EC-Seminar "Animal Welfare", August 24th - Sept. 2nd, 1999, Dublin

Humane Stunning of Poultry
- Part I: Electrical Stunning -

Ingrid Schütt-Abraham

Structure
	1.	Introduction
	2.	Water bath stunning
	3.	Requirements for an effective stun
	4.	Criteria for assessing the depth of a stun: 4.1 Epileptic fit / 4.2 Evoked potentials (EP)
	5.	Definition of an effective electrical stun in poultry
	6.	Currents required for an effective stun
	7.	Current flow durations ensuring an effective stun
	8.	Depth of immersion
	9.	Assessment of stunning efficiency under practical conditions: 9.1 Muscle tone / 9.2 Respiration / 9.3 Reflexes / 9.3.1 Cornealreflex / 9.3.2 Comb reflex
	10.	Clinical symptomatology of a stun with ventricular fibrillation in poultry
	11.	Technicalities of water bath stunning: 11.1 Head-to-body-stunning / 11.2 Group stunning / 11.3 Setting the stunner to the required amperage / 11.4 Minimizing the resistance/impedance / 11.4.1 Leg-shackle-contact / 11.4.2 Resistance of the water
	12.	Welfare problems related to water bath stunning: 12.1 Shackling / 12.2 Premature shocks / 12.2.1 Premature shocks due to overflowing water / 12.2.2 Pre-mature shocks due to the position of the bird / 12.3 Incomplete or delayed immersion of the heads
	13.	Requirements for sticking
	14.	Electrical head-only stunning with tongs
	15.	Electrical stunning and carcase quality: 15.1 Bleed-out / 15.2 Fractures and hemorrhages / 15.2.1 Fractures in the pectoral bones / 15.2.2 Breast muscle hemorrhages / 15.3 Other carcase defects / 15.4 Meat quality
	16.	Responsibility of the competent authorities as to water bath stunning
	17.	Literature

1. Introduction

The basic requirements for the welfare of poultry at the time of slaughter are laid down in Directive 93/119/EC (in the following only called the Directive). Methods for stunning shall instantaneously render the animals unconscious until death by bleeding supervenes and should not cause undue pain or anxiety themselves.

In commercial poultry slaughter plants processing thousands of birds per hour automatization of the slaughtering process was required at a very early stage. Electrical stunning in a water bath thus emerged as the method of choice: it provided the necessary immobilisation of the birds prior to automatic neck cutting, fitted nicely into the automatization process itself, and was cost-effective. Up to the present, the water bath stunner is the most widespread stunning method for poultry in a commercial abattoir.

2. Water bath stunning

With this electrical stunning method the birds have to be taken out of their transport crates and suspended upside down by fitting their legs into the metal shackles of a conveyor chain. Their heads are then dragged through a water bath which is connected to a voltage source. The electrical circuit is closed as soon as an animal's head is immersed in the water and its shackle contacts a metal bar attached to the stunner.

This procedure differs in three substantial aspects from electrical stunning methods used with other slaughter species:

1. Poultry is not stunned head-only, but head-to-legs, i.e. in any case the current will simultaneously flow through the heart.
2. Poultry is not stunned individually, but in groups, thus the particularities of parallel connections in an electrical circuit have to be taken into account.
3. Poultry is not stunned in intervals, but while continuously passing the stunner, which complicates the assessment of the stunning current.

3. Requirements for an effective stun

According to the Directive, the stun shall cause immediate loss of consciousness which lasts until death. Unconsciousness can be defined as a state in which the animal is impassible, i.e. does neither feel pain nor is able to experience stress, anxiety or fear.

Electrical currents which do not lead to an immediate stun may be experienced as painful even if the brain was included in the current path. Pigs that are not stunned by a head-only current lasting for 1 second will vocalize and show vigorous escape behaviour (Simmons, 1995). Chickens will leave the water bath stunner vocalizing and wildly flapping their wings in a futile attempt to fly away (Ali et al., 1995; Wormuth et al., 1981). This obviously painful experience of an ineffective stun can only be demonstrated with rather short exposure times, as prolonged whole-body exposure will have an immobilizing side effect. Prolonged exposure might also lead to a delayed onset of an effective stun which is accompanied by retrograde amnesia. Nevertheless, even a delayed stun accompanied by retrograde amnesia would only wipe out the memory, not the experience of the first painful seconds. In view of this reasoning with electrical stunning the onset of the stun has to be immediate.

4. Criteria for assessing the depth of a stun

As early investigations into electrical stunning in rabbits described an after-shock phase during which the animals showed a heart response to painful stimuli while in an outwardly unconscius state (Croft, 1952) the depth and duration of unconsciousness could not be reliably assessed from the behaviour of the animals alone. To overcome this problem laboratory experiments recording the electroencephalogram (EEG) were carried out. The EEG serves to detect changes in the functional properties of the central nervous system which are incompatible with the concept of consciousness.
Evaluating the EEG predominantly two criteria have been used: the occurrence of an epileptic fit, and the loss of evoked potentials.

4.1 epileptic fit

Passing an electrical current of sufficient strength through the brain of a slaughter animal will cause an epileptic fit. In the spontaneous EEG of a chicken this is demonstrated by a series of high amplitude low to medium frequency polyspike bursts, which are often, but not always, followed by electrical silence (Fig. 1). During a generalized epileptic fit consciousness is lost as is known from investigations in man. This method has been used by the working group of my institute.

4.2 Evoked Potentials (EP)

A more recent approach to assess as to what extent a stunning method disturbs brain function is the examination of evoked potentials. This method has been used by the working group at the University of Bristol/UK. It establishes whether the brain is functioning at the level of receiving and processing stimuli from its environment. Such stimuli are e.g. light flashes (investigation of visual evoked potentials, abbreviated VEP) or electrical stimulation of the body periphery (investigation of somatosensory evoked potentials, abbr. SEP). Such responses are immediately abolished in an instantaneous stun (Gregory et al., 1990c) like one due to concussion (Fig. 2).

In chickens there is a reasonably close association between the presence of epileptiform activity in the EEG and the absence of evoked potentials following stunning (Gregory and Wotton, 1989), although in a small percentage of birds evoked potentials may be present during a fit. The presence of such VEP or SEP does not necessarily prove that the animal is conscious as they can also be elicited in anesthetised animals. Their loss, however, indicates a severe functional disturbance of the central nervous system which is incompatible with the assumption of consciousness (Gregory and Wotton, 1990).

5. Definition of an effective electrical stun in poultry

Although an epileptic fit may be triggered in single chickens with currents less than 30 mA (Gregory and Wotton, 1987, Schütt-Abraham et al., 1983, Wormuth et al., 1981) it can be observed on a regular basis only with currents exceeding 65 mA (Schütt-Abraham et al., 1983; Wormuth et al., 1981). An associated loss of evoked potentials can only reliably be expected with currents of 120 mA and more (Gregory and Wotton, 1990).

Moreover, in chickens the polyspike bursts of an epileptic fit last for only 11 to 24 sec (Wormuth et al., 1981) and are on average terminated in water bath stunned birds 17 sec after the onset of current flow (Gregory and Wotton, 1987). A subsequent electrical silence is not always observed, and is usually terminated within 20 to 30 sec after the onset of current flow (Schütt-Abraham et al., 1983, Wormuth et al., 1981). The average time to complete loss of spontaneous brain activity if both carotid arteries are cut, however, was found to be 60 seconds in chickens (Gregory and Wotton, 1986).

With reversible electrical water bath stunning the epileptic fit is thus likely to be terminated before unconsciousness from bleeding can be ensured. However, if the stun triggers also ventricular fibrillation and thus causes cardiac arrest brain failure will be irreversible (Schütt-Abraham et al., 1983, Wormuth et al., 1981).

Ventricular fibrillation is the quickest method to abolish evoked potentials in chickens and ducks, and a quick one in turkeys (Gregory and Wotton, 1986; Gregory and Wotton, 1988). It irrevocably terminates spontaneous brain activity in chickens and ducks within 20 - 30 sec (Gregory and Wotton, 1986; Schütt-Abraham et al., 1983, Wormuth et al., 1981) (Fig. 3).

To guarantee instantaneous and irrevocable loss of consciousness both working groups have therefore recommended to stun poultry with currents causing not only an epileptic fit, but also ventricular fibrillation. This recommendation can also be found in the 1997 Report of the Scientific Veterinary Committee (SVC). Only the simultaneous induction of ventricular fibrillation ensures an uninterrupted change from reversible unconsciousness caused by the electrical stun into irreversible coma and death.

People have been concerned that bleeding a fibrillating chicken means killing a bird already in agony, and that meat hygiene legislations forbid their use for human consumption. Inconsistently, however, they don't apply this reasoning to captive bolt stunning after which the animal will likewise be in agony. The reasoning behind legislation is to keep animals from entering the food chain that are dying from an unknown etiology which might render their meat unsafe for human consumption. As with electrocution and captive bolt shot the agony is a result of an intentional action for the purpose of slaughter the use of these methods does not contradict legal principles. Article 5 of Directive 93/119/EC explicitly permits the use of killing methods prior to slaughter. Also the SVC recommends methods which not only stun but kill. The Directive also approves the use of the water bath stunner for poultry. As in a water bath stunner the current inevitably passes the heart with traditional sinus 50 Hz currents the risk of cardiac arrest can never be excluded. Even with as little as 30 to 45 mA passing through the individual chicken the fibrillation rate may vary between 5 and 10 % (Gregory and Wotton, 1987; Wormuth et al., 1981). Cardiac arrest may slow the process of bleeding in poultry, but does not adversely affect bleeding efficiency as will be shown later.

6. Currents required for an effective stun

The current thresholds for an epileptic fit are generally lower than those for ventricular fibrillation. Thus the latter would be the limiting factor. The following minimum currents (root mean square = rms) per bird have been recommended (BGA, 1990; Gregory and Wotton, 1990; Gregory and Wotton, 1991; Gregory and Wotton, 1991b; Knauer-Kraetzl, 1991; Schütt-Abraham and Wormuth, 1991; Wormuth et al., 1981):

In quails head-to-body-stunning with currents of 50 mA or more regularly lead to ventricular fibrillation (Gregory et al., 1991). Guinea-fowl was regularly killed with currents exceeding 80 mA (Schütt-Abraham et al., 1987; Schütt-Abraham and Wormuth, 1988).

The recommended amperages are mainly based on research using alternative currents of sinus 50 Hz - the European mains. They will achieve ventricular fibrillation in at least 90% of the birds (BGA, 1990; Fricker and Müller, 1981; Gregory and Wotton, 1987; Gregory and Wilkins, 1989c; Gregory and Wotton, 1990; Gregory and Wilkins, 1990; Gregory et al., 1991; Knauer-Kraetzl, 1991; Schütt-Abraham et al., 1987) and correspond well to the currents required for instantaneous and maintained loss of evoked potentials in chickens (Gregory and Wotton, 1990) and turkeys (Gregory and Wotton, 1991b).

For various reasons, eg to exclude ventricular fibrillation in case of ritual slaughter or to improve carcase quality, increasingly frequencies are used today which substantially deviate from sinus 50 Hz. With frequencies exceeding 100 Hz the rate of ventricular fibrillation declines. 350 Hz reduced the incidence of ventricular fibrillation to less than 10 %, and with 1500 Hz no cardiac arrest was observed (Gregory et al., 1991), making immediate sticking severing both carotid arteries mandatory. Also in turkeys the incidence of cardiac arrest after stunning with a constant current of 150 mA per bird declined. All examined hens fibrillated with 50 Hz, 60% with 300 Hz, 30 % with 480 or 550 Hz, while all survived 600 Hz. In turkey toms the corresponding rates were 53 % with 50 Hz and 38 % with 300 Hz, while at the higher rates no bird fibrillated (Mouchoniere et al., 1999). These percentages do only confirm the trend, as the number of birds per group was small (between 10 to 17 birds only).

In pigs stunned with 1.6 to 2.3 A (which is considerably higher than the 1.3 A recommendation) the quality and duration of an epileptic fit as expressed in the EEG were the same with up to 7000 Hz as with 50 Hz. While convulsive activities were more violent and occurred much quicker with high frequency stunning, sometimes wihile the animal was still under current flow, the return of corneal reflex and rhythmic breathing was not affected (Simmons, 1995). However, with high frequency currents lower than or near the recommended 1.3 A an earlier return of corneal reflexes and rhythmic breathing was observed in pigs, making sticking within 6 sec mandatory (Anil and McKinstry, 1992). This leads to the conclusion that with high frequency currents a higher amperage might be necessary to guarantee the occurrence of an epileptic fit. In fact it was reported that the threshold for triggering an epileptic fit was raised with increasing frequencies. Stunning pigs with 2000 Hz required roughly 50 % more current than with 50 Hz (Simmons, 1995).

Few investigations have been published on the welfare aspects of high frequency stunning in poultry. A 350 Hz pulsed direct current of 120 mA (rms) abolished evoked potentials in chickens (Gregory and Wotton, 1991). This demonstrates an effective stun although the rate of ventricular fibrillation declined to less than 10 % (Gregory et al., 1991; Gregory and Wotton, 1991). At 1500 Hz the fibrillation rate dropped to zero (Gregory et al., 1991), but the minimum currents required to abolish evoked potentials at this frequency were not reported. The results of investigations in turkeys suggest that increasing the frequency of a constant current stun decreases the time to recovery as judged from physical recovery criteria (Mouchoniere et al., 1999).

To ensure animal welfare in view of the absence of experimental evidence it should be assumed that also in poultry higher currents might be necessary than with sinus 50 Hz to produce an equivalent stun.

Directive 93/119/EC requires the competent authorities to choose adequate stunning currents and durations of current flow (Annex C II. 3. B. No. 1 and 2). The above mentioned recommendations which have been adopted by the Council of Europe have been transposed into the German "Tierschutz-Schlachtverordnung" (Regulation for the Protection of Animals at Slaughter or Killing) of March 1997. The 1997 Report of the SVC supports the recommended minimum currents for ducks, geese and turkeys to be included in the Directive and recommends 120 mA for chickens (with 100 mA considered the minimum for this species).

7. Current flow durations ensuring an effective stun

For fibrillating the heart the strength of the electric current is far more important than its duration. Within the normally used range of 2 to 12 sec of exposure in a water bath stunner currents insufficient for triggering ventricular fibrillation cannot be compensated for by prolonging the stunning time (BGA, 1990; Knauer-Kraetzl, 1991). As long as the stun is instantaneous a few seconds delay in reaching the threshold for ventricular fibrillation, however, seems negligible. Amperages will reach their appropriate maximum in water bath stunned turkeys and broilers within 2 sec and in layers within 4 sec (BGA, 1990; Knauer-Kraetzl, 1991; Schütt-Abraham et al., 1987; Wormuth et al., 1981), while in water fowl this takes several seconds longer (BGA, 1990; Knauer-Kraetzl, 1991). The current should therefore not flow less than 4 sec in chickens and turkeys resp. 6 sec in ducks and geese.

Prolonged current flow durations may improve the immobilising effect of the current. However, using them one has to ensure they will not mask an insufficient stun or adversely affect carcase appearance or meat quality.

It is also necessary to differentiate between the duration of current flow and the time of passage through the water bath stunner. They should not be used as synonyms! Under practical conditions the time the birds are exposed to the current may be considerably shorter than the time required for passing the stunner.

8. Depth of immersion

Only proper immersion of the birds' heads will guarantee sufficient current flow through the brain. A specification hereto is made in Annex C II B No.1 of Directive 93/119/EC. We recommend immersion up to the wing base to ensure good water contact also for the occasional undersized bird.

Deep immersion will also facilitate reaching the current thresholds for ventricular fibrillation. Geese stunned with 250 V received about 20 mA more if immersed up to their wings than if only immersed up to their eyes (Schütt-Abraham et al., 1992). Fears that deep immersion would impair stunning efficiency could not be substantiated (Gregory and Wotton, 1991c).

9. Assessment of stunning efficiency under practical conditions

During routine slaughter conditions in a commercial plant it would be quite impossible to monitor either EEG or ECG. Therefore one has

9.1 Muscle tone

The onset of current flow results in an instantaneous contraction of the skeletal musculature due to direct electrical stimulation. The bird becomes rigid, the wings are drawn to the body and the neck is stiff. If the current triggered an epileptic fit the bird will for some time remain in a tonic spasm after cessation of current flow. Birds who lose this rigidity the moment the current contact breaks are likely to have missed epilepsy. However, if the duration of current flow grossly exceeds the tonic phase the birds may leave the stunner flaccid. As long as the stun is instantaneous and the birds develop ventricular fibrillation prolonged current flow would be of no welfare concern.

Suppression or even absence of the clonic phase (jerks or wing flapping of the bird following tonic rigidity) can be attributed to the current passing the spinal cord, as during head-only stunning in chickens the clonic phase is strongly expressed (Gregory and Wotton, 1990; Gregory and Wotton, 1990b; Wormuth et al., 1981).

With currents below the immobilisation threshold chickens will leave the stunner vocalising and flapping their wings. This would happen with currents less than 20 to 25 mA per bird (Schütt-Abraham et al., 1983, Wormuth et al., 1981).

Chickens which develop ventricular fibrillation will not regain muscle tension after the end of the epileptic fit and may thus be easily recognized by their neck plumage "bristling up" 30 to 40 sec after current contact. Chickens with maintained heart function will smoothen their neck plumage within this time (Schütt, 1982; Wormuth et al., 1981). They will on average be able to withdraw their heads from an uplifting hand within 1 min, although some may regain this ability as early as 5 to 25 sec after the onset of current flow if currents are considerably less than recommended (Gregory and Wotton, 1990). Another means to differentiate between birds with and without ventricular fibrillation are hypoxemic convulsions which frequently occur during bleeding in non-fibrillating birds (Gregory and Wilkins, 1989c; Griffiths et al., 1985; Richards and Sykes, 1964 Schütt, 1982).

9.2 Respiration

During exposure to the current and under the influence of the epileptic fit respiration is inhibited. Birds which survived the stun will resume breathing within 30 - 40 seconds (Gregory and Wotton, 1990; Schütt-Abraham et al., 1987). In chickens, guinea fowl, ducks and quail with ventricular fibrillation cessation of respiration will be irreversible (Schütt-Abraham et al., 1987). In turkeys (and geese), respiratory movements may temporarily return even in birds which do not recover from the stunning process (Schütt-Abraham et al., 1987). This is in line with the observation that it takes turkeys considerably longer than chickens to die from hypoxemia (Raj, 1994).

9.3 Reflexes

Reflexes involving the brain stem are not necessarily evidence of consciousness as they can be elicited in an anesthetised animal. Their return after electrical stunning, however, demonstrates the beginning of the functional recovery of the central nervous system which is a prerequisite for the return of consciousness.

9.3.1 Corneal reflex

The corneal reflex is usually lost for a short period following the current flow. In birds which have just left the stunner the nictitating membrane is often covering the eyes and the reflex cannot be tested. Its regular elicitability right after cessation of the current flow, however, would indicate an insufficient stun.

The return of the corneal reflex after electrical stunning like the return of rhythmical breathing signals the beginning recovery of brain functions. In fibrillating turkeys and geese a transient return of the corneal reflex may be observed before it is abolished for good (Schütt-Abraham et al., 1987). In fibrillating broilers, layers, guinea fowl and ducks, however, it is irreversibly lost and a negative response to touching the cornea 30 - 40 seconds after the onset of current flow may be used to diagnose a humane stun with ventricular fibrillation. In survivors, it can be regularly elicited at this time (Schütt, 1982; Schütt-Abraham et al., 1987), in some birds for up to 3 1/2 min after neck cutting (Wormuth et al., 1981). In decapitated birds it persisted on average for 23 sec (Wormuth et al., 1981). In slaughter plants a reasonably close association was found between the percentage of chickens with negative corneal reflex ca. 30 sec after neck cutting and the percentage of birds with ventricular fibrillation verified by electrocardiogram (Weise et al., 1988).

9.3.2 Comb reflex

The comb reflex is generally lost right after the electrical stun and may not be elicited for more than 3 minutes even in birds which by then have obviously regained consciousness (Wormuth et al., 1981). Whether this shows a true analgetic effect as has been demonstrated in electrically stunned sheep (Gregory and Wotton, 1988b) or has to be attributed to a stun-induced state of immobility (Croft, 1952) remains to be determined. On the other hand, a positive comb response right after stunning would clearly indicate a failed stun and was associated with obvious conscious behaviour of the bird (Richards and Sykes, 1964).

10. Clinical symptomatology of a stun with ventricular fibrillation in poultry

In chickens and guinea fowl the clinical appearance of an effective and thus humane stun associated with ventricular fibrillation can be summarized as follows (Fig. 4): tonic spasms lasting clearly longer than the current contact, complete relaxation of the birds within 30 - 40 seconds after the onset of current flow ("bristling up" of neck plumage), irreversible loss of corneal reflex, irreversible cessation of respiration and absence of hypoxemic convulsions (wing-flapping) during bleeding.

These symptoms are less distinct in ducks. In turkeys and geese they cannot be used to distinguish between fibrillating and non-fibrillating birds on the line as corneal reflex, respiration movements and muscle tone although lost after an effective stun, may temporarily recur even in fibrillating birds (Schütt-Abraham et al., 1987). If in doubt whether the killing was effective, one has to take the birds off the line and lay them down for observation, looking for signs of recovery like permanent regular breathing or righting movements.

Judging the effectiveness of the stun by clinical symptoms under practical conditions one should look at the batch rather than at individuals. Even in turkeys where judging the depth of stunning in an individual by corneal reflex and respiration is not as reliable as in chickens or ducks improvements of the stunning conditions resulting in a more efficient stun will be demonstrable by a reduced percentage of birds responding to touching the cornea or resuming breathing between exiting the water bath and sticking.

Fig. 4: Signs of a stun with ventricular fibrillation in chickens

• complete relaxation of the bird ('bristled-up' plumage within 30 - 40 seconds after onset of current flow
• irreversible loss of corneal reflex
• irreversible cessation of respiration
• no convulsions ('wing-flapping') during bleeding

On-the-spot inspections in poultry slaughter plants to assess whether the stunning and bleeding process is in accordance with the requirements of the Directive will generally have to rely on the spontaneous behaviour and the reactions of the birds at various points along the slaughter line. As water bath stunning of poultry should lead to immediate loss of consciousness which lasts until death from bleeding, and as the birds should be dead or at least in deep coma when entering the scalding tank, inspection points would be the stunner entrance, the stunner exit, the point of sticking, the bleed-out rail, and the point of entering the scalding tank.

11. Technicalities of Water Bath Stunning

11.1 Head-to-body stunning

Water bath stunning is head-to-body stunning. As the heart is included in the current path with currents not deviating substantially from sinus 50 Hz the risk of ventricular fibrillation - even with low currents - is thus unavoidable (Fig. 5).

11.2 Group stunning

Water bath stunning means stunning in groups. All birds in contact with the water and the metal bar are simultaneously exposed to the current. Electrotechnically, the result is a parallel connection of the individual birds. As all birds are exposed to the same voltage the currents they receive are limited by their individual resistances. These individual currents sum up to the total current delivered by the stunner.

This deviation from the stunning of red meat animals is important for the supervising authority. Ammeters connected to electrical stunning devices for pigs measure the current which is supposed to flow through the animal's head. Ammeters connected to a water bath stunner for poultry show the sum of the currents flowing through all simultaneously immersed birds. To estimate the current the individual bird receives, the indicated amperage has to be divided by the number of simultaneously immersed birds.

The resulting value, however, gives only the average amperage per bird! The current which actually passes through an individual bird may well be higher or lower than the estimated average and could only be calculated from the voltage in the stunner and the individual impedance of the bird. The latter consists roughly of the body's inner resistance, skin resistance and the transition resistance between bird and equipment and would be unknown to the supervisor.

11.3 Setting the stunner to the required amperage

The total current the water bath stunner must provide to efficiently stun all birds can be calculated by multiplying the number of the simultaneously immersed birds with the required minimum current per bird. The voltage has to be adjusted to maintain this total current even under full load of the conveyor chain:

Imin = minimum current per bird
n = number of simultaneously immersed birds
Itot = total current in the water bath stunner

If at the beginning of slaughter one has no hint as to the voltage necessary to maintain the required currents or how many birds will be simultaneously immersed, the first birds of a batch may be passed through the stunner one at a time and the voltage adjusted until the necessary minimum current is ensured. Under full load of the chain, however, the voltage is bound to drop, so with this procedure one has to re-check the voltage immediately under full load and readjust it where necessary.

Recording currents from a water bath stunner during stunning will not give a straight line but a graph oscillating around the average amperage. These oscillations derive from two sources: first from the different resistances of the individual birds causing frequent changes in total resistance, second from the continuous change in the number of simultaneously immersed birds which insynchroneously enter and exit the water bath stunner (Fig. 6).

Checking the current one should also take into account that most commercially available volt- and ammeters are designed for measuring alternating currents of sinus 50 Hz. To correctly register currents with deviating wave forms or frequencies more sophisticated devices are required. Additionally it has to be ensured that the measured current is actually flowing between the electrodes, i.e. water bath and shackles, and not - due to faulty insulation or other shortcuts - leaking through secondary pathways within the stunner system itself. If the ammeter shows current flow without a bird being in the stunner the current leaks should be found and mended. It is in any case advisable to ask a competent technician for help in evaluating the stunning equipment.

Electrical stunning equipment including water bath stunners has recently been standardized by the International Electrotechnical Commission (IEC). The international standard has been approved as EN 60335-2-87:1999 on Jan 1st 1999 and may be obtained from the respective national standardisation organisation in the Member States which also provide translations. IEC Standards aim at guaranteeing the operators' safety, but shall also ensure proper functioning under adverse field conditions.

11.4 Minimizing the resistance/impedance

The inner body resistance of the bird cannot be influenced. However, of much greater practical importance is the bird/equipment resistance. This should be minimized by measures in the slaughter plant.

11.4.1 Leg/shackle contact

While the contact between the bird's head and the water is profuse and hardly adds to the bird/equipment resistance, a good contact between legs and shackle is of major importance. A high transition resistance due to loosely fitting shackles, soiled feet or dry, scaly legs may reduce the amperage below the level for an effective stun (Fig. 7). Application of a few drops of water will bring this transition resistance down. In some plants it may even suffice to hung the birds into shackles still dripping wet after cleaning. Hence the requirement of Directive 93/119/EC to wet the contact between legs and shackles (Annex C II 3. B. No. 3). One has to be careful, however, how and when to apply the water. In one turkey slaughter plant the birds' legs got splashed in the direct vicinity of the stunner entrance. This startled the birds, and sometimes wing flapping erupted which led to delayed immersion.

Not only a faulty legs/shackle contact but also maladjustment of the metal bar touched by the shackles while passing the stunner can markedly increase the transition resistance: in a turkey slaughter plant we once observed that not the shackles but the birds' legs were rubbing along this bar. Current density at the contact points was high, and some legs showed burns. Follow-up experiments in geese demonstrated that under these circumstances the individual currents could be reduced by 50% and more (Schütt-Abraham et al., 1992). Jamming the birds into the shackles might notably improve the legs/shackle contact by shedding scales and rubbing off skin, however, wetting the contact site with the bird's own body fluid cannot be considered a humane means to achieve a better stun.

11.4.2 Resistance of the water

Water conducts the electrical current rather well. It's own specific electric resistance depends on the amount of ions solved. Increasing the conductivity of the water by adding salt would be an easy means to reduce its specific resistance, however, would also pollute the environment - and quite unnecessarily. Experiments have shown that as long as the distance between the current delivering electrode and the animal's head is kept low (<20 cm) the drop in voltage between electrode and birds' heads due to the specific resistance of the water would be negligible (Schütt-Abraham et al., 1991). Moreover, taking into account the continuous pollution by the passing birds the conductivity of the water is bound to rapidly increase anyway.

Instead of using brine the design of the water bath should rather ensure that at any given point during passage of the stunner the birds' heads will be no more than 20 cm off the live electrode. This is why the Directive requires the live electrode to extend the full length of the stunner (Annex C II No. 3 para B No. 4). An electrode covering the bottom of the trough would be an optimal solution as it automatically places the head in the path of the current.

12. Welfare problems related to water bath stunning

Electrical stunning with the above recommended currents will lead to instantaneous and lasting unconsciousness in poultry. The method as such has therefore to be considered humane. However, water bath stunning has some welfare disadvantages: it requires prior shackling of the birds, immersion of the birds' heads cannot be guaranteed in some cases, and premature shocks may occur.

12.1 Shackling

Poultry is exempted from the general prohibition of the Directive to suspend animals by their legs before slaughter (Annex B No. 2). Shackling of poultry prior to stunning, however, has recently been criticised on welfare grounds as probably stressful (Sparrey and Kettlewell, 1994), derived from substantial increases in heart rate and breathing frequency after shackling (Zeller et al., 1989). However, heart rates in excess of 350 beats per minute which are common in shackled birds have also been reported in suspended chickens which were anaesthetized (Richards and Sykes, 1967). Moreover, similar responses in heart rate were observed in anaesthetized and thus unconscious birds in which pinching did not trigger a cardiac pain reflex whenever their position was changed from normal to upside down and back (Fig. 8). These changes were admittedly much smaller than in wake birds, however, this could have been due to a general depression of heart rate which comes as side effect of the anesthetics used (Rompun ® / Nembutal ®).

Although the reported findings in anaesthetized birds suggest that the rise in heart rate after suspension is a physiological response to the inverted position and not necessarily a sign of fear-induced stress they do not imply absence of the latter. Inverted handling resulted also in higher Cortisol concentrations than upright handling of broilers, although this effect was overridden by crating (Kannan and Mench, 1996).

Not only the inverted position as such, but also the weight of the animal pulling on its legs has been of concern, especially with heavy birds (turkeys may weigh up to 25 kg) or birds with leg problems. A survey showed 90% of broilers in intensively reared flocks to be affected by detectable, 26% by severe gait abnormalities (Kestin et al., 1992). It has also been discussed whether tight-fitting shackles would be experienced as painful because they were pinching the legs. This thought would be corroborated by the fact that the amplitude of the SEP were greatly reduced in shackled chicken compared to free-standing birds (Gregory and Wotton, 1989).

Although in view of these concerns the time of inverted suspension should be kept as short as possible, the birds should get a little time to calm down and stop wing flapping before they enter the stunner. For this purpose 12 sec have been recommended for chickens (Gregory and Bell, 1987). 3 min are legally allowed in Germany and the UK. During our early experiments layers were kept suspended by their feet for up to 60 min without obvious problems, while turkey stags tended to develop circulatory failure if shackled for 7 minutes. In a turkey slaughter plant where the birds had to travel a very long distance from the unloading bay to the stunner, many of the stags appeared already semi-comatose when entering the water bath. In another plant turkeys from the adjacent and the opposite side of the truck were alternatingly hung into the shackles. At the stunner entrance a closer look at the birds detected a pattern of alternating pink and bluish heads reflecting this fact, although the longest hanging to stunning interval did not exceed 5 minutes (BgVV, unpublished data).

12.2 Premature shocks

Premature electric shocks present a major welfare problem, and they may occur because of various reasons. They are caused by overflowing water or the positioning of the bird.

12.2.1 Premature shocks due to overflowing water

At the entrance of the water bath stunner even birds which unto then hung quietly may suddenly start to flap their wings. This is often caused by overspilling live water giving intermittent and painful electric shocks to the arriving birds.

To prevent this the Directive requires that by design of the stunner any contact between the entering birds and overflowing water should be excluded (Annex C II. 3. B. No. 4). This can be achieved by covering the stunner entrance with an extra (dry) ramp (Thomas, 1990; Wotton and Gregory, 1991), or by providing an overflow outlet in a less harmful position, e.g. at the exit ramp of the stunner or well underneath the stunner entrance (Fig. 9).

12.2.2 Premature shocks due to the position of the bird

Shackled turkeys tend to keep their necks stretched out more or less horizontally while letting their wings drop. Thus with a conventional sideways stunner passage the initial contact with the live water is usually made by the preceding wing. The resulting electric shock triggers off severe wing flapping, leading to intermittent and obviously painful current contact for up to 10 seconds (Fig. 10). Only then the bird's head is finally immersed and stunning prevails (BGA, 1990; Knauer-Kraetzl, 1991). Lowering the line over the water will considerably reduce this flapping interval (depending on line speed) but cannot entirely solve the problem as still the wings of the turkeys would hit the water first. Re-designing the stunner entrance seems the only way to overcome this problem.

In one plant a live metal ramp protruded from the stunner entrance providing current contact with the featherless parts of birds' heads. Contact of the wings with the live ramp would in a fully feathered bird not lead to current exposure as dry plumage provides a very effective insulation (Schütt-Abraham et al., 1992). The birds plunged into the live water head first with the wings tucked to their body due to their current-induced tonic state. Amperage recordings of single birds under these conditions showed a current peak due to the head-metal-contact followed by a current plateau after immersion of the bird (Fig. 11). The design worked surprisingly well with stags. Hens, however, rather often started wing flapping. Due to their higher transition resistance at the leg/shackle contact the initial currents were reduced and thus probably failed to produce an instantaneous epileptic fit (BGA, 1990).

Also a non-live ramp has been designed to keep the wings up to ensure the birds plunge head first into the water (Wotton and Gregory, 1991).

In another slaughter plant we found the turkeys hanging so close that only a few found room to move their wings or let them drop. Very little wing-flapping was observed at the stunner entrance.

12.3 Incomplete or delayed immersion of the heads

Incomplete or delayed immersion of the head may occasionally occur with all species, especially where birds are startled into head-raising and wing-flapping, but seems a predominant problem in water fowl.

Ducks often pass the stunner with only their lower beaks and throats immersed. As eyes and crowns protrude from the water a substantial part of the current might bypass the brain. Investigations have indeed shown that incomplete head immersion tends to give a lighter stun. Where only 20% of the birds maintained VEPs after stunning if their heads were fully immersed this rate increased to 70% with incomplete immersion (Gregory and Wotton, 1992).

Geese often erupt into head-lifting and wing-flapping, especially when they are dragged over the crates or contact the stunner entrance. Due to their long and flexible necks geese are able to withdraw their heads completely from immersion. In this case the resulting neck-to-legs passage will trigger ventricular fibrillation in fully conscious birds. Although we experimented in Berlin with quite a few stunner design changes including the use of cones head immersion could only be improved by lifting the geese over the stunner entrance and letting the line drop over the water (Knauer-Kraetzl et al., 1992).

In many countries slaughtering geese is a seasonal event which for economic reasons has to make do with existing poultry slaughter facilities. Stunners designed for chickens, however, enable geese (and large turkeys) to uplift themselves by pressing their wings against the stunner walls (BGA, 1990; Knauer-Kraetzl, 1991). The Directive requires water baths to be adequate in size and depth for the type of bird being slaughtered (Annex C II 3. B No. 4).

There will always be the chance that the occasional bird escapes immersion. Thus manual back-up will always be necessary and must therefore be available (Annex C II 3. B No. 5).

13. Requirements for sticking

Birds that develop ventricular fibrillation will not recover. Bleeding them would not be necessary from the welfare point of view (except, perhaps, for turkeys and geese which take considerably longer to die). However, as nobody can guarantee a 100% killing effect under commercial slaughter conditions all birds should be bled immediately, at least within 15-20 sec after the onset of current flow. To achieve a rapid and optimal blood loss the neck cut should sever both carotid arteries and jugular veins. Where the heart is not fibrillating and in the case of turkeys and geese, immediate bleeding severing both carotid arteries would be the only means to prevent recovery of brain functions (Gregory and Wotton, 1986) and is therefore strongly recommended by the SVC (1997).

14. Electrical Head-only stunning with tongues

Like red meat species poultry can be stunned head-only using tongues. Head-only stunning of chickens with 90 V led to an epileptic fit the isoelectric period of which lasted on average 36 sec (Richards and Sykes, 1967). Bitemporal application of 100 - 120 Volts for >1 sec would result in currents of 400 mA and more in chickens (Fricker, 1974; Gregory and Wotton, 1990b; Wormuth et al., 1981) and turkeys (Gregory and Wotton, 1991b). With currents of 400 mA the electrical silence following the epileptic fit lasted for at least 30 seconds after the onset of current flow (Fricker, 1974; Wormuth et al., 1981) and in some cases for up to 2 min (Fricker, 1974). The currents lead to an immediate loss of evoked potentials (22, 31). Moreover, the method would allow individual application of the current and thus lead to a more uniform stun. To ensure correct placement of the electrodes the birds' heads have to be well restrained.

V-shaped electrodes into which the birds' heads are pressed for head-only stunning of smaller slaughter numbers are on the market. As the voltages pose a risk to the operator the chickens are hung into the V by their heads before a button is pressed to activate the electrodes. Due to the weight of the birds their heads are dragged into the V providing good electrode contact, and if the birds are placed in a cone immediately after application of the stun and bled few will show signs of recovery. Nevertheless, the initial seconds of dangling by its head would seem to be far more stressful for the bird than the normal shackling routine.

As ventricular fibrillation does not occur bleeding within 10-15 sec severing both carotid arteries would be mandatory (Gregory and Wotton, 1990b; Gregory and Wotton, 1991b). However, clonic convulsions will be fully expressed, which might interfere with prompt sticking. Fixation of the birds, however, could be achieved using cones (Hillebrand et al., 1996) or providing for an immobilizing current along the spine. Combining head and head-to-body stunning in layers by applying 140 - 220 V across the head with an additional electrode on the legs led to 3/4 of the current passing the head and 1/4 passing the body (Wormuth et al., 1981). Reliable automatic head-only stunning equipment is not yet on the market, although various prototypes in different stages of development have already been tested under commercial conditions.

15. Electrical stunning and carcase quality

Correctly applied electrical stunning is an easy and highly effective means for stunning poultry. However, the method may lead to side effects which adversely affect carcase quality. As commercial slaughter plants tend to blame electrical stunning for any problem arising with product quality one should be able to sort out the facts. Electrical stunning may enhance certain fractures and hemorrhages, however, it does not interfere with bleed-out or lead to "exploding" organs. Moreover, variations due to origin of flock, rearing and fattening regimes, the physiologic condition of the bird, catching, crating and transport to slaughter, decrating, shackling, methods of bleeding, plucking, scalding and further processing after stunning do also play an important role in promoting carcase defects.

15.1 Bleed-out

Water bath stunning is head-to-body stunning with brain and heart included in the current path. Even with low currents around 45 mA (sinus 50 Hz) about 10 % of the birds will develop ventricular fibrillation (Fig. 5), (Wormuth et al., 1981). The traditional belief that animals with ventricular fibrillation (which is physiologically equivalent to cardiac arrest) will not properly bleed has long since been refuted by scientific investigations. Provided they are prompt and properly cut fibrillating birds will bleed as completely as non-fibrillating birds (Fig. 12).

However, bleeding is slower with cardiac arrest, and comparisons between fibrillating and non-fibrillating chickens carried out within the first 90 seconds of bleeding may thus show a significant difference in blood loss (BGA, 1990; Raj and Gregory, 1991; Schütt, 1982). Nevertheless, total bleed-out is not impeded and the ultimate blood loss within the range of birds with maintained heart function or gas stunned birds (Fig. 13) (Ali et al., 1995; BGA, 1990; Dickens and Lyon, 1993; Fricker and Müller, 1981; Gregory and Wilkins, 1989; Griffiths et al., 1985; Knauer-Kraetzl, 1991; Papinaho and Fletcher, 1995; Raj and Gregory, 1991; Schütt, 1982; Weise et al., 1982). No difference was found either in the amount of blood loss between chickens stunned with 50 Hz (associated with heart fibrillation) or 1500 Hz (excluding heart fibrillation) (Raj et al., 1997), nor between birds stunned with 50, 100, 150 or 200 mA (associated with respective increases in ventricular fibrillation) (Papinaho and Fletcher, 1995). The residual blood content in the musculature of electrocuted and bled chicken was in the same range as that of reversibly stunned and bled birds (Griffiths et al., 1985).

The significant differences not only in rate of bleeding but also in ultimate blood loss found in turkeys after unilateral neck cut and attributed to cardiac arrest (Mouchoniere et al., 1999) might in fact rather stress the importance of cutting both carotid arteries in turkeys to achieve a good bleeding. In our own investigations using a lateral stab severing both carotid arteries and at least one jugular vein no differences in blood loss (corrected for live weight) were found between fibrillating and non fibrillating baby turkeys, hens or toms (Knauer-Kraetzl, 1991).

To achieve rapid and complete bleeding both carotid arteries and jugular veins should be severed (SVC, 1997). Where birds are bled manually, efficiency of the bleeding cut may depend on the skill of the operator and the behaviour of the birds. Thus in one turkey slaughter plant we found in nearly 100% of the birds both carotids severed, while additionally both jugulars had been cut in 63% of the stags but in only 5 % of the hens. A reason for this difference was not easily found as both genders had been bled by the same operator.

However, sticking took place right after the stunner at an U-bend of the line. The stags left the stunner relatively relaxed. Gripping their necks and sticking could easily be carried out while they were approaching the operator. The hens, on the other hand, left the stunner often flapping their wings, making it difficult for the operator to grip their necks and often delayed the cut until the birds were moving away from him. This explanation was corroborated by the observation that in all stags where only one jugular had been cut this happened to be on the same side, while there was an equal distribution of severed left and right jugulars in hens.

Proper bleeding should be mandatory for all slaughtered birds. If fibrillating birds are not properly bled, especially if they are not bled at all, by the time they exit the plucker blood may have drained from the upper parts of the body, and the lower half of the suspended carcase may appear reddish while the rest shows a normal pale skin. This discolouration especially hits the necks, wings and feather tracts over the breasts (Gregory and Wilkins, 1989d). However, other factors seem to contribute, as in other investigations the processing of electrocuted unbled chickens failed to produce carcases that were visually different from normally processed ones (Griffiths, 1985; Heath et al., 1983).

The red discolouration of the lower half of normally processed, unbled birds has to be differentiated from the so-called "Redskin" condition in which the whole carcase appears bright red. Redskins seem to be caused by an inflammatory reaction of the skin whenever non-fibrillating birds escape bleeding and are scalded alive (Griffiths, 1985; Heath et al., 1983).

15.2 Fractures and Hemorrhages

Of all carcase damages attributed to electrical stunning only fractures in the pectoral bones and hemorrhages in the pectoral muscles have been demonstrated beyond doubt to be predominantly promoted by electrical stunning. While in the age of marketing frozen whole birds these defects did not bother companies as they did not show on the carcase, nowadays a substantial part of the production is sold as parts. Muscle hemorrhages are therefore easily detected by the client or consumer, and rejection will lead to economic losses.

15.2.1 Fractures in the pectoral bones

The onset of current flow stimulates instantaneous and maximal contraction of the entire skeletal musculature. This sudden contraction may cause fractures in the pectoral bones. Under quiet laboratory conditions one can even hear the bones crack the moment the bird becomes tonic. Fractures of the pectoral bones occur especially in heavily muscled birds like turkeys or broilers, or birds with extremely fragile bones like layers at the end of lay. In ducks or geese these fractures seem comparatively rare. None were observed in our own experiments (BGA, 1990; Knauer-Kraetzl, 1991), although broken coracoids have been reported in ducks (Gregory and Wilkins, 1990).

While the currents required to stun are generally blamed for a high incidence of bone fractures, results of investigations do not unequivocally corroborate this view. In a Danish study subjecting broilers to 0, 50, 100 or 200 V (AC 50 Hz) the rate of birds with broken bones increased from 7 to 21 % when the voltage was raised from 50 to 200 V. However, conveying the birds through the stunner without applying any voltage at all gave the highest rate with 25%, probably due to severe wing flapping observed in the stunner. As wing flapping was prevented by exposure to the current the percentage of broken legs or wings even decreased with increasing stunning voltage (Ali et al., 1995). Also, this study spotted bone fractures mostly in the clavicles, while scapulae and coracoids remained intact in all birds. British investigations using the same voltage range, however, found the rate of broken scapulae to be equally high or even exceeding those of broken clavicles (Gregory and Wilkins, 1990d). In an American study comparing 200 V and 50 V AC in a total of 72 chickens in a water bath stunner bone fractures occured in only 2 birds both of which had been stunned with 50 V (Dickens and Lyon, 1993). In ducks, the rate of broken coracoids rather tended to decline with higher currents (Gregory and Wilkins, 1990).

In another experiment the incidence of broken pectoral bones in turkeys was low and did not raise by increasing the stunning current from 75 to 250 mA per bird (Gregory and Wilkins, 1989c). Also, the wide variation in the percentage of fractures found under comparable stunning conditions in different plants or on different slaughter days leads to the assumption that other factors like susceptibility of the bird must be involved which facilitate or inhibit the occurrence of fractures. While in our laboratory investigation about 3/4 of heavy turkey stags got broken pectoral bones, the same currents, frequency and waveform applied to stags of similar weight in a commercial plant fractured only 20 % (BGA, 1990).

In spent layers stunned with currents of less than 100 mA (AC sinus 50 Hz) the rate of broken bones was reported to be around 70%. It dropped to 64% with currents exceeding 100 mA. Stunning at 1500 Hz further reduced this rate but still resulted in 54% affected carcases (Gregory et al., 1990). The rate of palpable fractures in the pectoral bones of culled breeding hens in one commercial plant was only 11% after plucking (BGA, 1990). In caged layers catching, crating, decrating and shackling may considerably contribute to fractures in pectoral bones. In 30% of caged layers we palpated fractured pectoral bones before the stunner entrance (BGA, 1990), which corresponds well with a 29% incidence reported by Gregory and Wilkins (Gregory and Wilkins, 1989b). According to their investigation removing layers from the transport crates and hanging them on shackles alone resulted in a 44% increase in the incidence of broken bones.

Although variations in wave form and frequency have been used to prevent bone fractures, no differences were found between 50, 200 and 350 Hz pulsed DC (Gregory et al., 1991). Variations in wave form (clipped sinus versus full sinus) for stunning broilers did not have the desired effect of cutting the rate of fractures but increased them instead (Gregory et al., 1995). Bone fractures in turkey stags were significantly reduced in one plant from 34 to 20 % by revoking the frequency from 108 Hz to 50 Hz at 150 mA per bird (BGA, 1990). Using 1500 Hz instead of 50 Hz managed to reduce the rate of broken bones in layers only from 64 to 54% (Gregory et al., 1990).

As broken pectoral bones are rarely found with other stunning methods it would be safe to conclude that electrical stunning as such promotes these fractures. However, in view of the conflicting results of the many published studies high rates of fractures should not automatically and solely be linked to high stunning currents at low frequencies.

15.2.2 Breast muscle hemorrhages

Electrical stunning has also been generally found to promote breast muscle hemorrhages which may as such occur in gas stunned (Raj and Whittington, 1990; Raj, 1994), captive bolt stunned (Hillebrand et al., 1996) or even birds having found dead on arrival (Gregory and Wilkins, 1990b). Susceptibility to hemorrhages is influenced by pre-harvesting factors such as rearing and the physiological condition of the slaughtered birds, catching, transport, stunning bleeding and processing and may thus vary widely between slaughter plants, slaughter days, slaughter batches, transport distances and farms of origin. Moreover, these individual factors interact with each other blurring the contribution each one may have and leading to contradictory results in different studies.

Haemorrhage scores in breast muscles depended on an interaction of stock, day of scoring and temperature during rearing (Kranen et al., 1998). In some plants the basic level of haemorrhages was so high that variations in the strength and amount of current would not show (BGA, 1990; Weise et al., 1982).

Breast muscle haemorrhages mainly occurred in the cranial third of the superficial and the profound pectoral muscle according to investigation in ducks and turkeys (BGA, 1990; Gregory and Wilkins, 1989c; Knauer-Kraetzl, 1991). They were a rare event and difficult to spot in the darker muscles of geese (BGA, 1990). In ducks we found predominantly the left side affected, which could not be explained as the current (sinus 50 Hz) had been equally applied to both sides of the head by a head-phone like device replacing the water bath (BGA, 1990; Knauer-Kraetzl, 1991).

In broilers, raising the current from 80 to 115 mA per bird had little effect on the incidence of breast muscle hemorrhages. Clipping the sinusoidal AC resulted in a higher rate of breast meat hemorrhages (Gregory et al., 1995). Increasing the average current over a range of 45 to 220 mA first increased, then decreased the rate of hemorrhages in the deep breast muscles of broilers, while hemorrhages in the superficial breast muscles seemed unaffected (Gregory and Wilkins, 1989e). No increase in breast muscle hemorrhages was found in ducks if the current was raised from 85 to 250 mA (Gregory and Wilkins, 1990). In turkeys, hemorrhaging in the deep but not the superficial breast muscle was found to be related to the current increasing from 150 or 250 mA per bird (Gregory and Wilkins, 1989c). No significant influence of current frequency (50 vs 200 Hz or 350 Hz) was found in broilers (Gregory et al., 1991; Hillebrand et al., 1996). In batches with a general high rate of moderate to severe blood spots in breast fillets this rate could be reduced from 70 to 47% replacing the automatic dorsal neck cut which severed both vertebral plus one carotid artery by a manual ventral cut severing both carotids and jugulars. Only a slight reduction (from 21 to 17 %) was observed, however, where the occurrence of blood spots was generally low (Gregory et al., 1999).

15.3 Other carcase defects

Other carcase defects often attributed to electrical stunning like red wing tips or pygostyles, red feather tracts, wing hemorrhages or engorgement of wing veins have been reported associated to the stunning current in some studies while others failed to show an association. Some have been associated with plucking, especially, if the birds were not properly bled (Gregory and Wilkins, 1990b). In some plants these defects occurred only occasionally even with as much as 150 V used for stunning chickens (Weise et al., 1982).

Red wing tips were related to wing flapping in chickens and turkeys, and the incidence of wing hemorrhages in turkeys more than doubled if the cut was applied unilaterally instead of bilaterally (Gregory, 1988; Gregory et al., 1989). No differences were found during visual inspection of carcases stunned with 200 or 50 V AC (Dickens and Lyon, 1993).

In chickens first an increase and then a decrease in the incidence of red wingtips, haemorrhages of the wing veins and the shoulder joint was reported, and the incidence of carcase defects was lowest when either less than 130 or greater than 190 mA was used to stun the birds (Gregory and Wilkins, 1989e). Compared to unstunned birds red wingtips and red feather tracts were considerably higher with 200 V, while hemorrhagic wing veins and shoulder hemorrhages peaked at 100 V compared to unstunned, 50 V stunned and 200 V stunned chickens (Ali et al., 1995). Inducing a ventricular fibrillation at stunning with currents around 81 mA increased the incidence of red wingtips and haemorrhaging at the humerus-radius joint in bled chickens, but there were no other obvious effects of inducing a ventricular fibrillation on carcase downgrading (Gregory and Wilkins, 1989d).

In ducks the incidence of red pygostyles, red wingtips, wing haemorrhaging, engorgement of wing veins, and leg muscle haemorrhaging was not affected by stunning currents between 85 and 250 mA (Gregory and Wilkins, 1990b).

Stunning turkeys with 75, 150 or 250 mA per bird did not affect the rate of skin and muscle haemorrhaging, or engorgement of veins with blood (Gregory and Wilkins, 1989c).

No relation was found between electrical current measured at stunning and the occurrence or severity of thigh haemorrhages in European investigations (Gregory and Wilkins, 1989e). Lambooij et al. (1999) found the incidence of thigh muscle hemorrhaging significantly reduced when broilers were restrained in a cone compared to being shackled. In the US a significantly higher incident score of haemorrhages in electrically stunned compared to unstunned broilers was found (Walker et al., 1993).

In our own investigations no association was found between the voltage (and corresponding current) level and haemorrhages visible on the whole carcase. Layers, ducks, light turkeys, turkey hens, heavy turkey stags and geese were closely inspected for red wing tips and pygostyles, haemorrhages on the breast skin, engorged wing veins or haematoma in the wings after shackling, after stunning, after scalding and after plucking. Engorged wing veins and fresh wing haemorrhages were detected in roughly 10% of ducks and turkeys prior to stunning, and must thus have been caused by handling the birds. 15 to 30 % of the turkeys showed also wing tip haemorrhages prior to stunning. No significant increase in visible blemishes was found during close inspection directly after stunning. The noticed defects were either present prior to stunning or detected first after plucking (BGA, 1990; Knauer-Kraetzl, 1991).

This is in accordance with other studies which showed a strong influence of the severity of plucking on these carcase defects (Gregory and Wilkins, 1990b).

The conclusion to be drawn from these contradictory results would be that bruising in poultry carcases depends on susceptibility of the bird and its physical condition at the time of slaughter and is promoted by stunning and post mortem processing. Especially harsh plucking of poorly bled birds is likely to produce particularly severe bruising. In view of this observation Gregory and Wilkins concluded it would be unwise to blame the stunner whenever there were an outbreak of bruising at a poultry slaughter plant (Gregory and Wilkins, 1990b).

15.4 Meat quality

Compared to red meat animals the maturation process in poultry is speedy and normally finished within a couple of hours. Conditions like PSE or DFD have not yet become a common problem as in pigs or cattle. According to more recent investigations the quality of poultry meat as such, e.g. determined by pH-value, tenderness, texture, colour or the overall sensory value after cooking, seems to be rather independent of the strength or duration of the currents used.

In comparison to unstunned chicken electrical stunning inhibited post mortem glycolysis as indicated by higher pH values and lower R-values at 15 min postmortem, but not after 24 h (Papinaho and Fletcher, 1995). Stunning amperages between 0 and 200 mA affected the rate of early rigor development but had no consistent effect on final breast meat quality (Papinaho and Fletcher, 1995). 75 min post mortem chickens killed by on average 132 mA showed a slightly, but not significantly higher pH and lower R-values than reversibly stunned chickens, and had elevated shear values in breast muscles deboned 75 min post-mortem (Lyon and Dickens, 1993). Varying stunning durations from 5 to 40 s at 100 V AC had no significant effect on postmortem rigor development, or meat quality (Papinaho and Fletcher, 1995b). Results indicated that the main effect of electrical stunning on early rigor development may be due primarily to inhibition of peri-mortem struggle (Papinaho et al., 1995).

Up to 4-6 hrs high currents resulted in significant greater shear values, but no significant differences were found between electroimmobilized (10.5 V / 500 Hz) and stunned (125 mA / 50 Hz, 5sec) chickens at 24 hours post mortem (Craig and Fletcher, 1997; Papinaho and Fletcher, 1996; Poole and Fletcher, 1998). Overall, stunning had no consistent effect on shear values (Craig and Fletcher, 1997; Papinaho and Fletcher, 1995, 57). However, if filleting took place within one hour longer stunning resulted in greater shear values (Young and Buhr, 1997; Young et al., 1996). In the case of turkeys, the texture of electrically stunned birds measured at 18 hrs after stunning varied considerably between the bird types (Raj, 1995). Stunning had no apparent effect on colour values of cooked chicken meat (Young et al., 1996) or cook loss (Papinaho and Fletcher, 1996).

Comparing the results of 50 V AC and 200 V AC it was concluded that 200 V AC for stunning broilers could be used without any detrimental effect on the cooked meat as long as the standard deboning trime of 4 h was maintained (Dickens and Lyon, 1993). Electrical stunning with 200 mA applied by an electrical knife for 5 s had no effect on the final muscle pH, raw muscle colour, cooked meat pH, cooked meat colour, cook loss or shear force in turkeys compared to carbon dioxide stunned or unstunned animals if the meat had aged for 24 hours on the carcase (Northcutt et al., 1998).

The varieties of stunning, bleeding and processing variables used in these studies makes drawing conclusions a difficult task. However, the general picture evolves that an influence of electrical stunning on meat quality is detectable only during the first four hours after killing and will wear out within 24 hours. This can be said in comparison to other stunning methods as well as to different currents and exposure times. Thus as long as deboning or filleting is delayed until 4 hours post mortem the use of electrical stunning methods which meet welfare requirements is not likely to have a negative effect on the final quality of the meat.

16. Responsibilities of the competent authority as to waterbath stunning

Under Directive 93/119/EC the competent authority has to ensure compliance of the slaughtering process with the requirements of the Directive (Chapter I Article 6 No. 1). In Germany the competent authority in charge of supervising animal welfare in an approved poultry slaughter plant would be the Official Veterinary Surgeon.

The EC has meanwhile started inspections in poultry slaughter plants in Member States, mostly in conjunction with inspections dominated by the poultry meat hygiene Directive 71/118/EEC. The inspection reports will soon be published in the internet.

The responsibilities of the competent authority with regard to the supervision of electrical stunning of poultry in a water bath stunner as stated or implied by Directive 93/119/EEC have been compiled in the Annex, including suggestions for supervising checks and their documentation.

17. Literature

• Ali, A.S.A., Jensen, F. and Jensen, J.A. (1995): Effect of different stunning voltages on blood loss, carcass quality and breast muscle pH - Archiv fuer Gefluegelkunde 59, 126-131
• Anil, M.H., and McKinstry, J.L. (1992): The effectiveness of high frequency electrical stunning in pigs. Meat Science 31, 481-491
• Bundesgesundheitsamt (BGA) (1990): Auswirkungen längerer Durchströmungszeiten und geringerer Spannungen auf die tierschutzgerechte elektrische Betäubung von Schlachtgeflügel. (Effects of prolonged stunning times and decreased voltages on the humane electrical stunning of poultry). Final Report, Research Project 88 HS 002 of the Federal Ministry of Nutrition, Agriculture and Forests, May 16, 1990
• Craig, E.W. and Fletcher, D.L. (1997): A comparison of high current and low voltage electrical stunning systems on broiler breast rigor development and meat quality. Poultry Science 76, 1178-1181
• Croft, P.G. (1952): Problems of electrical stunning. Veterinary Record 64, 255-258
• Dickens, J.A. and Lyon, C.E. (1993): Effect of two stunning voltages on blood loss and objective texture of meat deboned at various post-mortem times. Poultry Science 72, 589-593
• Fricker, C. (1974): Die elektrische Betäubung von Hühnern vor dem Schlachten mit Wechselstrom von 50 Hz. Vet. med. Thesis, Zurich
• Fricker, C. and Müller, H.R. (1981): Die elektrische Betäubung von Hühnern mit Wechselstrom von 50 Hz, deren Einfluß auf die Herzaktivität und die Bedeutung der Herzaktivität für den Ausblutungsgrad. (The electrical stunning of chickens with AC 50 Hz, its influence on heart function and the importance of heart function for bleed-out). Berliner und Muenchner tieraerztliche Wochenschrift 94, 108-111
• Gregory, N. G. and Wotton, S.B. (1986): Effect of slaughter on the spontaneous and evoked activity of the brain. British Poultry Science 27, 195-205
• Gregory, N.G. and Bell, J.C. (1987): Duration of wing flapping in chickens shackled before slaughter. Veterinary Record 121, 567-569
• Gregory, N.G. and Wotton, S.B. (1987): Effect of electrical stunning on the electroencephalogram in chickens. British Veterinary Journal 143, 175-183
• Gregory, N.G. (1988): Turkey stunning. Turkeys, June 1988, 29-30
• Gregory, N.G. and Wotton, S.B. (1988): Turkey slaughtering procedures: time to loss of brain responsiveness after exsanguination or cardiac arrest. Research in Veterinary Science 44, 183-185
• Gregory, N.G. and Wotton, S.B. (1988b): Sheep slaughtering procedures. V. Responsiveness to potentially painful stimuli following electrical stunning. British Veterinary Journal 144, 573-580
• Gregory, N.G., Austin, S.D. and Wilkins, L.J. (1989): Relationship between wing flapping at shackling and red wingtips in chicken carcases. Veterinary Record 124, 62
• Gregory, N.G and Wilkins, L.J. (1989): Effect of slaughter method on bleeding efficiency in chickens. Journal of the Science of Food and Agriculture 47, 13-20
• Gregory, N.G. and Wilkins, L.J. (1989b): Broken bones in domestic fowl: handling and processing damage in end-of-lay battery hens. British Poultry Science 30, 555-562
• Gregory, N.G. and Wilkins, L.J. (1989c): Effect of stunning current on downgrading in turkeys. British Poultry Science 30, 761-764
• Gregory, N.G. and Wilkins, L.J. (1989d): Effect of ventricular fibrillation at stunning and ineffective bleeding on carcase quality defects in broiler chickens. British Poultry Science 30, 825-829
• Gregory, N.G. and Wilkins, L.J. (1989e): Effect of stunning current on carcase quality in chickens. Veterinary Record 124, 530-532
• Gregory, N.G. and Wotton, S.B. (1989): Effect of electrical stunning on somatosensory evoked potentials in chickens. British Veterinary Journal 145, 159-164
• Gregory, N.G. and Wotton, S.B. (1990): Effect of stunning on spontaneous physical activity and evoked activity in the brain. British Poultry Science 31, 215-220
• Gregory, N.G. and Wotton, S.B. (1990b): An evaluation of the effectiveness of handheld stunners for stunning chickens. Veterinary Record 126, 290-291
• Gregory, N.G. and Wotton, S.B. (1990c): Comparison of neck dislocation and percussion of the head on visual evoked responses in the chicken's brain. Veterinary Record 126, 570-572
• Gregory, N.G. and Wilkins, L.J. (1990 ): Effect of stunning current on downgrading in ducks. British Poultry Science 31, 429-431
• Gregory, N.G. and Wilkins, L.J. (1990b): The role of electrical stunning, bleeding and plucking efficiency on the downgrading of chicken carcases. Veterinary Record, 127, 331-333
• Gregory, N.G. and Wilkins, L.J. (1990d): Broken bones in chickens: effect of stunning and processing in broilers. British Poultry Science 31, 53-58
• Gregory, N.G., Wilkins, L.J., Eleperuma, S.D., Ballantyne, A.J. and Overfield, N.D. (1990): Broken bones in domestic fowls: effect of husbandry systems and stunning method in end-of-lay hens. British Poultry Science 31, 59-69
• Gregory, N.G., Wilkins, L.J. and Wotton, S.B. (1991): Effect of electrical stunning frequency on ventricular fibrillation, downgrading and broken bones in broilers, hens and quails. British Veterinary Journal 147, 71-77
• Gregory, N.G. and Wotton, S.B. (1991): Effect of a 350 Hz DC stunning current on evoked responses in the chicken's brain. Research in Veterinary Science 50, 250-251
• Gregory, N.G. and Wotton, S.B. (1991b): Effect of electrical stunning on somatosensory evoked responses in the turkey's brain. British Veterinary Journal 147,270-274
• Gregory, N.G. and Wotton, S.B. (1991c): Effect of depth of immersion in the waterbath on the effectiveness of electrical stunning in chickens. Research in Veterinary Science 51, 200-202
• Gregory, N.G. and Wotton, S.B. (1992): Effect of incomplete immersion of the head in waterbath stunners on the effectiveness of electrical stunning in ducks. Research in Veterinary Science 53, 269-270
• Gregory, N.G., Wilkins, L.J., Wotton, S.B. and Middleton, A.L.V. (1995): Effects of current and waveform on the incidence of breast meat haemorrhages in electrically stunned broiler chicken carcases. Veterinary Record 137, 263-265
• Gregory, N.G., Robins, J.K. and Stewart, T.A. (1999): Blood spots in chicken meat after slaughter - A humane alternative. New Zealand Veterinary Journal 47, 77-78
• Griffiths, G.L. (1985): The occurrence of red-skin chicken carcasses. British Veterinary Journal 141, 312-314
• Griffiths, G. L., McGrath, M., Softly, A. and Jones, C. (1985): Blood content of broiler chicken carcases prepared by different slaughter methods. Veterinary Record 117, 382-385
• Heath, G.B.S., Watt, D.J., Waite, P.R. and Meakins, P.A. (1983): Further observations on the slaughter of poultry. British Veterinary Journal 139, 290
• Hillebrand, S.J.W., Lambooy, E. and Veerkamp, C.H. (1996): The effects of alternative electrical and mechanical stunning methods on hemorrhaging and meat quality of broiler breast and thigh muscles. Poultry Science 75, 664-671
• Kannan, G. and Mench, J.A. (1996): Influence of different handling methods and crating periods on plasma corticosterone concentrations in broilers. British Poultry Science 37, 21-31
• Kestin, S.C., Knowles, T.G., Tinch, A.E. and Gregory, N.G. (1992): Prevalence of leg weakness in broiler chickens and its relationship with genotype. Veterinary Record 131, 190-194
• Knauer-Kraetzl, B. (1991):Zur Optimierung der Betäubung von Puten und Enten vor der Schlachtung mit Berücksichtigung fleischhygienischer Belange. (Optimizing the stunning of turkeys and ducks prior to slaughter with regard to meat hygienic requirements) Vet. med. Thesis, FU Berlin
• Knauer-Kraetzl, B., Schütt-Abraham, I. and Wormuth, H.-J. (1992): Tierschützerische und technologische Probleme bei der Schlachtung von Gänsen. (Welfare and technological problems during the slaughter of geese). Rundschau fuer Fleischhygiene und Lebensmittelueberwachung 44, 3-6
• Kranen, R.W., Scheele, C.W., Veerkamp, C.H., Lambooy, E., Van Kuppevelt, T.H. and Veerkamp, J.H. (1998): Susceptibility of broiler chickens to hemorrhages in muscles: The effect of stock and rearing temperature regimen. Poultry Science 77, 334-341
• Lambooj, E., Pieterse, C., Potgieter, C.M., Snyman, J.D. and Nortje, G.L. (1999): Some neural and behavioural aspects of electrical and mechanical stunning in ostriches. Meat Science 52, 339-345
• Lambooij, E., Pieterse, C., Hillebrand, S.J.W. and Dijksterhuis, G.B. (1999): The effects of captive bolt and electrical stunning, and restraining methods on broiler meat quality. Poultry Science 78, 600-607
• Lambooij, E., Potgieter, C.M., Britz, C.M., Nortje, G.L. and Pieterse, C. (1999): Effects of electrical and mechanical stunning methods on meat quality in ostriches. Meat Science 52, 331-337
• Lyon, C.E. and Dickens, J.A. (1993): Effects of electric treatments and wing restraints on the rate of post-mortem biochemical changes and objective texture of broiler pectoralis major muscles deboned after chilling. Poultry Science 72, 1577-1583
• Mouchoniere, M., Le Pottier, G. and Fernandez, X. (1999): The effect of current frequency during water bath tunning on the physical recovery and rate and extent of bleed out in turkeys. Poultry Science 77, 485-489
• Northcutt, J.K., Buhr, R.J. and Young, L.L. (1998): Influence of preslaughter stunning on turkey breast muscle quality. Poultry Science 77, 487-492
• Papinaho, P.A. and Fletcher, D.L. (1995): Effect of stunning amperage on broiler breast muscle rigor development and meat quality. Poultry Science 74, 1527-1532
• Papinaho, P.A. and Fletcher, D.L. (1995b): Effects of electrical stunning duration on postmortem rigor development and broiler breast meat tenderness. Journal of Muscle Foods 6, 1-8
• Papinaho, P.A. and Fletcher, D.L. (1996): The effects of stunning amperage and deboning time on early rigor development and breast meat quality of broilers. Poultry Science 75, 672-676
• Papinaho, P.A., Fletcher, D.L. and Buhr, R.J. (1995): Effect of electrical stunning amperage and peri-mortem struggle on broiler breast rigor development and meat quality. Poultry Science 74, 1533-1539
• Poole, G.H. and Fletcher, D.L. (1998): Comparison of a modified atmosphere stunning-killing system to conventional elecrical stunning and killing on selected broiler breast muscle rigor development and meat quality attributes. Poultry Science 77, 342-347
• Raj, A.B.M. and Whittington, P.E. (1990): Effect of delayed bleeding on carcase appearance in gaseous stunned broilers. Veterinary Record 127, 308-309
• Raj, A.B.M. and Gregory, N.G. (1991): Efficiency of bleeding of broilers after gaseous or electrical stunning. Veterinary Record 128, 127-128
• Raj, A.B.M. (1994): An investigation into the batch killing of turkeys in their transport containers using mixtures of gases. Research in Veterinary Science 56, 325-331
• Raj, A.B.M. (1995): Poultry stunning / killing methods: Influence on carcase and meat quality. Proc. XII European Symposium on the quality of poultry meat, 25-29 September, Zaragoza
• Raj, A.B.M., Wilkins, L.J., Richardson, R.I., Johnson, S.P. and Wotton, S.B. (1997): Carcase and meat quality in broilers either killed with a gas mixture or stunned with an electric current under commercial processing conditions. British Poultry Science 38, 169-174
• Richards, S.A. and Sykes, A.H. (1964): Observations on the electrical stunning and slaughter of poultry. Veterinary Record 76, 835-839
• Richards, S.A. and Sykes, A.H. (1967): Physiological effects of electrical stunning and venesection in the fowl. Research in Veterinary Science 8, 361-368
• Schütt, I. (1982): Auswirkungen einer tierschutzgerechten Elektrobetäubung bei Schlachtgeflügel auf den Ausblutungsgrad und die pH-Wert-Entwicklung der Tierkörper. Vet. med. Thesis, FU Berlin
• Schütt-Abraham, I., Wormuth, H.-J., and Fessel, J. (1983): Electrical stunning of poultry in view of animal welfare and meat production. In: G. Eikelenboom: Stunning of Animals for Slaughter, Martinus Nijhoff Publishers, ISBN 0-89838-598-9
• Schütt-Abraham, I., Wormuth, H.-J., and Fessel, J..- (1987): Vergleichende Untersuchungen zur tierschutzgerechten Elektrobetäubung verschiedener Schlachtgeflügelarten (Comparative experiments on the humane electrical stunning of different types of slaughter poultry). Berliner und Muenchner tieraerztliche Wochenschrift 100, 332-340, (translated into English by Silsoe Research Institute No. 37 (New Series) in 1992)
• Schütt-Abraham, I. and Wormuth, H.-J. (1988): Cardiac arrest stunning in poultry. Proc. 34th International Congress of Meat Science and Technology, Workshop on Stunning of Livestock, 29 August - 2 September 1988, Brisbane, Australia
• Schütt-Abraham, I., Knauer-Kraetzl, B., Wormuth, H.-J. and Gregory, N.G. (1991): Effect of salinity in a water bath stunner on the amperage obtained during electrical stunning of poultry. Fleischwirtschaft 71, 1309-1310
• Schütt-Abraham, I. and Wormuth, H.-J. (1991): Anforderungen an eine tierschutzgerechte elektrische Betäubung von Schlachtgeflügel. (Requirements for a humane electrical stun of poultry). Rundschau fuer Fleischhygiene und Lebensmittelueberwachung 43, 7-8
• Schütt-Abraham, I., Knauer-Kraetzl, B. and Wormuth, H.-J. (1992): Effects of some stunning conditions on the currents obtained during the electrical stunning of geese. Fleischwirtschaft 72, 298-300
• Scientific Veterinary Committee (SVC) (1997): Report on the slaughter and killing of animals. European Commission, DG VI / BII.2, Ref. VI/1719/97
• Simmons, N.J. (1995): The use of high frequency currents for the electrical stunning of pigs. Phil. Thesis, University of Bristol
• Sparrey, J.M. and Kettlewell, P.J. (1994): Shackling of poultry: is it a welfare problem? Worlds Poultry Science Journal 50, 167-176
• Thomas, E. (1990): Design of water bath stunners in broiler plants. (Letter). Veterinary Record 127, 483
• Walker, J.M., Buhr, R.J. and Fletcher, D.L. (1993): Investigation of processing factors contributing to hemorrhagic leg syndrome in broilers. Poultry Science 72, 1592-1596
• Weise, E., Schütt, I. and Levetzow, R. (1982): Auswirkungen einer tierschutzgerechten elektrischen Betäubung auf die Fleischqualität geschlachteten Geflügels (Effects of stunning of broilers in accordance with animal welfare requirements on meat quality). Berliner und Muenchner tieraerztliche Wochenschrift 95, 241-247
• Weise, E., Wormuth, H.-J., Schütt-Abraham, I. and Levetzow, R. (1988): High voltage stunning of chickens and its effect on the quality of the meat. Fleischwirtschaft 68, 1416-1420
• Wormuth, H.-J., Schütt, I. and Fessel, J. (1981): Tierschutzgerechte elektrische Betäubung von Schlachtgeflügel. (Humane electrical stunning of poultry) VetMed Berichte 2/1981, Dietrich Reimer Verlag, Berlin, ISBN 3-496-02088-3
• Wotton, S.B. and Gregory, N.G. (1991): How to prevent pre-stun electric shocks in waterbath stunners. Turkeys 39 (2), 15 + 30
• Young, L.L. and Buhr, R.J. (1997): Effects of stunning duration on quality characteristics of early deboned chicken fillets. Poultry Science 76, 1052-1055
• Young, L.L., Northcutt, J.K. and Lyon, C.E. (1996): Effect of stunning time and polyphosphates on qualtiy of cooked chicken breast meat. Poultry Science 75, 677-681
• Zeller, W., Mettler, D., and Schatzmann, U. (1989): Untersuchungen zur tierschutzgerechten Betäubung des Schlachtgeflügels mit Mikrowellen (2450 MHz). Deutsche tieraerztliche Wochenschrift 96, 311-313
• Council Directive 93/119/EC
of 22 December 1993 on the protection of animals at the time of slaughter or killing
Official Journal of the European Communities No. L 340/21, 31.12.1993
• COUNCIL OF EUROPE: Recommendation R (91) 7 on the slaughter of animals
, approved by the multilateral consultation and adopted by the Committee of Ministers on 17 June 1991
• Tierschutz-Schlachtverordnung (TierSchlV)

Verordnung zum Schutz von Tieren im Zusammenhang mit der Schlachtung oder Tötung (Tierschutz-Schlachtverordnung - TierSchlV) vom 3. März 1997 (BGBl I S. 405), geändert durch Verordnung vom 25. November 1999 (BGBl I S. 2392)

< >

Copyright Dr. Ingrid Schütt-Abraham

---

Remarks and critical comments are welcome anytime.

---

Article summaries
the TSE-Hypertext-Project
Homepage