A good mineral and good grass hay if they don't have anything to pick around on if its only going to be a couple of weeks. No use in spending anything that you don't have to now. If they back up some, you'll get it back pretty quick on grass. This assumes that its your grass and not a gain deal. Also assumes that you're not talking about a very long ride for them. Just my opinion, but costly inputs such as those out of a sack remain on the wrong side of the balance sheet until they're hanging in a cold place.