©2012 Poultry Science Association, Inc.
A mobile modified-atmosphere killing system
for small-flock depopulation
A. B. Webster*1 and S. R. Collett†
*Department of Poultry Science, College of Agricultural and Environmental Science,
and †Department of Population Health, College of Veterinary Medicine,
The University of Georgia, Athens 30602
Primary Audience: Emergency Poultry Disease Containment Personnel, Veterinarians,
Equipment Manufacturers
SUMMARY
A modified-atmosphere killing system mounted on a trailer was tested for the purpose of
humanely depopulating backyard or other small poultry flocks in emergency situations. It was
evaluated using CO2 and N2 to kill spent layer breeders, spent laying hens, turkey broiler hens,
and broiler chickens of several ages. The system performed well with either gas when the atmosphere in the killing chamber was controlled automatically by sensors, or when it was manually guided by the operator’s observation of bird behavior. With CO2, birds could be loaded
continuously into a killing atmosphere because it was possible to maintain sufficiently high
concentrations of the gas during the process. As a result, the conscious experience of each bird
in the chamber was short. Automated control of the gas delivery system was helpful to hold
CO2 at a preset level, ensuring a quick kill of birds while minimizing CO2 consumption. It also
eliminated the need for an operator skilled at interpreting bird behavior to judge the quality of
the atmosphere in the chamber. With N2, birds must be killed in batches because the very low
levels of residual O2 necessary to cause death cannot be maintained during loading. As such,
automation of gas delivery is unnecessary when using N2 and the gas valve can be opened
manually after a batch of birds is loaded to allow continuous gas injection until the birds expire.
The chamber was able to hold more than 1,200 lb (544 kg) of carcasses, which numbered from
79 turkeys weighing 15.6 lb (7.1 kg) to almost 600 broiler chickens weighing 3 lb (1.4 kg) in
the tests of this study. This capacity is more than enough to hold most backyard flocks in 1 load,
and the fully loaded trailer can be towed without difficulty by a half-ton pickup truck.
Key words: backyard flock, carbon dioxide, depopulation, gassing, nitrogen, poultry
2012 J. Appl. Poult. Res. 21:131–144
http://dx.doi.org/10.3382/japr.2011-00375
DESCRIPTION OF PROBLEM
In the last decade, considerable effort has
been devoted to the development of effective,
humane systems for depopulating poultry flocks
to control outbreaks of highly pathogenic, po1
Corresponding author: [email protected]
tentially zoonotic avian disease. The primary
focus has been on how to kill thousands of birds
in large commercial flocks in a short time while
protecting workers from exposure to potential
pathogens. Various means have been tried, for
instance, by constructing sealable enclosures
132
into which birds can be driven and gassed [1];
by covering birds with fire-fighting foam [2]; by
placing transport modules of birds into specially
designed containers and injecting gas [3]; or by
whole-house gassing [4].
It has been reported that small mobile types
of gas-killing units have also been tried for depopulation of commercial and noncommercial
flocks to control avian influenza, but these units
were judged to be inadequate for the task of
killing large numbers of birds [5]. Nonetheless,
there is a need to have a viable mechanism of
depopulating small flocks of poultry in the event
of a disease outbreak. These flocks range from a
few birds in a backyard setting to a few hundred
birds kept as a small commercial venture, and
are scattered geographically. Backyard flocks
seldom are provided good biosecurity and may
be exposed to pathogens through a variety of
vectors, such as human traffic, pets, wild birds,
rodents, and insects. They typically are not given close veterinary attention and have the potential to become reservoirs of disease from which
pathogens can be transmitted to other backyard
or commercial flocks. Any attempt to eradicate
an infectious poultry disease by flock depopulation could be nullified if the depopulation
effort did not include backyard flocks. Given
the uncontrolled circumstances associated with
backyard flocks, rather than attempting to dispose of potentially contaminated carcasses on
site, as might be done with a commercial flock,
it would be better to deliver carcasses to a designated disposal location designed for disease
containment. Thus, the small-flock depopulation method should provide for safe transportation of carcasses off site.
The most critical small flocks to depopulate
in a disease outbreak would be those located near
large commercial poultry facilities. The USDA
National Animal Health Monitoring System
surveyed backyard and small production flocks
located in a 1-mile (1.7-km) radius around 349
large commercial poultry operations in 18 major
poultry states [6]. Fifty-five percent of these operations had 1 to 5 backyard flocks within that
radius. Six percent had 6 to 19 flocks within the
same distance. Sixty-two percent of the birds in
these flocks were chickens, but the remainder
constituted a variety of species, including birds
as large as turkeys and geese. Eighty-one per-
JAPR: Field Report
cent of the flocks were mixed species. Ninetytwo percent of the flocks had fewer than 100
birds, and 59% had fewer than 20.
We set out to develop a mobile modified-atmosphere killing (MAK) small-flock depopulation system, building on knowledge gained from
an MAK cart originated at the University of
Georgia for removal of spent commercial laying
hen flocks [7, 8]. The MAK cart is now commercially marketed and has been endorsed by
the United Egg Producers in the United States
for humane on-farm depopulation of spent laying hens as part of routine flock replacement [9].
On the basis of the numbers of birds in typical
backyard flocks, the killing chamber should be
able to hold more than 100 chickens to allow
depopulation of most small flocks by 1 unit in
a single visit. In addition, the unit should be
easy to operate, provide a humane kill, accommodate birds up to the size of mature turkeys
or geese, be easy to clean, be designed to close
up for transportation, and allow for easy carcass
removal at the disposal location.
A variety of configurations are possible for
how the MAK chamber might be mounted and
transported and for how the gas injection system might be controlled. Because we needed
sensors to monitor gas concentrations inside the
chamber during tests, we took the opportunity
to evaluate sensor-controlled automation of the
gas delivery system to maintain the interior atmosphere. We chose to mount the MAK chamber on a trailer with a built-in hydraulic lift so
that the unit could be moved to test locations
and unloaded after tests without the need for additional equipment other than a towing vehicle.
The MAK small-flock depopulation system was
built at the University of Georgia Instrument
Shop. The results of tests with several types of
poultry are reported below. The work done was
in accordance with an animal use protocol approved by the University of Georgia Institutional Animal Care and Use Committee.
MATERIALS AND METHODS
The MAK chamber and controls were built
on a 12-ft (3.66-m) tandem-wheeled trailer.
The chamber, constructed of 16-gauge stainless
steel, was placed at the back of the trailer, with
a slight overhang to allow the unloading door
Webster and Collett: SMALL-FLOCK DEPOPULATION
to swing out freely when the chamber is tipped
back to dump a load of carcasses. Figures 1 and
2 show side-view and rear-view schematics of
the MAK unit. Separate gas delivery systems
were installed so that CO2 or N2 could be injected into the chamber from cylinders mounted
at the front of the trailer. Gas sensors for CO2
[10] or O2 [11], electronically connected, respectively, to the CO2 or N2 delivery system,
sampled the atmosphere inside the chamber on
a continuous basis during operation. Each sensor controlled a solenoid valve [12], triggering it
to release gas to modify the internal atmosphere
according to a preset target gas concentration. A
toggle switch determined which automated gas
delivery system was operated. Bypass valves on
each gas system made it possible to deliver gas
manually. Gas was delivered from commercial
gas cylinders through pressure regulators capable of releasing gas at 90 psi. A small in-line
block heater [13] was mounted on the CO2 cylinder ahead of the pressure regulator to reduce
chilling of the regulator and gas line when CO2
133
was released. After the valves, the 2 gas systems
joined to a common gas line that ran around the
interior of the chamber at the top of the vertical
walls of the lower portion of the structure and
fed gas injection nozzles on each side and at the
corners (i.e., 8 nozzles total). A 30-gal (113.6
L) tank was mounted on the trailer ahead of the
chamber, with a pump, hose, and spraying wand
to allow the trailer and accompanying vehicle
to be sprayed down with disinfectant solution.
The MAK chamber had a hydraulic lift to tip
it backward, making it easy to dump carcasses
through a free-swinging unloading door (Figure
3). This door was latched closed during operation, except when unloading the chamber. A portable gasoline generator powered the electrical
systems on the trailer [14].
The floor inside the kill chamber was 57 in.
(145 cm) wide × 58.5 in. (149 cm) long. The
interior was divided functionally at the height of
the gas nozzles (17.5 in. or 44.5 cm) into a lower
section to hold carcasses and an upper section
housing the loading doors. The spring-loaded
Figure 1. Side-view schematic of the modified-atmosphere killing (MAK) unit. Dimensions are in inches [centimeters]. The killing chamber is at the back of the trailer. The gas cylinder at the front of the trailer is fixed to an angleiron stand. The tank for holding disinfectant solution is posterior to the control stand. The portable electric generator
would be mounted in the space just anterior to the chamber when the system is in use. The loading doors, which
also serve as windows, can be seen in the upper section of the MAK chamber.
134
Figure 2. Rear-view schematic of the modified-atmosphere killing (MAK) unit. Dimensions are in inches
[centimeters]. The loading doors (out of view) are
mounted in the angled portion of the upper section of
the MAK chamber.
doors were made of clear Lexan and served in
addition as observation ports. The lower section had a volume of 34 ft3 (0.96 m3). The total
interior volume was 75 ft3 (2.13 m3). A small
airlock near the top of the chamber at the rear
allowed air to escape from the chamber as gas
was injected so that the interior did not become
pressurized.
It was not feasible to test the MAK system
with backyard poultry flocks, so we obtained
access to flocks being removed from commercial poultry farms or from university research
flocks at the end of experiments. This also gave
us the opportunity to test the limits of the system
in terms of loading rate, capacity, and durability. The following report discusses the results of
6 trials. The trials involved different types and
ages of poultry and were, in order, floor-housed
spent commercial layer breeders, cage-housed
spent commercial laying hens, finished turkey
broiler hens, and 37-, 29-, and 51-d-old broiler
chickens. Different numbers of personnel were
available to catch and load in each trial, but the
number was never less than 2 individuals. Carbon dioxide was tested in all trials. Nitrogen was
JAPR: Field Report
tested with the 51-d-old broilers. A load was
considered full when carcasses reached the level
of the gas injection nozzles. As this level was
approached, the birds that had just been loaded
would begin to interfere with the doors, making it necessary to move them aside to allow the
doors to close.
Carbon dioxide consumption during tests
was determined by weighing cylinders before
and after. The volume of N2 used was calculated
from standard reference tables [15] based on the
pressure decrease in the cylinders during tests.
Because CO2 can induce rapid unconsciousness in poultry at concentrations of 40 to 50% in
air [16, 17], it would be possible when using this
gas to load birds continuously until a chamber
is filled, provided the gas delivery system can
maintain these concentrations throughout the
process and the loading rate is suitably matched
to the floor area such that birds would not be
overlain by others before becoming unconscious. Continuous loading would be the fastest
process for killing poultry; therefore, all trials
with CO2 used continuous loading to evaluate
the performance of the MAK system. The target
CO2 concentration was set at 50% for trials with
chickens. The target level was 40% for turkeys
to determine if this lower level would reduce
gas consumption while still being effective for
killing poultry. When a load was completed, the
chamber was left undisturbed for 5 min after
convulsive wing flapping had ceased to ensure
that all birds were dead before dumping. Convulsive wing flapping is an involuntary action
resulting from hypoxia that occurs primarily or
exclusively after unconsciousness occurs [18].
When the interior atmosphere was controlled as
described above, no surviving birds were found
after a load was dumped.
Nitrogen, or another inert gas such as Ar,
must dilute atmospheric O2 to a very low concentration (approximately 2%) to kill poultry
[19]. Because air enters the chamber as birds
are loaded, it would be virtually impossible to
maintain such low levels of residual O2 during
loading. For this reason, trials using N2 were
conducted in stages, with birds loaded and killed
a layer at a time to avoid smothering. Although
some degree of awareness may still be present
during the initial convulsions that occur when a
chicken is stunned with N2- or Ar-based anoxia,
Webster and Collett: SMALL-FLOCK DEPOPULATION
135
Figure 3. Modified-atmosphere killing (MAK) chamber tilted using the hydraulic lift to demonstrate how carcasses
are unloaded. The tank for disinfectant solution and portable generator are mounted anterior to the chamber. The
electrical switches for the gas delivery, disinfectant spray, and hydraulic lift systems, and the solenoid valves and
manual gas bypass valves are attached to the metal frame holding the gas cylinders. The gas sensors are mounted
on the back of the chamber above the unloading door. Color version available in the online PDF.
birds are unconscious in the latter stages of the
behavior [20, 21]. To ensure that all birds in the
prior layer were unconscious before being buried under other birds, a 30-s period was allowed
after convulsive wing flapping ceased before
the next layer of birds was loaded. As with CO2,
a 5-min waiting period after cessation of wing
flapping ensured that all birds were dead before
the load was dumped.
The digital displays of the gas sensors were
videorecorded during trials to monitor changes
in CO2 or O2 concentrations in the chamber. On
some occasions, particularly during the earlier
trials when the electrical systems were still being worked out, the gas sensors did not operate
and recording of gas levels in the chamber did
not occur.
A second camera was placed against one of
the loading doors to videorecord the behavior
of birds in the chamber. With CO2, several short
videos were recorded over the duration of the
loading period because the time from entry to
unconsciousness for a bird was much shorter
than the loading period itself. The times to first
manifestation (latencies) of specific behaviors
were recorded. Deep breathing, head shaking,
and loss of posture were as described previously
[22]. Subsiding was the behavior called stillness
described by Webster and Fletcher [22]. Wing
flapping was the convulsive behavior mentioned
above. The significance of these behaviors in regard to the welfare of poultry has been discussed
by Webster and Fletcher [22] and Gerritzen et al.
[23]. Three additional behaviors, namely, neck
back (backward flexion of the neck so the head
approaches or touches the back), step, and fall,
were recorded for turkeys because, with their
long necks and legs and tendency to stand, these
behaviors were more prevalent and distinct than
in chickens. Wing flapping was not recorded for
turkeys.
With N2, a single video was recorded for the
duration of gas injection into the chamber. As
with CO2, the latencies of specific behaviors
JAPR: Field Report
136
Table 1. Performance of the modified-atmosphere killing system during trials using CO2 with poultry of different
sizes and types1
Bird
type
Layer breeder
Caged hen
Turkey hen
Broiler
37 d
29 d
51 d
Loads
Birds/
load
Load time,
min
Birds/
min
Weight/bird,
lb
Weight/bird,
kg
Load
weight, lb
Load
weight, kg
2
1
2
335
440
79
15
52
14
22
9
6
3.8
3.0
15.6
1.72
1.38
7.06
1,257
1,340
1,230
570
608
558
2
4
1
296
595
172
9
20
8
33
31
22
4.5
3.0
8.6
2.04
1.38
3.91
1,327
1,812
1,483
602
822
673
1
Mean values are presented when there were multiple loads.
were recorded. The series of behaviors observed
for individual birds were as follows: mandibulation (movements of the beak as if the bird was
responding to sensations in the mouth), eye closure (apparently associated with reduced alertness), neck relaxation (apparent diminishment
of muscular control of the neck and subsidence
of the head downward), head wagging [abnormal wagging of the head by side-to-side movements of the neck (distinct from head shaking, a
normal behavior pattern involving quick, oscillating movements of the head on the axis of the
neck)], loss of posture, and wing flapping.
From the videos, an attempt was made to
record latencies for a complete sequence of behavior for individual birds from the start (i.e.,
the time the bird was placed into the chamber
with CO2 or from the beginning of N2 injection) until unconsciousness occurred. However,
the activity of other birds in the chamber often
blocked our view of a bird being observed so the
initiation of all behaviors could not be recorded.
Thus, the sample sizes for the different behaviors were not necessarily the same. A data logger [24] was hung inside the MAK chamber to
record temperature and RH during the trials with
turkeys, 29-d-old broilers, and 51-d-old broilers.
RESULTS AND DISCUSSION
MAK System Performance
CO2 . The performance variables for 6 trials using CO2 with different sizes and types of
poultry are presented in Table 1. The number
of birds the MAK chamber could hold varied
widely, from 79 turkeys to roughly 600 broiler
chickens at 29 d of age, which was primarily a
function of their size. The amount of feathering
on the birds may also have affected the number
that could be loaded. Fewer spent caged laying
hens constituted a load compared with 29-d-old
broilers, which had less developed plumage,
even though the birds were approximately the
same weight. In addition, the judgment of a full
load was somewhat subjective, and the decision
concerning when to stop loading no doubt varied between trials.
When measured during the trial with 29-dold broilers, it took an average of 4.2 ± 0.2 min
(mean ± SD) to prefill the chamber with CO2 to
a concentration of 50% before loading. Thereafter, the time required to obtain a full load was
primarily a function of the distance and number
of individuals available to carry poultry from
the catch site to the MAK unit. The times in
Table 1, which range from 14 to 20 min (turkey
hens, layer breeders, 29-d-old broilers), reflect
relatively realistic scenarios for the removal of
small flocks in that the birds had to be carried
some distance from the point of catching by
2 to 3 individuals, with 1 to 2 others catching
and handing them the birds. Turkeys had to be
placed into the MAK chamber individually because of their size. The cage-housed spent laying hens were carried from a high-rise house,
along a crosswalk, and down stairs to the MAK
unit by 3 individuals, who caught the birds
from the cages themselves. The time taken to
have a full load (52 min) represents a difficult
catching scenario with limited personnel. The
fastest times to load the MAK chamber, 8 and
9 min, respectively, for 51- and 37-d-old broilers, occurred when the broilers were cooped and
stacked near the MAK unit before loading. It is
unlikely that these times would be realistic in a
Webster and Collett: SMALL-FLOCK DEPOPULATION
137
Table 2. Performance of the modified-atmosphere killing system using N2 with 51-d-old broilers
Load
Load
1
2
3
4
Mean
SD
Load
1
2
3
4
Mean
SD
Layer 1 + 22
Weight,
lb
Weight,
kg
N2 fill,1
min
N2 used,
ft3
N2 used,
m3
6.6
6.8
7.3
7.0
6.9
0.3
105
111
117
122
114
7.8
3.0
3.1
3.3
3.5
3.2
0.2
3.1
2.8
3.0
3.0
3.0
0.1
276
301
283
216
269
37
5.4
5.5
5.4
5.2
5.4
0.1
92
91
80
72
84
9.7
2.6
2.6
2.3
2.0
2.4
0.3
2.7
2.3
2.7
2.8
2.6
0.2
544
12.3
Layer
Birds
1
1
1
1
64
72
72
72
70
4
542
637
617
624
605
43
246
289
280
283
275
19
2
2
2
2
72
77
72
54
69
10
609
664
624
476
593
81
139
1,198
198
5.6
Final
O2%
NA
1
Time from the start of N2 injection into the chamber until the end of wing flapping after loss of posture + 30 s.
2
Sum of means for load layers 1 and 2.
real-world scenario of small-flock depopulation.
Nonetheless, these trials provided good opportunities to test the limits of the performance of the
MAK systems.
Average load weights, calculated from bird
weights and numbers of birds per load (except
for caged hens, when the load was weighed on
the company’s feed mill weigh scale) varied
from a low of 1,230 lb (558 kg) for turkeys, the
largest birds, to a high of 1,812 lb (822 kg) for
29-d-old broilers, the smallest birds. Bird size
may influence load weight in that it is easier to
continue loading small birds into the chamber
when the level of carcasses inside nears the lower edge of the loading doors. However, the resolve of the depopulation crew to continue loading can also influence load weight. For instance,
the second heaviest load was obtained with the
second largest birds tested (51-d-old broilers) in
a situation where 1 load was sufficient to depopulate all the birds by means of some extra effort
near the end of the load to keep birds clear of
the sweep of the doors so they could be closed.
No difficulties were noted towing the trailer up
to 60 mph (100 kph) with the chamber unloaded
or loaded.
N2 . Loading performance data when using N2
to kill birds are presented in Table 2. These data
are from the trial using 51-d-old broilers. For
this size of bird, it happened that 2 layers filled
the chamber. It is not clear why the average load
weight turned out to be almost 300 lb less than
that for birds from the same flock killed with
CO2, but conscious, active birds may give the
appearance of filling more space than do birds
quickly subsiding into unconsciousness, as takes
place with CO2, and may have encouraged an
earlier decision to stop loading.
The difference in time required to depopulate
flocks could influence the choice of which gas
to use. With CO2, the process has 3 stages per
load, namely, prefill the chamber to the target
CO2 concentration, load the birds, and wait 5
min after convulsive wing-flapping stops to ensure no survival. With N2, the stages would be
the number of repetitions of the cycle (load a
layer of birds, inject N2 until convulsive wingflapping ceases, and wait 30 s to ensure unconsciousness), followed by the last layer of birds,
which would have a 5-min waiting period after
wing-flapping ceased to ensure no survival. Assuming the times to load birds and to ensure
no survival are the same regardless of the gas
used, the extra time required to use N2 would
be as follows: number × (N2 injection time + 30
s) + final N2 injection time − CO2 prefill time.
In this trial, the birds were large enough to fill
the MAK chamber in 2 layers, so the calculated
JAPR: Field Report
138
increase in time needed to depopulate a load of
birds of this size (i.e., average of 8.6 lb or 3.91
kg) using N2 was 6.9 min + (5.4 min − 0.5 min)
− 4.2 min = 7.6 min. (Note: 0.5 min was subtracted from the mean for layer 2 in Table 2 to
deduct the 30 s included in that value.) Trials
with birds small enough to require more than 2
layers to fill the chamber could be expected to
add an additional 5 to 6 min per layer. As seen
in Table 2, each successive layer would require
less time for gas injection because the headspace
of the chamber would be reduced by the volume
of birds already loaded, provided the rate of gas
delivery remained unchanged.
Gas Consumption
CO2 . Because the MAK system injected CO2
into the chamber to maintain a preset concentration while birds were loaded, the amount of gas
required per load was influenced by the volume
of air brought into the chamber with them, which
would be influenced by the number of times the
doors were opened to deposit birds. The amount
of CO2 used was variable among the trials in
which gas consumption was measured (Table
3), being lowest for turkeys (79 door openings/
load) and highest for 29-d-old broilers (approximately 200 door openings/load). The number of
door openings per load for the 51-d-old broilers was similar to that for turkeys, but the target
CO2 concentration was less for turkeys, so it was
to be expected that CO2 consumption would be
lower.
The quantity of CO2 consumed per load was
well within the capacity of the gas cylinders (50
lb or 22.7 kg of liquid CO2) used in the tests.
During times of sustained CO2 release from the
cylinders, condensation and frost would develop
on the regulator and adjacent gas lines, but the
heaters mounted before the regulators were suf-
ficient to keep the lines from becoming blocked.
Under most circumstances, 1 cylinder would
supply enough gas for at least 2 loads of birds.
However, during trials with high loading rates,
particularly during cold weather, the cylinders
themselves would chill until there was too little
gas pressure to continue dispensing CO2 at a rate
sufficient to hold the chamber concentration at
50%, even though a considerable amount of liquid CO2 remained in the cylinders. These had
to be replaced with fresh cylinders and set aside
to warm up before they could be brought back
into use, which took a considerable time. If it is
anticipated that 2 or more loads in quick succession would be required in a depopulation event,
replacement cylinders should be kept on hand.
On one occasion, which was not one of the trials
in this project, the cylinder and gas lines became
covered with ice when the MAK system was
used to depopulate a flock in the rain and near
freezing temperature.
N2 . The volumes of N2 used to kill 4 loads of
51-d-old broilers are presented in Table 2. The
amount of N2 required was less for the second
layer than for the first, as was reflected above in
the times to inject the gas into the chamber. All
told, approximately 200 ft3 (5.7 m3) of N2 was
needed to kill a load of broilers. The cylinders
used in this study had sufficient capacity (300
ft3 or 8.5 m3) to supply single loads, but replacement cylinders were required for additional
loads. Because gas injection was required only
after each layer of birds had been loaded, there
was no need for an automated system to control
gas injection, although it was used in these trials. A system with a manual valve would work
just as well. Despite ambient temperatures in
the low to mid 30°F range (0 to 2°C) during the
trial, there were no problems with the gas lines
or cylinders becoming excessively chilled during use.
Table 3. Carbon dioxide used during trials of the modified-atmosphere killing system
Bird type
Turkey hen
Broiler
29 d
51 d
1
CO2/tonne,1
kg
Total
weight, lb
Total
weight, kg
CO2/load,
lb
2152
3,347
1,518
10.8
4.9
17.4
8.7
2,381
2522
7,248
2,212
3,287
1,003
26.9
17.9
12.2
8.1
29.6
23.7
14.8
11.9
CO2 used per ton or tonne of BW.
Includes a partial load not recorded in Table 1.
2
CO2/load,
kg
CO2/ton,1
lb
Birds
killed
Webster and Collett: SMALL-FLOCK DEPOPULATION
139
Figure 4. Change of temperature, RH, and CO2 concentration in the interior of the modified-atmosphere killing
chamber during loading of the chamber with 29-d-old broilers (load 2).
Atmospheric Dynamics in the MAK Chamber
The injection of nonhumidified gas and the
release of humidity from the birds would have
interacting effects on interior RH. Ambient temperature and body heat would determine the
interior temperature. The ability of the gas delivery system to achieve or maintain the target
atmosphere inside the chamber would determine
the length of a bird’s experience of these things
in addition to its experience of the effects of the
gas itself before losing consciousness.
CO2 . Figure 4 shows an example of the variation in CO2 concentration, RH, and temperature
during a loading of the MAK chamber. As indicated above, slightly more than 4 min was needed to reach a 50% concentration of CO2. Thereafter, the automatic system did well at keeping
the interior atmosphere near the target level. In
the example shown, it can be seen that the CO2
concentration declined slowly from 50% during
loading to approximately 45% at the end and
then returned quickly to the target level when
loading stopped. In this example, the loading
rate of 583 birds in 17 min evidently strained
the capacity of the gas delivery system. All the
trials with broilers sought to test the ability of
the MAK system to maintain the target CO2 concentration at high loading rates, and it was not
unusual to observe a slight decline toward the
end of a loading episode. The performance of
the MAK system in these trials, as evidenced by
the time to loss of consciousness, is discussed
below. These loading rates would be unrealistically high in most real-world depopulation
situations. In trials with lower loading rates, the
MAK system maintained the target interior atmosphere without difficulty.
Interior temperature varied in the mid to high
70°F range (mid 30°C range) in the example
shown. For the 7 loads in which temperature was
measured when CO2 was used, the average temperature change during loading was 4.6 ± 2.9°F
(2.6 ± 1.6°C). If the ambient temperature was
less than the upper 70°F range (approximately
26°C), the interior temperature increased during loading, but did not necessarily do so if the
ambient temperature was at or above this level.
In all the tests, the interior temperatures were
within the range of 62 to 82°F (17 to 28°C).
In Figure 4, RH began at 90%, which is typical of a late August day in Georgia in the morning. Injection of CO2 lowered the RH to 57%
just after the beginning of loading, and then
it increased to 70% before declining back to
the mid-50% range at the end of loading. This
pattern was typical of trials with CO2, that is,
a decline in RH during chamber prefilling, an
RH increase as birds were loaded, and an RH
decline as the chamber approached a full load.
The reason for the RH decline near the end of
loading is unclear, but may have had to do with
the decreased volume of the interior atmosphere
at that time and the amount of CO2 that had to
JAPR: Field Report
140
be injected to maintain a 50% concentration in
relation to the volume of air brought in with the
birds. It is unlikely that RH had an effect on bird
welfare in the time before they became unconscious. Although the interior RH varied between
trials, particularly those conducted on different
dates, it was almost exclusively in a range somewhere between 40 and 90% during the loading
and killing of birds. On one occasion, the RH
declined below 30% during the last minute of
loading broilers.
N2 . Figure 5 shows the changes in O2, temperature, and RH for 1 load of broilers when
using N2. Residual O2 in the chamber declined
steeply when the gas delivery system was turned
on, reaching levels of 3% or less in an average of
6.9 and 5.4 min for layers 1 and 2, respectively
(Table 2).
The starting chamber temperature for a load
reflected the ambient temperature on the day of
the study (Figure 5), but the interior temperature increased to 60°F (16°C) by the end of gas
injection on the second layer. The highest interior temperature (77°F or 25°C) was measured
for the fourth load during a warmer part of the
day. Injection of N2 was not associated with a
change in interior temperature. Because the
chamber temperature is influenced by ambient
conditions, it would be best to position the MAK
chamber in a shaded location on hot days.
In the example shown in Figure 5, RH was
low at the beginning, reflecting ambient conditions, and increased to as high as 95% in the
chamber before trending down to the mid-70%
range. It was typical for the final RH to be somewhat lower than the peak, which occurred in the
first layer. Figure 5 shows the most exaggerated
RH variation of all the tests with N2. In all tests,
the minimum postpeak RH was above 57%.
Injection of N2 into the MAK chamber did not
appear to be greatly associated with changes in
RH.
Behavior
CO2 . The progression of behavior of chickens and turkeys in the MAK chamber, depicted
in Table 4 and Figure 6, respectively, was characteristic of poultry in CO2-enriched atmospheres [22, 23]. Loss of posture (i.e., loss of
muscular control and physical collapse), which
is closely associated with the loss of consciousness [25], occurred in approximately 20 s in the
trials with chickens. This was evidence that effective stunning and killing atmospheres were
achieved by both automated control (laying
hens, 29-d-old broilers, 51-d-old broilers) and
manual control (layer breeders, 37-d-old broilers) of the gas delivery system. The target CO2
concentration for turkeys had been set at 40%,
Figure 5. Change of temperature, RH, and O2 concentration in the interior of the modified-atmosphere killing chamber during loading of the chamber with 51-d-old broilers and injection of N2 (load 1, both layers of birds). The gas
profiles began slightly after N2 injection was started because of tardy initiation of videorecording.
Webster and Collett: SMALL-FLOCK DEPOPULATION
141
Figure 6. Behavior of turkey hen broilers exposed to CO2 in the chamber of the modified-atmosphere killing unit.
Average latency to the first appearance of each behavior ± SD (number of birds indicated above the bar for each
behavior). Target CO2 was 40%. LOP = loss of posture.
so it was to be expected that the time to loss of
posture (average 28 s) would be a little longer
than was observed with chickens. Nonetheless,
even with this lower target level for CO2, the
conscious experience of the turkeys within the
MAK chamber was only approximately 0.5 min,
and there was no survival.
N2 . Unlike with CO2, in which a bird was immersed in the modified atmosphere upon being
placed into the chamber of the MAK system,
with N2, a bird placed into the chamber had to
wait until the layer was complete and then experience increasing hypoxia during injection of N2
until it became unconscious. Judging from the
loading rates in this study, the waiting time for
the first birds loaded before the start of gas injec-
tion would be approximately 10 min or longer.
Once N2 injection began, loss of posture (approximate time of unconsciousness) occurred in
approximately 250 s (Figure 7). Even for a bird
put into the chamber immediately before injection of N2, its conscious experience was roughly
12 times longer than the time to loss of posture
for a bird in a 50% CO2 atmosphere. Although
the time required to kill poultry using N2 might
not be considered appreciably longer from the
standpoint of the overall job, the experience of
the individual bird was much more extended.
Nitrogen was the inert gas used in this study.
Argon, another inert gas, could similarly be used
because it would produce hypoxia by displacement of air in the same way as N2.
Table 4. Behavior of chickens placed into CO2-enriched atmospheres (target CO2 concentration = 50%) during
trials with the modified-atmosphere killing system1
Behavior
Deep breathe
Head shake
Subside
Loss of posture
Wing flap
1
Layer
breeder2
Laying
hen
Broiler,2
37 d
Broiler,
29 d
Broiler,
51 d
2.5 ± 0.5 (8)
4.0 ± 0.0 (2)
11.4 ± 1.6 (7)
18.5 ± 1.9 (4)
23.3 ± 3.5 (3)
2.7 ± 1.3 (9)
7.8 ± 3.7 (6)
11.0 ± 3.3 (7)
19.6 ± 5.3 (8)
28.4 ± 9.1 (7)
2.2 ± 0.8 (46)
3.5 ± 2.9 (31)
11.8 ± 5.1 (12)
22.1 ± 8.4 (17)
23.7 ± 11.6 (16)
2.6 ± 0.9 (60)
4.6 ± 2.4 (30)3
11.7 ± 3.0 (62)
17.9 ± 4.6 (54)
25.3 ± 6.1 (55)
2.0 ± 0.7 (15)
1.8 ± 1.3 (10)
12.1 ± 2.5 (15)
19.1 ± 4.4 (12)
22.3 ± 6.7 (14)
Average latency (s) to begin behavior ± SD. Number of birds observed is given in parentheses.
Manual control of CO2 injection into chamber.
3
An additional 33 birds did not perform head shaking.
2
142
JAPR: Field Report
Figure 7. Behavior of broilers exposed to N2 in the chamber of the modified-atmosphere killing unit. Average latency to the first appearance of each behavior ± SD (number of birds indicated above the bar for each behavior).
Mand = mandibulation; LOP = loss of posture.
Given the brevity of the conscious experience of the bird, CO2 would be preferred for use
in the MAK small-flock depopulation system on
the grounds of animal welfare. Nonetheless, despite the longer time required to kill birds using
N2, the broiler chickens tested did not manifest
agitation or appear to suffer significant distress
during the process. In situations in which CO2 is
not available or when its use might be difficult,
such as at ambient temperatures below freezing,
N2 would be effective and, in the judgment of
the authors, appropriate to use for emergency
flock depopulation.
As mentioned earlier, we constructed the
MAK small-flock depopulation unit as a selfcontained system with automated gas control
so that its use would require minimal poultry
expertise and no other equipment except a vehicle to tow it. This configuration provides a
nice combination of emergency readiness and
animal welfare protection, but is relatively expensive. Emergency response organizations that
must stockpile equipment to deal with projected
depopulation needs may have to trade off equipment quality and function against cost if faced
with limited funds. The small-flock MAK system could be simplified by removing certain
components, provided what was lost by doing
so was judged to be ethically acceptable and did
not cause undue inconvenience. For instance, it
might be possible to build the MAK chamber
using a self-dumping hopper as a base to allow
carcasses to be unloaded without the need for
a hydraulic lift. However, the slanted floor of a
self-dumping hopper would make it a challenge
to use a gas such as N2, which requires batch killing, because the birds would pile up in the lower
angle of the floor during loading. The trailer
could be avoided if the MAK chamber were designed to fit on a pickup truck. A forklift would
be needed to load the chamber on the pickup,
and steps would probably be necessary for a person to reach the loading doors comfortably. The
gas sensors on the MAK system used for automated control of the atmosphere in the chamber
are expensive. Although the O2 sensor allowed
monitoring of residual O2 in the chamber during tests with N2, it would not be necessary on
an MAK depopulation unit in which birds were
killed in batches using an inert gas. On the other
hand, leaving off the CO2 sensor would require
an ethical decision to be made if CO2 were to be
used. To avoid the possibility of causing distress
to birds by irritation of mucous membranes,
Webster and Collett: SMALL-FLOCK DEPOPULATION
it would be desirable to keep CO2 concentrations in the chamber from becoming too high.
The surest way of doing this would be to use
an automated, sensor-based gas control system.
Manual control of CO2 injection without measurement of interior gas concentrations could
not guarantee that the threshold of nociception
of a bird was not exceeded. The reduced cost
and greater simplicity of an MAK depopulation
unit without an automated gas control system,
on the other hand, would make it more accessible to agencies with limited budgets. Although
control of the chamber atmosphere would be
more variable than with an automated gas control system, a trained individual could manually
operate an MAK unit quite well [7], and the risk
of exposing birds to concentrations of CO2 that
might be irritating might not be excessive [8].
Chickens do not appear to find concentrations
of CO2 as high as 60% to be greatly aversive
[26, 27]. The most feasible design for a mobile
MAK small-flock depopulation unit, therefore,
may differ somewhat from the unit tested in this
study, depending on the trade-off of costs, convenience, and ethical considerations as worked
out between the manufacturer and customer.
CONCLUSIONS AND APPLICATIONS
1. The MAK small-flock depopulation
system can effectively and humanely
depopulate small flocks of poultry in
emergency situations. It is designed to
carry and dump carcasses at an off-site
disposal location.
2. One loading of the MAK chamber should
accommodate most small or backyard
poultry flocks that might be encountered. With larger flocks, multiple loads
can be killed in a short time, provided an
alternate means of carcass disposition is
available.
3. In warm weather or with single loads,
CO2 gives the fastest loading time and
quickest kill of individual birds. In cold
weather or with multiple loads, loss of
gas pressure caused by excessive cylinder chilling may be a problem, requiring
replacement of partially emptied cylinders. Ice may build up on cylinders and
143
gas lines when using CO2 in cold, inclement weather.
4. Nitrogen (or Ar) can also be used, and
may be the preferred gas to use in cold
weather because chilling of the cylinders during gas release is not problematic. With large loads, loading time is increased compared with CO2 because the
birds must be killed in batches (layers).
The time required for individual birds to
die is much longer than with CO2 in this
system.
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Acknowledgments
This study was supported by a grant provided by the
USDA, Animal and Plant Health Inspection Service, Animal
Care (Riverdale, MD).