The Welfare of Laying Hens

Quantifying the Welfare Impact of the Transition to Indoor Cage-free Housing Systems

The global production of eggs has reached 83 million tonnes in 2019, up from 19 million tonnes in 1970. This amounts to over 1.5 trillion eggs produced every year by a population of approximately 7.5 billion laying hens – a group of animals now as abundant as humans.

Productivity gains in the egg industry have stemmed predominantly from the selection of highly productive breeds, along with the manufacturing of highly specialized diets and additives designed to maximize feed conversion. Commercial layer lines now lay about 320 eggs over a period of 70-80 weeks, compared to about 80 eggs per year in 1900. The industry’s goal goes further, as it aims to reach an average production of 500 eggs per laying hen in about 100 weeks.

In addition to massive increases in output per animal within a few decades, major shifts in raising methods included the confinement of animals at high densities in closed and barren environments. In the case of laying hens, confinement has reached extreme levels through the adoption of cage systems (also known as battery cages). This system restricts hens’ space provided to approximately 430 to 450 cm2 per bird, an area smaller than an A4 sheet of paper. As of 2021, most laying hens still spend their entire lives in these cages.

Extreme confinement and over-selection for early, elevated and extended egg production takes a heavy toll on the health of laying hens, with a high incidence of metabolic and bone disorders, among other physical and psychological challenges associated with acute and chronic pain.

However, public attention to the welfare of laying hens over the previous decades has been fostering a transition away from conventional battery cage systems, with an increase in the proportion of furnished cages (which provide hens with some additional space, a nest, and perches) and cage-free systems in some countries, either through legislation or corporate commitments for the sourcing of cage-free eggs. 

In this project, we apply our analytical framework to quantify the welfare impact of cage-free reforms, as measured by the cumulative time in pain (i.e. affective states of negative valence) potentially averted for every laying hen raised in an aviary instead of a cage. 

Our analysis focuses on conventional cages, furnished cages and indoor aviary systems (single-tier and multi-tier), considered by the industry as the preferred cage-free alternative for large scale egg production. For each housing system, we estimated the total time an average laying hen spends in different categories of pain intensity based on the intensity and duration of the pain endured from key welfare harms and their typical prevalence in commercial flocks. In all cases, the analysis is focused on the laying phase, which was assumed to last 40 to 60 weeks (hence the assumed lifespan, considering a pre-laying phase of about 20 weeks, is approximately 60 to 80 weeks for all systems). 

Instead of considering an exhaustive list of every possible harm affecting commercial layers, we examined key sources of physical and psychological pain arising from the most serious welfare problems, particularly those likely to be affected by the transition from cages to cage-free aviaries. Among these are keel bone fractures, injurious pecking (e.g. skin wounds, vent wounds, cannibalism), the deprivation of a number of highly motivated behaviors and egg peritonitis syndrome (the most common production disease in modern breeds of laying hens and the leading cause of mortality). We also quantify the cumulative time in pain resulting from the process of depopulation and transport, as it remains one of the most serious welfare problems in the poultry industry. Estimates of time in pain should be therefore viewed as the minimum time in pain endured by laying hens, during the laying phase, in the different systems analysed.

DEFINITIONS

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 the 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 ongoing stimuli and surroundings are 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.

MAIN FINDINGS

  • Cage-free aviaries were found to be clearly superior to conventional cages and furnished cages even soon after a transition to cage-free environments. Overall, an average of at least 275 hours of disabling pain, 2,313 hours of hurtful pain and 4,645 hours of annoying pain are prevented for each hen kept in an aviary instead of CC during her laying life, and 1,410 hours of hurtful pain and 4,065 hours of annoying pain prevented for each hen kept in an aviary instead of a FC during her laying life.
  • The longer time in pain in caged systems stems predominantly from the deprivation of highly motivated behaviors;
  • Most pain endured in cage-free production results from ailments associated with intense breeding for productivity;
  • Keel bone fractures are the most important welfare concern in cage-free aviaries;
  • The time in pain due to the direct physical consequences of injurious pecking (skin and vent wounds, cannibalism, pain from feather removal) was substantially lower than that resulting from other challenges (the analysis, however, does not consider potentially higher levels of fearfulness as a consequences of injurious pecking or the discomfort from plumage damage);
  • Approximately 2,000 hours of excruciating pain are expected for every flock of 50,000 laying hens (1-3 hours per individual affected)
  • Egg peritonitis syndrome, a production disease kept out of the research scope, was the main source of intense pain in all systems given its possible evolution to sepsis and the chronic nature of the pain endured by many of those hens surviving the disease.

WHY ESTIMATES OF THE BENEFITS OF CAGE-FREE AVIARIES ARE CONSERVATIVE

The analysis primarily aimed to estimate the minimum welfare improvement associated with transitioning to cage-free housing. In this sense, we preferred to err on the side of caution than potentially overestimated reform benefits in any particular aspect. Thus, it is reasonable to anticipate that the actual benefits of this transition may surpass the estimates. This expectation is supported by the following factors:

  • The analyses assume the same lifespan for all systems, but hens in cages have a potentially longer lifecycle, with the last weeks of life being particularly challenging. For example, in many countries outside Europe producers may still subject hens to a practice known as “induced molting” to extend their productive life. This is achieved by withdrawing their feed for several days (e.g., in the US 60–80% of caged layers are molted compared to <15% in cage-free production)
  • The estimates are based on the prevalence of key welfare conditions during laying as reported for commercial settings. They reflect a situation where the technological, genetic and managerial maturity of cage-free housing is not yet on par with that already achieved by caged systems; as producers acquire more experience and genetics are optimized to cage-free environments, the welfare benefits are expected to be even higher;
  • We did not consider the welfare impacts of the perception of lack of control in cages. A sense of control over adverse conditions has been shown to be one of the most critical components of mental well-being, both in humans and animals. Animals unable to escape aversive conditions are known to develop severe emotional distress in the form of helplessness, a depressive-like state. Caged hens are virtually prohibited from expressing any form of agency, motivated behaviors, to distance themselves from cage mates, resolve conflicts or adopt postures needed for comfort and thermoregulation. Often, they are kept in extremely low light intensity, compounding this effect.
  • Lack of control can also potentiate fear. Fear can be extremely damaging if intense or persistent. Confinement in cages can generate a sustained state of fear as a result of helplessness and deprivation. Where differences are present, hens have been shown to be more fearful in cages.
  • Positive states, other than eating and drinking, are very much limited in cages, hence preventing any type of compensation for the negative states endured
    We did not analyze the rearing phase. Should caged hens be reared in cages too, the challenges emerging from confinement at this younger age would add to the estimated time in pain in cages;
  • We have considered a higher prevalence of keel bone fractures in cage-free systems. However, studies of the prevalence of keel fractures in cage-free settings using more precise methods (dissection) that typically lead to a higher prevalence. Additionally, a recent review highlights that the link between housing system and keel bone fracture prevalence may not be supported.

INTERACTIVE CHARTS WITH MAIN FINDINGS

PAIN-TRACKS AND SENSITIVITY OF THE ESTIMATES

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 “Quantifying Pain in Laying Hens” (see below). 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/hens. In this web application, all parameter values can be altered, with the immediate update of all estimates.

BOOK: QUANTIFYING PAIN IN LAYING HENS

All methods, assumptions, justifications and results are discussed thoroughly in the book “Quantifying pain in laying hens”, freely available for download.

RESEARCH GAPS

  •  The need to quantify the intensity and duration of the pain emerging from multiple injuries, diseases, and deprivations has highlighted different research areas where knowledge is still lacking, detailed in the book. Surprisingly little research has been dedicated to the understanding of the impact of different welfare challenges at the individual level (where suffering occurs). Little is known about clinical evolution of the various welfare issues affecting commercial layers (e.g. healing times, duration of exposure), the likelihood of different clinical outcomes, rates of recurrence, and how adversely welfare harms are perceived by the individuals affected. Similarly, knowledge on case-fatality rates, comorbidity patterns and the prevalence of different conditions over the laying cycle is scant.
  • Longitudinal studies targeting individual birds in commercial conditions are very much needed. They can help understand (1) the extent to which each welfare harm disrupts the routine and behavior of the individuals affected, hence helping establish the degree to which conditions are perceived as aversive; (2) the recurrence of harms (how many wounds, attacks, fractures, or disease episodes an individual animal endures over the laying cycle); (3) the temporal evolution of each harm, including fatal conditions; (4) the rates of spontaneous healing in the case of injuries and diseases; (5) patterns of comorbidity and (6) the extent to which behavioral opportunities are realized in cage-free settings, and how this changes over time.
  • Comprehensive necropsy studies based on the random and systematic sampling of apparently healthy hens kept in different production contexts can help determine the prevalence of a number of non-fatal conditions in layer flocks, and the extent to which these conditions are correlated. Data collected in this way can shed light on potentially neglected sources of suffering in layer flocks, and on the risk factors for the development of important conditions (such as keel fractures).
  • The possibility that in environments where animals can fulfil important behavioral needs the salience of pain and healing times are reduced relative to that experienced in barren and confined conditions should be investigated further. Should it be supported, it would have important implications for policies and reforms in all areas of animal welfare