In 2021 we focused most of our work on understanding the impact of welfare reforms for broiler chickens, particularly the adoption of slower-growing breeds and alternative stunning methods. We are very grateful to all collaborators and reviewers who joined us in this effort.
This was the second application of the Cumulative Pain Framework (the first is summarized in the Laying Hens book), an approach that has ‘time’ spent in different affective states at its heart (although time is an essential part of how individuals experience life, it has been frequently overlooked in the animal welfare sciences in general).

The outcomes of this work are available on this website and now on Amazon.

As always, the work involved an in-depth review of the many challenges affecting the animals along their lives, including lameness, ascites, sudden death, heat stress, the deprivation of potentially motivated behaviors (foraging, exploration, perching, dustbathing), chronic hunger in breeders and the many issues associated with standard stunning procedures.

When we started the research, we were unsure on the extent to which adoption of slower-growing breeds would affect broiler welfare, considering their longer lifespan hence the possibility that these birds would endure negative welfare states for a longer period. In that sense, we were positively surprised with the large net positive effect that such a transition was estimated to cause, especially given our very conservative assumptions (discussed in Chapter 7). Adoption of slower-growing breeds was shown not only to reduce the incidence of offenses such as lameness but also to delay their onset. So instead of spending a longer time in pain, ‘affected’ slower-growing individuals are expected to spend a disproportionately shorter time in pain, mostly by the end of their lives. Overall, even small reductions of growth rate are expected to have a net positive welfare effect, though the slower the growth rate, the higher the expected welfare impact.

Another unexpected finding was the observation that chronic hunger from feed restriction in broiler breeders is likely the greatest source of physical pain that any individual chicken will endure over her life. The time in pain breeding females endure due to chronic hunger was estimated to be in fact substantially higher than that experienced by egg-laying hens as a result of any of the challenges they face over their life. Therefore, the benefits achieved with adoption of slower-growing strains can only be fully realized if both males and females from parent lines are also slower-growing. Commitments based on the use of crosses (e.g. Cobb-Sasso) would be still associated with severe suffering in parent birds.

The importance of high standards of operation and control in slaughter plants also came to light in our assessment of the welfare impact of controlled atmosphere stunning with CO2. For example, the expected welfare benefits of CO2 stunning systems are lost if exposure times or gas concentrations are insufficient to ensure that birds do not regain consciousness after leaving the gas stunner – something to consider as slaughter plants may not be willing to leave birds in the stunner for a long time given their prioritization of speed and high throughput rates. This makes it critical that reforms focusing on the implementation of CO2 stunning ensure the adoption of mechanisms to guarantee that birds are killed in the stunner.

Other findings are available in the book chapters and at the broiler and stunning report pages. As before, our assumptions can be checked (and changed) at https://www.pain-track.org/broilers. Feedback is most appreciated!