‘Stunning’ prior to slaughter is legally required in many countries including Australia. It is intended to cause unconsciousness so that slaughter may be carried out without avoidable fear, anxiety, pain, suffering, or distress. The most common methods to stun pigs are electric stunning and exposure to high concentrations of carbon dioxide gas (CO2).
Stunning with CO2 gas offers benefits over electric stunning including the ability to stun animals in groups, with minimal restraint, less handling, and therefore potentially less stress before stunning. There is also less reliance on the skills of the people operating the equipment.
However, recent studies have revealed a number of welfare issues with CO2 stunning. These include that:
- the gas is very unpleasant for pigs (highly aversive)
- there is variability between pigs’ responses to CO2
- pigs are not rendered unconscious immediately
- high concentrations of CO2 gas can cause significant pain and distress to pigs when inhaled (by causing acute respiratory distress)
Studies of pigs’ behaviour have found that most pigs will avoid high concentrations of CO2 gas if possible, and that almost 90% of pigs preferred to go without water for 72 hours than experience exposure to CO2 gas.
Further research is urgently needed to develop stunning systems which retain the benefits of CO2 stunning while minimising the disadvantages. Evidence suggests that potential alternatives to be investigated may include:
- non-aversive (not unpleasant and do not cause pain) gas mixtures such as argon, nitrogen, or nitrous oxide
- a combination of argon with CO2
- anaesthesia with non-aversive gases, followed by killing by CO2 or electrical methods
- genetic selection for pigs which do not find CO2 to be painful.
The RSPCA recommends that stunning/killing pigs with high concentrations of CO2 should be phased out quickly, and replaced with a more humane alternative. The development of more humane gas mixtures and suitable equipment should be urgently prioritised.
For further information, see: Model Code of Practice for the Welfare of Animals: Livestock at Slaughtering Establishments