In piggery abattoirs and slaughterhouses, pigs are stunned before they are killed for meat. While carbon dioxide stunning is the most common method used in Australia and in Europe, electric stunning is also used in some facilities.
CO₂ Stunning in a Gas Chamber
Mechanical - with bolt shot
Carbon dioxide gassing (CO₂ stunning) is mainly used on large piggery farms. For a pig factory farm, this type of stunning is cost-effective and makes it possible to stun several animals at the same time.
Over five million pigs are slaughtered in Australia each year for the pork industry, with around 85% killed using CO₂ gas stunning.
In March 2023, an investigation by the Farm Transparency Project and ABC’s 7.30 revealed an inside look into the use of CO2 stunning in Australian slaughterhouses.
How is carbon dioxide CO₂ stunning done?
With this form of stunning, small groups of pigs (approx. 4-6 animals) are driven into a gondola which is lowered into a CO₂-lake, if necessary with an automatic driving shield and sometimes with the use of electric shocks. Since carbon dioxide is heavier than air, the systems used always have a shaft with a CO₂ pool.
The pod with the pigs enters a so-called controlled atmosphere in which the CO₂ concentration must be at least 80%1. Many slaughterhouses already operate with a 90% CO₂ concentration or higher, as a higher concentration combined with a longer dwell time can achieve a longer lasting stun.
The most common systems are the dip-lift and the paternoster system.
- Dip lift: A cage that can be loaded with up to six pigs is lowered into the CO₂ lake. The maximum CO₂ concentration is thereby in the upper area of the pit2.
- Paternoster system: These systems consist of up to seven cages, each of which can hold up to six pigs. These rotate down a slope through a pit three to eight metres deep and stop at certain intervals to load the live animals on one side and unload the stunned animals on the other.
This form of stunning allows a slaughter output of 60 to 1,200 pigs per hour, depending on the size of the slaughterhouse.
The key parameters established for CO₂ stunning, which must be regulated by law, are the carbon dioxide concentration (min. 80%), the duration of exposure (100 seconds) and in the case of simple stunning: the maximum duration between stunning and bleeding cut (45 seconds), the gas quality and the gas temperature3.
CO₂ exposure causes pain in the respiratory tract
CO₂ has been shown to have a strong irritating effect on the respiratory tract and is extremely painful when inhaled4. CO₂ exposure causes intracellular acidosis in the mucous membranes, resulting in stabbing pain in the nose, throat and pharynx5.
CO₂ atmosphere causes respiratory distress
Even a slight increase in the CO₂ concentration in the breathing air results in an increase in the depth of breathing, a feeling of breathlessness develops, which leads to panic6. In human research, low CO₂ concentrations (5% to 35%) are used to induce panic attacks7.
Studies investigating the process of stunning observed massive defensive reactions in almost all animals, such as pushing back in the pod, throwing against the wall, climbing on other pigs, etc., as well as signs of fear and vocalisations8. The literature speaks of different time spans, but it can be assumed that it can take up to 30 seconds (significantly longer at lower concentrations) until all animals are completely stunned and become unconscious9.
Insufficient stunning performance if used improperly
Other problems that occur in practice and have a massive impact on the animals occur when the method is not used properly.
- Too many animals in the gondola: If more animals than the permitted number are driven into the gondola, so that they are close together or even lying on top of each other, adequate stunning can no longer be guaranteed. The animals cannot breathe deeply enough due to the confinement, the duration of the subsequent stunning is shortened, which can lead to premature awakening from the stun.
- Too short exposure time in the CO₂-lake: If the animals are not left in the CO₂-lake for the prescribed 100 seconds, but are brought up earlier, e.g. to save time or for cost reasons, this also has a negative effect on the stunning performance.
- Temperature of the CO₂ gas does not comply with the specifications: The temperature of the gas must be checked for correctness, i.e. for the manufacturer's specifications, before slaughtering begins. Temperature differences and also fluctuations in relative humidity have an effect on the CO₂ concentration in the pit. Deviations may occur during the course of the day due to technical circumstances. If this is not corrected, it will have an effect on the stunning performance.
- Stun-to-stick time - time between stunning and bleeding sting is too long: CO₂ stunning is a reversible type of stunning. For each animal, it is of utmost importance that the prescribed maximum time between stunning and bleeding sting is respected in order to prevent animals from regaining consciousness before further measures are taken. With six animals per gondola, this can be tight, as the stitching operation is normally carried out by only one person. The Animal Welfare Slaughter Ordinance therefore specifies an exemption for the time interval mentioned, but this can only be given if the gas concentration is increased and the dwell time in the CO₂ lake is extended10.
Documented findings on CO₂ stunning has been known for a long time
The problems that occur with CO₂ stunning, which have been documented several times, have also been known at EU level for a long time.
The EFSA (European Food Safety Authority) has recommended in its studies since 2004 that CO₂ stunning should be replaced by another form of stunning11.
Regulation (EC) No 1099/2009, which is over 10 years old, states the following regarding EFSA's 2004 recommendation:
"The recommendations to phase out the use of carbon dioxide in pigs ... will not be incorporated in this Regulation, as the impact assessment has shown that such a recommendation is currently not economically viable in the EU". However, it is important to continue this discussion in the future."
With this stunning method, 1,000 animals or more can be stunned and slaughtered per hour. In addition, CO₂ is a cheaper resource compared to, for example, inert gases, which are being considered as an alternative. This makes CO₂ stunning economically interesting.
As is so often the case, economic interests are put above the welfare of the animals.
Nowadays, electric stunning is carried out in commercial facilities. This is a more humane alternative to carbon dioxide stunning.
How is electric stunning done?
Depending on the size of the farm:
- in groups in a stunning pen (with manually applied stunning forceps)
- in individual animal traps with hand-guided electrodes for head flow and
- manual or automatic heart flow (semi-automatic installations)
- as well as in fully automatic systems in combination with chest band conveyors are carried out12.
Current-carrying electrodes are applied to defined points on both sides of the head for at least four seconds to transmit the current to the animal's brain (head-only stunning). Within tenths of a second, the current causes a state in the brain similar to that of an epileptic seizure, rendering the animal unconscious and insensible13.
The head-only anaesthesia can be extended by an additional cardiac current, which is performed either immediately afterwards or in combination with the brain current (head-body). This also exposes the heart to the current, causing ventricular fibrillation14. This method ensures that a prolonged state of numbness and sensory deprivation occurs immediately15.
If the anaesthesia is correct, the animal collapses immediately and it is so deeply anaesthetised for about 30 to 60 seconds so they do not perceive the intense pain of the current flow and the bleeding incision. The stabbing and bleeding can take place hanging or lying down, whereby the stabbing must happen after no more than 10 seconds lying down and after no more than 20 seconds hanging down16.
Animals have to be brought in individually to be stunned
Pigs react insecurely and usually refuse to move on if they are to be separated from a group with other pigs. For electric stunning, however, separation is necessary in order to be able to stun the individual animal. In large slaughterhouses operating at high speed, singling is the most critical point during the slaughter process. Animals refuse to move on without other conspecifics, turn around or rear up. At this point, electric shocks are often used or even the stunning forceps themselves are used to bring the pig into the necessary position for stunning, which is extremely painful18.
Falsely stunned animals are more difficult to recognise
As not only the brain but also the spinal cord is perfused, this type of stunning leads to current-induced cramps and paralysis of the animal. If a false anaesthetic occurs, e.g. due to incorrectly positioned electrodes or too short an application time, the animal only becomes immobile but remains fully conscious19. Falsely stunned animals are more difficult for staff to recognise due to the paralysis and there is a risk that the animal will enter further processing20,21.
Incorrect application of the electrodes, poor contact of the electrodes and other misapplications
Placing the electrodes in the correct location to immediately expose the brain to the current requires a practised, steady hand. Any contact with the stunning forceps is extremely painful. Problems arise when the animal is already restless due to, for example, the driving and refuses to go into isolation or when the person, the stunner tires after a large number of already stunned animals and no longer works with as much concentration as required.
Stunning in automatic stunning systems
The slaughter industry requires animals to be as uniformly sized as possible in order to ensure smooth processing of the animals and subsequently the carcasses in the slaughterhouse. Too much variation in the size of pigs can lead to great animal suffering. If animals that are too small are herded into automated systems, this can lead not only to incorrect stunning but also to unwanted electric shocks, which causes great animal suffering for the animals concerned. The slaughterhouse must react in such a case and stun animals that do not meet the standard with other equipment22.
The lack of industry action to address the suffering of pigs during slaughter is indicative of a broader problem with the way animal welfare science is funded and governed in Australia.
In 2022, Australian Pork Ltd received $5.5 million for Research and Development from levies and a further $6.5 million in matched government funding. Yet the APL 2020-25 Innovation Plan makes no reference to improving welfare outcomes at slaughter but lists ‘reduced cost of production and processing’ as a key area of research.
“The pig industry should be immediately investing in R&D on alternatives to carbon dioxide stunning and working with overseas researchers to urgently tackle this serious and insidious problem." – Australian Alliance for Animals Co-Director, Dr Bidda Jones, who featured in the ABC 7.30 report.
The fact that gas stunning is legal in Australia highlights yet again the urgent need to fix Australia’s broken animal welfare system.
3 Federal Ministry of Health. (2014). Good practice guide to animal welfare at slaughter. WG "Animal Welfare During Slaughter" (LK AT, WKO AT).
4, 5, 7 Steiner, AR, Flammer, SA, Beausoleil, NJ, Berg, C., Bettschart-Wolfensberger, R., Meyer, R., . . . Ghent, TC (2019). Humanely Ending the Life of Animals: Research Priorities to Identify Alternatives to Carbon Dioxide. Animals 9, 911.
6, 9 Remien, D. (2001). Gas measurements during carbon dioxide stunning of pigs in a selected slaughterhouse. inaugural dissertation; EFSA. (2020). Welfare of pigs at slaughter. The EFSA Journal 18(6).
8 Atkinson, S., Larsen, A., LIonch, P., Velarde, A., & Algers, B. (2015). Group stunning of pigs during commercial slaughter in a Butina paternoster system using 80% nitrogen and 20% carbon dioxide compared to 90% carbon dioxide. research gate.
10 Working Group on Animal Protection of the Federal State Working Group on Consumer Protection (LAV). (2017). Manual of animal welfare monitoring at slaughter and killing. SL: LAV.
12 by Wenzlawowicz, M., Holmes, R., Schwarzlose, I., Maurer, B., Marahrens, M., Bucher, M., & Opitz, C. (2017, 4). High-voltage electrical stunning in pigs for slaughter - problem definition and conclusions for enforcement practice. Official Veterinary Service and Food Control, Volume 24, 4/2017, pp. 227-229.
13 LGL. (2022, 04 08). electro stunning. From the Bavarian State Office for Health and Food Safety 2021: https://www.lgl.bayern.de/tiergesundheit/tierschutz/schlachten/elektrobetaeubung/index.htm; EFSA. (2020). Welfare of pigs at slaughter. The EFSA Journal 18(6). Last accessed on April 8th, 2022
15, 19 Briese, A. (2000). Study on the behavior of pigs for slaughter after electrical stunning with special consideration of the electrode position, stun stick time and the types of current used. Berlin: Free University of Berlin, veterinary clinic Schwarzenbek.
16 Ostalbkreis regional council office, veterinary services and food monitoring. (2022). Leaflet on animal welfare-friendly stunning and bleeding in artisanal butchers. From Ostalbkreis.de: https://www.ostalbkreis.de/sixcms/media.php/26/merkblatt-betaeubungkontrolle.pdf
18 Fötschl, H. (2013). Animal welfare offenses at the slaughterhouse. Animal welfare: Claim - Responsibility - Reality. Conference report of the 4th ÖTT conference (pp. 29-35). Vienna: Johannes Baumgartner.
20 LGL. (2022, 04 08). electro stunning. From the Bavarian State Office for Health and Food Safety 2021: https://www.lgl.bayern.de/tiergesundheit/tierschutz/schlachten/elektrobetaeubung/index.htm. Last accessed on 04/08/2022.
24, 26 Trockenbacher, A. (2017). Electronic recording of findings as part of slaughter animal and meat inspection in Austrian slaughterhouses. Vienna: Department for Livestock and Public Health in Veterinary Medicine at the University of Veterinary Medicine, Vienna.