I feed round bales. It's best to get some type of grass hay or mixed bale (lucern/grass mix). If you have 2-3 horses eating on it, you don't need to worry about tarping. They'll eat it up in about 2 weeks or less.
I would suggest using a round bale feeder though, to reduce waste. They will end up stomping on it, laying on it, and soiling the hay if you don't use a feeder. 20-30% of the bale will go to waste on the ground, and then you'll have to clean it up or burn it.
I use a plastic molded hay ring. They're affordable, durable, and VERY easy to move around. I use the "adjustable hay ring" on this page. HAY RINGS
Another platic model, a bit more sturdy. O'Neill Bale Feeders - Products
You don't want to use a feeder that has a top ring on it as they will rub a horse's mane out. If you go with a metal feeder, you want a "tombstone" style feeder, like these: BarnWorld Farm and Ranch Superstore Horse Round Bale Feeder