This report was completed on October 8, 2003 by People for the Ethical Treatment of Animals (PETA). Please direct any questions or comments to Cem Akin at 757-622-7382, extension 1492, or email CemA@peta.org.

Section II: The Controlled-Atmosphere Killing Model
- Improved Welfare
- Inert Gases, Such as Nitrogen and Argon, Are the Most Humane
- Carbon Dioxide Can Be Detrimental to Welfare
- Close Monitoring Needed to Ensure That Birds Are Killed

Section III: Advantages of the Controlled-Atmosphere Killing Model for Producers
- Improved Carcass and Meat Quality
- Increased Revenue

Section IV: Costs of the Controlled-Atmosphere Killing Model
- Start-Up Costs
- Operating Costs
- Payback Period

Section V: Conclusion

References

Introduction

The practice of electrically stunning poultry, which is standard in North American slaughterhouses (Boyd 1994), results in both welfare and carcass-quality problems. In terms of welfare, the many variables and frequent process failures associated with electrical stunning methods make it difficult to ensure adequate stunning and result in pain and suffering for birds who experience pre-stun shocks, have their necks sliced open, and are dipped in scalding hot water, often while still fully conscious. Furthermore, the uncrating, shackling, and conveying of live birds that always precedes electrical stunning has been shown to cause severe stress and injury, which lead to a decrease in meat quality. On the other hand, the controlled-atmosphere killing of poultry—using a mixture of inert gases in air with no more than 2 percent residual oxygen by volume, preferably 80 percent nitrogen and 20 percent argon—helps to significantly alleviate many of these animal-welfare and carcass-quality issues and actually leads to increased revenue. Thus, this method should be adopted by producers immediately.

Section I: The Electrical Stunning Model

Dumping
Serious animal-welfare problems associated with the electrical stunning of poultry are painfully evident before stunning even takes place. In order to facilitate the process, birds are dumped from transport vehicles onto a conveyer before being subjected to a stressful procedure called “shackling.” Birds awaiting shackling are sometimes overloaded onto the conveyer and end up suffocating to death as other birds are dumped on top of them. Overcrowding and rough handling during unloading and dumping lead to a relatively high number of dead-on-arrivals.

Shackling
During shackling, birds are hung upside-down by their legs on a line of shackles moving so fast—approximately 182 birds per minute at some plants—that it is impossible to humanely handle the birds. Leg deformities and other injuries typical of large broilers may exacerbate the pain as their sensitive periostea are pinched and compressed by the metal shackles. Gregory and Wilkins (1989) found that, after shackling, 3 percent of broilers had broken bones and 4.5 percent had dislocations. Another study by the same authors (1990), which looked at hens before and immediately after shackling, found a 44 percent increase in newly broken bones following shackling. Others conclude that shackling can, indeed, be both a physiologically and psychologically painful experience (Sparrey and Kettlewell 1994; Gentle and Tilson 1999).

Improper Stunning and Temporary Immobilization
After enduring the stress of being dumped and shackled, the birds proceed to the stunning area, where they are passed through an electrically charged water bath before having their throats cut. “Humane slaughter,” as defined by law for most species in many developed countries, requires that animals be rendered unconscious and, thus, insensible to pain prior to slaughter. In order to make claims that slaughter is humane, it is critical that this be accomplished—without exception and with minimal stress to the animals involved.  However, when using electrical stunning methods on chickens, it is almost impossible to ensure that every animal is rendered unconscious because the varied nature of the birds hinders the effectiveness of the electrical settings. In other words, because each bird has a different weight, fat content, age, number of feathers, level of cleanliness, brain resistance, and leg size (which determines shackle-to-leg contact)—all of which influence the effectiveness of an electrical stun—it is nearly impossible to ensure proper stunning unless the settings are changed to accommodate each individual bird. Boyd (1994, p 227) concludes that “[t]he high occurrence of improper stuns is testimony to the difficulty of controlling all these variables,” and as a result, “under many commercial conditions in poultry slaughterhouses, we have little reason to believe that proper electrical stunning is achievable consistently.” This was confirmed by a Farm Animal Welfare Council report (Heath 1984) to the British minister of agriculture, which surveyed facilities in the United Kingdom and found that one third of chickens were improperly stunned and not rendered insensible to pain during electrical stunning.

Although it has been argued that settings in excess of 120mA may induce unconsciousness in chickens if applied properly, others have called this theory into question. The most accurate indication of insensibility to pain is an isoelectric (flat) EEG pattern. Electrical stunning, however, does not immediately produce such a pattern. It has been hypothesized that the epileptiform brain activity that electrical stunning does induce in some animals is akin to a human grand mal epileptic seizure wherein the subject is unconscious. And while this argument may be appropriate for sheep and pigs, who display the high frequency polyspike activity found in grand mal seizures after being electrically stunned, chickens present a markedly different reaction. In fact, in 90 percent of chickens, electrical stunning produces low frequency polyspike activity that is “associated with petit mal epilepsy in humans and is not necessarily associated with unconsciousness” (Boyd 1994, p 224); this was also suggested by Gregory and Wotton (1987). Higher voltage settings do not necessarily remedy the problem by causing higher frequency polyspike activity, which implies that regardless of the electrical settings, chickens may not be rendered unconscious as a result of stunning unless they are killed (Gregory 1986; Gregory and Wotton 1987). 

Furthermore, research presented at a recent symposium on the humane slaughter of farm animals (Gregory 1986) suggests that birds may still be able to experience pain after electrical stunning but are not able to display a pain reflex because of temporary paralysis. A study authored by four British poultry slaughter supervisors (Richards et al. 1967, p 361, cited in Heath et al. 1981) concluded that electrical stunning is fraught with problems and acknowledged that “electrical paralysis may occur under certain conditions in man and other animals, during which pain can be perceived but reaction to it is impossible.” Other researchers (Katme 1986; Gerlis 1986) at the aforementioned symposium presented evidence that even the shock, which is supposed to immediately and painlessly render the animals unconscious, is, in fact, intensely painful.

In addition to the general efficacy problems of electrical stunning, even when conducted as planned, frequent system failures cause further complications. Pre-stun shocks are both painful and common, occurring, for example, when a bird’s wing comes in contact with the stun bath before the bird’s head.  Testimony from the “McLibel” lawsuit revealed that, according to Dr. Neville Gregory, up to 13.5 percent of broilers at one particular slaughterhouse were being shocked before fully entering the stun bath (CIWF §3.2.3). Chief Justice Bell, who presided over the case, concluded that the pre-stun shocks were indeed cruel (Wolfson 1999).

Other birds are able to completely avoid the stun bath by lifting their heads or flapping their wings, and these birds are fully conscious when they are moved to the next area. Raj (1998b, p 1) explains that there is, indeed, “pain and distress experienced by some conscious birds which miss being stunned adequately (due to wing flapping at the entrance to the water bath stunners).” During the McLibel lawsuit, Dr.  Gomez Gonzales, a meat-management technician for the McDonald’s Corporation, testified that between 1 and 2 percent of chickens miss the stun bath in the company’s U.S. slaughterhouses (Wolfson 1999).  This has serious welfare consequences for birds who are conveyed to the killing area while still fully conscious.

The tendency for improper electrical stunning is even more pronounced in the United States where, despite studies showing that higher electrical settings do not “adversely affect the carcass appearance” (Griffiths and Purcell, 1985; p 382), most producers insist on keeping settings that are too low—significantly lower than the 120mA used at most facilities in the United Kingdom—to achieve anything more than temporary paralysis. A meta-study of electrical stunning methods (Boyd 1994, p 221) verifies that in North America, “the development and application of [electrical] poultry stunning had more to do with facilitating processing than with humane slaughter.” In fact, one U.S. manufacturer of electrical stunning equipment wrote that “[t]he typical amperage used in stunning by our pulsating direct current pre-stunner is approximately 12 to 15 mA” (Austin 1994, cited in Davis 1996, p 167). Such low electrical settings have particularly dangerous consequences for birds who are paralyzed but still alert after passing through the stun bath and for those who miss the “killing machine” altogether and fully recover by the time that they reach the “killer” or scald tank.

Throat-Cutting
After being stunned—or rather, temporarily immobilized or even entirely conscious after completely missing the stun bath due to avoidance behavior—birds are conveyed toward an automated spinning blade, commonly referred to as the “killing machine,” which is designed to cut their necks. Some conscious birds are able to avoid this blade, as well, by lifting their heads or flapping their wings. Recent research (Boyd 1994, p 226) that examines the electrical stunning of poultry verifies that “birds dodge the knives, some completely, some partially, because they are not fully stunned.” Gregory (1991) wrote that “problems associated with inefficient neck cutting [are] only too common in poultry processing plants.” The McLibel lawsuit highlighted the high number of occasions during which broilers were still fully conscious during neck-cutting. For example, Chief Justice Bell estimated that based on the evidence presented during the trial, more than two birds per minute in the U.S. were fully conscious as their throats were cut (McSpotlight 1997, p 31). When Dr. Gomez Gonzales’ estimate that between 1 and 2 percent of chickens miss the stun bath in McDonald’s U.S. slaughterhouses (see previous section) is applied to U.S.  Department of Agriculture statistics (USDA 2003) for a typical year (e.g. 8,716,099,000 chickens were slaughtered in the U.S. in 2002), it can be concluded that every year, up to 175 million chickens completely miss the stun bath and have their throats cut while they are still fully conscious. 

After going through the “killing machine,” birds usually pass by a knife-wielding manual killer, commonly referred to as simply “the killer,” but with such fast-moving lines, it is impossible to ensure that every bird is dead, let alone unconscious, before proceeding to the scald tank.

Scalding
Birds are dipped into the scald tank, which contains scalding hot water, to facilitate feather removal. Heath et al. (1981) determined that sentient birds are, indeed, sometimes scalded. Another survey (Griffiths and Purcell 1984), which examined various chicken processing plants in Australia, also concluded that some birds are “not killed before they reach the scald tank.” At least two studies have concluded that “red-skin” chicken carcasses, commonly found when electrical stunning methods have been used, are caused by a physiological response to heat when live birds enter a scald tank (Heath et al.  1983; Griffiths and Purcell 1984). Perhaps the most compelling evidence that live birds reach the scald tank on a routine basis in the U.S. comes from the USDA itself. On its Food Service Inspection Service (FSIS) Web site and in instructional materials used to train inspectors (FSIS 2001, p 12), the USDA states, “Poultry that die from causes other than slaughter are condemned under the cadaver category.  These birds are not dead when they enter the scald vat. When submerged in the water, they drown.” Furthermore, under a U.S. federal regulation (9 CFR, Ch III, Part 381, §381.90) titled “Cadavers,” “carcasses of poultry showing evidence of having died from causes other than slaughter [i.e. in the scald tank] shall be condemned.” According to USDA poultry slaughter statistics for a recent year (2002), more than 3.7 million chickens were classified as “cadavers” and had been either scalded to death or drowned in the scald tanks.

Section II: The Controlled-Atmosphere Killing Model

Improved Welfare
Clearly, electrical stunning methods result in severe welfare problems for billions of birds each year in the U.S. alone. On the other hand, the controlled-atmosphere killing of birds in their transport containers (prior to shackling)—using a mixture of inert gases, such as nitrogen or argon in air with less than 2 percent residual oxygen—has proved to be both far more humane and less likely to cause carcass degradation, two convincing reasons for producers to immediately adopt such systems. Under the most humane controlled-atmosphere killing model, birds are taken directly from the transport vehicles in their crates or modules, which are inserted into a chamber where controlled-atmosphere killing occurs. The dead animals are then shackled, cut, bled, scalded, and eviscerated. At no point during this process do the animals have any chance of experiencing pain or suffering.

The switch from electrical stunning to controlled-atmosphere killing would result in such vast welfare improvements that according to one of the world’s foremost experts on the subject (Raj 1998b, p 1), doing so would eliminate the “stress and trauma associated with removing conscious birds from their transport containers, in particular, under the bird handling systems which require tipping or dumping of live poultry on conveyors; the inevitable stress, pain and trauma associated with shackling the conscious birds, i.e. compression of birds’ hock bones by metal shackles; the stress and pain associated with conveying conscious birds hanging upside down on a shackle line which is a physiologically abnormal posture for birds; the pain experienced by some conscious birds that receive an electric shock before being stunned (pre-stun shocks); … the pain and distress experienced by some conscious birds which miss being stunned adequately (due to wing flapping at the entrance to the water bath stunners) and then pass through the neck cutting procedure; [and] the pain and distress associated with the recovery of consciousness during bleeding due to inadequate stunning and/or inappropriate neck cutting procedure.” The Canadian Food Inspection Agency (1999a) also recently issued a news release stating that an “advantage of using [controlled-atmosphere killing] for poultry is that it eliminates uncrating and shackling of conscious birds and thus contributes to reduce stress to the birds. The procedure is fast, painless, efficient and there is no risk of recovery from unconsciousness.”

The figure on the following page compares electrical stunning to controlled-atmosphere killing and gives an overview of the welfare consequences of each.

Inert Gases, Such as Nitrogen and Argon, Are the Most Humane
Research shows that inducing anoxia with inert gas mixtures—such as nitrogen or argon in air with no more than 2 percent residual oxygen—which can be breathed, undetected, by animals under the right circumstances, is the most humane controlled-atmosphere killing method available for poultry and can be used to create a non-aversive atmosphere where birds die painlessly. Raj (1994) reported that in one study, researchers observed that 100 percent of the tested hens voluntarily entered a feeding chamber filled with 90 percent argon, where they were killed by the gas without any detrimental effects. Raj (1996, p 593) also found that “… because argon is an inert gas with no taste or odour, most of the turkeys did not detect its presence, and they didn’t show any signs of respiratory discomfort before they lost consciousness.” And after visiting a chicken slaughterhouse that employed a controlled-atmosphere killing system using inert gases, Duncan (1997 p 9) was compelled to write, “In my opinion, this is the most stress-free, humane method of killing poultry ever developed. The birds are quiet throughout the operation. They remain in the transport crate until dead and the killing procedure itself is fast, painless, and efficient. There is no risk of recovery from unconsciousness.”

While both nitrogen and argon have been found to be suitable, nitrogen is gaining popularity because it is typically less expensive and easier to distill from atmospheric air than argon. In fact, some plants are able to separate nitrogen from air on their own, enabling them to avoid buying it from external sources. In fact, even though argon is heavier and easier to contain, nitrogen’s relative cheapness has enticed producers in Europe to the point that it has become the standard in poultry slaughterhouses that use gas. In the U.K., the Department for Environment, Food & Rural Affairs (DEFRA) recently amended legislation to allow the use of nitrogen in poultry slaughterhouses, and according to the European Commission (2003, p 288), “[T]here are at least 4 companies in the UK using a predominantly nitrogen based gas mixture for killing chickens and turkeys.” In Canada, the Canadian Food Inspection Agency has also approved the use of inert gases to kill poultry (CFIA 1999b).

Carbon Dioxide Can Be Detrimental to Welfare
When inhaled, carbon dioxide has been shown to be highly aversive to humans (Gregory et al. 1990) and birds. Raj (1998a, p 1818) states that “[c]arbon dioxide is an acidic gas and is pungent to inhale at high concentrations. It is also a potent respiratory stimulant that can cause breathlessness before the loss of consciousness. The welfare implication of this is that birds could experience unpleasant sensations either during initial inhalation of carbon dioxide or during the induction phase.” The European Commission’s Scientific Committee on Animal Health and Animal Welfare (1998, p 1) explains that “while CO2 is able to stun or kill, it is also [an] irritant, for example, to mucous membranes of the nose and mouth due to the formation of carbonic acid.” In fact, in one study, Raj (1994) reports that researchers observed that while 100 percent of tested hens voluntarily entered a feeding chamber filled with 90 percent argon, where they were killed by the gas, fewer than half would even set foot in a chamber containing carbon dioxide. Raj (1996) also found that with a mixture containing high levels of carbon dioxide, turkeys displayed discomfort via head-shaking and gasping. The UK’s Department for Environment, Food & Rural Affairs (2001) confirms these findings by reporting that “[o]bservational studies have shown nitrogen and other inert gases to be less aversive to birds than carbon dioxide.”

Other studies confirm that birds, as well as humans, can only tolerate carbon dioxide levels up to 30 percent (Gregory et al. 1990; Raj 1998a). Therefore, if producers insist on using some carbon dioxide, the concentration must not exceed 30 percent, and it must be used in conjunction with inert gases, such as nitrogen or argon in air. Indeed, according to a European Commission draft document (EIPPCB 2003, p 288), this is by no means the optimal concentration as “research during 2001 indicated that the adoption of a gas mixture consisting of 80% by volume nitrogen and 20% by volume argon, is considered to be better than the carbon dioxide-argon mixture from bird welfare and meat quality points of view.”

Close Monitoring Needed to Ensure That Birds Are Killed
The residual oxygen levels in an inert-gas-based system must be carefully maintained at less than 2 percent to ensure rapid brain-function loss, as several researchers have found that trapped air between birds or crates can raise the residual oxygen to levels that can prevent proper killing. Also, in order to ensure that recovery of consciousness does not occur, it is crucial that the birds be killed by the gas, not merely stunned, before being shackled. Studies examining the batch stunning of chickens using various gas concentrations found that many birds rapidly regained consciousness, suggesting that mere stunning may be unsuitable on welfare grounds. Raj and Gregory (1990, p 366) have recommended that “birds should be killed rather than stunned by the stunning gases” and that this “will not only obviate the recovery of consciousness, but subsequent operations such as uncrating and shackling of the birds and neck cutting would be performed more easily on the dead and hence relaxed carcasses.”

Section III: Advantages of the Controlled-Atmosphere Killing Model for Producers

Improved Carcass and Meat Quality
In addition to the welfare benefits, controlled-atmosphere killing also provides producers with improved quality when compared to different types of electrical stunning methods, which are “frequently criticised on … meat quality grounds” (Raj et al. 1997, p 169). The European Commission’s Scientific Committee on Animal Health and Animal Welfare (1998, p 3) agrees, writing that “[a]nother advantage of gas stunning or gas killing methods, in comparison with electrical stunning, is that they may improve carcass and meat quality.” These improvements include fewer broken bones, less hemorrhaging, reduced bruising, reduced internal and external contamination, improved shelf life and quality, and unimpeded bleed-out rates:

Fewer broken bones, less hemorrhaging, and reduced bruising: Researchers at the University of Bristol (Raj et al. 1997, p 173) compared the carcasses of gas-stunned broilers with those of electrically stunned broilers and concluded that the incidence of broken bones and breast muscle hemorrhaging would be “substantially reduced by gas killing of broilers.” Raj and Gregory (1991, p 127) also found this to be the case and concluded that “the advantages of gaseous stunning include improved meat quality, fewer broken bones and less muscle hemorrhaging.” Another study at the University of Bristol (Raj et al. 1990, p 725) found that “gaseous stunning of broilers produced relatively better quality carcasses and meat than electrical stunning and therefore may have commercial advantages.” Specifically, gassed broilers had a lower incidence of broken bones and breast- and leg-muscle bruising. The authors suggested that the increased incidence of leg-muscle bruising during electrical stunning was a direct result of shackling live birds. Even industry journals recognize this problem; a recent article in Poultry (McGuire 2003, p 2) reports that “[d]uring processing, shackles can be too tight and the hanging of the bird too rough, which causes more severe bruising in the thigh areas.” And the Canadian Food Inspection Agency (1999a) found that “the use of controlled atmosphere stunning in poultry reduces the incidence of broken bones, bruises and hemorrhages in muscle, all of which are commonly associated with electrical stunning.”

Farsaie et al. (1983) report that bruising may be found on up to 25 percent of broilers processed in the U.S., and according to the USDA (2002), in a recent year, almost a million carcasses were condemned.  Controlled-atmosphere killing would significantly reduce both these problems, and the resulting reduction in bruising would have important implications for the producer because it would “improve the yield and the value of products” (EIPPCB 2003, p 288) and almost completely eliminate blood stains (Raj 2003). 

Reduced internal and external contamination: During electrical stunning, chickens tend to defecate and inhale water during the initial spasm from being electrically shocked. Gregory and Whittington (1992) examined this tendency by including a radioisotope in the stun bath and then looking at carcasses to determine whether internal radioactivity was detected. The results clearly showed that “chickens can and do inhale water during electrical stunning in a waterbath and that no remedy is available at the moment” (p 362). The authors suggest that the respiratory tract could, thus, be contaminated with bacteria from the stun bath, which could leak onto the edible portions of the carcass during evisceration.

When using electrical stunning, chickens commonly enter the scald tank while they are still alive (see “Scalding” section above). When this happens, external contamination is a concern because of live birds’ tendency to defecate in the scald tank. Subsequent birds are then dipped into the contaminated water, which necessitates excessive rinsing with water later down the line.

Furthermore, the dumping of live birds onto the conveyor under the electrical stunning model leads to scratches and wounds because the birds land on each other or otherwise struggle or panic as they try to regain their bearings. Raj (1998b, p 3) speculates that these skin wounds not only reduce the value of the carcass, but “can become a potential site for microbial attachment.”

The controlled-atmosphere killing model would almost completely eliminate all three forms of potential contamination because birds would be killed in their transport containers rather than being dumped and would, therefore, be unable to inhale in the stun bath or defecate in the scald tank. This has significant implications for producers since, according to the USDA (2002), in a recent year, almost 5.5 million chickens were condemned for being contaminated.

Improved shelf life and quality: Raj (1998b, p 3) explains that using inert gases induces anoxia on the cellular level in carcass muscles, which can “change the oxidation/reduction (radox) potentials” and, thus, lead to “increased shelf-life of meat due to a slow rate of development of off-odours … and discoloration …” The Canadian Food Inspection Agency (1999a) states that controlled-atmosphere killing “is also reported to produce more tender breast meat than when electrical stunning is used.” Taken together, these statements mean that controlled-atmosphere killing produces better quality meat that lasts longer, in terms of smell and color, than electrically stunned birds.

Unimpeded bleed-out rate: Raj et al. (1997) looked into the concern that the bleed-out rate of controlled-atmosphere-stunned birds is not as good as that of electrically stunned birds and found that after one minute, the differences were “not sufficient to impede the bl