Friend used to do this in a Barley Boiler in the winter for all the horses. Walking in to the ban on a cold winters morning and smelling the barley, then dishing it up was heaven. The same in the evening.
the smell is devine I usually just boil it on the stove but as Im a dozy mare, Im sick of forgetting about it and welding it to the bottom of the pan!! Tried it once in a pressure cooker...that was a disaster...still got bits stuck to the kitchen ceiling! So I thought that maybe a slow cooker would gently cook it through the day. I only feed it to one horse now so dont need a vast amount.
I've always used a pressure cooker, but all I do is let it come up to pressure, give it 5 minutes and then turn it off and leave it overnight. It's always been cooked through with no problems. Can't see why the slow cooker wouldn't do the same job though.