PART THREE: Methods of stunning and killing | Viva!

PART THREE: Methods of stunning and killing

This report deals primarily with mainstream slaughter methods, which require animals to be stunned before they are killed. Animals slaughtered by a religious method are exempt from stunning requirements under UK legislation. Religious slaughter is dealt with separately in Viva!'s 'Going for the Kill' report.

Pre-slaughter stunning was originally introduced to protect abattoir personnel, rather than the animals themselves. The idea was to immobilize the animal to facilitate killing procedures. However, the principle of stunning has now evolved to encompass the idea that animals should be rendered unconscious before they are killed.

UK legislation states that stunning means, 'Any process which causes immediate loss of consciousness which lasts until death.' (5)

The following are stunning techniques used in UK abattoirs. How each stunning technique affects different animals will be examined in more detail in sections four to seven.

The Captive Bolt Pistol

This stunning method is widely used for all farmed animals. There are two types of captive bolt pistol: penetrative and non-penetrative. Penetrative stunners drive a bolt into the skull and cause unconsciousness both through physical brain damage and the concussive blow to the skull. The bolt on a non-penetrative stunner is 'mushroom-headed' and impacts on the brain without entering the skull. Unconsciousness is caused by the concussive blow.

The bolt is described as 'captive' because it flies out of the barrel but remains attached to the pistol. The pistol is placed on the centre of the animal's forehead and is either trigger-fired or fires automatically on contact with the animal's head.

Percentage of plants using the captive bolt pistols (penetrative and mushroom-headed) according to species and type of plant (10)

Cattle (captive bolt/pith): 71.1%

Cattle (captive bolt only): 24.9%

Sheep & goats (captive bolt only): 38.3%

Sheep & goats (captive bolt/pith): 1.1%

Pigs (captive bolt): 20.5% (mainly low throughput premises)

Pigs (captive bolt/pith): 1.1%


Pithing is carried out in the majority of cattle slaughterhouses. The practice involves inserting a wire or rod through the hole in the head made by the captive bolt. The rod is slid up and down to destroy the lower part of the brain and the spinal cord.

The Farm Animal Welfare Council say, 'From purely hygiene considerations, the practice is not favoured. On animal welfare grounds, provided the animal has been effectively stunned, there is no evidence that the practice increases suffering; by hastening brain death there may well be advantages.' (15). NB. Pithing is due to be banned in the UK during 2001. Studies show that this process may risk BSE infected brain material entering the animal's carcass.

Electric head-only stunning

Electric head-only stunning with tongs is used to stun cattle, calves, sheep, goats, pigs, rabbits and ostriches. The operator places a pair of electric tongs on either side of the animal's head and passes an electric current through the brain - supposedly causing a temporary loss of consciousness.

Percentage of plants using electric head-only stunning according to species (10) and type of plant


Cattle: 0.5%

Sheep & goats: 56%

Pigs: 73.9%

The RSPCA say that, 'There is increasing scientific evidence that some animals stunned electrically using tongs regain consciousness before they die from loss of blood.' (20) There are two reasons for this: either insufficient electrical current passes through the brain to stun the animal, or the time interval between stunning and sticking exceeds 20 seconds and the animal starts to regain consciousness.

The Scientific Veterinary Committee of the EU says that, 'Under the commercial conditions, a considerable proportion of animals are either inadequately stunned or require a second stun. This is mainly because of poor electrode placements, bad electrical contacts and long stun-to-stick intervals.' (17)

The Committee also expresses concern that, 'The strength of electric current used should be high enough for the species to induce a stun within one second of application. Otherwise, the animals could suffer a potentially painful electric shock before being stunned.'

The Welfare of Animals at Slaughter Regulations 1995 state that electrodes should not be used to stun animals unless the stunning apparatus incorporates a device which, 'measures the impedance of a load and prevents operation of the apparatus unless a current can be passed which is sufficient to render an animal of the species being stunned unconscious until it is dead.' (5)

In other words, electrical stunning equipment should not be used unless a device is attached which disables the equipment if a strong enough current cannot be achieved. This law is being openly flouted because according to the Meat Hygiene Service, 'such a device is not currently commercially available.' (9)

Waterbath stunning

The electric waterbath is widely used to stun chickens, turkeys, ducks and geese. Birds are shackled upside down on a moving conveyor which carries them to an electrified waterbath into which their heads are supposed to be immersed. The shackles contact a bar which is connected to earth. The strength of the electrical current has risen in recent years - with the aim of ensuring that birds suffer a cardiac arrest and die when they enter the waterbath. The Meat Hygiene Service report that in 1997/8 the average electric current applied to chickens stunned in an electric waterbath was 157 mA. However not all birds will suffer a cardiac arrest and scientific papers show that there are serious flaws with the waterbath system (see part 7: Poultry Slaughter - The Electric Waterbath).

Stun to kill techniques

Traditionally, animals are stunned before their throats are cut but the stun does not actually kill the animal. Animals die from loss of blood after their throats are cut. Stunning techniques do not kill animals outright because it has always been assumed that the heart needs to continue functioning so that as much blood as possible can be pumped out of the animal before s/he is eaten.

However in their 1984, 'Report on the Welfare of Livestock (Red Meat Animals) at the Time of Slaughter', the Farm Animal Welfare Council point to scientific research undertaken on pigs at the Meat Research Institute which shows that if animals die from a heart attack before they are knifed and bled out there is 'no effect on the amount of blood lost, the rate of loss or the residual content of blood in the meat.' (15)

FAWC conclude that, 'the release of blood from the animal need not necessarily occur prior to death... and should a change of attitude come about variations could advantageously be made in the design and operation of stunning techniques.'

The following table shows that the number of abattoirs using stunning methods which kill the animal outright are very low (10):

(Source: Meat Hygiene Service Animal Welfare Survey Report, 1997/8)


Sheep and goats - 3.5% (7% of the national kill)

Pigs - 1.9% (not including CO2 gas stun/kill)

Chickens - 1.1% (gas stunning)

Cattle - 0.5%

CO2 Gas Stunning

Four high throughput slaughterhouses stun and then kill pigs by exposing them to a mixture of carbon dioxide and air. The Meat Hygiene Service say that, 'The killing of pigs by exposure to CO2 is used in only four slaughterhouses but these premises process 25% of the total number of pigs slaughtered each year.' (10) 16.3 million pigs were killed in the UK in 1998, so over 4 million were stunned using CO2 gas.

The severe welfare problems associated with this stunning technique are discussed in part 5.

Cardiac arrest stunning

Cattle, sheep, pigs, rabbits and goats can be stunned and simultaneously given a cardiac arrest. However, as the table above shows, very few abattoirs actually use these methods. An electric current is either sent through the head and body at the same time to span the brain and heart or is sent though the head first to cause unconsciousness and then across the chest to cause a cardiac arrest.

If administered correctly, these methods do at least remove the risk of animals regaining consciousness while they are bleeding to death as the heart attack should kill the animal outright. However, the Scientific Veterinary Committee of the EU say that when the second method is used, 'a considerable proportion of animals are either inadequately stunned or require a second stun. This is mainly because of poor electrode placements and bad electrical contacts. Measures shall be taken to avoid these practices. Otherwise, when using method 2, the animals could suffer a potentially painful cardiac arrest.' (17)

Killing birds with gas

Researchers at the Department of Food Animal Science in Bristol have developed gas-based techniques which can be used to stun birds while they are still in their crates. When using this system, birds should not be shackled and knifed until they are dead. (21)

Viva! believes that the whole process of cramming live birds into crates for transportation is abhorrent. But now that this killing technique has become available, there is no excuse for subjecting the hundreds of millions of birds who arrive at UK abattoirs each year to the additional stress of being unloaded, shackled up and electrically stunned before they are killed. However, stunning with gas mixtures containing carbon dioxide is wholly unacceptable (see below).

The following gas mixtures are permitted under UK law (5):

a) A maximum of 2% total oxygen by volume and 90% argon (or other inert gas) by volume in atmospheric air.

b) 25% to 30% carbon dioxide by volume and 60% argon (or other inert gas) by volume in atmospheric air.

Stunning poultry with a gas mixture does not result in an immediate loss of consciousness. Poultry scientist Mohan Raj says that when carbon dioxide is present in the stunning chamber, turkeys exhibit, 'head shaking and gasping'. (22)

Carbon dioxide gas is lower in price than argon - meaning that abattoirs could be tempted to opt for the carbon dioxide/argon mixture rather than using 90% argon. Abattoirs should not be placing commercial interests above animal welfare.


Sticking is the term used to describe sticking a knife into an animal's throat or chest with the aim of causing blood loss and brain death. When the neck is severed, the killing is described as a 'neck stick' and when the major vessels near the heart are severed, the killing is described as a 'thoracic stick'. After being stuck, an animal's blood pressure is supposed to fall quickly, resulting in a rapid loss of blood supply to the brain. If the major blood vessels are adequately cut, animals should lose between 40 and 60% of their total blood volume. (23)

Researcher Steve Wotton explains that, 'Poor sticking, leading to inadequate or delayed exsanguination, can allow blood pressure to be maintained so that sensibility is regained before death supervenes.' (23)

In order to ensure that animals are not recovering from a stun, slaughtermen are supposed to check that animals have an absence of rhythmic breathing movements and an absence of a corneal (eye) reflex.