PROPER HAY STORAGE

Your hay needs to breathe as it naturally has a moisture content (even when cured and baled properly) that, when enclosed in a poorly ventilated area, can cause the growth of mold. Do not store your hay in unventilated or poorly ventilated areas.

Store your hay in a cool, well-ventilated area. Be sure the area in which you store the hay is dry. Storing hay in a loft is best, but if you do not have access to one, storing hay in a garage, shed, or any other structure is fine, as long as it's well ventilated and dry.

Keep the hay off concrete. Concrete can collect moisture causing the hay to rot or mold. If you need to store your hay in an area that has a concrete floor, put wood down first (pallets work well), ensuring the boards are spaced apart like railroad ties. By keeping the hay raised from the concrete, you ensure that it keeps.

It is particularly important in high-humidity areas, such as Western Washington and the southeastern costal states to have air frequently circulating in your storage area (fans can be placed strategically and turned on for short periods of time) to reduce the chance of growth of "storage mold" in and on the the bales.

Stagnant humid air can cause the growth of storage mold, especially during the months when humidity is high and air-circulation is low. Running a fan occasionally can be relatively inexpensive insurance on your hay investment.  Additionally leave sufficient gaps between bales to ensure air circulation throughout the stack.

OUTDOOR STORAGE

While it is not a good idea to just lay a piece of plastic or a tarp over the top of hay (moisture will condense on the bottom side of the plastic and saturate your hay causing it to degrade), it can be a good idea to "tent" plastic over your hay supply if you have any moisture issues from your roof or if you have a particularly dusty storage area. The operative word here is "tent" the plastic so as to cover the hay while allowing airflow around the hay.