Minecraft wiki entry for Animal_farming, associated text: There are limits to how many animals you can keep in a given pen. In modern versions of Minecraft, if animals are packed too tightly (24 in a single block space), they will begin dying of overcrowding. If there is a mix of adult and baby animals, the adults will specifically be killed off first, which allows for construction of a "cow crusher" (popularized by the youtuber Wattles). This does work for pigs and sheep, but for various reasons (leather) cows are the usual inmates.