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Adopting slower-growing breeds of chicken would reduce animal suffering significantly

Fast growth rates result in significant pain and suffering for chickens. Selecting slower-growing breeds would reduce some of this suffering.

Every year, almost ten chickens are slaughtered for every person on Earth.1 We raise and kill around 74 billion every year. That’s more than ten times the number in 1960 and almost twice as many as in 2000.

Part of this rise has come from gains in the ‘efficiency’ of chicken production. Chickens today are much bigger than in the past, and are much more efficient at converting feed into meat.

This push for increased efficiency has increased the time and intensity each chicken spends in pain.

In this article, I look at estimates of pain duration in fast-growing chicken breeds. The research shows that even opting for slightly slower-growing breeds – that reach the same final weight – would provide large welfare benefits.

The size and growth rates of chickens have increased a lot over the last 50 years

You might have already seen the well-known graphic below, showing how the size of chickens has changed since the 1950s. It’s from this paper, where the researchers looked at the growth rate of different strains of chicken; those typically used in 1957, in 1978, and modern fast-growing breeds that we use today.2

The strains from 1957 and 1978 came from the University of Alberta; it has maintained 10 heritage lines of chicken strains that were typical in the past. The breed shown for 2005 is called the ‘Ross 308’. It’s a popular breed that is still used for commercial production today. It’s the same breed that will be the focus of our comparisons later.

As you can see, the typical broiler chicken in 2005 reached up to 4.2 kilograms after 56 days, compared to 1.8 kilograms in the 1970s; and just under a kilogram in the 1950s. That means the typical weight doubled from 1957 to 1978, then doubled again by 2005.

The weight of different breeds of chicken over their lifespan. Modern breeds tend to grow much faster, but at the cost of animal welfare. Source: Zuidhof et al. (2014).
Image of chickens at different life stages, in the 1950s, 1970s and 2005. Chickens in 2005 are much bigger than those in the 1970s and 1950s.

You can see, from an economic perspective, why modern breeds would be more favorable. Each chicken produces more meat, and the authors report that the feed conversion efficiency – how much meat you get for a given amount of feed – is higher.

Most of this increase in growth rates is not due to changes in the composition of feed, but because of genetic selection for fast-growing breeds. It’s estimated that almost 90% of the increase between the 1950s and 1990s was the result of genetic selection.3

Improvements in feed conversion efficiency also improve the environmental footprint of chicken: we need fewer crops to get a kilogram of chicken in return. But, as we’ll see later, this has come at the cost of animal welfare. Fast-growing chickens experience various additional health problems.

Opting for slower-growing breeds of chicken improves animal welfare significantly

Quantifying levels of pain in other animals is extremely difficult. It often relies on drawing lessons from various disciplines, including evolutionary biology, neuroscience, and pharmacology. Researchers Wladimir J. Alonso and Cynthia Schuck-Paim have written a detailed book on the analytical framework that is used to assess pain and discomfort felt by broiler chickens, which are those raised for meat (it’s available for free).4

The researchers attempt to quantify the amount of time spent in pain for breeds of chickens that grow at different rates.

The researchers tried to quantify the amount of time that each chicken would spend in pain over their lifespan.

This is an important question. While fast-growing breeds might experience more pain on a given day, slower-growing breeds could experience more over their lifespan since they live an extra 15 days. There would be the chance that by raising slower-growing breeds, we would just extend the time that a chicken suffers.

But it is not just the total hours of pain that matters. The intensity is just as – if not more – important. Many of us would rather spend 5 hours in mild pain than 1 hour in intense pain.

This pain can be physical or psychological. For their analysis, the researchers broke it down into four levels of pain, from least to most intense:

In the chart, we can see the amount of time spent at different intensities across the systems, as estimated by Schuck-Palm and Alonso. Estimates represent the total time in pain endured by the average hen due to each of the different harms and deprivations experienced. The researchers only considered a period between 14 and 20 hours per day that chickens were awake. So when converting hours to days, I have divided by 17 rather than 24. That means the number of waking days in pain is shown. The one exception to this is ‘excruciating pain’, which is measured in seconds.

Grouped bar chart showing levels of pain in fast- and slower-growing chicken breeds. For all levels of pain, except 'annoying' pain, fast-growing breeds experience more days.

As we can see, slower-growing breeds experience much less excruciating, disabling, and hurtful pain. Excruciating pain is reduced by 80%, disabling pain by 67%, and hurtful pain by 25%.

The only exception is ‘annoying pain’, where they experience slightly more. This is probably because all chickens experience long stretches of minor discomfort, and slower-growing breeds would live for 15 more days on average to reach the same slaughter weight.

This data suggests that opting for slower-growing breeds would lead to significant welfare benefits.

What causes pain in broiler chickens?

In the chart below, we see the contributors to each level of pain for each chicken breed.

One of the biggest causes of pain in chickens is ‘lameness’ – shown in orange. This is an impairment of the ability to walk. Lameness is extremely common in modern broiler chickens because of the extremely fast rates of weight gain. You might have seen images of chickens struggling to stand and hold their weight. That is what ‘lameness’ refers to.

Growth rates have increased by over 400% since the 1950s, so chickens reach maturity within 5 to 6 weeks. The problem is that bone development is slower than muscle deposition, which puts a lot of stress on its skeleton and joints. The rapid growth of breast weight also changes the distribution of weight. Chickens will often try to change their gait to accommodate, leading to skeletal imbalances.

Slower-growing breeds significantly reduce pain from ‘lameness’ because their slower weight gain means that bones have more time to develop.

Breakdown of the causes of pain in broiler chickens. ‘Conventional’ means fast-growing breeds, and ‘Reformed’ means slow-growing breeds. Source: Welfare Footprint.
Bar chart showing the hours that broiler chickens spend in pain, by the cause of this pain.

The other big difference between the breeds is the time spent by breeders (the parents of the chickens who are produced for their meat) in hunger, shown as a black bar.

Why is this?

The problem with fast-growing chickens is that they grow so quickly that they will often not survive into adulthood and have issues with reproduction. While meat chickens are slaughtered at a very early age, their parents (who have the same genetics for fast growth) must live long enough to reach adulthood and reproduce. Obesity in breeder chickens increases the risk of a range of health problems and reduces fertility.

So, producers want meat chickens to grow quickly to get a good ‘yield’ at slaughter time, but for their parents to survive to older age and reproduce, producers will often drastically restrict the amount of feed they eat. This feed restriction can be severe – where chickens are only fed once every two days – leading to pain and discomfort. It reduces their immune response to infections and diseases. It can also lead them to be more aggressive to other chickens, sometimes, it leads to them eating others in their litter (cannibalism). 6

Feed restriction is less of an issue for slower-growing breeding chickens because the risk of obesity – and its complications – is lower.

Heat stress — shown in pink in the chart — occurs when chickens struggle to lose body heat, often because of high densities where the ambient temperature is higher, and there is less room for adjusting their posture by moving their wings away from their body. Heat stress is higher in fast-growing breeds because they have a higher metabolic rate and increased thermogenesis (when the body produces heat by breaking down food).

Raising slower-growing breeds improves animal welfare, but eating less meat is the best way to reduce suffering

Research into the intensity and duration of pain in livestock is still emerging. Researchers are trying to develop various techniques to evaluate animal behavior and its relationship to pain. One example is using camera surveillance and flow patterns to understand aggressive or distressed behavior in pigs or chickens. But there are a range of unanswered questions.

The research from Schuck-Paim and Alonso suggests that moving from fast-growing to slower-growing breeds would offer significant benefits to animal welfare. The reductions in pain for individuals are already large. But considering we raise and kill more than 70 billion chickens every year, even small gains at the individual level lead to a massive reduction in animal suffering.

Slower growth rates are one of several policies that form part of the Better Chicken Commitment that many European countries have signed up to. This suggests that there is growing support for adopting better practices.

The other obvious conclusion from this research is that slower-growing breeds still experience significant amounts of suffering. That’s why the largest welfare benefits come from eating less meat overall.


I would like to thank Cynthia Schuck-Paim, Edouard Mathieu, Pablo Rosado, and Max Roser for their valuable comments and suggestions on this article.


  1. The human population is approximately 8 billion. Ten times this figure would be 80 billion. That’s only a bit more than the 74 billion chickens killed each year. In some countries, the ‘per person’ numbers are even higher: in Brazil, it’s 29 chickens per person, 28 in the United States, and 26 in Australia.

  2. Zuidhof, M. J., Schneider, B. L., Carney, V. L., Korver, D. R., & Robinson, F. E. (2014). Growth, efficiency, and yield of commercial broilers from 1957, 1978, and 2005. Poultry science, 93(12), 2970-2982.

  3. Havenstein, G. B., Ferket, P. R., Scheideler, S. E., & Larson, B. T. (1994). Growth, livability, and feed conversion of 1957 vs 1991 broilers when fed “typical” 1957 and 1991 broiler diets. Poultry science, 73(12), 1785-1794.

  4. The authors stress that this is an evolving area of research with existing knowledge gaps and uncertainty around various input parameters.

  5. This is the average growth rate in Europe and the US, but they can reach much higher growth rates.

  6. Wilson, K. M., Bourassa, D. V., McLendon, B. L., Wilson, J. L., & Buhr, R. J. (2018). Impact of skip-a-day and every-day feeding programs for broiler breeder pullets on the recovery of Salmonella and Campylobacter following challenge. Poultry science, 97(8), 2775-2784.

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    journal = {Our World in Data},
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