Online GuideKilling poultry

Gaseous methods

Gas mixtures which are anoxic, that is lacking in oxygen, can be used to stun and kill poultry. Gas killing is possibly the most feasible option for the mass culling of poultry on-farm during a disease outbreak. Compared to the alternative methods discussed in previous sections, using gas mixtures requires less handling and a higher number of birds can be processed in a relatively short time.

A number of gas mixtures can be used to stun and kill poultry.  For welfare reasons the HSA suggests:

  • any mixture of argon, nitrogen or other inert gases (maximum of 2% total oxygen by volume) OR
  • A mixture of argon, nitrogen or other inert gases and CO2 (provided the CO2 does not exceed 30% and maximum free O2 2% by volume). Some commercially available welding gas mixture may be suitable+.

+When selecting an appropriate pre-mixed gas the best option is to select one with no oxygen in it. Many welding gas mixes include 2% oxygen meaning when you are trying to purge the chamber down to low oxygen levels you can never get below 2%, and would have to purge ALL the atmospheric air to do so.

Birds can be exposed to these gas mixtures by three methods:

  1. The first method, controlled atmosphere stunning/killing, is performed by exposing animals to a predetermined mixture of gas in a gas-filled container or apparatus i.e. the animals are introduced after the gas is present
  2. The second method also relates to the killing of poultry using a controlled atmosphere. In this method birds are placed in crates or transport modules and put in a sealed container into which the gas mixture is introduced i.e. the animals are put in the container then gas turned on.
  3. Method 3 involves the introduction of gas in to a sealed poultry house. This includes the use of Nitrogen filled foam.

When stunning and killing using gas, it is essential that the equipment allows the oxygen concentration to be constantly and reliably monitored. It is also advisable that, where possible, a viewing window allows operators to visually inspect birds while they are in the apparatus or house. To prevent disease spreading, if equipment such as crates, gas containers and hoses are transported between farms, or even different locations on the same farm, it is essential that these are fully cleaned and sterilised.

Method 1 – Animals placed in container or apparatus pre-filled with gas

Large metal containers, such as skips, can be covered and the gas mixtures introduced. When the desired concentration of gases is achieved, birds are introduced through openings in the top.  The birds are added into the container by hand after being carried from their housing. When introducing birds from the top of a container, care should be taken to prevent broken bones and bruising while they are still conscious. Birds should have sufficient space while in the container to ensure they are not overcrowded which may lead to them unintentionally suffocating. The rate at which birds are added to the chamber must also be carefully controlled to prevent smothering before the onset of unconsciousness

The gas concentration must be continually monitored, especially when doors or hatches are opened and closed allowing gas to escape. The desired concentration must be maintained for a sufficient time to allow all animals to die before being removed. It should take less than 2.5 minutes to kill chickens and turkeys, but five minutes for geese and ducks. However, the concentration should be maintained for at least five minutes after the last birds have been added to the container regardless of species. If the container has a viewing window this can be used to assess the state of the birds. Without a viewing window, the cessation of vocalisation and wing flapping sounds can be heard from outside the container and can be used to determine the birds are unconscious and after continued exposure will be dead. Birds leaving the container must be confirmed to be dead; any for which there is doubt must be killed using a back-up method such as non-penetrative captive-bolt. The carcases of birds from previous batches must be removed before birds from a subsequent batch are added.

Considerations when using pre-filled gaseous methods during depopulation due to disease control

There are a number of advantages of using this method during a disease outbreak. The sizes of the containers are known and therefore the volume of gas required can be estimated during the planning stages. As more birds are added to the containers, gas will be lost from the opening and therefore the actual amount of gas required will depend on how frequently the flaps are opened and closed. As the technique is non-invasive, meaning the skin remains unbroken, there should be no release of contaminated bodily fluids entering the environment. This ensures the disease transmission risk is kept to a minimum. These containers are dead bird disposal skips with specially designed tops. Once the skip is full, and all the birds dead, a telehandler is used to remove the lid which is replaced with a sheet. The skip is then pulled on to a lorry and removed. The containers are typically made of metal and can be easily disinfected after use. Although the apparatus and equipment used may differ from those used by catchers and handlers on a daily basis (skip rather than crate) it should not be hard for the skilled handlers to use the system.

Handler health and safety is generally maintained as the units are operated from the outside and the gas disperses quickly once the doors are opened. However, when adding birds in to the top of the containers the handlers must climb stairs. Adequate care and attention is required when walking up and down stairs, especially when carrying restless birds in wet weather.

There are some disadvantages to this method. From a practical perspective the containers and apparatus may be difficult to obtain, or may be expensive. The number of birds in a batch is limited by the number of people in a catching team and the size of the containers. When considering animal welfare, this approach raises some important concerns. The stocking density of the animals may increase the risk of suffocation due to overcrowding. The speed at which birds are added to the container needs to be carefully controlled to allow them to become unconscious before more birds are added in on top of them – for welfare reasons they should not be added in continuously. The act of dropping birds in through the roof of the container is also particularly concerning as it may lead to broken bones and bruising while the animals are conscious. Immediately entering an anoxic environment is known to be aversive and the animals are not rendered immediately unconscious. Confirmation of death should occur before the animals are released from the container, however, as the humans and animals are apart this is difficult and animals which are not killed may go unnoticed.  If any birds are still alive following removal from the container, a backup method such as a captive-bolt must be used immediately.

Conclusion – species and situations

Method 1 is suitable for use with poultry although, as there are serious welfare concerns, alternative methods should be considered first.

Method 2 – Poultry confined in crates or modules placed in chamber into which gas is then introduced

Birds may be killed by loading them into a chamber while confined in crates or modules. In contrast to method one, when the birds are initially placed in the chamber the oxygen levels are still normal (i.e. 20.7%) and lowered gradually as the oxygen in the air is replaced with another gas mixture. This seeks to reduce the aversiveness of the experience as the animals gradually lose consciousness as the gases infuse over a period of 60 seconds.

The birds should be caught gently and placed in crates or modules and the stocking density should allow all birds to sit down.

 The crates or modules are placed inside the container either by hand or by using machinery such as a forklift.  The door is then shut when the operator is ready to administer the gas. When the door is secured the gas is delivered until a minimal concentration of oxygen is achieved at the top of the container (5% O2 for chickens and turkeys, 2% for waterfowl when using inert gas/CO2 mix. 2% O2 for all species when using pure inert gas). The gas concentration must be checked using a gas meter and this level should be maintained until all birds in the group have been killed. If the container has a viewing window, it can be used to assess the state of the birds. Without a viewing window the cessation of vocalisation and wing flapping sounds can be heard from outside the container and be used to determine the birds are unconscious and, after continued exposure, will be dead.

Upon removal from the chambers, all birds should be inspected and if any are found to still be alive they should be killed immediately by use of an alternative method, e.g. captive-bolt.

Although at sufficient concentrations (see above) it should take less than three minutes to kill chickens and turkeys, and five minutes for geese and ducks, the concentration should be maintained for at least five minutes after the desired gas concentration has been achieved.

Considerations when using chambers into which gas is introduced during depopulation due to disease control

This method of killing birds has advantages. As with Method 1 the size of the containers are known and therefore the volume of gas required can be calculated during the planning stages. The containers are typically made of metal and can be easily disinfected after use. Handler health and safety is maintained as the units are operated from the outside and the gas disperses quickly once the doors are opened. The apparatus and equipment used is the same as, or similar to, those used on a daily basis and so skilled handlers should be able to perform these tasks easily. As with Method 1, this technique is non-invasive. Thus, there should be no release of contaminated body fluids in to the environment and the disease transmission risk is kept to a minimum.

Compared to Method 1 there are some improvements in terms of animal welfare. As the birds are placed in transport crates or modules uniformly they are less likely to smother each other and are unable to pile on top of one another. They are introduced into the chamber before additional gas is introduced. The slow introduction of anoxic gases is believed to be better for the birds’ welfare. This system can also be scaled to suit the needs of the operator from a backyard smallholder to a large commercial unit.

There are some disadvantages to this method. Animal welfare may be compromised by the need to catch and handle birds. As poultry are not habituated to this experience it is likely to cause fear and distress. Improper handling is also likely to cause physical injury, particularly when being lifted out of cages or into the crate/module.  From a practical perspective the containers and apparatus may be difficult to obtain, or may be expensive. When used for depopulation on a commercial scale, this method also requires machinery to load the crates or modules into the container, such as forklift trucks. As with all three methods it is difficult to assess welfare if the container does not have a window.

Conclusion – species and situations

Method 2 is suitable for use with adult poultry, pullets and chicks during an on-farm kill for disease control. In theory there is potential for use with neonates such as piglets, but recommendations for durations or gas concentration for neonatal mammals are currently unknown. Although some work has been performed in continental Europe with piglets further research is required.  

Method 3a – Gas (CO2) introduced to sealed poultry house

As an alternative to placing birds in a container pre-charged with a gas mixture, gases can be introduced directly in to the birds’ house. In order to maintain birds’ welfare, the house should be only be sealed immediately before the introduction of gas to avoid overheating or unintentional suffocation. If the poultry shed has a ventilation system fitted, this should be shut off immediately before gas delivery. There is a possibility that water pipes may freeze during the gas administration and therefore the water should be turned off at the mains and drained. The feeders and drinkers inside the houses with floor reared birds should be raised or removed to prevent obstruction to the gas delivery or injury to birds.

The house should be gradually filled with gas so that all birds are ultimately exposed to a concentration of >45% CO2, measured 50cm above the level of the highest bird, until they are dead. The most common method is to use a source of liquid CO2 which is pumped into the house and released. Preferably the liquid carbon dioxide should be passed through a vaporiser before entering the house. This not only removes the risk of exposing birds to sub-zero temperatures (potentially       -80°C), but also speeds up the dispersal of gas throughout the shed. Care should be taken to ensure the birds will not be directly hit by very cold gas delivered at high pressures. Nets or mesh may be used to create a distance between the gas delivery pipes and the animals. Devices should be placed inside the shed at the maximum height of accommodation of the birds.

Considerations when using gas in sealed poultry houses during depopulation due to disease control

There are advantages when using this non-invasive method to depopulate birds in a poultry house. CO2 is readily available and the application of the gas to birds in their housing eliminates the need to manually remove live birds. This is beneficial to animal welfare as it does not require handling and inversion, which is known to be stressful for birds, and the birds remain in familiar surroundings. The handler also benefits, as reduced handling minimises disease spread. It is believed the gradual raising of CO2 concentrations minimises the aversiveness of the induction of unconsciousness. As the skin is unbroken there should be no release of contaminated body fluids entering the environment, ensuring the disease transmission risk is kept to a minimum.

There are disadvantages to this method which may be detrimental to the birds’ welfare. Unlike the containers described in Methods 1 and 2, the size and area volume of a poultry house may be unknown. This means it may be difficult to determine the volume of gas required to achieve adequate concentration of CO2 in some poultry houses. It is also difficult to accurately determine the point at which all animals in the house are dead, which may lead to some animals requiring additional killing methods. The very low temperature of CO2 entering the house, and the formation of dry ice, may cause concern for bird welfare if they come in to contact with it.

Conclusion – species and situations

This method is suitable for use with poultry in a closed environment such as their rearing sheds. The OIE suggests that this method may also be suitable for housed pigs, although further work is required before this can be recommended.

Method 3b –Nitrogen-filled foam introduced to a poultry house

Foam containing bubbles of anoxic gas has been researched and developed as a method of killing housed poultry during a disease outbreak. A benefit to this is that the house does not need to be sealed as when using gas. Even naturally ventilated, partially open sheds can be depopulated using this method. It is much more flexible and time saving as the foam can “plug” small holes in the house structure when it is pumped into the shed and fills the building.

In the USA, medium density fire-fighting foam created with atmospheric air has been approved for the depopulation for housed poultry. This foam has a high water content and the small bubbles do not break and are ingested by the birds, which are killed due to the obstruction of the airways by the liquid components of the foam. However, this is not recommended as death as occlusion of the respiratory tract is not considered to be humane.

A better alternative is a foam containing nitrogen.  The movement of the birds breaks the bubbles and releases the inert gas. This foam differs from the medium density fire-fighting foam as it has a low water content and is sometimes referred to as “dry foam”. This method is preferred as birds do not suffocate due to their airways being obstructed. Death is caused by exposure to the pure nitrogen gas contained within the bubbles which break around the birds.

Figure 11: Use of nitrogen filled foam during depopulation of a poultry house. Note how the foam provides a seal for the building. (Photo credit: Julian Sparrey, Livetec Systems Ltd).

Considerations when using nitrogen foam during depopulation due to disease control

Using gas-filled foam to kill birds in their houses has a number of advantages. As the technique is non-invasive, there is no contamination of the area with body fluids. When used during a particularly pathogenic strain of disease, such as avian influenza, it may be possible to add anti-viral compounds to the foam to assist in the decontamination and clean-up process, although further research is required in this area.  No handling of live birds is required, which is beneficial as it prevents the spread of zoonotic diseases. As handling only occurs once the birds are dead it means this job does not require skilled personnel and, as such, can be performed by anyone. The procedure and time to cull a farm is quicker as sheds do not need to be sealed (see Figure 11). It is also possible that handling can be avoided entirely, as a catching machine could be used once the foam has subsided. From a practical perspective it is beneficial to Method 3a, as the foam can fill gaps in the house structure. This allows the shed to be filled even if small gaps between the walls and roof are present, which is not possible with whole-house gassing and reduces the need for people to enter the shed to make repairs prior to the kill. An additional benefit is that, as the foam covers the animals and contains the pockets of nitrogen gas, the foam does not need to fill the shed up to roof level. This allows the killing staff to view the shed through a window or gap in the roof to assess the coverage. The birds do not appear to be startled by the presence of the foam, nor do they try to escape from it once in contact. Additionally, the birds are killed in their home environment, which is likely to be beneficial to their welfare compared to methods which take place outside of the familiar environment. However, a disadvantage to this method is the cost. It is expensive compared to whole house gassing by direct liquid nitrogen injection.

Conclusion – species and situations

Method 3b is suitable for the killing of large numbers of poultry during a disease outbreak. It has previously been used on-farm with chickens and successfully trialled with waterfowl and turkeys.

Next: Addition of anaesthetics to feed or water

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