On any day, over 200 million chickens are slaughtered for food, a figure that excludes unproductive laying hens killed in the egg production chain and the mass-killing of male chicks deemed unsuitable for meat production. It amounts to over 8 million chickens killed per hour, representing more than 95% of all land animals slaughtered for food on this planet.
With millions of animals slaughtered daily, even small differences in the magnitude of the pain associated with the different manners of death that befall these birds become crucial to inform strategies to reduce suffering in the poultry industry. In this project, we address the welfare impacts of different slaughter methods used for poultry.
Although originally developed as a means of immobilising the animal for ease of processing, recognition that the pain endured during slaughter should be alleviated has become the main driver for the implementation of slaughter methods that include an initial stunning stage to render the animal unconscious and insensible to pain 一 ideally in an immediate and painless way 一 until the time of death. For instance, since 1974 European legislation requires that all animals to be killed for human consumption must be stunned before they are slaughtered. Similarly, most countries in Latin America have some legislation in place that makes it compulsory to use methods to stun animals prior to their slaughter for consumption.
Unfortunately, pre-slaughter stunning regulation is still lacking in many parts of the world and, where present, enforcement is poor and legal loopholes abound. For example, European legislation on welfare at the time of killing states that animals “must be spared any avoidable pain, distress or suffering during their killing”. Yet, the choice of the term ‘avoidable’ in several paragraphs of the regulation leaves room for interpretation and flexibility in the implementation. In the United States, poultry are not listed under the Humane Methods of Slaughter Act, hence there is no legal requirement for pre-slaughter stunning 一 its practice is merely voluntary. The observed frailties in the legislation in Europe and the US are an indicator of welfare during slaughter in other regions.
Even where pre-slaughter stunning is widely implemented, stunning methods will differ in their effectiveness and welfare impacts. Existing methods mostly fall in three categories: mechanical, electrical, and controlled atmosphere stunning (CAS). While mechanical stunning (e.g., cervical dislocation) is predominantly used at small-scale or as a back-up, electrical and, to a lesser extent, CAS stunning are the methods of choice in commercial slaughterhouses.
In this project it was our goal to provide a quantitative basis upon which to evaluate the welfare impacts of existing and potential policies on the stunning of poultry prior to slaughter. To this end, we use the Cumulative Pain Framework to estimate the loss of welfare endured by birds exposed to electrical waterbath stunning (the most commonly used method for poultry stunning globally) and multi-stage controlled atmosphere stunning with CO2 (the most commonly applied stunning gas under commercial conditions).
The results are thoroughly described and justified in Chapter 8 (‘Quantitative assessment of the welfare impacts of methods for the commercial slaughter of broilers‘) of the book Quantifying Pain in Broilers.
Experiences of pain perceived as aversive, but not intense enough to disrupt the animal’s routine in a way that alters adaptive functioning or affects the behaviors that animals are motivated to perform. Similarly, Annoying pain should not deter individuals from enjoying pleasant experiences with no short-term function (e.g., play) and positive social interactions. Sufferers can ignore this sensation most of the time. Performance of cognitive tasks demanding attention are either not affected or only mildly affected. Physiological departures from expected baseline values are not expected to be present. Vocalizations and other overt expressions of pain should not be observed.
Experiences of pain in this category disrupt the ability of individuals to function optimally. Different from Annoying pain, the ability to draw attention away from the sensation of pain is reduced: awareness of pain is likely to be present most of the time, interspersed by brief periods during which pain can be ignored depending on the level of distraction provided by other activities. Individuals can still conduct routine activities that are important in the short-term (e.g. eating, foraging) and perform cognitively demanding tasks, but an impairment in their ability or motivation to do so is likely to be observed. Although animals may still engage in behaviors they are strongly motivated to perform (i.e., exploratory, comfort, sexual, and maintenance behaviors), their frequency or duration is likely to be reduced. Engagement in positive activities with no immediate benefits (e.g., play in piglets, dustbathing in chickens) is not expected. Reduced alertness and inattention to ongoing stimuli may be present. The effect of (effective) drugs (e.g., analgesics if pain is physical, psychotropic drugs in the case of psychological pain) in the alleviation of symptoms is expected.
Pain at this level takes priority over most bids for behavioral execution and prevents all forms of enjoyment or positive welfare. Pain is continuously distressing. Individuals affected by harms in this category often change their activity levels drastically (the degree of disruption in the ability of an organism to function optimally should not be confused with the overt expression of pain behaviors, which is less likely in prey species). Inattention and unresponsiveness to milder forms of pain or other ongoing stimuli and surroundings is likely to be observed. Relief often requires higher drug dosages or more powerful drugs. The term Disabling refers to the disability caused by ‘pain’, not to any structural disability.
All conditions and events associated with extreme levels of pain that are not normally tolerated even if only for a few seconds. In humans, it would mark the threshold of pain under which many people choose to take their lives rather than endure the pain. This is the case, for example, of scalding and severe burning events. Behavioral patterns associated with experiences in this category may include loud screaming, involuntary shaking, extreme muscle tension, or extreme restlessness. Another criterion is the manifestation of behaviors that individuals would strongly refrain from displaying under normal circumstances, as they threaten body integrity (e.g. running into hazardous areas or exposing oneself to sources of danger, such as predators, as a result of pain or of attempts to alleviate it). The attribution of conditions to this level must therefore be done cautiously. Concealment of pain is not possible.
Although pain inherently concerns individuals, we operationally accept that the collective welfare of the members of a population can also be determined. Measuring cumulative pain at the population level is also necessary to account for the heterogeneity in the exposure of population members to different challenges. For example, while lameness is experienced by a large fraction of broiler chickens, fatal cases of ascites are only experienced by a few. Therefore, measurement efforts must consider the prevalence of each welfare challenge, so that pain is determined for the average member of the population (which may not necessarily correspond to any real organism). At the population level, the time spent at each level of pain intensity as a result of each challenge is determined by multiplying it by its prevalence. For example, if a condition causes 10 hours of Disabling pain and 70% of the population are affected, then the average member of this population could be said to experience 7 hours of Disabling pain due to the condition. Measurements at the population level enable comparing the impact of different practices and conditions across demographics, geographies, and time.
All parameter values used for the analysis (estimates of duration, intensity, number of occurrences, and the prevalence of each welfare offense) are justified in the book in Chapter 9 of the book “Quantifying Pain in Broilers” (Chapter 9). However, we invite the reader to examine the extent to which the estimates presented here are sensitive to our choices and assumptions, at the platform https://pain-track.org/broilers/stunning. In this web application, all parameter values can be altered, with the automatic update of all estimates.
