Sometimes you can’t keep them from eating sand. We have very sandy land and the horses pick up sand by picking up “scraps.” I was concerned about this myself so asked my vet. He suggested rice bran or wheat bran in the feed. Not very expensive and adds fiber to diet. This is what he does for his horses so I do it. So far with great results. My horses are lucky enought to have a large pasture to graze in for 12 hours a day. I believe this helps also.