E. Hay, too, suffers the loss of vitamins when it is exposed to air and light. All hay is best stored under a roof, and a three-sided hay shed often will pay for itself within a few years in terms of hay salvaged from losses due to weather. The increased fire hazard must be weighed against the convenience of being able to easily grab and feed hay bales without going to a separate outside storage area. If you do store hay in the same barn in which your horses are housed, pay particular attention to the ventilation in your loft. Air circulation is crucial in preventing heat buildup, so leave some room between stacks and avoid stacking right up to the ceiling.
Many farms find that ventilation fans, cupolas, and vents are beneficial as well.
You can monitor the heat between your hay bales by inserting a thermometer deep into the stack periodically. Any dramatic rise in temperature is a danger sign, and you should dismantle the stacks and break open any suspiciously warm bales outside the barn, well away from your buildings. If your storage space is limited and you are forced to store round bales of hay outside, consider purchasing bales wrapped in plastic, or those treated with a spray of liquid tallow. Furthermore, it's relatively inexpensive, although not every farmer is equipped to apply this treatment. Even under the best conditions, hay loses much of its vitamin content over time.