In Australia, it is legally required that pigs are stunned prior to slaughter. The purpose of stunning is to ensure an animal is unconscious and unable to experience pain, suffering or distress before slaughter. The most common stunning methods used for commercial slaughter pigs are electrical stunning and exposure to high concentrations of carbon dioxide gas. Both electrical and carbon dioxide gas stunning methods have animal welfare issues associated with their use and there is an urgent need for further research to identify more humane stunning methods for pigs.
Carbon dioxide gas stunning systems
In carbon dioxide gas stunning systems, pigs are moved into a stunning chamber (known as a gondola). The gondola is lowered directly or in stages into a high concentration of carbon dioxide gas (>90%). Pigs are not rendered unconscious immediately; instead, as pigs inhale the gas, their blood carbon dioxide levels gradually increase and blood oxygen levels decrease, which eventually causes unconsciousness due to loss of brain function . After an adequate period of time, (several minutes) and once pigs are unconscious, they are removed from the stunning chamber and then bled to ensure death.
What are the animal welfare issues associated with carbon dioxide stunning?
Carbon dioxide gas at high concentrations is aversive (very unpleasant) and activates chemoreceptors in the body (which detect changes in carbon dioxide, oxygen and pH in the blood) which results in respiratory distress. Pigs have been reported to find concentrations of more than 30% carbon dioxide aversive . During stunning with carbon dioxide gas, pigs perform behaviours consistent with pain and distress, such as attempting to escape, gasping, head shaking, and high-pitched vocalisations [3–8]. The response of pigs to carbon dioxide has also been shown to differ depending on pig genetics, age, stress levels, and handling prior to stunning [2,9,10].
Carbon dioxide gas stunning systems are one of the only commercial stunning methods that allows pigs to be handled and stunned in groups. Pigs, unlike cattle and sheep, prefer to walk side by side and in groups rather than in single file, and individual handling and restraint is very stressful for pigs. There are two types of carbon dioxide gas stunning systems that either load pigs individually (side-loaders) or in small groups (back-loaders). Side-loader systems require pigs to walk in single file up a narrow raceway and then one by one into the gondola prior to stunning. The individual handling of pigs required to move them up the raceway and into the gondola is stressful and increased stress has been shown to cause increased aversion to carbon dioxide gas. Back-loader systems allow pigs to walk in small groups through a wide raceway and into the gondola using automatic backing gates. Back-loader system reduce the pre-slaughter handling stress for pigs because it is designed to work with pigs’ natural behaviour to walk in groups and minimises the need for human interaction and the use of handling aids .
Electrical stunning systems
The main commercial method of electrical stunning for pigs is head-only stunning. For electrical stunning, pigs are usually individually restrained using a V-restraint. Tongs with electrodes are then placed either manually or automatically on the pig’s head for stunning . The tongs pass an electrical current through the brain of the pig which causes immediate unconsciousness. Once pigs have been stunned, they are released from the restraint and bled immediately to ensure death before consciousness can be regained.
What are the animal welfare issues associated with electrical stunning?
Pigs are very resistant to individual handling and restraint because it goes against their natural behaviour to move about in groups. Therefore, to move pigs into the restraint required for electrical stunning, there is an increased risk of excessive force, or an electric prodder being used, which also increases likelihood of pigs experiencing pain and distress before slaughter [2,11–13]. In addition, the placement of electrodes and the electrical parameters used are critical to ensuring stun effectiveness. The incorrect placement of electrodes and/or use of inappropriate electrical parameters can result in pigs receiving pre-stun shocks and being ineffectively stunned. Electrical stunning systems where electrodes are placed automatically and where pigs are inadequately restrained have an increased risk of incorrect electrode placement and therefore increased risk of pigs being ineffectively stunned [14–16]. Electrical head-only stunning systems induce a shorter period of unconsciousness compared to carbon dioxide stunning, which means there is an increased risk of pigs regaining consciousness during bleeding if there is any delay after stunning [13,16–20].
Penetrating captive bolt stunning
Pigs may also be stunned using a penetrating captive bolt gun. The captive bolt gun is usually applied to the forehead of a pig while they are individually restrained and causes irreversible unconsciousness through physical damage to the skull and brain. When applied correctly, pigs should not regain consciousness, however, at slaughter, they are still bled after stunning to ensure death.
What are the animal welfare issues associated with penetrating captive bolt stunning?
Similar to electrical head-only stunning systems, to ensure correct placement of the captive bolt gun, pigs must be individually handled and restrained, which is very stressful for pigs . Due to the shape and thickness of the skull of some pigs (e.g., boars and sows have very thick skulls), correct placement of the captive bolt gun is challenging and increases the risk that pigs will be ineffectively stunned . Effective penetrating captive bolt stunning relies on staff competency and is prone to human error because there is no automated system currently available .
What is RSPCA’s position on stunning methods for pigs?
Stunning systems that require individual pig handling, such as electrical head-only, penetrating captive bolt, and side-loader carbon dioxide gas stunning, increase the risk of pigs experiencing pain and distress before slaughter. Stunning pigs with high concentrations of carbon dioxide gas is aversive, however systems that allow pigs to be handled and stunned in groups can improve pig welfare prior to stunning compared to systems that require individual handling and restraint. The RSPCA is opposed to carbon dioxide stunning systems where pigs are handled or loaded into gondolas individually, such as side-loader gas stunning systems. There are animal welfare issues associated with all commercial stunning methods currently available for pigs. Further research is urgently needed to develop alternative and humane group stunning systems for pigs that are also commercially feasible.
 Sindhøj E, Lindahl C, Bark L (2021) Review: Potential alternatives to high-concentration carbon dioxide stunning of pigs at slaughter. Animal 15(3), 100164
 Jongman EC, Woodhouse R, Rice M, Rault JL (2021) Pre-slaughter factors linked to variation in responses to carbon dioxide gas stunning in pig abattoirs. Animal 15(2), 100134
 Atkinson S, Algers B, Pallisera J, Velarde A, Llonch P (2020) Animal welfare and meat quality assessment in gas stunning during commercial slaughter of pigs using hypercapnic-hypoxia (20% CO2 2% O2) compared to acute hypercapnia (90% CO2 in air). Animals 10, 2440
 Llonch P, Dalmau A, Rodríguez P, Manteca X, Velarde A (2012) Aversion to nitrogen and carbon dioxide mixtures for stunning pigs. Animal Welfare 21(1):33–9
 Llonch P, Rodríguez P, Gispert M, Dalmau A, Manteca X, Velarde A (2012) Stunning pigs with nitrogen and carbon dioxide mixtures: Effects on animal welfare and meat quality. Animal 6(4):668–75
 Llonch P, Rodríguez P, Jospin M, Dalmau A, Manteca X, Velarde A (2013) Assessment of unconsciousness in pigs during exposure to nitrogen and carbon dioxide mixtures. Animal 7(3):492–8
 Rault JL, Lai A, Hemsworth L, Le Chevoir M, Bauquier S, Gates RS, Lay Jr DC (2020) Wireless ‘under the skull’ epidural EEG and behavior in piglets during nitrous oxide or carbon dioxide gas euthanasia. Physiology & Behavior 227, 113142
 Verhoeven M, Gerritzen M, Velarde A, Hellebrekers L, Kemp B (2016) Time to loss of consciousness and its relation to behavior in slaughter pigs during stunning with 80 or 95% carbon dioxide. Frontiers in Veterinary Science 3, 38
 Lechner I, Léger A, Zimmermann A, Atkinson S, Schuppers M (2021) Discomfort period of fattening pigs and sows stunned with CO2: Duration and potential influencing factors in a commercial setting. Meat Science 179, 108535
 Terlouw EMC, Deiss V, Astruc T (2021) Stunning of pigs with different gas mixtures: Behavioural and physiological reactions. Meat Science 175, 108452
 Wallgren T, Wallenbeck A, Berg C (2021) Stunning methods for pigs at slaughter – Report 56. Report to the Swedish Board of Agriculture
 Anil MH, McKinstry JL (1998) Variations in electrical stunning tong placements and relative consequences in slaughter pigs. Veterinary Journal 155(1):85–90
 Von Wenzlawowicz M, Von Holleben K, Eser E (2023) Identifying reasons for stun failures in slaughterhouses for cattle and pigs: A field study. Animal Welfare 21(S2):51–60
 Gregory NG (2001) Profiles of currents during electrical stunning. Australian Veterinary Journal 79(12):844–5
 May K, Hartmann L, von Wenzlawowicz M, Bühler C, König S (2022) Key parameters of head-heart electrical stunning need to be adapted to improve stunning effectiveness and meat quality in pigs of different genetic lines. Meat Science 190, 108829
 Velarde A, Gispert M, Faucitano L, Manteca X, Diestre A (2000) Survey of the effectiveness of stunning procedures used in Spanish pig abattoirs. Veterinary Record 146(3):65–8
 Nodari SR, Polloni A, Giacomelli S, Vezzoli F, Galletti G (2014) Assessing pig welfare at stunning in Northern Italy commercial abattoirs using electrical method. Large Animal Review 20:87-91
 Grandin T (2001) Solving return-to-sensibility problems after electrical stunning in commercial pork slaughter plants. Journal of American Veterinary Medical Association 219(5):608–11
 McKinstry JL, Anil MH (2004) The effect of repeat application of electrical stunning on the welfare of pigs. Meat Science 67(1):121–8
 Vogel KD, Badtram G, Claus JR, Grandin T, Turpin S, Weyker RE, Voogd E (2011) Head-only followed by cardiac arrest electrical stunning is an effective alternative to head-only electrical stunning in pigs. Journal of Animal Science 89(5):1412–8
 Anderson K, Ries E, Backes J, Bishop K, Boll M, Brantner E, Hinrichs B, Kirk A, Olsen H, Risius B, Bildstein C, Vogel KD (2019) Relationship of captive bolt stunning location with basic tissue measurements and exposed cross-sectional brain area in cadaver heads from market pigs. Translational Animal Science 3(4):1405–9