Is it normal for horses to get annual scratches?

I have three horses. I've been noticing every two weeks or so they get little scratches on them. They aren't very serious and heal in about a month. I apply ointment to them daily, too, so they heal faster and do not get infected. Is this regular, or is their something wrong about my pasture (i.e. sharp edges) I keep them in their pasture 24-7.