stunning methods for poultry

Stunning methods for poultry

MOHAN RAJ’ and ANGELIKI TSERVENI-GOUSI’

‘Division of Food Animal Science, Department of Clinical Veterinary Science, University of Bristol, Langford BS40 5DU, UK and ’Department of Animal Production, Aristotle University, 54006 Thessaloniki, Greece

Electrical waterbath stunning is the most common method used to stun poultry under commercial conditions. The voltage supplied to a multiple bird waterbath stunner must be adequate to deliver the required minimum current to each bird. High frequency ( > 300 Hz) electrical waterbath stunning needs further investigation to determine its efficiency. It should always be followed by a prompt neck cutting procedure where all the major blood vessels in the neck are severed. Irrespective of the waveform or frequency of the currents employed, constant current stunners should be installed under commercial conditions to ensure that the minimum currents are delivered to individual birds in waterbath stunners. Head only electrical stunning of poultry is being investigated in detail and there is scope for commercial development. Important features include (a) a constant current capable of delivering a preset current, (b) a bird restraining conveyor and head presentation devices enabling the stunning tongs to be accurately placed, (c) more effective electrical stunning tongs in terms of delivering necessary currents while using low voltages, and (d) induction of cardiac arrest immediately after stunning to eliminate wing flapping. Stunning/killing of poultry still in their transport containers using gas mixtures would appear to be the best future option as far as bird welfare is concerned. However, birds can also be stunned/killed on a conveyor using gas mixtures, thereby eliminating the stress associated with the shackling of live birds before electrical stunning. Under the conveyor system birds should be presented to the gas mixtures in a single layer. Within gas mixtures a minimum of 90% argon in air would appear to be the first choice. A mixture of 30% carbon dioxide and 60% argon in air is better than using a high concentration of carbon dioxide in air, and is therefore considered to be the second choice. A two stage system that involves firstly stunning broilers with a low concentration of carbon dioxide and then killing them with a high concentration of carbon dioxide can be used by those who wish to use this gas for economic reasons. The two stages should be distinctly separated so that the birds are stunned well before exposure to a high concentration of carbon dioxide in air. In comparison with carbon dioxide alone, a mixture of 30% oxygen and 40% carbon dioxide in air prolongs the induction of anaesthesia and the exposure time required to kill the birds. The addition of

All correspondence should be addressed to Dr M. Raj ([email protected])

0 World’s Poultry Science Association 2000

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Stunning methods f o y poultry: M . X a j and A. Tsemeni-Gousi

oxygen to carbon dioxide may therefore not have any benefit to bird welfare or the processors. Mechanical stunning of poultry using penetrating captive bolts or non-penetrating mushroom headed bolts has been developed. However, stunning with these devices results in very severe wing flapping and further research is necessary to find ways of alleviating this problem.

Keywords: Methods; poultry; stunning

Introduction Stunning of poultry has been practised either to meet the legislative requirements for humane slaughter, induce relaxation and thereby facilitate automatic neck cutting, or to reduce wing flapping during bleeding, a response which adversely affects carcass and meat quality. With increasing consumer concerns associated with the welfare of livestock during production and processing, and the meat trade becoming a global business, the poultry industry needs to take some proactive measures to sustain its growth and prosperity. One of those could be the implementation of an appropriate stunning method to protect the welfare of the millions of birds slaughtered annually for human consumption. A minimum welfare standard under any stunning system warrants that the duration of unconsciousness and insensibility induced by the stun should be longer than the sum of the times taken to perform neck cutting following stunning and that taken for brain death to occur from blood loss. Some stunning methods can induce an immediate loss of consciousness whereas others take several seconds. It is essential that stunning methods which do not induce an im-mediate loss of consciousness do not cause distress to the birds.

This paper reviews the welfare implications of some existing and other promising stunning methods (TubZe 1). The carcass and meat quality aspects of some of these methods have been previously reviewed (Raj, 1999).

Electrical stunning It is assumed that the ’near threshold’ depolarised state of neurones in the brain, especially in the thalamus and cerebral cortex, is a necessary condition for consciousness and the perceptual processes. Electrical stunning therefore needs to disrupt the depolarised state of the neurones in the brain and render the animals

Table 1 Stunning methods for poultry

Electrical Mechanical Gas mixtures

1. Head only electrical

2. Waterbath electrical 2. Non-penetrating

1. Penetrating captive bolt 1. Argon, nitrogen or other inert gas in air

2. Carbon dioxide and argon, nitrogen or

3. Carbon dioxide in air

4. Carbon dioxide and oxygen in air

concussion bolt other inert gas in air

~~

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Stunning methods for poultry: M. Raj and A. Tserveni-Gousi

or birds unconscious and insensible. The extent of disruption caused by electrical stunning can be measured using an electroencephalogram (EEG), and this is normally recorded from the surface of the brain. In humans the occurrence of grand ma1 epilepsy, characterised by the high amplitude (about 100 pV), low frequency (8-13 Hz) activity in the EEG, is associated with unconsciousness. It should be noted that there are some types of seizure which are not always associated with the loss of consciousness. Typical examples are the myoclonic form of petit ma1 and partial epilepsy.

Glutamic and aspartic acids are the major excitatory amino acid neurotransmitters in the central nervpus system (Meldrum, 1975). When they are released into the extracellular space as a consequence of electrical stunning they facilitate epileptiform activity in the brain. Gamma amino butyric acid (GABA) is a major inhibitory neurotransmitter which is also released into the extracellular space following electrical stunning. GABA-nergic effects can occur independently of the excitatory amino acid responses (Cook et al., 19921, and an enhanced GABA-nergic effect can prevent manifestations of epilepsy (Meldrum, 1984). This implies that the presence or absence of epileptiform activity in the brain following electrical stunning depends upon whether the excitatory or inhibitory pathways are activated by the current, the amount of neurotransmitter released, and/or the uptake by the receptors.

However, in birds, in contrast to the situation in mammals, the neurones that are homologous to the mammalian neocortex are located deep within the cerebral hemisphere instead of on the surface (King and McLelland, 1984). As a result, it can be assumed that the release of other neurotransmitters in the subcortical structure of the brain could also influence the manifestation of epilepsy following electrical stunning of poultry. In addition, noradrenaline and dopamine are present in abundance in the brain stem. The release of noradrenaline could enhance the glutamate induced excitation (Radisavljevic et al., 1994), whereas dopamine could decrease the amplitude of depolarisation and action potentials induced by glutamate, i.e. inhibit glutamate induced excitation. These effects have been seen in mice (Cepeda et al., 1992) but have yet to be investigated in poultry.

WATERBATH STUNNING The most common method of stunning poultry before slaughter is in an

electrical waterbath where the current flows from the bath through the birds to the shackle line, the earth. The electrical frequency, the waveforms (sine or square wave), the minimum current applied to each bird and the blood vessels cut at the time of exsaguination vary widely.

Some of the basic requirements of this approach are as follows: (1) the waterbath should be supplied with a voltage sufficient to ensure that every bird receives the recommended minimum current; (2) the electrode placed in the waterbath must extend the entire length of the bath; (3 ) there should be good contact between the legs of the birds and the shackle line; (4) the birds’ heads must be completely immersed in the bath; and (5) the water should not overflow at the entrance to the bath.

However, a universal problem with the multiple bird electrical waterbath stunning system is that the electrical impedance varies considerably between birds; thus, some birds can receive more than the necessary current while others receive an inadequate dose. This is because it is not possible to deliver the current required to induce an effective stun in each bird in a waterbath when a constant

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voltage stunner is used. Bilgili (1992) and Sparrey et al. (1992) have described the basic concepts and models of current pathways.

When a constant voltage stunner is used the current starts to rise from zero to the maximum depending on the supply voltage and the electrical impedance in the pathways. Because of this, there is a delay between the start of the application of the stun and the passage of the recommended current through the brain. On the other hand, however, a constant current stunner would expect infinite impedance in the pathway and therefore start with the maximum available voltage (e.g. 620 V). Under these conditions the recommended current would flow through the birds within 0.25s of the start of the stun (Sparrey et al., 1993). However, a basic requirement for implementing constant current stunning is that each bird in the waterbath should be electrically isolated.

The minimum currents necessary to achieve an effective stun can be derived in the laboratory using either the spontaneous EEG or loss of brain responsiveness determined using somatosensory evoked potentials (SEPs). However, one of the problems in determining the minimum currents required to stun poultry is that birds do not always show grand ma1 epileptic activity in the EEG following electrical stunning. Instead, at least five different EEG manifestations have been reported. Gregory and Wotton (1987) interpreted the polyspike activities shown by some broilers to be similar to that of petit ma1 activity in humans and not always associated with unconsciousness. However, it has been suggested that, if chickens show this kind of activity immediately followed by a quiescent phase in their EEG, it can be assumed that they have been adequately stunned (Schutt- Abraham ef a!., 1983). This can be achieved by either inducing cardiac arrest at the point of stunning or severing both the carotid arteries immediately after stunning.

It has also been shown that some chickens fail to develop an epileptiform activity in the EEG following electrical stunning in a waterbath with currents higher than l00mA per bird (Gregory and Wotton, 1987) but nevertheless lose SEPs in the brain (Gregory and Wotton, 1989). This indicates that neurotransmis- sion can be disrupted following stunning with high currents, even when epilepsy is not induced. However, a current of more than 120mA per bird is required to induce a sustained loss of SEPs until death supervenes from exsanguination following electrical stunning. This minimum current has been recommended while using either a 50Hz sinusoidal alternating current (AC) (Gregory and Wotton, 1990a) or 350Hz pulsed direct current (DC) (Gregory and Wotton 1991a). In the case of turkeys, the minimum current necessary to abolish SEPs immediately following electrical waterbath stunning has been found to be greater than 250mA per bird (Gregory and Wotton, 1991b). The minimum currents necessary to abolish SEPs have not been determined in other species of poultry (ducks, geese and quail) and the values recommended are based on the requirements for the induction of cardiac arrest at stunning.

Indeed, inducing cardiac arrest at the point of stunning in electrified waterbaths has advantages, but is not a prerequisite, as far as the welfare of the birds is concerned because any delay between the end of the stunning process and neck cutting (the efficiency with which the neck is cut) becomes less important. Induction of cardiac arrest at stunning will result in the cessation of the supply of oxygenated blood to the brain and thus eliminate the potential problem of recovery of consciousness in the birds. It has been shown that a sine wave (full, clipped or rectified) up to 125 Hz is more efficient in inducing cardiac arrest than the other waveforms.


Stunning methods for poultry: M . Raj and A. Tserveni-Gousi

When a 50 Hz AC with a sinusoidal waveform is used, the current necessary to induce cardiac arrest in 99% of chickens is 148 mA per bird (Gregory and Wotton, 1987). Stunning chickens with 120mA per bird (50Hz sine wave AC) induces cardiac arrest in about 90% of broilers. A stunning current of 105mA per bird provides at least 26 s and 52 s of apparent insensibility (based on the time taken for the neck tension to return) in non-fibrillated hens and broilers, respectively (Gregory and Wotton, 1990a). It is worth noting that it takes less time for the tension in the neck muscle to return in hens than in broilers and this can be attributed to the difference in the breast muscle volume. The minimum currents necessary to induce cardiac arrest in the majority (90%) of birds is 150mA per turkey, 130 mA per duck and goose and 45 mA per quail (Gregory and Wotton, 1988; Schutt-Abraham and Wormuth, 1988; Gregory et al., 1991). However, because the minimum current necessary to induce cardiac arrest is lower than that required to abolish brain responsiveness in poultry, welfarists argue that cardiac arrest stunning may be painful to birds. This is probably because, under the systems based on electrical waterbath stunning, only 10-28% of the applied current flows through the brain, the major proportion passing through the heart (32-4096) and pectoral muscles (42-63%) (Wooley et al., 1986a, 1986b).

In recent years there have been new developments in the area of electrical stunners for poultry using high frequencies and modified waveforms. Although the relationship between the various electrical parameters (frequency, waveform and current) at stunning and the effectiveness of the stun have yet to be well established, recent research with turkeys (basing the resumption of consciousness on the time taken for the neck tension to return) has shown that, as the frequency of the stunning current increases, the time taken to resume consciousness decreases (Mouchoniere et al., 1999). However, a major concern with the use of the time taken for neck tension to return is that consciousness may return well before that of neuromuscular coordination. The time taken for muscle tone to return after the application of an electric current may be influenced by the muscle mass and the amount of current flowing through it. For example, at a given current broilers weighing >2.70kg took longer to regain neck muscle tension than those weighing < 1.68 kg (Wilkins et al., 1998). This interpretation is also supported by another report involving two different weight groups of turkeys (Mouchoniere et al., 1999) where it was suggested that the greater the volume of skeletal muscle, the greater the proportion of current flowing through it. Another problem associated with the use of the time taken for a physical reflex to return is that it is subjective and could therefore depend on the experience of the observer. For example, Wilkins et al. (1998) found in broilers that, at the time of return of neck tension, 52% were able to balance when placed on their feet. These concerns highlight the importance of using more objective parameters such as changes occurring in the spontaneous EEG and/or the loss of SEPs to determine the effectiveness of the stunning current.

A potential welfare problem associated with the electrical stunning of ducks and geese in waterbaths is that their heads are not always immersed in the bath. Instead, the bill and crop may make contact with the water (Gregory and Wotton, 19921, leading to a suspicion that the current is flowing through the body and inducing cardiac arrest without rendering the birds unconscious. Although the currents necessary to abolish SEPs in these species of birds have not yet been established, because of the tendency of an electric current to flow through the heart and breast muscles it is very likely that the currents required to induce cardiac arrest in these birds could be lower than those necessary to achieve an effective stun.



Table 2 Major welfare concerns associated with the waterbath electrical stunning

0 The stress and trauma associated with removing conscious birds from their transport containers, particularly bird handling systems which require tipping or dumping of live poultry on conveyors.

compression of birds’ hock bones by metal shackles.

shackle line, a physiologically abnormal posture for birds.

stunned (pre-stun shocks). T h s occurs either as a result of wing flapping caused by shackling and hanging upside down or, in turkeys and geese, when the birds are hung upside down with their wings hanging lower than their heads. Consequently, they tend to make contact with the electrified waterbath before the immersion of their heads.

0 The pain and distress experienced by some conscious birds who miss being stunned adequately (as a result of wing flapping at the entrance to the waterbath stunners) and then pass through the neck cutting procedure.

inadequate stunning and/or inappropriate neck cutting.

upside down on the shackle line which can lead to injury and suffering.

0 The inevitable stress, pain and trauma associated with shackling conscious birds, i.e.

0 The stress and pain associated with conveying conscious birds hanging upside down on a

0 The pain experienced by some conscious birds who receive an electric shock before being

0 The pain and distress associated with the recovery of consciousness during bleeding from

A potential welfare problem at slaughter of game birds is that they attempt to fly while hanging

Considering all the potential welfare problems (Table 2) associated with the commercial stunning of poultry in waterbaths, it is hardly surprising that both scientists and the industry are seeking alternative stunning/killing methods to improve matters. On the other hand, because of the complexity of the electrical stunning systems in multiple bird waterbaths, some argue that it is unlikely that humane stunning will ever be achieved in these systems under commercial conditions (Boyd, 1994).

Nevertheless, investigations into the neurochemistry of electrical stunning in poultry and the impact of the magnitude of the current, the frequency (Hz), the nature of the waveform (sine or square wave) and the blood vessels cut during exsanguination on these are under investigation. This basic research project, funded by the Ministry of Agriculture, Fisheries and Food in the UK, should help to establish the electrical stunning conditions necessary to release threshold levels of neurotransmitters that are associated with the induction and maintenance of unconsciousness.

MEAD ONLY STUNNING This stunning method has not been implemented under commercial conditions

where high throughput rates are required. Some of the requirements are as follows: (1) birds must be restrained to facilitate the correct placement of the stunning electrodes; (2) electrodes mhst be kept clean and placed firmly on either side of the head so that they span the brain; (3) the voltage must be such that it is adequate to deliver the recommended minimum current; and (4) neck cutting and severance of all the major blood vessels in the neck should be performed within 15 s of stunning to prevent the resumption of consciousness in the birds.

Richards and Sykes (1967) reported that head only electrical stunning of chickens for 4 s with 90V, 50Hz AC followed by neck cutting resulted in the isoelectric EEG and remained so until death supervened through bleeding. Another study showed that the time taken for the neck tension to return (apparent duration of insensibility) after head only stunning of chickens (with a hand held

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stunner delivering 336 mA for 7 s) is 26 s (Gregory and Wotton, 1990b). It should be noted that this spell of apparent unconsciousness is only half that produced under the electrical waterbath stunning system. This implies that, following head only stunning, either a prompt severance of arteries supplying blood to the brain must be carried out or cardiac arrest should be induced immediately.

Nevertheless, minimum currents of 240mA for chickens and 400mA for turkeys have been recommended in the UK, having been determined using conventional electrical stunning tongs. The minimum currents necessary for stunning ducks and geese have not yet been determined, but will vary according to the density and type of feathers on the head, the skull thickness and porosity, the electrical impedance of the tongs and the electrode surface area that is in contact with the head.

A major problem following head only stunning is the occurrence of severe wing flapping which impedes prompt neck cutting. However, research carried out in the Netherlands has shown that the severity of wing flapping can be reduced by the application of a high frequency current through the spinal cord (Hillebrand et al., 1996). In that study, free standing chickens were stunned using a pair of tongs with either 50 Hz AC, 100 V for 4 s or 200 Hz AC, 100 V for 1 s followed by the application of a ’relaxation current’ of 100 000 Hz, 200 V for 4 s through the whole body. It was found that the two head only stunning treatments resulted in mild to severe (subjective scores) wing flapping and the application of a ’relaxation current’ reduced its severity. The hypothesis is that the application of a high frequency electric current through the spinal cord results in the depolarisation of the spinal neurones and thereby reduces wing flapping. Wing flapping following head only stunning can also be reduced by increasing the duration of the stunning process, for example to 15 s, and this has been practised by some processors.

Another approach is first to head only stun the birds using a pair of hand-held tongs and then kill them immediately by passing an electric current either from head to body or across the chest such that the electrical field spans the heart. This method is very similar to that practised under the head-to-back electrical stunning of pigs. Such an alternative stun/kill (ASK) method appears to be more humane than the induction of cardiac arrest in a waterbath stunner. Firstly, the stunning current is applied focally to the head in order to span the brain before the induction of cardiac arrest. This will enable the amount of current found to be adequate for any one species of bird on welfare grounds to be applied without compromising carcass and meat quality, a weakness of the waterbath system. Secondly, it is envisaged that the ASK method will be applied to birds which are restrained in a sitting posture using a pair of conveyors, thereby enabling shackling to be performed, either manually or automatically, on freshly killed carcasses. This will certainly eliminate the stress and pain associated with the shackling of conscious birds under the waterbath system.

With the support of funds from the Ministry of Agriculture, Fisheries and Food, an ASK system has been developed jointly by the University of Bristol and the Silsoe Engineering Research Institute in the UK. This system involves the following: (I) a constant current stunner that will deliver a pre-set current; (2) the development of a bird restraining conveyor and head presentation devices that enable accurate placement of the stunning tongs; (3) novel electrical stunning tongs that are more effective than the conventional tongs in terms of delivering necessary currents whilst using low voltages; and (4) the feasibility of inducing cardiac arrest immediately after head only stunning to eliminate wing flapping. The results of the studies in broilers so far indicate that this system is better than


Stunning methods fov poultry: M . Raj and A. Tsevveni-Gousi

the waterbath system on the grounds of bird welfare and of carcass and meat quality. We therefore wish to extend this research to evaluate welfare and quality benefits in turkeys, ducks and geese.

Gas mixtures In view of the stress associated with the handling of poultry before electrical stunning, it was suggested by the Farm Animal Welfare Council in the UK that research should be carried out to test the suitability of using carbon dioxide for stunning while the birds were still. in their transport containers (FAWC, 1982). However, it has been known that carbon dioxide is an acidic gas and is pungent to inhale in high concentrations. It is also a potent respiratory stimulant which could cause breathlessness before the loss of consciousness. The welfare implications of these features are that birds could experience unpleasant sensations either during initial inhalation of carbon dioxide or during the induction phase. This was illustrated by observations made on hens and a report involving turkeys (Raj, 1996). It was found that three out of eight hens and six out of 12 turkeys tested showed aversion to entering feeding chambers to obtain food and water when they contained 47% and 72% carbon diofide, respectively, in the atmosphere. Although there is no doubt that gas stunning of poultry in transport containers would eliminate some of the welfare problems associated with electrical stunning, it is important that the induction of anaesthesia with the stunning gas or gas mixture itself should not be distressing to the birds, and that it should be rapid. A potential problem is that birds that have just been stunned tend to regain consciousness rapidly when they exit from the gaseous atmosphere. For example, broiler chickens which were not killed with either argon induced anoxia or 45% carbon dioxide in air responded to a comb pinch as early as 15 s and 26 s, respectively, after returning to atmospheric air (Raj and Gregory, 19904.

Poultry can also be stunned/killed on a conveyor because it will at least eliminate the stress associated with live bird shackling which occurs with the conventional electrical stunning systems. Under a conveyor system a single layer of birds should be presented to the gas mixture in order to ensure that all are exposed to the mixture at the same time and not suffocated by layers of birds above them, and that they are all dead before exiting the gas mixture.

Some of the requirements are as follows: (1) the birds should be lowered into the gas mixtures within 15 s of leaving the atmospheric air; (2) gas concentrations should be continuously monitored and maintained at the recommended doses; and (3) the birds must be killed with the gas mixtures. Stunning of broilers with gas mixtures after they have been shackled is not acceptable on both welfare and humanitarian grounds.

ARGON, NITROGEN O R OTHER INERT GASES IN THE AIR Xenon, krypton and argon are chemically inert under most circumstances, yet

all have anaesthetic properties (Kennedy et al., 1992). Xenon is an anaesthetic gas under normobaric conditions, whereas argon and krypton have anaesthetic properties only under hyperbaric conditions. However, anoxia induced by argon or nitrogen at normobaric conditions can render animals and birds unconscious and insensible very rapidly. The atmospheric concentrations of argon, xenon and krypton are 0.94%, 0.05 ppm and 1.0 ppm, respectively. Understandably, the costs of xenon and krypton are prohibitive and therefore they have not been considered for stunning animals. In contrast, although nitrogen occurs in abundance (about


Stunning methods fur poultry: M . Raj and A. Tserveni-Gousi

79%) in the atmospheric air, because it is slightly lighter than air it is difficult to contain in a stunning apparatus. Argon, however, is heavier than air and it can be contained within an apparatus into which chickens can be lowered. Therefore argon was chosen as an anoxic agent to stun and kill poultry in this laboratory. The residual oxygen concentration in argon should be less than 2.0% (by volume).

The effects of anoxia on the brain are well documented. Ernsting (1965) reported that, under anoxic conditions, depression of activity in the brain extends progressively from the telencephalon to the diencephalon and then to the mesencephalon. Anoxic convulsions result from the release of the caudal reticular formation from the suppression by higher centres, particularly the cerebral cortex and rostra1 reticular formation (Dell et al., 1961; Ernsting, 1965). The implication is that the onset of anoxic convulsions themselves can be used as an indicator of the loss of consciousness. Research has shown that either epileptiform or polyspike activity occurs in the EEG of broilers during convulsions (wing flapping), suggesting that the convulsions originate from the brain. In this regard, it has been reported that anoxic depolarisation is associated with an increased rate of oxygen decline and reductions in the concentrations of cytochrome a and a3 in the brain (Raffin ef al., 1991). Raffin et a/. (1991) also found that the latencies of anoxic depolarisation and EEG suppression occurring during anoxia are inversely correlated with the latency to maximal cytochrome reduction. From the point of view of anoxic stunning, this would imply that the time to onset of unconsciousness (and, thus, convulsions) will depend upon the residual oxygen concentrations in the argon. For example, the time to onset would be shorter when the argon contained 0.5% rather than 2.0% residual oxygen.

Passive avoidance testing has shown that, because argon is an inert gas with no taste or odour, the birds did not detect its presence or feel any unpleasant sensations during the induction of anaesthesia. When argon with less than 2.0% oxygen was presented in a feeding chamber, all six hens tested entered spontaneously and were killed with the gas (unpublished data). When chickens and turkeys were exposed to 90% argon in air, the time taken to lose SEPs was also found to be rapid (Raj et al., 1991; Raj and Gregory, 1994). Although the stress of induction of unconsciousness with argon has not been determined in other poultry species, studies have shown that quail and ducks can also be killed with this gas while they are still in their transport containers (Raj ef al., 1998a; Tserveni- Gousi et a/., 1999).

CARBON DIOXIDE AND ARGON, NITROGEN OR INERT GAS IN AIR The recommended concentrations of carbon dioxide and argon should be

25-30% and a minimum of 60%, respectively. Although carbon dioxide or argon on its own will be able to stun poultry, a combination of these two gases has been recommended for two reasons. Firstly, when chickens and turkeys are exposed to this gas mixture the time to loss of posture or SEPs is faster than that taken during exposure to a high concentration of carbon dioxide or argon induced anoxia alone (Raj et al., 1992a, b; Raj and Gregory, 1994). Secondly, when this mixture is used for stunning/killing the concentrations of carbon dioxide and oxygen are less critical. For example, the carbon dioxide concentrations can vary between 25% and 30% and the oxygen concentrations can vary between 0% and 5%, and yet all the broilers contained in a transport container will be dead within 2 minutes of exposure. It is thought that this would provide some allowance for any inadvertent fluctuations in the gas concentrations resulting from atmospheric air being carried within the birds. A commercial advantage of using the carbon


https://www.researchgate.net/publication/21031172_Effect_of_carbon_dioxide_stunning_on_somatosensory_evoked_potentials_in_hens?el=1_x_8&enrichId=rgreq-dd8a084bc296ed002ee0ddf8c8e19c20-XXX&enrichSource=Y292ZXJQYWdlOzI0ODYyNjU1MjtBUzoxNTE4MTI2NDg3Mzg4MTdAMTQxMzIwNjM2NDY1NA==



dioxide-argon mixture is that the duration of exposure required to kill broilers is shorter than those with either argon or carbon dioxide alone.

Passive avoidance testing in turkeys showed that the majority ( > 80%) did not avoid a chamber to obtain food and water when it contained a mixture of 30% carbon dioxide and 60% argon in air (Raj, 1996). It is very likely that broilers also do not find this gas mixture aversive. Although gasping and head shaking behaviours are also exhibited during the inhalation of 30% carbon dioxide in argon, in comparison with higher concentrations of carbon dioxide these behaviours only appear in a small number of birds and then only for a short time. From the results of the study in turkeys it appears that any discomfort associated with the induction of anaesthesia with 30% carbon dioxide in argon was either tolerated by the birds or led to a rapid loss of consciousness before the birds could react to its presence. Humans inhaling various concentrations of carbon dioxide have also experienced low sensations of pungency and breathlessness from 30% carbon dioxide (Gregory et aI., 1990). By implication, it is suggested that, on welfare grounds, either 90% argon in air or a mixture of 30% carbon dioxide and 60% argon in air is acceptable for killing poultry, and these two gas mixtures have been approved for use in the UK (HMSO, 1995).

Although the stress of induction of unconsciousness with this gas mixture has not been determined in other poultry species, studies have shown that quail and ducks can also be killed in this way while still in their transport containers (Raj et aI., 1998a; Tserveni-Gousi et al., 1999).

CARBON DIOXIDE IN AIR The narcotic properties of carbon dioxide were recognised a century ago.

Kotula and Helbacka (1966) and Zeller et al. (1988) investigated the feasibility of using carbon dioxide for stunning broilers. A minimum of 55% carbon dioxide in air would be required to kill broilers in transport containers (Raj and Gregory, 1990a). However, because of the physical and physiological properties of carbon dioxide gas, welfarists believe that its use at high concentrations is unethical and therefore its application in this context is likely to remain controversial. Besides, increasing the concentration of carbon dioxide in the stunning atmosphere does not appear to reduce the time taken to lose consciousness (Raj and Gregory, 1990b). High concentrations of carbon dioxide seem to have no welfare advantage over either 90% argon in air or 30% carbon dioxide/60% argon in air in terms of rate of induction of unconsciousness. Carbon dioxide stunning/killing of poultry is therefore not permitted in the UK (HMSO, 1995).

The results of a preliminary study showed that an exposure time of 6 minutes to 50% carbon dioxide in air failed to kill ducks, and the birds which survived this treatment vocalised within 20 s and regained posture within 30 s after returning to atmospheric air (unpublished data). In this study it was also found that a 3 minute exposure to a minimum of 70% carbon dioxide in air was needed to kill them.

Nevertheless, if broiler and turkey processors in other parts of the world where poultry welfare is not protected by the law are keen to use carbon dioxide gas for economic reasons, then a low concentration could be used to induce unconsciousness in birds before killing them with a high concentration of the same gas. For example, broilers and turkeys could be exposed to 30% carbon dioxide for 1 minute and subsequently exposed to a concentration of >50% for a further minute to kill them. Under this two stage stun and kill system the two gas compartments must be distinctly separated to prevent conscious birds being



exposed to a high concentration of carbon dioxide in air. However, the feasibility of such a system needs further investigation.

CARBON DIOXIDE AND OXYGEN IN AIR This gas mixture has recently been considered for stunning broilers. However,

addition of oxygen to carbon dioxide may not be beneficial to bird welfare. For example, during the induction phase birds exposed to 50% carbon dioxide and 50% oxygen showed similar behaviour (i.e. showed signs of respiratory distress) to those exposed to carbon dioxide (Zeller et al., 1988). In addition, the presence of oxygen can prolong the time to loss of brain responsiveness and thus to unequivocal loss of consciousness. For example, the average times taken for broilers to lose posture during exposure to 40% carbon dioxide in air or 40% carbon dioxide/30% oxygen/40% nitrogen were found to be 29s and 35s, respectively. When a more objective measure of the loss of consciousness (such as the abolition of SEPs) was considered, it was found that some broilers retained their brain responsiveness for a period of longer than 2 minutes when exposed to a mixture of 40% carbon dioxide and 30% oxygen in air (Raj et al., 1998b). The duration of exposure required to kill broilers with this gas mixture has not been established. However, it has been reported that exposure of hens to 45% carbon dioxide in air results in the loss of SEPs, on average, after 30 s (Raj et al., 1990) and death in the majority of the broilers within 2 minutes (Raj and Gregory, 1990a). The presence of oxygen in carbon dioxide seems to prolong the time taken to loss of consciousness and reduces the duration of unconsciousness when the birds are just stunned. For example, when broilers were exposed to 45% carbon dioxide in air for 2 minutes and returned to atmospheric air they responded to comb pinching in two different manners (Raj and Gregory, 1990a). One group opened their eyes on average at 90 s (range 26-290 s); although the other group opened their eyes at 200-300 s, they did not respond to comb pinching performed for up to 6 minutes after exposure to the gas. This confirmed that the analgesic effect of carbon dioxide could be prolonged beyond the resumption of consciousness (Zeller ef al., 1988). The results from another study showed that broilers exposed to the carbon dioxide/oxygen mixture responded rapidly (30-62 s) to comb pinching after being returned to atmospheric air and they also opened their eyes when their combs were pinched (Raj et al., 1998a). Together, these results imply that the addition of oxygen to carbon dioxide reduced the time taken to resume consciousness as well as negating the analgesic effect of carbon dioxide. Therefore, if a carbon dioxide/oxygen mixture is to be used for stunning chickens, the birds should be exposed to the gas mixture for longer than 2 minutes and should then be immediately subjected to a killing procedure to prevent resumption of consciousness.

The welfare aspects of stunning/killing other species of poultry with this gas mixture have not been investigated.

Mechanical stunning Mechanical stunning of birds with a penetrating (percussive) or non-penetrating (concussive) captive bolt can be performed using either blank cartridges or compressed air. Spring loaded captive bolts have been used to stun poultry in both the USA and Germany. Captive bolts in general render animals and birds unconscious and insensible by inducing concussion in the brain. Penetrating bolts also induce structural damage to the brain.

Worlds Poultry Science Journal, Vol. 56, December 2000 301



Some of the requirements of this approach are as follows: (1) the birds must be restrained and their heads presented suitably; (2) the guns should be maintained in good working condition; (3) the power of the cartridges or the pressure of the compressed air line should be sufficient to achieve effective stunning; (4) the stunning should be achieved in a single shot without fracturing the skull; and (5) the birds (if they do not die) must be bled out immediately. The major welfare concerns associated with mechanical stunning are the stress associated with live bird handling (restraint), the incidence of failure to shoot in the appropriate position, the prevalence of mis-stunning (number of birds requiring more than one shot), delayed neck cutting and resumption of consciousness.

CAPTIVE BOLT Research carried out in the Netherlands involving broiler chickens and a bolt of

diameter 5 mm and length 25 mm (Hillebrand et al., 1996) showed that captive bolt stunning can be effective in inducing unconsciousness in birds. It was also found that the partial destruction of the left lobe of the brain by the captive bolt reduced the severity of wing flapping. More recently, a pneumatically operated penetrating captive bolt was evaluated for broilers in the University of Bristol. The results indicated that the ideal parameters for this method of stunning are a bolt of diameter 6 mm (minimum) driven at an air line pressure of 827 kPa and a penetration depth of 10 mm (unpublished data). Broilers stunned under these conditions died immediately but showed very severe wing flapping. Fur- thermore, it was found that, unless the bolt was fired perpendicular to the surface of the skull, it did not always stun the birds, a disconcerting feature from a welfare point of view.

It is not known whether applying a ’relaxation current’ through the bolt would help to reduce or eliminate the wing flapping. Alternatively, cardiac arrest may be induced in birds by the application of a 50Hz AC through the bolt to either a metal shackle or conveyor as the earth. This would certainly eliminate wing flapping and the potential problem of recovery of consciousness following stunning. Based on this concept, a prototype has been developed in this laboratory and further investigations are in progress.

CONCUSSION BOLT It has been reported that, based on their spontaneous behaviour, when birds

were shot using a bolt fitted with a plastic concussive head, effective stunning was achieved (Wotton and Hewitt, 1997). This bolt was fired using compressed air and, based on this and further research, an equipment manufacturer in the UK has produced a concussive device for casualty slaughter of poultry. Owing to the severe wing flapping that occurs following concussion stunning of poultry, this method may not be suitable for application under commercial conditions. However, like the penetrating bolt, it can be applied in combination with an electrical cardiac arrest treatment.

Acknowledgements This review was written as a part of an investigation into the development of alternative studkill techniques for broilers (MAFF Project Code MH 0114) funded by the Ministry of Agriculture, Fisheries and Food. Dr Tserveni-Gousi wishes to thank the University of Aristotle in Greece for their financial support to visit the University of Bristol.


Stunning methods ~ O Y poultry: M . X a j and A. Tserveni-Gousi

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