Behaviour of pigs exposed to mixtures of gases and the time required to stun and kill them: welfare implications.

Pigs were exposed individually to either 90 per cent argon in air (anoxia), a mixture of 30 per cent carbon dioxide and 60 per cent argon in air (hypercapnic anoxia) or 80 to 90 per cent carbon dioxide in air (hypercapnic hypoxia) and the times to loss of posture, the onset and duration of convulsions, vocalisation and cessation of gagging (respiratory arrest) were determined. The duration of convulsions and the time to onset of respiratory arrest were longer when the pigs were exposed to argon than when they were exposed to the mixture of carbon dioxide and argon or to the high concentration of carbon dioxide in air. A second experiment was carried out under commercial conditions to determine the duration of unconsciousness and insensibility based on the response to a nose prick, and the incidence of death induced by exposing pigs to either 90 per cent argon in air or a mixture of 30 per cent carbon dioxide and 60 per cent argon in air for different periods. The results showed that when pigs were exposed for three minutes to either argon or the mixture of carbon dioxide and argon they should be bled within 25 seconds from the end of exposure to the gas to prevent them regaining consciousness during bleeding. When the pigs were exposed to either argon or the mixture of carbon dioxide and argon for five minutes and bleeding out began within 45 seconds they did not regain consciousness or suffer convulsions while being bled. The majority of the pigs died when they were exposed to argon for seven minutes, and all of them died when they were exposed to the mixture of carbon dioxide and argon for seven minutes.