Start with ground that drains
More outdoor hay is lost to the ground than to the sky. A bale sitting on soil wicks moisture up into its base all winter and rots from the bottom while the top still looks sound. Choose the highest, driest spot you have, improve it with a pad of gravel or hardcore if you can, and keep bales on pallets or old tyres anywhere the ground stays soft. Site the stack where water runs away from it, never in the hollow where the yard drains, and close enough to the feeding area that moving a bale in mud season is a tractor job of minutes rather than an expedition you keep postponing.
Butt the flat ends together, in rows
A round bale's curved side sheds rain reasonably well; its flat ends drink it. Push bales tightly end to end in long rows so the flat faces touch and only the curved barrels meet the weather, and leave a gap between rows — around a metre — so air can circulate and you can walk the stack to inspect it. Rows running north to south dry more evenly on both sides. Net-wrapped bales tolerate outdoor storage better than twine-only bales, whose strings rot and let the bale slump; if you buy twine bales for outdoor storage, plan to feed those first.
Cover the tops, and accept the trade-offs
A tarpaulin or purpose-made cover over the top of the rows sheds the bulk of the rain and can save most of the outer layer that uncovered bales sacrifice. Anchor covers against wind so they do not flap themselves to ribbons, and leave the sides open enough for air to move, since a bale sealed in plastic sweats and moulds from its own moisture. However you store them, expect uncovered outdoor bales to lose their weathered outer few inches, and budget that loss when you buy rather than discovering it as a shortfall in February.
Feed oldest first, and inspect every bale
Work through the stack in order: oldest bales and the most exposed rows first, while they are still usable, keeping the best-sheltered hay for the animals who need it most. Every bale gets checked as it is opened, whatever the storage. Cut in and look and smell before it goes out: discard sections that are mouldy, musty, slimy or dusty, and never feed questionable hay to horses, whose airways and digestion punish mould far harder than cattle's do. Heating in the middle of a bale, or a caramel, tobacco-like smell, means that bale is feed for the midden or the cattle at best, not for horses.
Keep the stack away from the feeding mess
Store bales where they stay storage — not in the paddock corner where animals gather, trample the ground to soup and rub against the stack. A bale stored where it will also be fed out deteriorates from both ends at once. Fence the stack if it sits in a field, and you protect both the hay and the string and net your animals should never be chewing among.
Adding a weekly stack inspection to KinsLe's Care Teams — covers secure, no bale slumping or heating, oldest rows marked for use — means small storage problems get caught by whoever is on duty while they are still a tarpaulin adjustment, not a lost winter's hay.