Prevention of mould in hay for horses is important. Horse owners can do so by keeping a close eye on plant growth, the moisture content at harvest, and other baling considerations.
Kentucky Equine Research writes that common moulds that grow on hay include Aspergillus, Fusarium, Penicillium, and Rhizopus, among others. Moulds generate spores that can irritate respiratory tissues or cause colic. Moulds can also produce mycotoxins, Some problems stemming from mycotoxin consumption are acute (immediate), while others are chronic (long-term). It all depends on the type and amount of mycotoxin the horse ingests.