Moldy hay is a year-round concern but most of the reasons hay develops mold are related to problems at harvest. Dr. Sally Flis at Equi-Analytical Lab indicates there is a concern for moldy hay because the if mold develops during storage, there is a loss of dry matter in the hay plus a loss of total digestible nutrients because mold eats the hay before your horse can. The other issue is that moldy hay can cause major issues with the horse's respiratory tract and can cause allergic reactions. Some horses are extremely sensitive to the allergens in mold and this can lead to inflammatory airway disease. Many horses with heaves or COPD are very sensitive to environmental allergens in the hay and will have a difficult time breathing if fed moldy hay.
Lastly, most horses do not like to eat moldy hay so there may be a palatability problem. Mold can develop on hay through several methods, and one is that if the weather is cool and wet, mold can develop on grass before it is even cut to bail. If it is cool and wet, hay takes longer to dry because the soil is wet and the hay absorbs moisture from the wet soil. Also, many times hay gets rained on after being cut and before bailing and the longer it takes to dry, the greater the chance of mold developing. Raking the hay to facilitate drying soon after a rain will help decrease the amount of mold growth. Baling hay that has a moisture content greater than 14 percent will increase the chance of mold developing. If you feel your hay has mold but are not sure, you can have the hay tested to determine the risk of using it as not all moldy hay has to be discarded; depends on the level of mold.