During the warm months when the herd is out on pasture not getting hard feed we just put the magnesium in a bowl. If they need it, they'll eat it. If they don't, they won't. But they rarely refuse, even the picky eaters. We use MagOx, which is a powdery consistency. It's technically a cattle supplement, but works great for horses and is much cheaper than the special magnesium supplements for horses. We get it in a 50lb brown bag at the feed mill. Do the same thing with biotin (also the brown bag "cattle" version).