The present invention relates to a system and a method for stunning animals with gas, in particular, but not exclusively, a system and a method for stunning animals with carbon dioxide or a mixture of carbon dioxide with other gases. The present invention further relates to a system and method for stunning and slaughtering animals, particularly poultry. Inducing unconsciousness of animals, including poultry, before slaughter is a requirement and/or a convention throughout many countries. Accordingly, a significant number of jurisdictions have set guidelines and regulations relating to animal slaughter welfare which include, when appropriate, a list of known stunning methods suitable for different animal species and certain minimum standards for each method. Each stunning method has typically been reviewed by the competent regional, national or supranational authorities. In the European Union (EU), the agency responsible for carrying out studies of and supervising stunning methods under laboratory conditions is the European Food Safety Authority (EFSA). Further, each European jurisdiction also has a national body responsible for supervising the slaughter of animals. In the United Kingdom (UK) this body is the Department for Environment Food & Rural Affairs (DEFRA).

In accordance with Regulation (EC) 1099/2009 the stunning methods allowed in the EU are penetrating and non-penetrating captive bolt stunning, firearm stunning, head- only electrical stunning, head-to-body electrical stunning, electrical water bath stunning and gas stunning. Gas stunning includes phased carbon dioxide stunning, stunning with a mix of carbon dioxide and inert gases and inert gas stunning. Moreover, each state might retain national legislation predating Regulation (EC) 1099/2009 if said legislation offers higher standards of slaughter welfare and was in place at the time of adoption of the European Regulation.

Commercially, the most significant methods used for stunning poultry in the EU are electrical water bath stunning and gas stunning. The former is applied in 81% of broilers while the latter is applied in 19% of broilers. Gas stunning is also known as controlled atmosphere stunning (CAS) or CAS system. This type of stunning has become increasingly popular in the past decades as a result of consumer emphasis on animal welfare and the product quality advantage. For example, it is the method most often used in the US and is gaining popularity in the UK and other parts of Europe. Gas stunning induces unconsciousness gradually instead of immediately and can be achieved by using different gases or combinations thereof. Commercially, carbon dioxide (CO2) is the most commonly used gas, either alone or in combination with other gases. If only CO2 is used, stunning takes place in two or more phases. Birds are typically exposed to a relatively low concentration of CO2, less than 30% by volume in air or inert gas, during the first phase and once the birds are unconscious, they are exposed to a higher concentration of C0 ₂ , typically 80% to 90% by volume in air, in the second phase. This process can be extended to include three or four different phases to ensure poultry wellbeing. Birds and mammals have carbon dioxide chemoreceptors; as a result, exposure to carbon dioxide at high concentrations leads to physiological responses and stress. Consequently, direct exposure to high concentrations of carbon dioxide is likely to cause pain and distress and the gas stunning methods are therefore divided into at least two phases to minimise any adverse effects caused by the stunning gas.

As mentioned above, carbon dioxide may be used in combination with other gases, typically inert gases such as argon, nitrogen or both. Such inert gases act as anaesthetic properties under hyperbaric conditions and lead to progressive hypoxia in birds. However, as neither mammals nor birds have chemoreceptors responsive to inert gases, the adverse effects associated with carbon dioxide chemoreceptors are obviated while different adverse effects are encountered. Despite, the aforementioned disadvantages, gas stunning poultry is still considered to be advantageous from an animal welfare perspective because the requirement of inverting and shackling conscious birds. In addition, this stunning method has an improved performance across a variety of bird sizes in the same flock.

Nonetheless, current gas stunning systems and methods suffer from several drawbacks. Firstly, commercially available systems have relatively high implementations and running costs. Secondly, known systems are not environmentally desirable because the carbon dioxide used is typically emitted into the atmosphere. Thirdly, available systems often require specific transport container systems further increasing the carbon footprint of the method. For these reasons, gas stunning is primarily used by large slaughterhouses. In addition to the drawbacks already described, existing gas stunning systems are inconsistent and do not ensure that all animals in a chamber are exposed to a reliable mixture of gas and air. This problem is particularly significant for poultry which is usually transported and placed in a chamber in transport crates, which are often stacked.

The present invention seeks to improve known gas stunning systems and methods.

According to a first aspect of the present invention, there is provided a system for stunning an animal used for food comprising: a chamber for exposing the animal used for food to a gas or gas mixture; an inlet for enabling ingress of a gas or gas mixture into the chamber; gas exchange means arranged to allow gas to enter and/or exit the chamber; a plurality of gas combinations provided in respective pre-mixed gas combination containers which pre-mixed gas combination containers are connectable to the chamber; a gas reservoir containing a gas and connected to the pre-mixed gas combination containers; an outlet for removing a gas or gas mixture from the chamber; wherein, in use, the gas exchange means causes the chamber to be filled with any one of the plurality of pre- mixed gas combinations and/or the gas to allow an animal used for food to be exposed to any one of the plurality of pre-mixed gas combinations and/or the gas and then causes the gas in the chamber to be removed from the chamber and directed back to a recycled gas container and/or the relevant pre-mixed gas combination container and/or the gas reservoir to be re-used.

Advantageously, the system further comprises gas circulation means to improve gas circulation in the chamber and/or a de-humidifier. Preferably, the gas exchange means comprises extraction means. More preferably, the system further comprises a controller arranged to actuate the gas exchange means and thereby regulate ingress and/or egress of any one of the plurality of pre-mixed gas combinations and/or the gas into the chamber. In a preferred embodiment, the pre-mixed gas combination containers are bellow containers and the gas exchange means is at least one air cylinder or actuator arranged to activate the bellow containers. In a further preferred embodiment the chamber or the gas exchange means is a bellows. In another preferred embodiment the air circulation means is a fan or a bellows. Advantageously, the inlet and outlet are provided by the same pipe. In a preferred embodiment, the pre-mixed gas combination containers are expandable bladders. In a further preferred embodiment, the gas exchange in the chamber occurs in less than 10 seconds. In another preferred embodiment, the chamber and/or the pre-mixed gas combination containers comprise carbon dioxide sensors. In an advantageous embodiment, the system further comprises chamber climate control means. According to a second aspect of the present invention, there is provided a method for stunning animals used for food comprising the steps of: providing a chamber to stun animals used for food; filling the chamber with a first gas combination; placing animals used for food in the chamber to expose the animals used for food to the first gas combination for a first predetermined period of time; removing the first gas combination from the chamber and filling the chamber with a second gas combination; exposing the animals used for food to the second gas combination for a second predetermined period of time; wherein each one of the first and second gas combinations is mixed prior to introduction in to the chamber; and wherein the method further comprises the step of providing gas exchange means arranged to fill the chamber with chamber the first and second gas combinations and/or extract the first and second gas combinations from the chamber to allow said gas combinations to be re-used. In a preferred embodiment, the method further comprises the steps of: filling the chamber with a third gas combination; and exposing the animals used for food to the third gas combination for a third predetermined period of time. In a further preferred embodiment, the method further comprises the steps of: filling the chamber with a fourth gas combination; and exposing the animals used for food to the fourth gas combination for a fourth predetermined period of time. In yet another preferred embodiment, the method further comprises the steps of: filling the chamber with a fifth gas combination; and exposing the animals used for food to the fifth gas combination for a fifth predetermined period of time. Advantageously, the method further comprises the step of providing a de- humidifier to extract moisture from and/or lower temperature of the chamber. More advantageously, the method further comprises the step of circulating a first and/or second and/or third and/or fourth and/or fifth gas combination between the chamber and a first, and/or second, and/or third, and/or fourth, and/or fifth pre-mixed gas combination container to minimise gas concentration changes. In a preferred embodiment, the first predetermined period of time is between 2 and 3 minutes, and/or the second predetermined period of time is between 1 and 2 minutes, and/or the third predetermined period of time is between 1 and 2 minutes, and/or the fourth predetermined period of time is between 1 and 3 minutes, and/or the fifth predetermined period of time is between 1 and 5 minutes. Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings in which: Figure 1 is a diagram of a system for stunning animals in accordance with a first embodiment of the present invention;

Figure 2 is a diagram of a system for stunning animals in accordance with a second embodiment of the present invention;

Figure 3 is a diagram of a system for stunning animals in accordance with a third embodiment of the present invention; and

Figure 4 is a diagram of the steps followed to stun an animal in accordance with a method in accordance with a second aspect of the present invention;

Figure 5 is a flow chart of the exemplary controller logic used for the method of Figure 4;

Figure 6 is a diagram of the fail-safe steps followed to ensure that the chamber contains a precise combination of gases;

Figure 7 is a diagram of a system for stunning animals in accordance with a fourth embodiment of the present invention; Figure 8 is a diagram of a system for stunning animals in accordance with a fifth embodiment of the present invention;.

Figure 9 is a diagram of a system for stunning animals in accordance with a sixth embodiment of the present invention;

Figure 10 is a schematic representation of the valve network and sequence used in Figures 7 to 9; and

Figure 1 1 is a diagram of a system for stunning animals in accordance with a seventh embodiment of the present invention; .

Referring now to Figures 1 , 2 and 3, the system of the present invention comprises a chamber 6 arranged to allow poultry or other animals to be stunned by exposure to a gas mixture. The chamber 6 includes a fan 7 to allow the gas mixture to circulate and extraction means 14 to remove the gas mixture from the chamber 6. Pre-mixed gas combination containers 1 , 2, 3, 4 store different mixtures of carbon dioxide and oxygen or air; these containers 1 , 2, 3, 4 are connected to the chamber 6 via inlet gas pipes 5. A further gas reservoir 8 for storing low pressure carbon dioxide is provided and is in fluid communication with each container 1 , 2, 3, 4 to allow carbon dioxide levels to be raised in said container as required. The gas reservoir 8 is supplied with carbon dioxide from a pressurised carbon dioxide tank 9. The two-stage carbon dioxide storage is used because low-pressure apparatus or equipment is safer and more economical than high- pressure gas apparatus. The extraction means 14 allows the gas combinations to be removed from said chamber 6 and returned to the relevant expandable gas container 1 , 2, 3, 4 via outlet gas pipes 15 to be re-used or discharged for periodic replacement. A controller 100 is connected to valves 50, 150 in the inlet and outlet gas pipes 5, 15 and controls opening thereof. In addition, the controller 100 regulates operation of the fan 7 and a de-humidifier 12. In use, the controller 100 opens a valve 10 provided in a first pre- mixed gas container 1 causing the chamber 6 to be filled with a pre-mixed gas combination, described in further detail below, to allow poultry or other animals used for food to be exposed to said first pre-mixed gas combination for a predetermined period of time. The controller 100 further activates the fan 7, and/or a pump 13, to ensure that the first pre-mixed gas combination circulates inside the chamber 6 thereby avoiding one of the most significant problems found in prior art systems (i.e. carbon dioxide hotspots and/or reduced concentration areas). Once the poultry or other animals are exposed to the pre-mixed carbon dioxide mixture for a predetermined period of time, the controller 100 opens a valve in one of the outlet gas pipes 15 and a second valve 20 on the inlet pipe to allow the first pre-mixed gas combination to be replaced by a second pre-mixed gas combination, typically having a higher concentration of carbon dioxide and described in further detail below. The first pre-mixed gas combination is removed by the extraction means 14 and returned to the first gas combination container 1 to allow said gas combination to be re-used (i.e. re-cycled). Gas container sensors monitor the concentration of gases within the gas containers and allow carbon dioxide or oxygen to be added into the container as appropriate. This ensures that the pre-mixed gas combinations are kept at predetermined levels throughout the stunning process. Preferred gas stunning methods for poultry involve exposing a bird to carbon dioxide in four stages, the concentration of CO2 and duration of exposure depend on the species to be stunned and the size of the bird. Typically, the birds are exposed to between 8% and 92% by volume carbon dioxide and the remaining gas volume is made up of oxygen or air. Usually, the first stage includes exposing poultry to 10% to 16% by volume of carbon dioxide for 1 to 2 minutes, the second stage involves subjecting poultry to 17% to 29% by volume of carbon dioxide for 2 to 3 minutes, the third stage includes exposing birds to 30% to 40% by volume of carbon dioxide for 1 to 2 minutes and the fourth stage involves subjecting poultry to 50% to 90% by volume of carbon dioxide for 1 to 3 minutes. It should be clear to a skilled person that the number of stages, carbon dioxide concentration and duration of each step could be adapted or revised as required. For example, it is widely accepted that cycle duration varies significantly between chickens and turkeys. Accordingly, the system exposes poultry or animals to the relevant number of pre-mixed gas combinations for a predetermined period of time.

The main differences between the systems of Figures 1 , 2 and 3 are the type of chamber 6 which is chosen depending on the size and number of animals to be stunned in a stunning cycle, fan and/or pump, and gas extraction means. These features will be described in further detail below.

It is known that carbon dioxide (C0 ₂ ) molecules are denser and have a higher molecular mass than oxygen molecules (O2). Accordingly, under atmospheric pressure, carbon dioxide will eventually descend to the bottom of an area containing both gas molecules. In known gas stunning systems this fact leads to stunning gas dosage variability between livestock located on different levels of a chamber 6. This is thought to cause distress to the animals because animals on lower levels of the chamber 6 will be exposed to higher concentrations of carbon dioxide than the dose intended thereby triggering chemoreceptors before unconsciousness while animals on higher levels of the chamber 6 may not be exposed to sufficiently high carbon dioxide concentrations thereby staying conscious. A conscious animal is likely to be distressed on a subsequent stage as the carbon dioxide concentration goes up because, once again, carbon dioxide chemoreceptors would be activated before the animal is unconscious. In the present system, gas inside the chamber 6 is constantly moving, at least for the first two stages. As a result, the carbon dioxide dosage is not variable on different parts of the chamber 6.

Typically, poultry farming methods involve large number of birds living in a controlled environment. Said controlled environment is arranged to provide the birds with everything considered necessary for their wellbeing and growth including nutrition, medication and protection from the weather. Certain characteristics and requirements of poultry production have changed significantly in the last two decades or so, in particular, heat stress accompanied by higher humidity has become a more significant problem than it had been before the 1990s. Heat stress can considerably impact poultry production by reducing animal welfare and increasing death rates; as a result, production output is decreased. In birds, heat is lost mainly in three standard ways: radiation, convention and conduction. If the environment surrounding a bird is at a lower temperature than said bird, heat will simply be lost from the body by radiation. Heat loss in birds also occurs from the natural rise of warm air from around a hot body including a bird; accordingly, providing moving air (such as by performing a flapping action) assists with convection if the air moves fast enough to break down the boundary layer of still air that surrounds the body. Finally, during conduction, heat transfers from a surface in contact with another surface, for example, if a bird sits on a surface cooler than its body. Birds adapt their behaviour to stay in a "thermoneutral zone". Once birds can no longer maintain body heat balance by one of the three methods described above, i.e. an upper critical temperature is reached, they must use evaporative heat loss, or panting. Panting action enables a bird to lower body heat by evaporating water from the moist respiratory tract. The action eventually generates body heat and causes the bird to eliminate water from the body. In a confined environment such as a stunning chamber, poultry panting results in increased temperatures and humidity. The increase in both environmental temperature and humidity leads to poultry discomfort and/or anxiety as the birds are unable to reduce body temperature by evaporative cooling in such environmental circumstances. This reduces effectiveness of known stunning methods.

The fan 7 provided in the chamber 6 of the present invention is thought to assist birds in losing heat by convection as well as ensuring that the gas mixture is circulated in the chamber 6. Moreover, the system of the present invention is preferably provided with C0 ₂ and/or temperature and/or humidity sensors which sensors are connected to the controller 100. Accordingly, if a temperature sensor in the chamber 6 detects a rise or decrease in temperature, the controller 100 can increase or adjust the fan speed. Further, any CO2 produced by breathing can be extracted or removed by extraction means 14 to ensure that carbon dioxide levels inside the chamber 6 remain constant, this reduces the likelihood of animal chemoreceptors being activated and causing stress to the animal. Further, delays in sensor detection of increase in CO2 levels are also avoided because the stunning gas combination is pre-mixed to precise levels and therefore there is no need to provide the system with a mixing time window. Finally and as already mentioned, the fan 7 allows the stunning gas to be distributed evenly through the chamber 6. As a result, the stunning gas is administered in a reasonably constant and consistent dose throughout a cycle.

In addition, the system of the present invention is provided with a de-humidifier 12 arranged to remove excess moisture and heat from the chamber 6 to ensure that animals remain comfortable. As mentioned above, birds produce water by evaporative heat loss, as such, humidity inevitably increases throughout the stunning process in known systems. Thus, poultry become stressed when humidity increases to levels which prevent birds from losing heat by evaporative cooling. The de-humidifier 12 of the present system allows humidity to be kept constant thereby eliminating or reducing any stress caused to the birds as a result of humidity levels. Further, when connected to humidity sensors, the controller 100 can increase or decrease the rate at which moisture is removed from the chamber thereby responding to any humidity variations in the chamber 6.

It should also be mentioned that because the stunning gas dose is pre-mixed, it could be cooled or controlled (for example by reducing humidity) before reaching the chamber 6 to further improve chamber conditions.

Referring now specifically to Figures 1 and 2, extraction means 14 are combined with the fan 7 in an extractor and circulation fan 7, 14. Accordingly, one pre-mixed gas combination is removed from the chamber 6 as another pre-mixed gas combination enters said chamber. In other words, any fluid entering or exiting the chamber 6 does so via the extraction and circulation fan 7, 14. In these embodiments, the de-humidifier 12 is connected to the circulation fan 7, 14 to allow moisture to be removed from the fluid input or output. The chamber 6 shown in the embodiment of figure 1 comprises a large volume to enable animal holding cases or crates 16 to be stacked and placed side by side. This type of chamber 6 is preferred for smaller animals such as poultry. In contrast the chamber 6 of the embodiment of Figure 2 comprises a conveyor belt 60 which allows placing an individual animal holding container or crate 16 in the chamber. This type of chamber 6 is preferred for larger animals such as pigs.

In the embodiment of Figure 3, the fan 7 is a circulation fan connected to the inlet gas pipes 5 via a chamber inlet having an air valve 70 between the fan and the chamber 6. Each inlet gas pipe comprises a valve 10, 20, 30, 40 to ensure that a single pre-mixed gas combination is supplied. Extraction means 14 is a fluid pump which directs re-cycled fluid (i.e. gas or liquid) removed from the chamber 6 into a pre-mixed gas container 1 , 2, 3, 4 via the relevant outlet pipe 15 having a valve 11 , 21 , 31 , 41. As extraction means 14 is a fluid pump, it acts as a de-humidifier as well as an extractor. In this embodiment, the pump may be set to allow gradual change in dosage between different stages by decreasing or increasing pumping rates.

Although the different embodiments comprise different chambers 6 and/or extraction means 14, every embodiment described above allows the pre-mixed gas combinations to be recovered from the chamber 6 and re-used or re-cycled. Accordingly, the system of the present invention greatly reduces stunning gas usage. Specifically, the system of the present invention uses less than 25% of the gas used by known systems thus reducing reliance on expensive gases and significantly reducing the direct carbon footprint of known systems. Moreover, the system of the present invention reduces the carbon footprint of known systems indirectly by reducing the number of deliveries of C0 ₂ required by a plant comprising the system when compared to a plant with system having a single-use CO2 format. In addition, it reduces the likelihood of having to produce carbon dioxide as anything other than a by-product from other processes in the event gas stunning becomes mandatory or required. Unlike previous systems and methods, the present invention uses pre-mixed gas combinations to ensure that the birds or animals are exposed to the precise concentration required by each cycle. Further, the fan 7 ensures that the appropriate gas combination is circulated inside the chamber 6; thus, carbon dioxide hotspots and reduced concentration areas observed in known systems are absent in the present system. Moreover, the de- humidifier 12 ensures that the chamber is maintained at a constant humidity despite poultry panting. In addition, sensors, constantly monitor carbon dioxide levels and any other predetermined environmental characteristic and, if the carbon dioxide levels or other predetermined environmental characteristic monitored by the sensors are outside of a specified threshold, the controller generates a command to the fan, de-humidifier 12, carbon dioxide reservoir 8 and/or air-pump to adjust the relevant parameter and ensure that conditions are returned to optimal levels.

Referring now to Figure 4, a method in accordance with a second embodiment of the present invention comprises four stunning steps interspersed with four re-cycling steps and a final air-pumping step. Once the animals have entered the chamber, a first pre- mixed gas combination comprising between 10% and 16% by volume of C0 ₂ is pumped into the chamber from a first pre-mixed gas container while fresh air is vented out of said chamber. The first pre-mixed gas combination is circulated for a first predetermined time, for example between 2 and 3 minutes, inside the chamber by the fan once gas sensors have detected that all fresh air has been exhausted from the chamber. When the first predetermined time has elapsed, the first pre-mixed gas combination is vented through an outlet gas pipe connected to the first pre-mixed gas container and replaced with a second pre-mixed gas combination comprising between 17% and 29 % by volume of CO2, which second pre-mixed gas combination is pumped into the chamber from a second pre-mixed gas container. The gas sensors monitor replacement of the first pre-mixed gas combination with the second pre-mixed combination and when the first pre-mixed gas combination has been completely vented, the fan circulates the second pre-mixed gas combination inside the chamber for a second predetermined time, for example between 1 and 2 minutes. Once the second predetermined time has passed, the second pre-mixed gas combination is vented through the outlet gas pipe and re-directed to the second pre- mixed gas container for re-cycling while the gas in the chamber is replaced with a third pre-mixed gas combination comprising between 30% and 40% by volume of C0 ₂ from a third pre-mixed gas container. Once the chamber sensors detect that the third pre-mixed gas combination has replaced the second pre-mixed gas combination, the fan circulates said third gas combination for a third predetermined time, typically 1 to 2 minutes, to ensure that the animals in the chamber are exposed to the gas mixture. After the third predetermined time has elapsed, the third gas combination is pumped back to the third pre-mixed gas container via the outlet pipe while the gas inside the chamber is replaced with a fourth gas combination having between 50% and 90% by volume of C0 ₂ . As in the previous stages, the sensors monitor the gas composition inside the chamber and the fan circulates the fourth gas combination after the chamber concentration of CO2 reaches the required level. The animals are then exposed to the fourth pre-mixed gas combination for a fourth predetermined time, usually 1 to 3 minutes. Once the predetermined time has passed, the fourth pre-mixed gas combination is pumped back into its contained via the outlet pipe and the chamber is provided with some fresh air while the animals are removed from the chamber. It should be noted that the circulation rate of the pre-mixed gas combinations may be adjusted to optimise animal welfare. Throughout the stages, a controller, such as the one described in relation to Figures 1 to 3, performs a check of the chamber seal by using a chamber vacuum sensor. The chamber vacuum sensor detects a light vacuum generated, for example, by the extraction pump to ensure that fluid does not escape from the chamber. If a fluid leak is detected, the controller activates a time delay and the cycle is locked out until the seal sensor detects the light vacuum. Further, the controller is also connected to gas sensors inside the chamber, the pre-mixed gas containers and the carbon dioxide reservoir to ensure that the stunning gas concentration is appropriate. If the stunning gas concentration detected is different from the predetermined concentration, the controller actuates valves to allow air or carbon dioxide to be added to ensure that the stunning gas concentration is adjusted to the specified level. A time delay and cycle lock out is activated until the appropriate stunning gas concentration is detected.

As shown in Figure 5, the controller 100 is connected to one or more gas sensors 80 for detecting carbon dioxide concentrations. If the detected carbon dioxide concentration is below a predetermined threshold, the controller 100 allow a gas top-up solenoid valve to open for a specific period of time which is calculated based on the concentration differential. A delay timer 110 is also activated and the stunning cycle is interrupted as described above in relation to Figure 4. An audible or visual alarm 120 could be generated if the concentration adjustment takes longer than a predetermined period of time in order to alert an operator. If the level of carbon dioxide detected is within specified parameters, the time delay 110 is not activated and the stunning cycle continues. On the other hand, if the gas sensors 80 detect a carbon dioxide concentration above the specified parameters, the controller 100 opens an air top-up solenoid valve for a specific period of time which is calculated based on the concentration differential. If this occurs, the delay timer 110 is also activated and the stunning cycle is suspended. An audible or visual alarm 120 may also be generated if the concentration adjustment takes longer than a predetermined period of time.

Referring now to Figure 6, the system and method of the present invention has a fail-safe to ensure that the animals are not distressed. Typically, the animals would remain awake until approximately the end of the second stunning stage and would therefore be unconscious by the time the third stunning stage has commenced, this ensures that the animals' chemo-receptors are not stimulated while the animals are conscious to prevent the animals from reacting or stressing. The system and method allow an operator to manually actuate a lever and open a valve in the event that the sensors inside the chamber detect, for example by detecting movement, that the animals are conscious when the third stage starts. This operation allows fresh air to enter the chamber for a specific time to ensure that the animals' chemo-receptors are not activated and therefore that the animals are not subjected to unnecessary distress. Once a threshold stunning gas concentration is detected by the sensors, the manual lever is returned to the close position and the fan is activated to repeat the second stunning stage to ensure that the animals are unconscious. If necessary, the second stage can be maintained for an additional predetermined time period. In the event that the sensors detect, for example, any movement or any other indication that the animals in the chamber are conscious during after the third stage has begun or during any subsequent stages, an operator may actuate the lever which will open a valve and cause the chamber to be exposed to a high concentration of carbon dioxide from the carbon dioxide reservoir 8 or pure C0 ₂ from the pressurised carbon dioxide tank 9 to quickly slaughter the animals in the chamber as painlessly and/or peacefully as possible. As shown in Figure 7, in a further embodiment the system uses a pre-vacuumed or pressurised tank to charge and exhaust the system with a diaphragm; thus, in this embodiment, a single carbon dioxide source is provided instead of two. Specifically, pressurised carbon dioxide tank 9 supplies carbon dioxide in pre-mixed gas combination containers 1 , 2, 3, 4. Flow of the carbon dioxide supply is regulated by a valve in each container in the same manner as the non-pressurised supply described in relation to Figures 1 to 3 above. In use, pre-vacuumed or pressurised carbon dioxide is supplied directly to the pre-mixed gas combination containers 1 , 2, 3, 4 and each pre-mixed gas combination container 1 , 2, 3, 4 comprises a diaphragm 17, 27, 37, 47. A diaphragm is filled with fresh fluid (i.e. air or water) via an air inlet pipe 25 which supplies fresh fluid forced into the container 1 , 2, 3, 4 by inlet fluid pump 22 or emptied via an air outlet pipe connected to an outlet fluid pump 23. The fluid flow to and from each diaphragm is controlled by a fluid inlet valve 18, 28, 38, 48 and a fluid outlet valve 19, 29, 39, 49. Expansion or contraction of each diaphragm 17, 27, 37, 47 causes positive or negative pressure within the respective pre-mixed gas combination container 1 , 2, 3, 4 to push pre- mixed gas into or to extract pre-mixed gas from the chamber 6. For example, if diaphragm 17 is inflated with fresh air, pressure in the first pre-mixed gas in container 1 increases and the pre-mixed gas is vented out of the pre-mixed gas container 1 into gas inlet pipe 5 via valve 10. A fan 7 forces the pre-mixed gas combination into chamber 6 and ensures circulation of the pre-mixed gas within said chamber 6. Further, a recirculation pump may be provided as mechanical extraction means 14. If extraction means 14 are provided, valves 11 , 21 , 31 , 41 are provided to regulate the flow of pre- mixed gas from the gas outlet pipe 15 into the relevant pre-mixed gas container 1 , 2, 3, 4. Valves 10, 20, 30, 40 are provided on each inlet gas pipe comprises to ensure that a single pre-mixed gas combination is supplied to the chamber 6. This system is advantageous because it allows fresh air or water to be used to pump carbon dioxide mixtures into the chamber 6, this is much more economical than using gas. Further, it prevents heating the pressure pump thus ensuring that the system is as efficient as possible. Further, using a supply of pre-vacuumed or pressurised carbon dioxide increases the speed of gas exchange inside each pre-mixed gas combination container 1 , 2, 3, 4, as a result, the dose of pre-mixed gas supplied to the chamber 6 via gas inlet pipes is more even. In addition, the initial gas exchange within each pre-mixed gas combination container 1 , 2, 3, 4 is not limited to the pump capacity because the carbon dioxide supply is pre-charged. Moreover, leaks are less likely to occur and the carbon dioxide supply can be kept at a distance from the chamber 6 thereby increasing safety of the system. A settling tank (not shown) may be provided to keep gas which has been supplied to the pre-mixed gas combination container 1 , 2, 3, 4 and then excreted from said pre-mixed gas combination container 1 , 2, 3, 4 when carbon dioxide concentration is too high. In a further embodiment shown in Figure 8, the system comprises a single gas exchange means 107, a vent 115, three pre-mixed gas combination containers 1 , 2, 3 and a recycled gas container 102. In this particular embodiment, the chamber 6 is connected via a single pipe 52 to a gas exchange means 07 arranged to either push gas into the chamber 6 or to pump gas out of the chamber 6. The gas exchange means 107 is connected to the pre-mixed gas combination containers 1 , 2, 3, the vent 1 15 and the recycled gas container via pipe 52. The system is connected to a carbon dioxide tank (not shown) via a gas inlet pipe or pipes in fluid communication with the pre-mixed gas combination containers 1 , 2, 3. A valve (not shown) is provided in the gas inlet pipe or in each pre-mixed gas combination container 1 , 2, 3 to ensure that ingress of pressurised carbon dioxide into the relevant pre-mixed gas combination container is controlled. In use, pre-vacuumed and/or pressurised carbon dioxide is supplied to the pre-mixed gas combination container 1 , 2, and 3. Air, if necessary is supplied from the atmosphere directly into each pre-mixed gas combination container 1 , 2, 3 via an air inlet valve (not shown). Once the carbon dioxide has reached the required concentration in the relevant pre-mixed gas combination container 1 , 2, 3, a gas inlet valve (not shown) halts delivery of said gas into the pre-mixed gas combination container. In use, the gas exchange means 107 is operable to create a pressure differential in relation to the pre-mixed gas combination containers 1 , 2, 3. As a result, if there is a positive pressure differential relative to the containers 1 , 2, 3, a pre-mixed gas combination from a predetermined container is delivered to the gas exchange means 107 via pipe 52 whereas if there is a negative pressure differential relative to the containers 1 , 2, 3, the pre-mixed gas combination in the chamber 6 is extracted from said chamber 6 via pipe 52. In normal operation, the pre-mixed gas combinations extracted from the chamber 6 are stored in a recycled gas container. If the extracted pre-mixed gas combination is still within the predetermined concentration of carbon dioxide, the gas combination is returned to the relevant container 1 , 2, 3 via pipe 52. In the event the carbon dioxide inside the chamber 6 or pipe 52 increases and exceeds a predetermined level, pre-mixed gas is vented in an emergency procedure through vent 115 and air supplied via an air inlet valve in the chamber (not shown).

Referring now to the system of Figure 9, this system is largely the same as that described in relation to Figure 8; however, in this particular embodiment, the gas exchange means 107 comprises two gas exchange units 107a and 107b. In use, the first gas exchange unit 107a is connected to two of the pre-mixed gas combination containers 1 , 2 while the second gas exchange unit 107b is in fluid communication with the third the pre-mixed gas combination container 3 and a recycled gas container 102. A vent 115 is provided between the gas exchange units 107a and 107b to allow pre-mixed gas to be vented if the carbon dioxide inside the chamber 6 or pipe 52 increases and exceeds a predetermined level in an emergency situation. In this embodiment, carbon dioxide is supplied by a carbon dioxide tank (not shown) via a gas inlet pipe or pipes in fluid communication with the pre-mixed gas combination containers 1 , 2, 3. A valve or valves (not shown) are provided in the gas inlet pipe or pipes. The valve or valves control ingress of the pre-vacuumed and/or pressurised carbon dioxide into the relevant pre- mixed gas combination container. It should be noted that the embodiments described above in relation to Figures 8 and 9 vastly improve effectiveness of gas administration because they allow a dose of pre-mixed gas combination to be delivered within a short time, i.e. less than ten seconds, and preferably less than five seconds, whereas systems currently on the market which can take up to twenty seconds to be administered in each cycle. In these embodiments, the chamber and/or pre-mixed container valves are actuated either simultaneously or very closely and therefore gas exchange occurs quickly. Moreover, the systems described in relation to the above embodiments are reasonably straightforward and can be retrofitted on existing systems. Known systems retrofitted with the embodiments described above also benefit from more efficient and faster gas exchange. In addition, retrofitted systems are able to re-use or re-cycle carbon dioxide. Thus, the environmental advantages of the present invention are also incorporated when the above embodiments are installed in known systems.

Referring now to Figure 10, a valve network for use with the systems described above in Figures 7, 8 and 9 comprises a supply side and a return side. A main gas line having two one-way or isolation valves V6, V7 connects the gas sources and storage (gas containers, recycling storage, fresh air) with the chamber. Fluid communication from the pre-mixed gas containers, 1 , 2, 3, the recycled gas container 102 and fresh air via gas lines to the main gas line is controlled by valves V1 , V2, V3, V4 and V5 respectively. The chamber comprises a gas supply valve V12 and a gas return valve V13. Gas exchange is controlled via a vacuum line having a supply valve V8 and a return valve V9 and an air pumping line having a supply valve V10 and a return valve V11. In use, during the first cycle, air is extracted from the chamber when V8 opens. Concurrently, V1 and V6 are also opened to allow pre-mixed gas from the first pre-mixed gas container to be drawn into the chamber. The gas combination remains in the chamber for a predetermined period of time, in this example 24 seconds. Once the predetermined time elapses, valve V11 in the air pumping line opens thereby drawing the fluid out of the chamber via isolation valve V7 on the main gas line and into one of the containers. The gas exchange means are then preloaded for 6 seconds by opening of the return valve V9 on the vacuum line and the supply valve V10 on the air pumping line, followed by opening of chamber valves V12 and V13. During the second stage, opening of valve V1 is replaced with opening of valve V2, while during the third stage valve V3 is opened instead of V1. Finally, once a stunning cycle is completed, V5 opens and allows fresh air to be drawn into the chamber. The stun cycle time in this example is based on a 1000L chamber with a 140 litres per second air pump operating at incl. 60% efficiency. However, carbon dioxide levels may be adjusted as required.

Referring now to Figure 11 in an alternative embodiment, the pre-mixed gas combinations are provided directly in a bellows system comprising bellows 210, 220, 230, 240 each having air cylinders or electric actuators 90. The bellow containers 210, 220, 230, 240 are all connected to a single distribution pipe 105 which connects said bellow containers 210, 220, 230, 240 to a gas inlet pipe 5 and a gas outlet pipe 15. The gas inlet pipe 5 carries pre-mixed gas combinations from the distribution pipe 105 into the chamber 6 including crates 16. The gas outlet pipe 15 carries gas extracted from the chamber 6 to be returned to the single distribution pipe 105 and/or sent to a recycled gas container (not shown) via a recycling pipe 202. In this embodiment each bellow container 210, 220, 230, 240 can be pre-pressurised and/or pre-vacuumed prior to gas exchange occurring. In use, the relevant air cylinder or electric actuator 90 is activated to push or extract a gas mixture from one of the bellow containers 210, 220, 230, 240 into the distribution pipe 105 and thus into or out of the chamber 6 via gas inlet or outlet pipes 5, 15. A controller and valves, such as those described in relation to Figures 1 to 10 above, could be used to automate the system and/or achieve a specific bellow container 210, 220, 230, 240 activation sequence. Pre-vacuumed and/or pressurised bellow containers 210, 220, 230, 240 allow exchange pressures to be precisely set and can be more reliable than using standard gas combination containers. In addition, pre-mixed gas combinations can also be delivered in less than ten seconds, and preferably less than five seconds. Moreover, this system can be retrofitted on existing systems to improve performance of known systems and to allow recycling of carbon dioxide mixtures.

In an alternative embodiment (not shown) the de-humidifier could be replaced by an air-conditioning unit. In a further embodiment, the de-humidifier could be replaced or supplemented with an intercooler or a climate control unit. Moreover, it should be apparent that the sensors, controller, de-humidifier, air-conditioning unit, extraction means, fluid pump, fan or any other chamber climate control means could be combined or replaced with climate control means described in relation to another embodiment.

In a further embodiment, the fan and extraction means or gas exchange means could be replaced by bellows.

Although the system and method of the present invention has been described as having four stages, it should be clear to the skilled person that the system and method could have any number of stages between two and five depending on the bird species and the skilled person preferences. Depending on the number of stages, the pre-mixed gas combinations are referred to as first, second, third, fourth and fifth gas combination.

Moreover, it should be obvious to the skilled person that stun chambers different to the three types described above could be used. Further, although the pre-mixed gas combinations have been described as a blend of carbon dioxide and oxygen, it should be clear that other gases, such as inert gases including noble gases and nitrogen could be used as part of the mixture to increase the dose precision or instead of oxygen as appropriate. Further, other compounds such as medication or anaesthetic substances could be added to the mixture.

Additionally, it should be clear that pure carbon dioxide could be supplied directly from a pressurised tank.

It should also be apparent that the first and/or second and/or third and/or fourth and/or fifth gas combination could be circulated between the chamber and the relevant pre-mixed gas combination container to minimise or rectify gas concentration changes occurring as a result of use in the chamber and replacement with a different gas combination. In a further alternative embodiment, the pre-mixed gas containers could be expandable bladders and/or be pressurised to allow an operator to perform a manual emergency stun without electricity.

It should further be apparent that the term fan is intended to include any gas circulation means such as a pump or air conditioning unit.