Give an example of a negative effect of selective breeding

Birth problems are not the only unintended negative consequence of selective breeding in farm animals. ... This has resulted in intensive farming systems, such as in pigs and poultry farming, where the animal products should be produced with as little costs as possible. So fast growth or more eggs with less feed.

•shortened life spans of dairy cows
• roosters bred for muscle are top-heavy and cannot properly perform courtship dances
• chickens bred for large breasts cannot support the extra weight on their skinny legs
• chickens bred to lay more eggs have brittle bones since the eggs soak up the hens' calcium supply
• animals that are not profitable (such as male chicks) are killed