‘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 fear, anxiety, pain, suffering, or distress. The most common methods to stun pigs are electrical stunning and exposure to high concentrations of carbon dioxide gas (CO2).
Stunning with CO2 gas offers benefits over electrical stunning including the ability to stun animals in groups, with minimal restraint, less handling, and therefore potentially less stress before stunning.
What are the animal welfare benefits of group stunning?
Carbon dioxide stunning is one of the only commercial stunning methods that allows pigs to be handled and stunned in groups rather than having to be individually handled, restrained and stunned. Pigs, unlike cattle and sheep, do not have the natural instinct to walk in single file. Therefore, stunning systems that use single-file races, such as electrical and single-file carbon dioxide stunning systems, often require excessive force and coercion to move pigs. With the added excitability of pigs in a new environment such as an abattoir, the handling problems with single-file races are increased often resulting in regular use of handling aids, including electric prodders .
Group stunning allows pigs to stay and move in small groups, more closely mimicking pigs’ natural movement behaviour. This minimises the need for handling aids when moving pigs and removes the need for individual pig restraint prior to stunning.
What are the animal welfare issues with carbon dioxide stunning?
Commercial CO2 stunning involves pigs being exposed to high concentrations (>80% by volume in air) of CO2 resulting in gradual loss of consciousness. Recent studies have revealed a number of welfare issues with high concentration CO2 stunning. These include that [2, 3]:
- concentrations >30% are highly aversive (very unpleasant, painful) for pigs
- 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 (due to acute respiratory distress, i.e. difficulty breathing)
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 .
What is RSPCA’s position on carbon dioxide stunning?
The RSPCA recommends that stunning/killing pigs with high concentrations of CO2 should be phased out and replaced with a more humane alternative. Further research is urgently needed to develop stunning systems which retain the benefits of group CO2 stunning while minimising the disadvantages. Evidence suggests that potential alternatives to be investigated may include:
- non-aversive gas mixtures such as argon, nitrogen, or nitrous oxide
- a combination of argon with CO2
- stunning with non-aversive gases, followed by killing by CO2 or electrical methods
- genetic selection for pigs which do not find CO2 to be aversive.
For further information, see: Model Code of Practice for the Welfare of Animals: Livestock at Slaughtering Establishments
 Grandin T (2000) Grandin (ed) Livestock handling and transport, 2nd edition: handling and welfare of livestock in slaughter plants. Pp409-439.
 Raj A, Gregory N (1996) Welfare implications of the gas stunning of pigs 2 – stress of induction of anaesthesia. Animal Welfare 5:71-78.
 Grandin T (2018) Carbon dioxide stunning.
 Scientific Report of the Scientific Panel for Animal Health and Welfare on a request from the Commission related to welfare aspects of animal stunning and killing methods (2004) Welfare aspects of animal stunning and killing methods. The EFSA Journal 1-241.