I've read the "Wild Flock Management System" on Wind Ridge Farm's site - they say folks
should select against early heats in their animals so you can keep the males and females together.
On the other hand, I met a family whose pot belly pig turned out to be pregnant when they got her. They kept her son a bit too long and she was pregnant again by the time they sold him. It's been a never-ending stream of piglets for them since then. My pot belly piglet was practicing on his sister and mom when he was only 6 weeks old. They say they aren't very fertile at that age.
At least one book says the biblical law against eating pork was because pigs were worshipped for their amazing fertility.