Stunning and Slaughter of Poultry: Evolving Consensus

by Mohan Raj, BVSc MVSc PhD

Poultry do not have muscular diaphragms. Consequently when birds are hung upside down for shackling purposes, abdominal organs compress their hearts. Additionally, compression of leg bones by metal shackles is an extremely painful procedure. Since inversion and shackling are unavoidable using an electrical water bath stunning system—a universal method of poultry slaughter—those concerned with the welfare of these birds, including legislators, have until recently been compelled to accept these painful and distressing practices.

In 2009,the UK’s Farm Animal Welfare Council (FAWC)—an independent governmental advisory body—reported on the welfare at slaughter of white meat animals (poultry species) and recommended that current systems of pre-slaughter inversion and shackling associated with water bath stunning should be phased out (

In addition to the problems associated with inversion and shackling, there are other welfare concerns associated with electrical water bath stunning systems. Further pain and distress is caused to birds:

  • who are forcefully removed from their transport containers, and in particular when birds are tipped or dumped on conveyors;
  • who may receive electrical shocks before being stunned (pre-stun shocks);
  • who may miss being stunned adequately and then reach the neck cutting machines;
  • who may be immobilised, rather than stunned, by the use of inappropriate electrical parameters;
  • who may recover consciousness during bleeding; and
  • who may enter scald tanks while conscious.

Accordingly, there is a drive within the European community to phase out the use of electrical water bath stunning. In fact as far back as 1982, the FAWC reported that many of the welfare concerns above would be eliminated if poultry were killed in their transport crates using controlled atmosphere methods.

At that time carbon dioxide was utilized for stunning pigs in some EU slaughter plants, so this gas was suggested as an alternative to electrical water bath stunning. However the induction of unconsciousness with gas mixtures is not immediate. Bird welfare advocates wanted an alternative to water bath stunning, but one that was not distressing to the individuals. The problem is that all vertebrates have well developed chemoreceptors to detect and respond to carbon dioxide; they find this gas extremely aversive and given an alternative, avoid an atmosphere containing it. While the welfare issues of water bath stunning were fully accepted, no one wanted to replace this system with a new set of problems such as stressful induction of unconsciousness.

Inert gas such as argon or nitrogen is a potential alternative to the use of carbon dioxide. Stunning or killing with inert gases, especially argon, has been studied largely in poultry and pigs. Animals, including birds, do not have chemoreceptors to detect inert gases and therefore do not show any aversion during initial exposure to hypoxia/anoxia induced with nitrogen, argon or their mixtures.

It is worth mentioning that studies involving humans indicated that the induction of unconsciousness with inert gas (nitrogen) is free from distress. Scientific literature suggests that human volunteers described their experience with the inhalation of nitrogen as a “euphoric way of losing consciousnesses.” Therefore it is suggested that use of hypoxia/anoxia is far more humane than the other gas mixtures containing carbon dioxide. Some reports suggest that Controlled Atmosphere Stunning (CAS) is not humane because of the distress that will be caused by the feeling of being unable to breathe just before the bird becomes unconscious. From the points above it can be seen that this concern only relates to carbon dioxide stunning.

However, exposure of poultry to argon or nitrogen results in convulsions manifested as wing-flapping after the loss of consciousness. This wing-flapping has been interpreted by some as a sign of distress. On the contrary it demonstrates the success of this method in inducing unconsciousness. The wing-flapping occurs when depression of activity in the brain extends to the part that governs motor functions and consciousness. Basically this wing-flapping—or anoxic convulsion as it should more properly be called—has no welfare implication; in fact the onset of these convulsions could be used as an indicator of the loss of consciousness.

There have been a number of studies in recent years assessing the potential welfare benefits of CAS. Some reports make much of the fact that electrical stunning induces unconsciousness in milliseconds whereas CAS is a more gradual process. When a bird passes through a water bath which delivers the correct amount of current across the brain, this method of stunning is most efficient. However the variation in bird size, the problems of birds evading the water bath and the variation in current delivered by the bath all reduce the process’ efficiency. Even if the industry figures on the efficacy of the process are accepted, with the billions of birds processed each year the small percentage that are reportedly not effectively stunned may equal millions of birds. When assessing the welfare of CAS, it is therefore crucial to look at the comprehensive slaughter process. Aside from improper stunning of birds, a large concern about electrical water bath stunning—as discussed previously—relates to inversion and shackling of live birds. CAS eliminates all of these welfare concerns.

From the points discussed above, CAS has the potential to deliver much higher welfare at slaughter than electric water bath stunning. Inevitably, our lack of knowledge and understanding of science frequently leads to misconceptions. But those that doubt the efficacy and welfare benefits of CAS will find more than enough published, and peer reviewed evidence, to confirm that this is the route we should take for the humane slaughter of poultry.

Mohan Raj BVSc MVSc PhD is a reader in farm animal welfare for the Department of Clinical Veterinary Science at the University of Bristol, United Kingdom.

Reference: Gregory, N.G. "Recent concerns about stunning and slaughter"; Meat Science 70 (2005) 481-491

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