The weight of different breeds of chicken over their lifespan
EDIT: for some context on the problems this creates
The science is clear that fast-growing chickens like the Ross 308 are doomed by their genetics. These have been engineered to grow so incredibly fast, and their bodies just cannot handle it.”
Jackson said secret filming at broiler farms supplying big supermarkets has shown birds struggling to walk or collapsing under their own weight, or dying from heart failure, and dead birds were filmed lying among the flocks.
[...]
Andrew Knight, a professor of animal welfare and ethics at the University of Winchester, said: “With these really rapid growth rates, it can be difficult for the heart and circulatory system to keep up with the expanding body mass. A proportion of these animals suffer from heart failure. It’s also difficult for the bones, ligaments and tendons to keep up with the rapidly increasing body mass, meaning that a proportion of these birds become severely lame [inability to walk properly].”
The science is clear that fast-growing chickens like the Ross 308 are doomed by their genetics. These have been engineered to grow so incredibly fast, and their bodies just cannot handle it.”
Jackson said secret filming at broiler farms supplying big supermarkets has shown birds struggling to walk or collapsing under their own weight, or dying from heart failure, and dead birds were filmed lying among the flocks.
[...]
Andrew Knight, a professor of animal welfare and ethics at the University of Winchester, said: “With these really rapid growth rates, it can be difficult for the heart and circulatory system to keep up with the expanding body mass. A proportion of these animals suffer from heart failure. It’s also difficult for the bones, ligaments and tendons to keep up with the rapidly increasing body mass, meaning that a proportion of these birds become severely lame [inability to walk properly].”
While I can't attest to the veracity of this website, and my objection to the practice of raising chickens for meat has more to do with treatment than anything else, this website gives common reasons why the modern broiler chicken is an unhealthy breed
A good place to start is by changing consumption levels as not doing doing so would make things much harder. It'd be difficult to maintain current consumption levels with slow-growing birds as it'd require a much larger number of chickens to be slaughtered
Maintaining this level of consumption entirely with a slower-growing breed would require a 44.6%–86.8% larger population of chickens and a 19.2%–27.2% higher annual slaughter rate, relative to the current demographics of primarily ‘Ross 308’ chickens that are slaughtered at a rate of 9.25 billion per year.
[...]
In sum, without a drastic reduction in consumption, switching to alternative breeds will lead to a substantial increase in the number of individuals killed each year, an untenable increase in land use, and a possible decrease in aggregate chicken welfare at the country-level scale