When I hunted around horses it was the horses that were more alert of the deer than the deer of of the horses. Many times the horse would tip me off to an approching deer on the other side of the field comeing across it where I couldn't see it due to the rise of the field. The horse would stand still and stare at the deer till it passed. When you saw that horse perfectly motionless and staring in one direction you knew something was comeing across the field. The horses were penned up in a half acre or so section of the field that was off the woodline about 50 yrds and not allowed to roam the field.