United Poultry Concerns
5 January 2005
Poultry Stunning and Slaughter Seminar December 16, 2004
UPC Summary of Welfare Issues Presented by Dr. Mohan Raj

“If the humane goal fails, it doesn’t matter what the meat quality is.” – Dr. Mohan Raj speaking on December 16, 2004 at the U.S. Department of Agriculture

“You can’t control the amount of [electrical] current flowing through a bird.” – Dr. Raj

“I think the waterbath has to be replaced.” – Dr. Raj

“Slaughter [neck cutting] without stunning induces unconsciousness quicker than carbon dioxide. Both are painful and distressing, and could be avoided.” – Dr. Raj

“Humanitarian intentions of eliminating avoidable pain and suffering during water bath stunning could be seriously compromised by carbon dioxide stunning.” – Dr. Raj

“Use of anoxia [absence of oxygen] is far more humane than gas mixtures containing carbon dioxide.” – Dr. Raj

On December 16, 2004, Dr. Mohan Raj held a three-hour seminar on the “Welfare, Economic and Practical Implications of Gas Stunning Prior to Poultry Slaughter” at the U.S. Department of Agriculture in Washington, DC. Dr. Raj is Senior Research Fellow in the Farm Animal Division of the School of Clinical Veterinary Science at the University of Bristol, Langford, UK (England).

Phone/fax: (44) 0117 928 9241 Email: M.Raj@bristol.ac.uk

UPC President Karen Davis attended the seminar. Following is a summary of key welfare issues presented by Dr. Raj regarding the pre-slaughter stunning of chickens and turkeys including ways to reduce the unavoidable cruelty to birds inherent in the electrified waterbath “stunning” system – the system most widely used by the poultry industry. Dr. Raj first discussed electrical stunning. He then discussed gaseous stunning. 1

Dr. Raj:


Stunning is a procedure that induces an unequivocal pathological brain state that is incompatible with the persistence of consciousness and sensibility in order to perform slaughter without causing avoidable fear, anxiety, pain, suffering and distress. Ideally the method should induce immediate unconsciousness and insensibility. If not immediate, the method should prevent avoidable pain, anxiety, distress and suffering. The stunning method itself should not be stressful.

  • The duration of unconsciousness induced by the stunning method should be longer than the sum of the time interval between the end of the stun and neck cutting and the time it takes for blood loss to cause death or for bleeding to cease.
  • Birds need to loose sufficient blood so that they don’t regain consciousness.
  • Irrespective of the stunning method and species of animal, the stunning method shouldinduce no less than forty seconds of unconsciousness. Inadequately stunned birds may recover consciousness while bleeding out, and enter the scald tank alive.
  • “Slaughter” refers to the immediate severance of both carotid arteries [not just the jugular veins]. If all neck muscles are not cut, birds retain consciousness via oxygenated blood flowing to the brain through the carotids.


I. Inevitable pain: The electrical waterbath stunning method used by the poultry industry causes inevitable pain associated with:

  • Tipping birds out of crates to be shackled : Poultry are the only species of sentient farm animals that could be treated this way, and there is no sound scientific evidence to suggest this is not a welfare problem.
  • Shackling : The force of this hanging procedure [birds are hung upside down and clamped in the shackles at their ankles on a conveyer belt] causes pain because the affected bone surface is enriched with pain receptors causing over 90 percent of birds to flap their wings due to pain. There is enough scientific evidence to suggest that shackling live birds increases the prevalence of dislocated joints, broken bones and muscle bruising in conscious birds.
  • Painful pre-stun electric shocks : These painful shocks predispose birds to miss the electrified waterbath [they avert their heads and faces from it].
  • Miss-stunning : Birds who miss the waterbath stunners due to flapping their wings in pain, smaller than average size, head-raising, etc. are still physically in contact with adjacent birds and are thus likely to receive painful electrical shocks as a result of that contact.
  • Variation in electrical impedance (resistance) : Although the electrical current is supposed to flow through the whole bird in the electrified waterbath, various body parts including fat, muscle tissue, and bone ends impede the flow of electricity. Number of birds in the stunner also affects the amount of current each bird gets. Though the industry strives for uniformity, we can’t harmonize electrical resistance in broiler chickens. Exactitude using the electrified waterbath stun cabinet method is not possible.

II. Overall Perception:The complexity of multiple bird waterbath stunning is not conducive to maintaining good welfare. Effectiveness of the stun cannot be determined. The method, widely practiced because it is simple and cheap, cannot be controlled.

III. Effective electrical stunning criteria include inducing epileptiform activity in the EEG followed by spreading depression (profoundly suppressed EEG indicating lack of consciousness] indicative of generalized epilepsy.

IV. Problem with monitoring physical signs of stunning (unconsciousness and insensibility):

  • Both effectively and ineffectively stunned broiler chickens exhibit seizures and apnea (absence of breathing); therefore, these are not good indicators of unconsciousness and insensibility following electrical stunning.
  • In the absence of profound EEG suppression (“spreading depression” indicating lack of consciousness), an animal regains painful consciousness. If electrical stunning is incomplete, it is a painful experience to recover from that immobility.

V. Manufacturer, Industry Responsibility:

  • The poultry industry has a responsibility to consumers to ensure humane slaughter (one that occurs without causing animals avoidable fear, anxiety, pain, suffering and distress) – even when not legislated. But this responsibility is not being exercised. The poultry industry does not know what they are buying when they buy these machines, and manufacturers, who should be responsible for providing humane equipment to industry, fail to exercise that responsibility.


I. The intention and purpose of gaseous stunning is to eliminate uncrating of birds, hence the inevitable anxiety, pain, distress and suffering in conscious birds from uncrating till death. Gaseous stunning isintended to eliminate the problems inherent in multiple bird waterbath electrical stunning. 2

II. Welfare criteria for effective gaseous stunning:

  • Induction of unconsciousness should be non-aversive.
  • Induction of death should be rapid.
  • Birds should be killed, rather than stunned, in the transport crates because birds vary in their response to gaseous stunning and because it is necessary to prevent birds from waking up during the slaughter process.
  • For example, say each crate contains 35 birds, the 35 th bird has a much longer time interval than the first bird from initial gas exposure to shackling.

III. Bird Response to Recommended Gas Mixtures (Controlled Atmosphere Stunning, or CAS) Versus Carbon Dioxide (CO2)

  • Birds do not avoid (show aversion to) anoxia (lack of oxygen) created by argon or nitrogen, but birds do show aversion to high concentration (40% by volume or more) of carbon dioxide.
  • In experiments in the US and UK, turkeys and chickens made fewer stops and retreats when argon was present while showing an increased tendency to stop when carbon dioxide was present. When carbon dioxide levels are high (40%+), birds gasp, shake their heads, and stretch their necks to breathe. But in the presence of argon, there is no sign of birds gasping or stretching their necks to breathe.
  • The reason is that birds have chemical receptors in their lungs (intrapulmonary chemoreceptors, or IPCs) that are acutely sensitive to carbon dioxide but insensitive to anoxia (lack of oxygen) and hypoxia (subnormal levels of oxygen).
  • Chickens, like other birds, have no receptors to detect argon or nitrogen, no receptors to detect lack of oxygen.
  • The small amount of head shaking in chickens in an argon chamber indicates they are trying to ‘wake up,” rather than experiencing suffocation, hence suffering, as in a CO2 chamber.
  • Humans can breathe comfortably with 30% CO2. Beyond that, humans experience a feeling of suffocation and breathlessness.
  • Carbon dioxide (CO2) increases the rate and depth of breathing to expel the CO2 from the lungs, but breathing actually increases the intake of CO2; thus the desire to breath to expel CO2 causes suffocation.
  • Anoxia/hypoxia differ from suffocation/asphyxiation . Unlike anoxia, suffocation involves receptors that register the physical separation of the upper respiratory tract from the outside atmosphere resulting in dyspnea (shortness of breath, a subjective distress in breathing)
  • CO2 breathlessness is the same as pain. Dyspnea or breathlessness in humans activates regions associated with pain and induces an emotional response of panic.
  • The panic and pain of dyspnea induced by carbon dioxide in birds and nonhuman mammals compare to the experience in humans.
  • In experiments, whereas chickens and pigs resumed eating immediately upon recovery from argon or nitrogen induced unconsciousness, pigs, for example, required two or three days to eat again after recovery from CO2.
IV. Welfare Inferences
  • Induction of unconsciousness with anoxia is non-aversive and rapid, whereas carbon dioxide induces breathlessness, activating brain regions involved in the perception of pain: inhalation of carbon dioxide is distressing and evidently painful.

V. Conclusions

  • Use of anoxia is far more humane than gas mixtures containing carbon dioxide .
  • A mixture containing low concentrations (less than 30%) of carbon dioxide and an anoxic gas such as argon or nitrogen is probably better than using high concentrations of carbon dioxide in air.
  • Slaughter without stunning induces unconsciousness quicker than carbon dioxide.
  • Both are painful and distressing and could be avoided.
  • Humanitarian intentions of eliminating avoidable pain and suffering during waterbath stunning could be seriously compromised by carbon dioxide stunning.
  • Bird welfare is maximized by stunning/killing in crates prior to shackling.
  • Shackling of freshly killed birds improves operators’ health and safety.
  • Human hazards of argon/nitrogen-based gaseous mixtures are low. Such gases are already used for modified atmosphere packaging of food (meat, fruits, and vegetables).
  • Like electrical waterbath stunners, Controlled Atmosphere Stunning (CAS) systems are secured (caged).
  • Gas suppliers perform thorough risk assessment and ensure the health and safety of operators
  • Environmental and personal gas monitoring systems are commercially available.
  • Many gas stunning/killing systems involving inert gas mixtures (argon, nitrogen, and krypton) are being used.
  • Processors, who need a license to slaughter animals for food, should also be required to obtain a license to use stun/kill devices.
  • Equipment manufacturers should share corporate responsibility for ensuring animal welfare.


Raj, A.B.M. 2003. A critical appraisal of electrical stunning in chickens. World’s Poultry Science Journal 59: 89-98.



1 Dr. Raj’s discussion of AC versus DC electrical stunning is omitted from this summary. His point, based on physiological and other measured parameters, was that AC/DC electrical stunning is cruel and inhumane. Also omitted here is his discussion of constant current stunning. Constant current stunning involves electrically isolating individual birds in the waterbath stunner as opposed to the standard industry practice of dragging multiple birds (hanging head down from a moving conveyer) through a waterbath stunner. While constant current electrical stunning could possibly be less cruel than the standard AC/DC method, Dr. Raj states that “Owing to the complexity, constant current electrical water bath stunning is not practiced [whereas] many gas stunning/killing systems involving inert gas mixtures are being used.”

2 250-280 chickens are killed per minute in a modern chicken slaughter plant. – Dr. Raj

United Poultry Concerns is a nonprofit organization that promotes the compassionate and respectful treatment of domestic fowl. www.upc-online.org

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