Lameness from leg disorders is endemic in commercial broiler chickens, affecting 25-30% of all birds and representing one of the largest-scale chronic pain problems in any animal population on Earth.
The landmark study by Danbury et al. (2000) demonstrated that lame broiler chickens voluntarily consume carprofen (an NSAID) when it is mixed into feed, and then walk more normally while on the pain medication - providing direct evidence that these birds experience pain from their leg disorders. This finding has profound welfare implications: up to 30% of commercial broilers worldwide are experiencing chronic pain that could be meaningfully reduced by using slower-growing genetics and better housing conditions. The aggregate suffering represented by hundreds of millions of chickens with chronic leg pain is staggering. The fix is known - slower-growing breeds - but adoption requires consumer and regulatory pressure.