The most important thing to remember about storing hay is keeping it dry. When hay gets wet it will quickly grow mold. Never feed a horse moldy hay, it will make them very sick.
When storing hay outside use pallets to keep the hay off the ground and either place it under a well-protected shelter or use a tarp to cover the stacks.
Using pallets even when storing hay inside is also a good idea. Be sure to keep the area around your stacks as clean as possible. This will help keep the everything clean and tidy and dissuade rodents from making comfy nests around your hay!
(free photo from Canva - https://www.canva.com/ )
~ Saddle Up