Mycotoxin Poisoning in Horses: Moldy Feed and Hay Risks

Mycotoxin poisoning is illness caused by toxins made by certain molds growing on feed ingredients, grain, corn, hay, or other stored forage. A bale or feed bag does not have to look dramatically moldy to be risky. Some toxins are present even when the feed only smells musty or has a few discolored areas.

In horses, the biggest concern is often fumonisin exposure from contaminated corn or corn-containing feed, which can cause a serious neurologic disease called equine leukoencephalomalacia. Aflatoxins are another important group because they can damage the liver and affect appetite, weight, and overall health. Other molds may irritate the airways or contribute to digestive upset, poor performance, and reproductive problems.

The effects depend on the toxin, the amount eaten, and how long the horse has been exposed. Some horses become sick quickly. Others show vague signs over days to weeks, which can make the problem easy to miss at first. That is why a careful feed history matters so much when your vet is trying to connect the dots.

Mycotoxin poisoning starts with mold growth on feed or forage. Moisture, heat, poor ventilation, delayed drying before baling, damaged grain, and long storage times all increase risk. Corn and corn-based feeds are especially important because Fusarium molds can produce fumonisins, while Aspergillus molds can produce aflatoxins in warm, humid conditions or in poorly stored feed.

Hay can also become a problem when it is baled too wet, stored where rain or condensation reaches it, or develops hot spots inside the bale. Even if the main issue is respiratory irritation from mold and dust, some moldy hay and forage can also carry toxins that affect the gut, liver, immune system, or nervous system.

Not every moldy-looking feed contains dangerous toxin levels, and not every toxin can be seen or smelled. That is one reason horses should not be fed visibly moldy hay or feed. Mixing questionable feed with clean feed is risky because it dilutes the appearance, not necessarily the toxin.

Diagnosis usually begins with history and pattern recognition. Your vet will ask what your horse has been eating, whether a new hay lot or grain batch was started, how the feed has been stored, and whether other horses are showing similar signs. A physical exam helps your vet decide whether the main concern is digestive, liver-related, respiratory, or neurologic.

Bloodwork is often the next step. A complete blood count and chemistry panel may show liver injury, dehydration, inflammation, or other organ stress. Depending on the signs, your vet may also recommend clotting tests, urinalysis, or additional neurologic evaluation. These tests do not always identify the exact toxin, but they help measure how sick the horse is and guide supportive care.

Confirming mycotoxin exposure often requires testing the suspect feed, grain, or hay. Feed testing is most useful when samples are collected early and from the actual batch the horse was eating, because toxin levels can vary from one area of a bale or bin to another. In severe fumonisin cases, diagnosis may be supported by neurologic signs plus feed history, and confirmed after death with characteristic brain lesions. Because many illnesses can mimic mycotoxin exposure, your vet may also rule out colitis, liver disease from other causes, botulism, EPM, and other neurologic conditions.

The safest rule is straightforward: do not feed visibly moldy hay or feed to horses. Horses are especially sensitive to some mycotoxins, and trying to use up questionable forage can backfire. Check hay for musty odor, unusual heat, discoloration, clumping, excess dust, or damp spots. Inspect grain and pelleted feed for caking, moisture, off smells, insect damage, or obvious mold growth.

Storage matters. Keep hay and feed dry, well ventilated, and protected from leaks, condensation, and ground moisture. Rotate inventory so older feed is used first, and avoid buying more than you can store properly. Corn-containing feeds deserve extra attention because fumonisins are a major equine concern.

If a new batch of hay or feed seems suspicious, stop feeding it and contact your vet before more horses are exposed. Save labels, lot numbers, and representative samples. On farms with repeated forage quality issues, your vet may suggest working with a reputable feed supplier, improving storage conditions, or submitting suspect lots for laboratory testing before they are fed.