CAS System
Well designed and managed controlled atmosphere stunning systems have been shown to improve animal welfare rather than exacerbate distress. 

As early as 1853, a British anesthetist, Benjamin Ward Richardson, designed a gas stunning chamber to humanely stun slaughter animals. Today, controlled atmosphere stunning (CAS) is a slaughter method where small groups of animals are placed in a container with an atmosphere that consists of an asphyxiant gas, typically argon, nitrogen or carbon dioxide, causing them to lose consciousness.

Modern era CAS systems date back nearly 70 years, and in the last three decades a steady expansion of CAS technology in pig, turkey and broiler slaughter has occurred. In North American plants, carbon dioxide is the gas of choice due to ready availability and lower cost. For these species, CO2 stunning has proven to be one of the most reliable methods for attaining consistency in terms of good animal welfare and meat quality.

Unlike electrical stunning or concussion stunning, controlled atmosphere stunning does not produce instantaneous insensibility. The period of time between the initial exposure to CO2 and actual insensibility is referred to as “the induction of insensibility.” There has been controversy at times about the process when videos show birds or pigs in an agitated state in the chamber during the induction phase.

These observations are not considered as a rejection of the CAS process. On the contrary, well designed and managed CAS systems have been shown to improve animal welfare rather than exacerbate distress.

Terry Geertman, co-owner of Midwest Machine Technologies, a company that developed a CAS system for turkeys in 1999 and the early 2000s, said, “The induction phase of gas stunning, where the bird is taking in CO2 until it becomes unconscious, is the most critical. If the induction phase is done incorrectly, there is negative reaction from the bird, through wing flapping and signs of agitation. But when the induction phase is done correctly the bird easily becomes unconscious.”

Temple Grandin, professor of Animal Science at Colorado State Univ., reinforces that notion. “You have to have a good induction. You’ve got to ask ‘what is behavior like before loss of posture?’ A little beak opening or head shaking is an OK trade-off. If they try to get out of the box, an escape motion – that’s a bad induction. When chickens have a bad induction, they go berserk.”

The Trade Off

Well-designed CAS systems with consistently good induction have provided a platform for improving animal welfare. Grandin says the short period of minimal distress is worth the gains that a group stunning system provides.

“With all of these systems there’s a trade-off. With hogs, you’ve really improved handling. You don’t have to line them up, and you can eliminate electric prods with a CO2 system. A little discomfort during induction is a fair trade-off for better handling. It’s the same thing with chickens (and turkeys). You’ve eliminated putting live birds on the rail, which is super stressful.”

Although the design is different by the species, CO2 stunning systems for pigs, turkeys and broilers hold several things in common:

1) A successful induction to insensibility must be achieved by the CAS system. Observed behavior before loss of posture should include minimum excitability and no escape behavior.

2) CAS systems allow the movement of animals to the stunning equipment in groups. This eliminates the stressful task of moving pigs in a single line, or hanging sensible birds upside down on a shackle chain.

3) Moving pigs and poultry in groups requires careful planning of the method for delivering animals to the stunning equipment. Integrated systems that automate handling in the plant optimize the calm delivery of animals to CAS equipment.

4) There are tradeoffs with CO2 stunning. CO2 is an aversive gas for pigs and poultry. A good induction with the appropriate CO2 concentration is less stressful for the individual animal than the stress of handling prior to the stunner in other systems.

5) Human error of individual stunning is eliminated.

Leading the Way

Modern efforts at CO2 stunning for pigs can be traced back to Hormel and Oscar Mayer plants in the early 1950s. These systems did not gain widespread use, and were abandoned in the US by the early 1990s.

In Europe however, events followed a different course. CO2 stunning for pigs was introduced in 1970 by Wernberg Engineering in Denmark. By 1972, also in Denmark, Butina ApS was formed to produce a patented CO2 stunning system for pigs. Butina, in concert with the Danish Meat Research Institute improved on the Hormel CO2 stunning model. The updated and upgraded system gained rapid acceptance in pork plants in Denmark and Germany, soon achieving a global presence.

The initial goal of Butina’s sytem was to improve meat quality. Electrical and concussion stunning that were the industry standard for pigs in the 1970s and 1980s caused a number of quality issues including blood shot meat, broken backs and poor meat color.

Butina’s concept was that a calmer, quieter entry to insensibility would improve meat quality. That theory proved true in real life production situations, and soon the company couldn’t build systems fast enough. By 1988, the company had 100 installations, mostly in Europe. In 1999, Butina re-introduced CO2 stunning to the United States.

In the Butina process, pigs are loaded into a gondola in small groups (2 to 8 pigs). The gondolas are mounted on a Paternoster system (think Ferris Wheel). After loading, the gondola descends in to a CO2 dense pit with a concentration of approximately 90 percent CO2. In a timed journey through the pit, pigs undergo CO2 induction and become insensible. In the ascent phase from the bottom of the pit, the pigs are then slid out of the gondola onto a conveyer where they are bled, never regaining consciousness.