A mobile modified-atmosphere killing system for small-flock depopulation

Transcription

1 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 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 CO 2 and N 2 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 CO 2, 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 CO 2 at a preset level, ensuring a quick kill of birds while minimizing CO 2 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 N 2, birds must be killed in batches because the very low levels of residual O 2 necessary to cause death cannot be maintained during loading. As such, automation of gas delivery is unnecessary when using N 2 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 : 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, potentially 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 1 Corresponding author: bwebster@uga.edu

2 132 JAPR: Field Report 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 percent 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

3 Webster and Collett: SMALL-FLOCK DEPOPULATION 133 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 CO 2 or N 2 could be injected into the chamber from cylinders mounted at the front of the trailer. Gas sensors for CO 2 [10] or O 2 [11], electronically connected, respectively, to the CO 2 or N 2 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 CO 2 cylinder ahead of the pressure regulator to reduce chilling of the regulator and gas line when CO 2 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.

4 134 JAPR: Field Report 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 ft 3 (0.96 m 3 ). The total interior volume was 75 ft 3 (2.13 m 3 ). 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 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 N 2 used was calculated from standard reference tables [15] based on the pressure decrease in the cylinders during tests. Because CO 2 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 CO 2 used continuous loading to evaluate the performance of the MAK system. The target CO 2 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 O 2 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 O 2 during loading. For this reason, trials using N 2 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 N 2 - or Ar-based anoxia,

5 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 CO 2, 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 CO 2 or O 2 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 CO 2, 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 N 2, a single video was recorded for the duration of gas injection into the chamber. As with CO 2, the latencies of specific behaviors

6 136 JAPR: Field Report Table 1. Performance of the modified-atmosphere killing system during trials using CO 2 with poultry of different sizes and types 1 Bird type Loads Birds/ load Load time, min Birds/ min Weight/bird, lb Weight/bird, kg Load weight, lb Load weight, kg Layer breeder , Caged hen , Turkey hen , Broiler 37 d , d , d , 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 CO 2 or from the beginning of N 2 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 CO 2. The performance variables for 6 trials using CO 2 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 CO 2 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

7 Webster and Collett: SMALL-FLOCK DEPOPULATION 137 Table 2. Performance of the modified-atmosphere killing system using N 2 with 51-d-old broilers Load Layer Birds Weight, lb Weight, kg N 2 fill, 1 min N 2 used, ft 3 N 2 used, m 3 Final O 2 % Load Mean SD Load Mean SD Layer , NA 1 Time from the start of N 2 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. N 2. Loading performance data when using N 2 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 CO 2, but conscious, active birds may give the appearance of filling more space than do birds quickly subsiding into unconsciousness, as takes place with CO 2, 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 CO 2, the process has 3 stages per load, namely, prefill the chamber to the target CO 2 concentration, load the birds, and wait 5 min after convulsive wing-flapping stops to ensure no survival. With N 2, the stages would be the number of repetitions of the cycle (load a layer of birds, inject N 2 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 N 2 would be as follows: number (N 2 injection time + 30 s) + final N 2 injection time CO 2 prefill time. In this trial, the birds were large enough to fill the MAK chamber in 2 layers, so the calculated

8 138 JAPR: Field Report 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 N 2 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 CO 2. Because the MAK system injected CO 2 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 CO 2 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 CO 2 concentration was less for turkeys, so it was to be expected that CO 2 consumption would be lower. The quantity of CO 2 consumed per load was well within the capacity of the gas cylinders (50 lb or 22.7 kg of liquid CO 2 ) used in the tests. During times of sustained CO 2 release from the cylinders, condensation and frost would develop on the regulator and adjacent gas lines, but the heaters mounted before the regulators were sufficient 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 CO 2 at a rate sufficient to hold the chamber concentration at 50%, even though a considerable amount of liquid CO 2 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. N 2. The volumes of N 2 used to kill 4 loads of 51-d-old broilers are presented in Table 2. The amount of N 2 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 ft 3 (5.7 m 3 ) of N 2 was needed to kill a load of broilers. The cylinders used in this study had sufficient capacity (300 ft 3 or 8.5 m 3 ) 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 Birds killed Total weight, lb Total weight, kg CO 2 /load, lb CO 2 /load, kg CO 2 /ton, 1 lb CO 2 /tonne, 1 kg Turkey hen ,347 1, Broiler 29 d 2,381 7,248 3, d ,212 1, CO 2 used per ton or tonne of BW. 2 Includes a partial load not recorded in Table 1.

9 Webster and Collett: SMALL-FLOCK DEPOPULATION 139 Figure 4. Change of temperature, RH, and CO 2 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. CO 2. Figure 4 shows an example of the variation in CO 2 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 CO 2. 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 CO 2 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 CO 2 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 CO 2 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 CO 2 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 CO 2, 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 CO 2 that had to

10 140 JAPR: Field Report 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. N 2. Figure 5 shows the changes in O 2, temperature, and RH for 1 load of broilers when using N 2. Residual O 2 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 N 2 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 N 2. In all tests, the minimum postpeak RH was above 57%. Injection of N 2 into the MAK chamber did not appear to be greatly associated with changes in RH. Behavior CO 2. 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 CO 2 -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 CO 2 concentration for turkeys had been set at 40%, Figure 5. Change of temperature, RH, and O 2 concentration in the interior of the modified-atmosphere killing chamber during loading of the chamber with 51-d-old broilers and injection of N 2 (load 1, both layers of birds). The gas profiles began slightly after N 2 injection was started because of tardy initiation of videorecording.

11 Webster and Collett: SMALL-FLOCK DEPOPULATION 141 Figure 6. Behavior of turkey hen broilers exposed to CO 2 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 CO 2 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 CO 2, the conscious experience of the turkeys within the MAK chamber was only approximately 0.5 min, and there was no survival. N 2. Unlike with CO 2, in which a bird was immersed in the modified atmosphere upon being placed into the chamber of the MAK system, with N 2, a bird placed into the chamber had to wait until the layer was complete and then experience increasing hypoxia during injection of N 2 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 injection would be approximately 10 min or longer. Once N 2 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 N 2, its conscious experience was roughly 12 times longer than the time to loss of posture for a bird in a 50% CO 2 atmosphere. Although the time required to kill poultry using N 2 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 N 2. Table 4. Behavior of chickens placed into CO 2 -enriched atmospheres (target CO 2 concentration = 50%) during trials with the modified-atmosphere killing system 1 Behavior Layer breeder 2 Laying hen Broiler, 2 37 d Broiler, 29 d Broiler, 51 d Deep breathe 2.5 ± 0.5 (8) 2.7 ± 1.3 (9) 2.2 ± 0.8 (46) 2.6 ± 0.9 (60) 2.0 ± 0.7 (15) Head shake 4.0 ± 0.0 (2) 7.8 ± 3.7 (6) 3.5 ± 2.9 (31) 4.6 ± 2.4 (30) ± 1.3 (10) Subside 11.4 ± 1.6 (7) 11.0 ± 3.3 (7) 11.8 ± 5.1 (12) 11.7 ± 3.0 (62) 12.1 ± 2.5 (15) Loss of posture 18.5 ± 1.9 (4) 19.6 ± 5.3 (8) 22.1 ± 8.4 (17) 17.9 ± 4.6 (54) 19.1 ± 4.4 (12) Wing flap 23.3 ± 3.5 (3) 28.4 ± 9.1 (7) 23.7 ± 11.6 (16) 25.3 ± 6.1 (55) 22.3 ± 6.7 (14) 1 Average latency (s) to begin behavior ± SD. Number of birds observed is given in parentheses. 2 Manual control of CO 2 injection into chamber. 3 An additional 33 birds did not perform head shaking.

12 142 JAPR: Field Report Figure 7. Behavior of broilers exposed to N 2 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, CO 2 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 N 2, the broiler chickens tested did not manifest agitation or appear to suffer significant distress during the process. In situations in which CO 2 is not available or when its use might be difficult, such as at ambient temperatures below freezing, N 2 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 N 2, 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 O 2 sensor allowed monitoring of residual O 2 in the chamber during tests with N 2, 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 CO 2 sensor would require an ethical decision to be made if CO 2 were to be used. To avoid the possibility of causing distress to birds by irritation of mucous membranes,

13 Webster and Collett: SMALL-FLOCK DEPOPULATION 143 it would be desirable to keep CO 2 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 CO 2 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 CO 2 that might be irritating might not be excessive [8]. Chickens do not appear to find concentrations of CO 2 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, CO 2 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 gas lines when using CO 2 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 CO 2 because the birds must be killed in batches (layers). The time required for individual birds to die is much longer than with CO 2 in this system. REFERENCES AND NOTES 1. Kingston, S. K., C. A. Dussault, R. S. Zaidlicz, N. H. Faltas, M. E. Geib, S. Taylor, T. Holt, and B. A. Porter- Spalding Evaluation of two methods for mass euthanasia of poultry in disease outbreaks. J. Am. Vet. Med. Assoc. 227: Benson, E., G. W. Malone, R. L. Alphin, M. D. Dawson, C. R. Pope, and G. L. Van Wicklen Foam-based mass emergency depopulation of floor-reared meat-type poultry operations. Poult. Sci. 86: Raj, M., M. O Callaghan, K. Thompson, D. Beckett, I. Morrish, A. Love, G. Hickman, and S. Howson Large scale killing of poultry species on farm during outbreaks of diseases: Evaluation and development of a humane containerized gas killing system. World s Poult. Sci. J. 64: Sparks, N. H. C., V. Sandilands, A. B. M. Raj, E. Turney, T. Pennycott, and A. Voas Use of liquid carbon dioxide for whole-house gassing of poultry and implications for the welfare of the birds. Vet. Rec. 167: Gerritzen, M. A., E. Lambooij, J. A. Stegman, and B. M. Spruijt Slaughter of poultry during the epidemic of avian influenza in the Netherlands in Vet. Rec. 159: National Animal Health Monitoring System Poultry 04. Part 1. Reference of health and management of backyard/small production flocks in the United States, USDA, Animal and Plant Health Inspection Service, Veterinary Services. Accessed Jan. 12, aphis.usda.gov/animal_health/nahms/poultry/downloads/ poultry04/poultry04_dr_parti.pdf. 7. Webster, A. B., D. L. Fletcher, and S. I. Savage Humane on-farm killing of spent hens. J. Appl. Poult. Res. 5: Webster, A. B., D. L. Fletcher, and S. I. Savage Modified atmosphere killing of spent commercial layer flocks on the farm. Egg Ind. (April) :10, 12, 14, United Egg Producers Animal Husbandry Guidelines for U.S. Egg Laying Flocks ed. Accessed Oct. 24, Guardian Plus Model CO 2 Monitor, Edinburgh Instruments Ltd., Livingston, UK. 11. Air Check O 2 Monitor, PureAire Monitoring Systems Inc., Lake Zurich, IL. 12. Red Hat II valve, 3/8 in., Asco, Florham Park, NJ.

14 144 JAPR: Field Report 13. In-line Industrial Gas Heater, Model 120-3, CalCo, Cary, IL. 14. Honda EU2000i portable gasoline-powered generator (1.6 kw rated/2.0 kw maximum power output), American Honda Motor Company, Alpharetta, GA. 15. Younglove, B. A., and N. A. Olien Tables of industrial gas container contents and density for oxygen, argon, nitrogen, helium, and hydrogen. Techn. Note US Department of Commerce, National Bureau of Standards, Gaithersburg, MD. 16. Raj, A. B. M., N. G. Gregory, and S. B. Wotton Effect of carbon dioxide stunning on somatosensory evoked potentials in hens. Res. Vet. Sci. 49: Raj, A. B. M., and N. G. Gregory An evaluation of humane gas stunning methods for turkeys. Vet. Rec. 135: Woolley, S. C., and M. J. Gentle Physiological and behavioural responses of the domestic hen to hypoxia. Res. Vet. Sci. 45: Raj, A. B. M., and N. G. Gregory Investigation into the batch stunning/killing of chickens using carbon dioxide or argon-induced hypoxia. Res. Vet. Sci. 49: McKeegan, D. E. F., J. A. McIntyre, T. G. M. Demmers, J. C. Lowe, C. M. Wathes, P. L. C. van den Broek, A. M. L. Coenen, and M. J. Gentle Physiological and behavioural responses of broilers to controlled atmosphere stunning: Implications for welfare. Anim. Welf. 16: Coenen, A. M. L., J. Lankhaar, J. C. Lowe, and D. E. F. McKeegan Remote monitoring of electroencephalogram, electrocardiogram, and behavior during controlled atmosphere stunning in broilers: Implications for welfare. Poult. Sci. 88: Webster, A. B., and D. L. Fletcher Reactions of laying hens and broilers to different gases used for stunning poultry. Poult. Sci. 80: Gerritzen, M. A., E. Lambooij, S. J. W. Hillebrand, J. A. C. Lankhaar, and C. Pieterse Behavioral responses of broilers to different gaseous atmospheres. Poult. Sci. 79: Hobo Pro-Series, Onset Computer Corp., Bourne, MA. 25. Zeller, W., D. Mettler, and U. Schatzmann Untersuchungen zur Betäubung des Schlachtgeflügels mit Kohlendioxid. Fleischwirtschaft 68: As cited in Raj, A. B. M., and N. G. Gregory Effect of rate of induction of carbon dioxide anaesthesia on the time of onset of unconsciousness and convulsions. Res. Vet. Sci. 49: Webster, A. B., and D. L. Fletcher Assessment of the aversion of hens to different gas atmospheres using an approach-avoidance test. Appl. Anim. Behav. Sci. 88: McKeegan, D. E. F., J. McIntyre, T. G. M. Demmers, C. M. Wathes, and R. B. Jones Behavioural responses of broiler chickens during acute exposure to gaseous stimulation. Appl. Anim. Behav. Sci. 99: Acknowledgments This study was supported by a grant provided by the USDA, Animal and Plant Health Inspection Service, Animal Care (Riverdale, MD).