I feed rounds and only rounds. I actually have 30 of them sitting out by the pasture at the moment. If the bales are done correctly, the only mold you will get is on the outside and that is mostly where it sits on the ground unless you put them on pallets. All these bales weigh 800-1200 lbs. They are an bromegrass/alfalfa mix. The feeder we use is the tombstone style metal feeder.
I free choice feed the rounds to my horses. During the fall, my rounds tend to last a little under 2 weeks. This is feeding 3 horses (APHA mare, POA mare and Grade appy Mare), as well as two miniatures. During the winter, depending on how cold it is, the bales last a little under a week to a little over a week. Last winter it was routinely about -15 F here (was up and down a lot). None of my horses were blanketed or stalled either.
I have some wastage, but not bad during the winter. The hay doesn't sit out long enough to mold. The only issues are in the spring, but by then the horses have hay pounded into the mud around the bale feeder, which actually is helping build up the ground in that area.