Science behind the BCC

A white feathered, higher-welfare breed chicken, with a slimmer body than a fast-grow breed, taking a big stride across the floor of a shed.
Image credit – Wakker Dier.

The Better Chicken Commitment is a list of animal welfare standards that will stop the worst suffering of chickens bred for meat. These standards are evidence-based and some of the relevant scientific literature is listed below.


Replacing fast-grow breeds, with slower-growing, healthier breeds

Circular image with a black line drawing of a chicken, side-on. Text says "Slower-growing, healthier breeds"

The Better Chicken Commitment states:
Adopt breeds that demonstrate higher welfare outcomes: either the following breeds, Hubbard Redbro (indoor use only); Hubbard Norfolk Black, JA757, JACY57, 787, 957, or 987, Rambler Ranger, Ranger Classic, and Ranger Gold, or other breeds that meet the criteria of the RSPCA Broiler Breed Welfare Assessment Protocol.

In Aotearoa, chickens bred for meat (called broilers by the chicken industry) are all currently one of two breeds, Cobb or Ross, both genetically selected for extremely fast growth. There is a wealth of evidence on the negative welfare outcomes of genetic selection for fast growth, some of which are summarised here.

In 2020, animal welfare scientists K Hartcher and H Lum performed a scientific literature review, where the authors summarise peer-reviewed scientific research publications looking at the welfare consequences of genetic selection of chickens bred for meat. The authors identified several welfare issues including cardiovascular (heart) disease, musculoskeletal issues, reduced walking ability, foot pad and hock dermatitis (burn), and breeder bird chronic hunger.

The authors concluded that “Breeding goals in the broiler industry over the past 60 years have generally narrowly focused on production traits, leading to several welfare problems” and that “Two of the most serious welfare problems in the broiler industry – feed restriction in breeders and health issues in broilers grown for meat – are directly linked to genetic selection”.

Cardiovascular (Heart) Disease

There is a wealth of scientific research looking at heart disease, and its genetic basis, in chickens bred for meat. Sudden death syndrome, where chickens die suddenly without any prior clinical signs, and ascites, where fluid builds up in the body cavity, are both major cardiovascular diseases in ‘meat’ chickens. 

Olkowski (2007) did extensive research into the underlying mechanisms of heart disease in meat chickens. They found that meat chickens were much more likely to have a cardiac arrhythmia (abnormal heart rhythm) with up to 27% of fast grow birds having arrhythmias but only around 1% of slower-growing birds. They also found that fast-growing birds have lower oxygen blood concentrations and higher carbon dioxide blood concentrations than slower-growing birds. Post mortem examinations of apparently healthy fast-growing meat chickens revealed that many apparently healthy birds have subclinical heart disease. 

Findings on post mortem included weakening and thinning of the muscle of the ventricles (heart chambers), degeneration of the left atrio-ventricular valve (a valve between two of the heart chambers), and changes to the pericardium (the thin sac around the heart) that include a build-up of fluid in the sac, thickening of the sac, and adhesions between the sac and the outer wall of the heart. Additionally, the authors found that the heart rate of fast-growing birds was lower than that of slower-growing birds. 

Zhang, Schmidt, and Lamont (2018) looked at the genetic basis for heart disease in modern fast-growing chickens (breed Ross 708) and a slower-growing meat chicken (Illinois broiler). They found 321 genes that were expressed significantly differently between the two breeds by day 6, which increased to 819 genes by day 21. The fast-growing chickens had changes to their lipid (fat) transport genes. The study authors suggest that cell damage and inhibition of normal cell function results from these genetic changes and may be the cause of heart disease seen in these birds. Additionally, the authors found that the fast-growing birds had proportionally smaller hearts than the slower-growing chickens.

A white feathered chicken with dirty feathers lying on the litter on a shed floor, which is strewn with feathers. A red feed hopper is suspended above them. Their left leg is splayed out unnaturally to the side and both feet are swollen.
Chicken whose legs have collapsed [Image credit – Farmwatch]

Musculoskeletal Issues

Musculoskeletal issues cause pain, lameness, increased risk of injury, reduced access to food and water, and reduced ability to perform natural behaviours. 

Bacterial chondronecrosis with osteomyelitis (BCO) is a bacterial infection of the bone that often causes bone death and fractures. BCO is considered a common cause of leg disorders in chickens bred for meat. Wijesurendra and other authors (2017) examined birds that had died or been culled from 20 commercial meat chicken farms in Australia and found that 28% of these had signs of BCO. Wideman and others (2014) showed that there is a genetic predisposition for fast-growing meat chickens to develop BCO, and in 2013, these authors showed that fast-growing meat chickens were more likely to develop BCO than slower-growing meat breeds. 

There is also a genetic basis for other bone and leg deformities in fast-growing meat chickens. In 2012, Shim and others found that slower-growing birds have lower rates of tibial dyschondroplasia (TD) than fast-growing birds. 

Reduced Walking Ability

Reduced walking ability is genetically predisposed to in fast-growing meat chickens. Not being able to walk normally also contributes to abnormal development of the skeletal system. 

Wilhelmsson (2019) showed that Ross breed chickens, a fast-growing breed commonly used in Aotearoa, were significantly less able to walk and had more culls due to leg weakness than the slower-growing Rowan Ranger breed. Bizeray and others (2000) found that there were significant differences between walking behaviour and activities between slower-growing Label Rouge chicken breeds and young fast-growing chickens bred for meat. 

In 1992 Kestin and others developed a six-point gait scoring system which is commonly used to assess walking ability in broilers. Zero is a normal gait and five describes a bird that cannot stand or walk. This gait scoring system is used in the New Zealand Code of Welfare for Meat Chickens to assess walking ability. The Code recommends that all chickens with a gait score of 4 or 5 are culled.

Many authors think that chickens with a gait score of three or more have a welfare compromise. Many studies have been conducted looking at how common gait scores of three or more are, these have ranged from 5.5% to 55%. The most recent study conducted in Aotearoa by the Ministry for Primary Industries (MPI) was in 2013 and found that in all weight categories just over 30% of birds had a gait score of three or more, and in heavier birds (2.9-3.6kg) this was almost 56%. 

Kestin and others (1992) found that birds randomly bred for 11 generations (so not selected for rapid growth) had much better gait scores than three commercial breeds. Additionally, the same authors later (2001) found that slower-growing chickens bred for meat had better gait scores than fast-growing chickens. 

Caplen and others (2012) showed that fast-growing chickens bred for meat are unbalanced due to their large breast muscle and body mass, which has occurred due to genetic selection. 

Side-on of a white chicken, turning to look at the camera, with very dirty feathers and excrement on her leg. Her bottom and underside are red, bare skin. Her left leg is splayed out. Behind her is a red feed hopper. The litter on the floor of the shed is strewn with feathers and excrement.
Chicken with sores on foot and leg [Image credit – Farmwatch]

Foot Pad and Hock Dermatitis

Contact dermatitis is ulceration of the skin. It most commonly occurs on the footpad (FPD), hock (HB), and breast (BB). These lesions can be painful, increase the risk of infection, and worsen existing lameness issues. Estimates for the frequency of contact dermatitis range from 9.7% to 54.5% of birds at the time of slaughter. 

Environmental management (such as type and depth of litter, type of feed, lighting, and environmental enrichments) can contribute to contact dermatitis. However, there is clearly a genetic predisposition in some faster-growing breeds of chicken. 

Kjaer and others (2006) found that FPD and HB were much more common in a faster-growing strain of meat chicken when compared with a slower-growing strain. These authors found no FPD lesions and only a few mild HB lesions in the slower-growing strain but 44% of the fast-growing strain had FPD and 88% had HB. Allain and others (2009) found that a faster-growing strain had deeper FPD lesions when compared with a slower-growing strain.

Breeder Birds

Because of all the health issues that result from fast growth, and the strong genetic drive for this in modern chickens bred for meat, there exists a significant welfare issue called the ‘broiler breeder paradox’. Broiler breeders are the parents, grandparents, or great grandparents of the birds that are reared to be eaten.

Broiler breeders have the same genetics as their offspring, but they need to survive much longer so they can lay fertile eggs. To reduce the health issues these birds face, and to improve their reproductive performance, they are severely and consistently feed restricted, often being given only a third of what they would choose to consume. This leads to severe chronic hunger, frustration, boredom, chronic stress, aggression and abnormal behaviours. 

This ‘broiler breeder paradox’ has come about directly because of genetic selection. If the birds were allowed to eat as much as they feel they need to they will develop health issues, but if they have their food intake restricted they have other significant welfare issues. De Jong and Guemene (2011) noted that it is impossible to meet both desired rapid and efficient meat chicken production and to have good welfare in broiler breeders. 

Qualitative feed restriction – where food has a low energy bulk agent such as fibre added – is controversial. Some authors have found that metabolic hunger persists for these birds and that this is not a humane solution to the problem. 

Millman, Duncan, & Widowski (2000) looked into the behaviour of broiler breeder birds. They found that male birds had significantly more male to male and male to female aggression than a layer (a breed used for egg production) strain observed for comparison. Stopping the feed being restricted did not reduce this behaviour.

Aggressive sexual behaviours observed included chasing female birds, forced copulation, and lack of courtship behaviour. The authors conclude that broiler males are motivated to mate with female birds but not communicate with them. In commercial settings, broiler breeders are housed in large flocks of mixed male and female birds, providing the birds with no way to escape from these behaviours. 

White chickens crowded at the wall of a shed. Barely any ground is visible, and where it is it is brown. There is a small open part of the wall that gives chickens access to the outdoors. Chickens line the whole wall.

Conclusion on importance of breed change

There is a huge body of scientific evidence that demonstrates that fast-growing chickens bred for meat suffer a range of very significant welfare issues as a consequence of their genetics. Better welfare and improved quality of life – a life that is worth living – have been demonstrated in slower-growing breeds that have healthier genetics. 

  • Allain, V., Mirabito, L, Arnould, C., Colas, M., Le Bouquin, S., Lupo, C., & Michel, V. (2009). Skin Lesions in Broiler Chickens Measured at the Slaughterhouse: Relationships between Lesions and between Their Prevalence and Rearing Factors. British Poultry Science 50: 407–417. doi:10.1080/00071660903110901.
  • Caplen, G., Hothersall, B., Murrell, J., Nicol, C., Waterman-Pearson, A., Weeks, C., & Colborne, G. (2012). Kinematic Analysis Quantifies Gait Abnormalities Associated with Lameness in Broiler Chickens and Identifies Evolutionary Gait Differences. PloS One 7: e40800. doi:10.1371/journal.pone.0040800.
  • De Jong, I, & Guemene, E. (2011). Major Welfare Issues in Broiler Breeders. World’s Poultry Science Journal 67: 73–82. doi:10.1017/S0043933911000067.
  • Hartcher, K, & Lum, H. (2020). Genetic selection of broilers and welfare consequences: a review. World’s Poultry Science Journal, 76:1, 154-167, DOI: 10.1080/00439339.2019.1680025. 
  • Kestin, S., Gordon, S., Su, G., & Sorensen, P. (2001). Relationships in Broiler Chickens between Lameness, Liveweight, Growth Rate and Age. Veterinary Record 148: 195–197. doi:10.1136/ vr.148.7.195. 
  • Kestin, S., Knowles, T., Tinch, A., & Gregory, N. (1992). Prevalence of Leg Weakness in Broiler Chickens and Its Relationship with Genotype. Veterinary Record 131: 190–194. doi:10.1136/vr.131.9.190.
  • Kjaer, J., Su, G., Nielsen, B., Sorensen, P. (2006). Foot Pad Dermatitis and Hock Burn in Broiler Chickens and Degree of Inheritance. Poultry Science 85: 1342–1348. doi:10.1093/ps/ 85.8.1342.
  • Millman, S., Duncan, I., & Widowski, T. (2000). Male Broiler Breeder Fowl Display High Levels of Aggression Toward Females. Poultry Science, 79, 1233-1241.
  • Ministry for Primary Industries (2013). Survey of Lameness in New Zealand Meat Chickens. MPI Technical Paper No: 2013/45
  • Olkowski, A., (2007). Pathophysiology of Heart Failure in Broiler Chickens: Structural, Biochemical, and Molecular Characteristics. Poultry Science 86, 999–1005. 
  • Shim, M, Karnuah, A., Anthony, N., Pesti, G., & Aggrey, S. (2012). The Effects of Broiler Chicken Growth Rate on Valgus, Varus, and Tibial Dyschondroplasia. Poultry Science 91: 62–65, doi:10.3382/ps.2011-01599.
  • Wijesurendra, D., Chamings, A., Bushell, R., Rourke, D., Stevenson, M., Marenda, M., Amir, H., & Stent, A. (2017). Pathological and Microbiological Investigations into Cases of Bacterial Chondronecrosis and Osteomyelitis in Broiler Poultry. Avian Pathology 46 (6): 683–694.
  • Wilhelmsson, S., Yngvesson, J., Jönsson, L., Gunnarsson, S., & Wallenbeck, A. (2019). Welfare Quality® Assessment of a Fast-growing and a Slower-growing Broiler Hybrid, Reared until 10 Weeks and Fed a Low-protein, High-protein or Mussel-meal Diet. Livestock Science 219: 71–79.
  • Zhang, J., Schmidt, C., & Lamont, S. (2018). Distinct Genes and Pathways Associated with Transcriptome Differences in Early Cardiac Development between Fast- and Slow-growing Broilers. PloS One 13 (12): e0207715.

Stocking density and thinning

Circular image with a black line drawing of a chicken with outstretched wings. Text says "More space per bird"

The Better Chicken Commitment states:
Implement a maximum stocking density of 30kg/m2 or less. Thinning is discouraged and if practiced must be limited to one thin per flock.

Some of the papers on stocking density and chicken welfare are listed:
  • Simitzis, P. E. et al. (2012) Impact of stocking density on broiler growth performance, meat characteristics, behavioural components and indicators of physiological and oxidative stress. British Poultry Science. 53, 721–730. 
  • Gomes, A. V. et al. (2014) Overcrowding stress decreases macrophage activity and increases Salmonella Enteritidis invasion in broiler chickens. Avian Pathology. 43, 82–90. 
  • Tsiouris, V. et al. (2015) High stocking density as a predisposing factor for necrotic enteritis in broiler chicks. Avian Pathology. 44, 59–66. 
  • Velo, R. & Ceular, A. (2017) Effects of stocking density, light and perches on broiler growth. Animal Science Journal. 88, 386–393. ​​


Circular image with a black line drawing of a chicken standing in front of an open window with the sun visible outside. Text says "Natural light"

The Better Chicken Commitment states:
At least 50 lux of light, including natural light. At least six hours of darkness in each 24-hour period, with four hours of that darkness being continuous.

Some of the papers on light and chicken welfare are listed:
  • Bailie et al. (2013) Influence of the provision of natural light and straw bales on activity levels and leg health in commercial broiler chickens. Animal. 7(4). 
  • Blatchford et al. (2009) The effect of light intensity on the behavior, eye and leg health, and immune function of broiler chickens. Poultry Science. 88(1). 
  • Deep et al. (2010) Effect of light intensity on broiler production, processing characteristics, and welfare. Poultry Science. 89(11). ​​


Perches and substrates

Circular image with a black line drawing of a two chickens. One is standing on a white straw bale. Text says "Enrichments: objects to perch on and peck at"

The Better Chicken Commitment states:
​At least two metres of usable perch space, and two pecking substrates, per 1,000 birds.

Some of the papers on enrichments and chicken welfare are listed:​​
  • Bailie, C. L., Ball, M. E. & O’Connell, N. E. (2013) Influence of the provision of natural light and straw bales on activity levels and leg health in commercial broiler chickens. Animal. 7, 618–626. 
  • Baxter, M., Bailie, C. L. & O’Connell, N. E. (2018) An evaluation of potential dustbathing substrates for commercial broiler chickens. Animal. 12(9), 1933-1941. 
  • Kaukonen, E., Norring, M. & Valros, A. (2017) Perches and elevated platforms in commercial broiler farms: use and effect on walking ability, incidence of tibial dyschondroplasia and bone mineral content. Animal. 11(5), 864-871. 
  • Bailie, C. L. & O’Connell, N. E. (2015) The influence of providing perches and string on activity levels, fearfulness and leg health in commercial broiler chickens. Animal. 9, 660–668.

Air quality

Circular image with a black line drawing of a chicken, side-on. There is a white wave icon to represent air flow. Text says "Better air quality"

The Better Chicken Commitment states:
On air quality: the concentration of ammonia does not exceed 20 ppm and the concentration of carbon dioxide does not exceed 3000 ppm at the level of the chickens’ heads, regardless of stocking density.

20 ppm of ammonia is already a Minimum Standard under the NZ Meat Chickens: Code of Welfare (2018).


Circular image with a black cage and a red cross in front of it. Text says "No cages"

The Better Chicken Commitment states:
​No cages or multi-tier systems. ​

A paper covering the effect of cages on welfare is listed:​

Shields, S. & Greger, M. (2013) Animal Welfare and Food Safety Aspects of Confining Broiler Chickens to Cages. Animals (Basel). 3(2): 386–400.

Several white chickens in a shed with natural light. They look clean and healthy. One is sitting on a straw bale and the others are showing interest in the straw bale and the camera.
Slower-growing chickens in an enriched shed [Photo credit – Wakker Dier]

Impact of the Better Chicken Commitment on suffering


In this study, the Cumulative Pain Framework was used to investigate how the adoption of the BCC affects the welfare of chickens bred for meat. They compared welfare outcomes of typical fast-grow breeds used in Aotearoa (e.g. Ross 308 and Cobb 500) with slower-growing breeds acceptable under the BCC. 

The Cumulative Pain Framework translates evidence on the duration and intensity of the pain associated with growth rate, into time spent in four categories of pain intensity: Annoying, Hurtful, Disabling, and Excruciating. Pain is operationally defined as ‘any negative affective state’, thus encompassing negative affective experiences of both physical pain and more related to psychological pain. 

The results strongly support that adoption of BCC standards and slower-growing broiler strains have a net positive effect on the welfare of broiler chickens. 

Because most suffering of broiler chickens is strongly associated with fast growth, adoption of slower-growing breeds not only reduces the incidence of suffering, but also delays the onset of suffering. As a consequence, slower-growing birds are expected to experience a shorter time in pain before being slaughtered.

Adoption of the Better Chicken Commitment, with the use of a slower-growing breed reaching a slaughter weight of approximately 2.5 Kg at 56 days (average daily weight gain of 45-46 g/day) is expected to prevent at least 33 hours of Disabling pain, 79 hours of Hurtful and 25 seconds of Excruciating pain for every bird affected by this intervention (only hours awake are considered). These figures correspond to approximately 66%, 24% and 78% reduction, respectively, in the time experienced in Disabling, Hurtful and Excruciating pain relative to a conventional scenario due to lameness, cardiopulmonary disorders, behavioural deprivation and thermal stress. 

Adoption of slower-growing birds reaching a similar slaughter weight as conventional breeds is also likely to lead to a reduction in the number of individuals needed to produce the same amount of meat given reduced losses from mortality, as well as from disposal and rejection of carcasses and meat products (e.g. lower incidence of muscle myopathies) at the production and consumer sides.

Although the adoption of better management practices – including lower stocking density, longer resting times and the provision of enrichment – is beneficial and desirable for improving broiler welfare, their impact is limited if the negative welfare effects inherently associated with the genetics for fast growth are not addressed.