Bean bag made of animal feed....?

A friend of a friend of a friend..... gave one of my toddler son's a 'bean bag' filled with animal feed for Christmas. He's from Montana, and I guess they heat them up at night, and use it to warm their beds. He said they used them to warm the kids cribs, when they were little, before they'd put them down. My son LOVES it. We warm it in the microwave before he lays down, and he lays it across his stomach and goes right to sleep. Problem is, now my 18 mo old son wants it..... Does anyone have any idea what kind of feed goes into these things? I searched all over the internet and I can't find them. The guy who gave it to him said that if you get the wrong kind of feed, that it can get moldy, and get weevils.....! Has anyone heard of these?

Thanks