Most feed tags include a statement cautioning against offering moldy feed or hay to horses. Owners might suspect mold when they see white, blue, or greenish powdery substances in hay or feed; black spots on hay; or simply by detecting a musty smell in damp, heavy hay. It is crucial to notice mold and avoid feeding molded feedstuffs because the fungus itself—or organisms it produces—have the potential to cause illness and even death in horses.

So what exactly is mold? Mold is a type of fungus, and the number of different molds is estimated in the hundreds of thousands of species. Some molds are harmless; others are useful and even edible. For instance, blue cheese is made by adding a particular Penicillium mold culture to cheese, while another Penicillium species is used to produce antibiotics. And in Asia the mold Aspergillus oryzae has been used for many centuries to ferment a soybean and wheat mixture to make soy sauce or to break down the starch in rice to make sake and other distilled beverages. So why is moldy hay or feed harmful to horses?

Because molds consume nutrients from the host plant material, reducing the nutritive value

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