Sheep herding - food trough

Once we get sheep herding skill, we don't get the ability to build food troughs.
Does that mean that sheep just eat grass or whatever they find on the ground?
Or they use trough, but we have to get pig keeping in any case if we want to feed other animals? Is this intended anyway?
Does that mean that sheep just eat grass or whatever they find on the ground?
Or they use trough, but we have to get pig keeping in any case if we want to feed other animals? Is this intended anyway?