I learned last year that if you leave your tractor in the pen, they will rub on it. I also learned that red paint can be washed off antlers on the day the deer are knocked down to transport. However, I did have some 6' treated posts leftover from when the farm was built, and I tamped some of those into the ground. I also have some old treated 4x4's that I'm going to put in some more pens when I get around to it...They don't seem too picky. I have some T-posts that used to support some pipes where trees were protected-that is, until they started rubbing on the pipes and got them off. I also have some 5x9' sheets of heavy steel from some silo bins that I use for shade, and they like those, too.