When a horse eats moldy grain, the consequences range from mild feed refusal to fatal brain damage, depending on which molds are present and how much the horse consumed. Mold itself isn’t the core danger. The real threat comes from mycotoxins, invisible chemical byproducts that different mold species produce as they grow. A single batch of spoiled grain can contain multiple mycotoxins at once, each targeting a different organ system.
Why Moldy Grain Is Uniquely Dangerous for Horses
Horses are more sensitive to several common mycotoxins than cattle, sheep, or goats. Their digestive system processes grain differently, and they lack the rumen bacteria that help ruminants partially break down certain toxins before absorption. This means a contamination level that barely affects a cow can be lethal to a horse eating from the same bin.
The type of mold matters enormously. Fusarium species produce fumonisins and a toxin commonly called vomitoxin. Aspergillus species produce aflatoxins. Claviceps fungi produce ergot alkaloids. Each causes a distinct set of problems, and none of them can be detected by sight or smell alone. Grain can look and smell fine while carrying dangerous mycotoxin levels, and visibly moldy grain doesn’t always contain high concentrations. There’s no reliable way to judge safety without lab testing.
Brain Damage From Fumonisins
The most devastating outcome of moldy grain ingestion in horses is a condition called leukoencephalomalacia, which literally means softening of the brain’s white matter. Fumonisins B1 and B2, produced by several Fusarium mold species that commonly colonize corn, cause this. Horses are exceptionally vulnerable.
The toxin disrupts the normal metabolism of certain fats in cell membranes, triggering cell death in the brain. At necropsy, affected horses show flattened brain folds and softened tissue across the frontal and temporal regions, with widespread hemorrhage and swelling in the white matter.
The clinical signs reflect this progressive destruction: early on, a horse may seem lethargic, stop eating, and become unusually sensitive or agitated. As the damage worsens, you’ll see muscle twitching, a staggering gait, circling, and an exaggerated stepping motion called hypermetria. In advanced cases, horses lose the ability to swallow, go blind, develop fixed dilated pupils, press their heads against walls or fences, and suffer seizures. The progression from early neurological signs to death can happen within days, and there is no treatment that reverses brain tissue already destroyed.
Liver Failure From Aflatoxins
Aflatoxins, produced primarily by Aspergillus molds that thrive in warm, humid storage conditions, target the liver. Horses with aflatoxicosis show loss of appetite, depression, fever, tremors, uncoordinated movement, and sometimes coughing. Because the liver is the body’s main detoxification organ, damage there creates a cascade of problems: poor clotting, fluid buildup, weight loss, and eventually organ failure.
Aflatoxin poisoning can be acute from a single large exposure or chronic from weeks of eating moderately contaminated grain. Chronic exposure is insidious because the horse may simply seem “off” for a long time before liver damage becomes severe enough to cause obvious symptoms. By that point, significant irreversible damage has often already occurred.
Feed Refusal and Digestive Problems
Deoxynivalenol, often called vomitoxin or DON, is one of the most common mycotoxins in grain worldwide. Its primary effect in horses is straightforward: they stop eating. In controlled studies, horses fed contaminated grain at roughly 14 mg/kg reduced their grain intake from 2.8 kg per day down to just 1 kg, while still eating their normal 5 kg of hay. Even exercised horses dropped grain consumption from 3.5 to 2.3 kg per day at contamination levels around 11 mg/kg.
The European Food Safety Authority identified reduced feed intake as the most consistent adverse effect in horses, with levels around 6 mg/kg in the total ration enough to cause problems. Interestingly, horses seem to detect something wrong and selectively avoid the contaminated portion of their diet while continuing to eat clean hay normally. Prolonged exposure leads to weight loss and poor condition simply from inadequate calorie intake.
Reproductive Problems From Zearalenone
Zearalenone, another Fusarium toxin, mimics estrogen in the body. In mares, this can disrupt normal heat cycles, cause swelling of the vulva and uterus, and in severe cases lead to uterine prolapse and internal hemorrhage. One well-documented case involving feed contaminated at 2.7 ppm found that 15 of 37 mares showed signs of estrogenic toxicity, and 2 of 11 stallions on the same farm developed severe flaccidity of their reproductive organs.
Research on controlled zearalenone exposure has shown mixed results on pregnancy rates. In one study, only 4 of 7 mares on a low dose became pregnant, and just 3 maintained pregnancy with a detectable fetal heartbeat by day 30, compared to 5 of 6 mares in the control group. Both studies that examined ovarian activity found increases in the number of follicles after zearalenone exposure, suggesting the toxin actively interferes with normal reproductive cycling even when other measurable hormone levels appear normal.
Respiratory Damage From Mold Spores
Even before a horse swallows moldy grain, it inhales mold spores while eating. This is a separate and significant health risk. Aspergillus fumigatus, commonly found in moldy feed, is highly allergenic and contributes to equine asthma (previously called recurrent airway obstruction or “heaves”). Fungal spores in stable air trigger and worsen respiratory inflammation, and grain tends to carry higher counts of mold and bacteria than pelleted compound feeds. Grain stored below 86% dry matter content, meaning moisture above 14%, has significantly higher mold counts.
A horse with equine asthma develops chronic coughing, nasal discharge, labored breathing, and exercise intolerance. Once a horse becomes sensitized to mold spores, even low-level exposure can trigger flare-ups for life.
Ergot Alkaloids and Blood Flow
Ergot alkaloids, produced by Claviceps fungi that infect grain heads (particularly rye, wheat, and barley), cause blood vessel constriction. In cattle, this leads to a condition called fescue foot, where reduced blood flow causes tissue death and gangrene of the ears, tail, and sometimes hooves. Horses appear less susceptible to the gangrenous form of ergotism, but research has confirmed that ergot alkaloids do reduce blood flow in the arteries of the lower legs. The full implications of this reduced circulation, particularly during cold weather or in horses with existing hoof problems, remain a concern.
What to Do After Exposure
There are no antidotes for mycotoxin poisoning. The single most important step is removing the contaminated grain immediately. Every additional meal increases the toxin load and the risk of irreversible organ damage.
For certain mycotoxins, binder compounds added to feed can reduce absorption. Aluminosilicate clay products effectively bind aflatoxins in the gut before they reach the bloodstream, but they have little effect on other mycotoxin types. A resin called cholestyramine has shown effectiveness at binding fumonisins and zearalenone in laboratory and animal studies. Your veterinarian can recommend the appropriate binder based on which toxin is suspected.
Beyond removing the source and using binders when appropriate, treatment is supportive: fluids, nutritional support, and management of specific symptoms as they appear. Horses showing any neurological signs, jaundice, or persistent refusal to eat after known moldy grain exposure need veterinary evaluation promptly, because early intervention for liver or brain involvement can mean the difference between recovery and death.
Testing Grain Before Problems Start
You can have suspect grain tested through veterinary diagnostic laboratories. Purdue University’s Animal Disease Diagnostic Laboratory, for example, runs a mycotoxin screen covering aflatoxins, vomitoxin, zearalenone, and fumonisins for $110, with results in 5 to 10 business days. You’ll need about one pound of grain collected in a sealed plastic bag that represents the batch in question.
Prevention is more reliable than testing after the fact. Mold grows on grain stored above 14% to 15% moisture content, so proper drying and dry storage are essential. Store grain in clean, dry bins with good airflow. Inspect grain regularly for clumping, discoloration, musty odors, or visible mold growth, keeping in mind that dangerous mycotoxin levels can exist without visible signs. Buy from reputable suppliers, use grain within a reasonable timeframe rather than stockpiling, and discard any batch that looks or smells questionable. The cost of replacing a bag of grain is trivial compared to the veterinary bills and potential losses from mycotoxin poisoning.