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 of moldy feed or hay. Molds can also be allergenic, causing irritation to the lungs, digestive tract, or skin. Certain molds produce secondary metabolites called mycotoxins, which can be found in any animal or human foodstuff that has previously supported growth of specific fungi. Molds and their associated mycotoxins are present in varying amounts depending on climatic growth and storage conditions. Cool, wet growing seasons lead to increased fungi growth, especially of the genus Fusarium, in grains. High moisture levels support fungal growth, and cool temperatures can promote mycotoxin production. Hay baled with moisture content greater than 15% is more likely to develop mold, giving way to mycotoxin proliferation.

Pasture grasses, hay, straw, and grains can all support growth of various fungi. The fungi could be saprophytes, which live on the plant’s exterior, obtain nutrients from the plant and provide no benefit to the plant, or endophytes, which live within the plant, providing benefits to the plant in exchange for nutrients. Fusarium and Aspergillus are saprophytes, whereas ergot alkaloids found in some pasture grasses are endophytes. Endophytes make the plant hardier—they’re more able to withstand challenges such as harsh weather—than endophyte-free grass, but can harm animals. Endophytes are responsible for fescue toxicity, which causes prolonged gestation length, thickened or “red bag” placentas that separate prematurely; small, weak or dead foals; and often lack of milk production.

The research on effects of mycotoxins in horses is limited as compared to information available in cattle or swine. Scientists have studied the Fusarium and Aspergillus species the most. Mycotoxins from these fungi have been reported to cause feed refusal, decreased growth rates, poor feed conversion (pounds or kilograms of weight gain per pound or kilogram of feed fed), and increased restlessness. Some mycotoxins are associated with compromised immune function or damage to the cardiovascular, reproductive, and nervous systems.

In 1934, moldy corn disease in the Midwest caused the death of more than 5,000 horses. Fumonisin from Fusarium molds, which invade corn, fresh pasture, and hay, was the suspected cause of this incident. Low-dose (8–10 parts per million) chronic exposure to fumonisin for 30 days has been reported to cause brain degeneration (leukoencephalomalacia), with clinical signs including facial paralysis, excitability, blindness, depression, head pressing, and sudden death. Higher-dose, shorter-term exposures to fumonisin lead to liver damage, which manifests as jaundice, swelling of limbs, mental derangement, and death.

Aspergillus molds produce aflatoxins, which have the potential to be much more toxic than fumonison. Aflatoxins can be found in several grains including corn, cottonseed, barley, and peanuts, as well as in moldy hay. Ingesting very low concentrations of aflatoxins (200–300 parts per billion) consistently for several weeks can cause liver damage in horses. Clinical signs might include anorexia, depression, jaundice, and anemia. High doses of aflatoxins result in severe gastrointestinal distress, hemorrhage, and rapid death. Many factors influence the susceptibility of an animal to specific mycotoxins, including age, weight, and general health. Toxicities are often difficult to diagnose because many of the clinical signs are not specific to mycotoxin ingestion.

Visually detecting fumonisin and aflatoxins in feed and hay isn’t possible, making laboratory testing necessary. Mycotoxin-binding products can be added to feeds in an effort to reduce the potential harm from mycotoxins in the ingredients. However, these binders have not been proven effective against all the different mycotoxins, to bind all present mycotoxins, in all species of animals or across all ranges of gastrointestinal pH. Therefore, they are unable to prevent all potential health problems associated with mycotoxin ingestion. Even with the inclusion of mycotoxin binders in feeds, mycotoxins that might be present may still cause health issues in horses consuming that feed or in horses ingesting mycotoxins from hay or pasture sources.

The best protection you can provide for your horse is to buy feed that has been tested for mycotoxins and aflatoxins, only use hay that is properly cured, and keep pasture grasses mowed to minimize seed head (where most mycotoxins are found) development.