Partial correlation adjusting for the association of the alternate gait-scoring system with latency-to-lie.
1
though the average LTL for broilers scored as
2 in the 3PT system was not zero, the SE was
large enough to overlap with zero. The numbers of birds scored in the Kestin system as 4
or 5 or the 3PT system as 2 were small, being
5 and 6 birds, respectively, and a small difference in assignment of birds to gait score categories could produce a large apparent difference
in average LTL. Spearman rank correlations of
LTL with the 3PT and Kestin gait scores were
highly significant but did not indicate a strong
association of LTL with either gait-scoring system (Table 3). Spearman partial correlation of
LTL with 3PT gait score after adjusting for LTL
association with Kestin gait score was low and
not statistically significant (Table 3). Likewise,
the comparable Spearman partial correlation of
LTL with Kestin gait score was also low and
not significant. These results indicate that the 2
gait-scoring systems had overlapping associations with LTL so that when the association of
1 with LTL was accounted for, LTL had little remaining association with the other gait-scoring
system. Thus, it appears that the 3PT and Kestin gait-scoring systems were similar measures,
although not very precise measures, of the underlying factor affecting willingness or ability of
the bird to remain standing. The Spearman rank
correlation directly comparing the 3PT and Kestin scores from the university trials was highly
significant but not particularly strong (r = 0.53,
P < 0.0001).
Field Observations
The field observations allowed a better assessment of between-observer agreement within
systems and direct agreement between the 2
gait-scoring systems, because it was possible to
sample from large numbers of birds on multiple
farms and to take the samples in the same environment and with the methods used in commercial animal welfare audits. Figures 1 and 2
depict the between-observer agreement for the
3PT and Kestin systems, respectively. Weighted
κ values indicated substantial between-observer
agreement within the 3PT system. Betweenobserver agreement was also substantial for the
Kestin system but at a lower level (Table 3).
Similarly, Spearman rank correlations between
scores for observers within gait-scoring systems
were high but more so for the 3PT system. The
challenges of observing broilers in commercial
environments may have made it easier to apply
the 3PT system. Between-observer agreement
for the Kestin system as indicated by the Spear-
JAPR: Field Report
536
Figure 1. Between-observer agreement: 3-Point gait-scoring system—field observations.
man correlation (r = 0.74) was lower than that
reported by Garner et al. [9] for observations
made under more controlled circumstances (r =
0.89).
Figure 3 shows the association between the
3PT and Kestin systems in the field observations. The Spearman rank correlation comparing
the 3PT and Kestin gait-scoring systems indicates a good association between the 2 (r = 0.76,
P < 0.0001).
General Discussion
Although the relative infrequency of gait
abnormalities found in this study limited our
ability to evaluate the gait-scoring systems with
equal precision across the full spectrum of score
categories, the results allow some general conclusions. Overall, the 3PT and Kestin gait-scoring systems showed good observer reliability,
both having substantial between-observer and
within-observer agreement. Good observer reliability is essential for an auditing tool. For the
more comprehensive data set derived from the
field observations, the distribution of 3PT scores
across the Kestin scores was not perfectly balanced in that birds scored as Kestin 2 were most
frequently given a 3PT score of 0, and broilers
scored as Kestin 4 were most frequently scored
3PT 1. Some of this imbalance may be due to
Figure 2. Between-observer agreement: Kestin gait-scoring system—field observations.
Webster et al.: THREE-POINT GAIT-SCORING SYSTEM
537
Figure 3. Scoring agreement between the 3-Point and Kestin systems—field observations.
individual observer differences in interpretation of severity of gait abnormality, such as was
seen between observers using the Kestin system
in regard to birds scored as 1, 2, or 3. It should
be noted that the 3PT-Kestin between-system
agreement was as strong as the between-observer agreement within the Kestin system. The criterion in the 3PT system that a broiler be able to
walk 5 ft might influence an observer to interpret walking ability differently from an observer
using the Kestin system for birds with the more
severe walking abnormalities. Because many
broilers with serious leg problems can travel 5
ft if sufficiently motivated, it is important when
using the 3PT gait-scoring system not to drive
the birds. Modest efforts to get the birds to walk,
such as described earlier in this report, are sufficient. If a bird has an abnormal gait and must be
prompted several times to travel 5 ft, it should
be scored as 2 using the 3PT system even if it
ultimately goes the distance. This may require
some judgment, because it is not unusual to find
broilers without obvious gait defects that are reluctant to walk and sit down repeatedly before
traveling 5 ft.
As a tool for assessing bird well-being, a
gait-scoring system should separate birds that
are likely to experience discomfort or pain from
those that are not. Broilers scored as Kestin 3
have been found to show evidence of discomfort or pain [4, 5], and this score has come to be
viewed as the threshold at which animal welfare
becomes an issue in terms of leg problems. In
the present study on commercial farms, broilers given Kestin 3 scores were preponderantly
scored as 3PT 1, with little imbalance of scores
between the 2 systems. As such, the 3PT system
enabled identification of lame birds with similar sensitivity to the Kestin system, and the 3PT
1 category provided the needed separation of
sound birds from those with walking problems.
With the 3PT system, flock leg health would
be reflected by the percentage of broilers with
a score of 0. The remaining percentage would
represent birds having diminished welfare to
some degree. If this were the only objective of a
simplified gait-scoring system, a 2-point system
might be preferable to one with 3 categories.
However, we believe that a 3PT system better
serves the objectives of animal welfare audits
and other assessments of the quality of flock
management for the following reason. Broilers
classified as 2 are crippled. Because these birds
should be killed when found, a rigorous culling
program would keep the percentage of broilers
in this category low. If all is well, birds in the
2 category constitute an insignificant proportion
of the flock; however, retention of the 2 category
(and the 3PT system) would allow an auditor to
document the result of an inadequate culling
program or some other factor seriously affecting
leg health.
There was little imbalance of scores between
observers using the 3PT system on commercial
farms. The stronger between-observer agreement for the 3PT system may indicate that the
JAPR: Field Report
538
simplicity of the 3PT system promotes consistency of scoring in commercial environments
where there are limitations in the control of light
and bird behavior.
Good training is essential for the proper application of a gait-scoring system, including a
relatively simple one such as the 3PT system.
Butterworth et al. [17] demonstrated that the
background or culture of an assessor affects
gait-scoring accuracy and that proper training
improves scoring accuracy. As animal welfare
auditing becomes established in the commercial
poultry industry, flock assessments will be done
in many cases by professional auditors and others who may not come from backgrounds giving
them understanding of poultry welfare. The body
type of a commercial broiler produces a stride
that may appear ungainly compared with chickens that have not been bred for meat production.
Interindividual variation of body conformation
in a flock, particularly when sexes are mixed,
leads to a range of walking styles even among
birds that have good leg health. These broilers
must be distinguished from those with impaired
welfare caused by a leg or spinal abnormality
that affects ability to walk. The welfare impairment may be due to pain or decreased ability to
access resources. The critical distinction to be
made using the 3PT gait-scoring system, and
others, is whether or not a gait is sufficiently abnormal to negatively affect function (i.e., score
0 vs. score 1 in the 3PT system). Gait-scoring
training must ensure that auditors are able to
make this distinction accurately and with good
interauditor reliability so that audits done using
the 3PT system at diverse locations and times
have comparable meaning. This should include
teaching the various types of walking abnormalities that typically occur and how to conduct
a gait-scoring assessment. A standard training
program would help achieve these objectives. To
this end, gait-scoring educational material based
on the 3PT system is being developed under the
auspices of the American Association of Avian
Pathologists for use in training prospective poultry welfare auditors and others [18]. Although
video clips are very helpful to illustrate different
categories of walking ability, there is no substitute for live bird observations to learn how to
gait score. Training should include sessions in
flocks of live birds led by an experienced trainer,
in which trainees conduct gait-scoring assessments as would be done on commercial farms
until they reach a required standard of proficiency. It is important that at least 2 individuals
conduct a farm gait-scoring audit, one to handle
the birds and the other to record gait scores. The
observer must view the broilers from the side,
because, as noted by Garner et al. [9], it can be
difficult to judge gaits from above, as would be
done if the handler was also the observer.
CONCLUSIONS AND APPLICATIONS
1. The 3PT and Kestin (6-point) gait-scoring systems had good intersystem agreement when used on commercial farms.
2. Both the 3PT and Kestin gait-scoring
systems were found to have substantial
observer reliability, but between-observer agreement was somewhat greater for
the 3PT system on commercial farms.
3. The simplicity of the 3PT system may
promote observer reliability for gait
scoring commercial poultry flocks.
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Acknowledgments
This research was supported by a grant from the US Poultry & Egg Association (Tucker, GA).