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Fast-growing chickens can't grow old
A barnyard chicken can live up to 10 years. A broiler is taken only to 38 or 39 days, which is as far as farms want to go for the product people want to buy.
The gap shows what selective breeding at a global scale has done to the bird. Broilers' bodies can't cope with that rapid weight growth beyond the point at which they are normally slaughtered, and the pressure on their bodies is huge.
For a farm raising animals for meat, the reasoning is to take the animal to its optimal point and then turn it into food for people.
Why do fast-growing birds struggle with their own weight?
Muscle in a broiler grows faster than the skeleton, heart, and lungs that have to support it. Veterinary and poultry science research describes leg disorders, because young bones and joints carry loads they were not built for. It also describes ascites, a buildup of fluid in the abdomen that occurs when the heart and lungs cannot deliver enough oxygen to such a large body. Both conditions become more common the longer a fast-growing bird is kept past market age.
Why do farms stop at 38 or 39 days?
A growing bird needs feed just to maintain its body, and that maintenance cost rises as the bird gets heavier. Past a certain age, each extra kilogram of weight takes more feed than the one before it, so the cost of gain climbs. Processing plants also buy to weight specifications, and a flock is scheduled so that birds arrive at the plant near the target size. Poultry economists call the measure of feed eaten per kilogram of weight gained the feed conversion ratio.
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