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Industry and Veterinarians Debate Results on Gas Stunning

by 5m Editor
17 February 2009, at 10:56am

US - The latest National Chicken Council statement highlights that scientists have found no advantages for gas stunning over conventional (electrical) stunning for chickens. The American Association of Avian Pathologists and American College of Poultry Veterinarians conclude that both are humane and viable methods.

Statement by the National Chicken Council (NCC)

According to scientists, there is no advantage in terms of animal welfare for gas killing systems for poultry compared to the conventional stunning systems used by the United States chicken industry, says NCC. The industry feels that while gas systems are worthy of further study, there is no proven reason yet to move away from conventional stunning systems.

The NCC statement reads as follows:

According to scientists, there is no advantage in terms of animal welfare for gas killing systems for poultry compared to the conventional stunning systems used by the United States chicken industry. The industry feels that while gas systems are worthy of further study, there is no proven reason yet to move away from conventional stunning systems.

Conventional stunning, as used by nearly all chicken processing plants in the United States, is both effective and humane. Chickens are made unconscious and insensible to pain before they are humanely killed.

Conventional stunning is based on the fact that animals can be put into an unconscious state by a low-level electrical current. This is usually facilitated by contact with water or an aqueous mist. Moments after being stunned, the birds are passed by a blade that opens an artery, resulting in rapid death. The entire process takes seconds rather than minutes.

There is some interest in this country and in Europe in systems that employ carbon dioxide or argon gas in a cabinet or chamber to displace oxygen and render the birds unconscious or even to kill them. These are sometimes called Controlled Atmosphere Killing (CAK) systems. About one-fifth of chicken processing plants in Europe use gas stun-to-kill systems. Only a few plants in the United States use these systems, however. Gas systems are more popular in Europe because electrical systems in Europe are required to actually kill the birds, not just stun them. A much higher level of electricity is needed to accomplish the kill step, and this sometimes results in discoloration of the meat and other product quality issues. The ability of gas systems to avoid these product quality problems is a main reason for the popularity of gas systems in Europe.

According to a statement issued by the American Association of Avian Pathologists and the American College of Poultry Veterinarians: "Physiologic evaluation has failed to demonstrate any welfare advantage of any CAS system over other accepted poultry electrical stunning methods in the United States . . . Specifically, pulsed DC or AC low voltage stunning (the current U.S. industry standards) allows plants to achieve instant electro-anesthesia at rates exceeding 99.95 percent efficiency when properly applied, as denoted by EEG monitoring and physical examination . . . The alternative CAS systems, while viable, do not offer any known animal welfare advantages and may in fact be associated with poultry excitation and injury prior to loss of consciousness."

While the use of gas is sometimes depicted as 'putting chickens to sleep', the process is not always a pleasant experience. As Dr Bernard Rollin, a professor at Colorado State University and a noted writer on animal welfare, put it, "There is no distress as severe as the feeling of not being able to breathe. This feeling of suffocation is not only a result of lack of oxygen but also the inability to blow off carbon dioxide. Carbon dioxide drives respiration. Even though CAS creates unconsciousness, there must be a period when the animal feels a sense of suffocation. For this reason, I do not accept CAS as a humane method of euthanasia.”

Some poultry scientists are willing to accept gas stunning but say it offers no significant advantages over conventional processing in terms of humaneness.

"The main animal welfare issue with poultry slaughter is the ability to induce instantaneous insensibility," said Dr Yvonne Vizzier Thaxton, professor of poultry science at Mississippi State University. "The requirement is that all animals be insentient when slaughtered. Both electrical stunning and CAS fulfill this requirement when properly administered. We are continually examining potential technologies that may be equal to, or improve upon, those in current use."

The stunning and slaughter process is covered by the National Chicken Council Animal Welfare Guidelines and Audit Checklist, which is widely followed within the industry. The Guidelines state: "Stunning and killing equipment should be constantly monitored to insure proper functioning for humane processing. Birds should be insensible to pain when killed. A post-stun posture that includes arched neck and wings tucked in is visual evidence of an effective stun. Back-up personnel should be employed at the killing station to euthanize manually any bird not properly killed by the equipment."


The National Chicken Council represents integrated chicken producer-processors, the companies that produce, process and market chickens. Member companies of NCC account for approximately 95 percent of the chicken sold in the United States.

American Association of Avian Pathologists (AAAP) and American College of Poultry Veterinarians (ACPV) position statement on stunning of poultry

The AAAP and ACPV have also issued a statement on stuning methods:

A significant amount of discussion and research has been generated regarding the humane stunning of poultry in the United States. To this end, The American Association of Avian Pathologists (AAAP) and the American College of Poultry Veterinarians (ACPV) have generated a position statement which reflects the current understanding and application of the technologies that are in existence today. Specifically, the two technologies used in commercial poultry slaughter facilities include the electrical stunning method and the controlled atmosphere stunning (CAS) method.

Direct comparisons of low voltage electrical stunning systems in the U.S. to European electrical stunning systems are inappropriate as the two are quite different. In the U.S., electrical stunning systems utilize low voltage to produce electro-anesthesia while the European electrical stunning systems apply high voltage electrical stunning to electrocute the birds, or 'stun to kill'. For that reason, Europe is more inclined to use CAS systems to avoid the tissue damage associated with electrocution.

Physiologic evaluation has failed to demonstrate any welfare advantage of any CAS system over other accepted poultry electrical stunning methods in the United States. Specifically, pulsed DC or AC low voltage stunning (the current U.S. industry standards) allows plants to achieve instant electro-anesthesia at rates exceeding 99.95 percent efficiency when properly applied, as denoted by EEG monitoring and physical examination.

Controlled atmosphere stunning includes several variations of gaseous mixtures given to induce an anesthetic state. One method of CAS involves exposure of poultry to a mixture of inert gases, principally nitrogen and argon, to produce anoxic loss of consciousness. Another approach is to subject birds to an atmosphere enriched with successively higher concentrations of carbon dioxide which results in a two-stage progressive hypercapnic-hypoxic anesthesia. The alternative CAS systems, while viable, do not offer any known animal welfare advantages and may in fact be associated with poultry excitation and injury prior to loss of consciousness. However, in the case of extremely large poultry, the CAS systems can offer some human ergonomic advantages for processing plant employees.

Based on current research, it is the current position of AAAP and ACPV that pulsed DC or AC low-voltage stunning and controlled atmosphere stunning (CAS) are all viable and acceptable systems for humane stunning of poultry.


The two organisations recommend continued research on stunning physiology and the refinement of humane stunning applications in poultry processing.