Animal Welfare Evaluation of Gas Stunning (Controlled Atmosphere Stunning) of Chickens and other Poultry

Temple Grandin
Dept. of Animal Science
Colorado State University

(Updated January 2013)

Controlled atmosphere stunning has several welfare advantages. Handling stress is reduced because live chickens no longer have to be hung on the shackles. Another welfare advantage is that when a gas system runs correctly, 100% of the chickens will be rendered insensible before shackling and bleeding. Small runt chickens that may miss a water bath stunner will be effectively stunned by the gas. Some systems may also improve meat quality.

A major disadvantage of gas stunning is high installation and operating costs. Another disadvantage is that insensibility is not instantaneous. There has been much controversy about the effect of different gasses on discomfort to the birds during the induction of insensibility. How chickens may react to a certain gas mixture in the research laboratory may be different compared to how they react in a commercial system in a slaughter plant. Specific gas concentrations and mixtures that can be easily maintained in a laboratory may be difficult to maintain in a commercial facility. Other factors such as overloading of the equipment or changing exposure times may effect how the birds react when they first contact the gas.

When gas stunning systems are compared to electrical stunning, the trade off between stressful reactions to the gas and a reduction in handling stress by eliminating live shackling must be considered. Eletrical stunning provides instant insensibility, but live shackling is definitely bad for bird welfare. It is my opinion that overall bird welfare would be improved with gas stunning even if there is some discomfort before the bird looses consciousness. If most of the birds constantly flap or try to escape from the container, electrical stunning would provide better welfare. Reactions that are limited to gasping and head shaking only would probably be less distressful to the birds than live shackling.

Therefore I recommend that commercial systems should be evaluated by direct observations of the birds when they first enter the gas until they fall over (lose posture) and become insensible. Research done with an EEG shows that when a bird loses posture and the ability to stand, it has become unconscious (Benson et al., 2012). Commercial systems should have either windows or video cameras so that the birds can be observed while the system is in operation. Direct observation is the only way to verify that a commercial system is inducing insensibility with a minimum of discomfort. In any type of controlled atmosphere stunning system, there is a likely to be some discomfort before insensibility is induced. There may be a trade off between some discomfort during insensibility induction but the plus side is much less pre-stunning handling stress.

Birds reactions to induction of insensibility should be rated. Wing flapping AFTER the birds fall over (lose posture) is not a welfare concern and should be ignored. It may be a commercial concern due to wing breakage, but it is not a welfare concern if it occurs after the bird is insensible. Broken wings that occur before the birds become insensible are a major welfare concern.

Welfare Ranking of Bird Reactions During Insensibility Induction
Behavior Ranking
No change in behavior from the time the birds enter the gas until they fall over (lose posture) Excellent
Gasping only, with no other change in behavior from the time the birds enter the gas until they fall over (lose posture); Most birds with no wing flapping and a few birds with weak intermittant flapping Acceptable
Gasping, combined with continuous wing flapping from the time the birds enter the gas until they fall over (lose posture) Not Acceptable
All birds flap continuously or attempt to climb out of the container from the time the birds enter the gas until they fall over (lose posture) Serious Problem

The above recommendations for evaluating bird behavior during induction would also apply to chamber methods where containers full of birds are rendered insensible with low atmospheric pressure. In these systems automated controls must be used to gradually remove air during a three minute cycle. Low atmosphere pressure systems must also have either windows or video cameras in insure that the system is operating correctly. The best systems for either gas stunning or low pressure stunning are designed so that the birds remain in the transport containers during stunning. This provides an additional advantage of greatly reducing opportunities for plant employees to abused live birds, because people do not handle the individual birds until after they are dead.


Benson, E.R. et al. 2012. Evaluation of EEG based determination of unconsciousness vs. loss of posture in broilers. Research in Veterinary Science. 52:960-964.

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