Pigs raised in factory farms have their tails docked (i.e., cut off) because they are weaned from their mothers at about 12 days.  They would normally wean at about three weeks, but the feed they are given causes them to grow at a much faster rate than their mother’s milk would.  However, pigs who are weaned early retain an urge to suck on and bite things, which in a confined operation means the tail of whatever neighboring pig is closest. Normally when this happens the pig in question would retaliate in order to make it stop, but in a CAFO they experience a kind of learned helplessness which demotivates them to do anything about it.  Hence, tail removal for demoralized pigs. 

I could really go for some bacon right now…