Originally Posted by Left Hand Percherons
I would not cover the hay if it's stored inside. In a humid climate, you will get condensation under the tarp and get spoilage. If your barn is metal, don't stack it up against the wall as well for the same reason. Stack it on pallets and leave some air space all around the stack.
Storage is the most important factor (assuming high quality, properly cured, etc). As mentioned, in hot (90+), humid (60%+) summers like we have, if you don't have lots of good ventilation, it will start to mold in less than a week from just condensation/dew/humidity. If stored well, it lasts for a very long time (we've fed 2 year old hay that our mares cleaned right up).