Humaneness of Killing Birds with Carbon Dioxide is Disputed by Science
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Humaneness of Killing Birds with Carbon Dioxide is Disputed by Science

Alberta's new system for destroying "spent" laying hens is unlikely to be painless and quick, despite claims

From United Poultry Concerns (UPC)
March 2005

On March 21, Alberta Egg Producers announced support for a system of destroying large numbers of end-of-lay (“spent”) hens by dumping hundreds of the birds at once into deep bins that can hold 650 birds at a time, and killing them with carbon dioxide (CO2).

The egg producers claim that, in addition to being cheap and efficient, CO2 is “a very humane way to handle the birds.” However, this claim conflicts with scientific evidence showing that CO2 causes extreme suffering.

In a seminar presented at the US Department of Agriculture on December 16, 2004, Dr. Mohan Raj, Senior Research Fellow in the Farm Animal Division of the School of Clinical Veterinary Science at the University of Bristol in England, described the effects of CO2 on the body. CO2 induces breathlessness – a subjective distress in breathing known as dyspnea. According to Dr. Raj, dyspnea in both birds and mammals “activates brain regions associated with pain and induces an emotional response of panic.”

This is because, while CO2 increases the rate and depth of breathing to expel CO2 from the lungs, breathing actually increases the intake of CO2, and thus the desire to breathe to expel the noxious CO2 gas causes slow, painful suffocation.

In contrast, a gaseous stun/kill system based on the use of the inert gases argon or nitrogen, known as Controlled Atmosphere Stunning, eliminates or greatly reduces the suffering caused by pure CO2. Whereas CO2 induces painful suffocation, gases like argon and nitrogen induce a lack of oxygen, or anoxia, resulting in a painless death, according to Dr. Raj.

The crucial difference between anoxia (lack of oxygen) and dyspnea (breathlessness) is that, unlike anoxia, for which birds and mammals lack chemical receptors, suffocation involves receptors that register the physical separation of the upper respiratory tract from the outside atmosphere. In experiments in North America and the UK, chickens and turkeys exposed to high (40 percent or more) levels of CO2 “gasp, shake their heads, and stretch their necks to breathe.”

In an email dispatch to animal protectionists on March 26, Dr. Raj reiterated that exposure to carbon dioxide is “distressing and painful to animals and therefore should be avoided.” Alternatively, he said that end-of-lay-hens could be killed with less suffering in systems supplied with a mixture of 80% by volume argon and 20% by volume carbon dioxide – a mixture “universally available as a welding gas mixture.”

Welfarists agree that killing the hens on-farm is preferable to trucking them to slaughter, without food and water, the majority suffering from broken bones and other injuries incurred in the course of being cramped in cages and roughly handled.

However, a key welfare requirement for such on- farm disposal is that the hens be killed in their cages rather than pulled from the cages, thrown into front- end loaders, and dumped into huge bins while fully conscious, there to be suffocated with carbon dioxide.

“The system may be cost effective, but it cannot be called humane,” says Karen Davis, President of United Poultry Concerns, an organization that promotes the compassionate treatment of domestic fowl. “If producers really want to reduce the suffering of the birds, they will kill the birds in the cages using argon or nitrogen. That is not only what conscience but science dictates.”

United Poultry Concerns produced a report on the extermination of 19 million birds in British Columbia to control the avian influenza outbreak in 2004. “The Avian Flu Crisis in Canada: Ethics of Farmed-Animal Disease Control” is available at www.upc-online.org/poultry_diseases


United Poultry Concerns is a nonprofit organization that promotes the compassionate and respectful treatment of domestic fowl.  

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