Quantity
Mycotoxin toxicity is cumulative for many toxin types. Low-level exposure over weeks or months can be as damaging as a single large dose. Even hay that smells "a little dusty" may be delivering a constant mycotoxin load. There is no safe level of mycotoxin consumption — the goal is zero exposure.
Notes
Hay that got rained on before baling, hay stored in damp conditions, and grain stored in humid environments are the primary culprits. Visual inspection catches obvious mold but misses mycotoxin contamination — hay can look clean and still contain dangerous toxin levels. Corn and corn-based feeds are the highest risk for fumonisin contamination. Small square bales are easier to inspect than large rounds, which can harbor mold in the core without visible external signs. Dust from moldy hay also causes chronic respiratory disease (heaves/recurrent airway obstruction).
Negative Signs
* Refusal to eat or picking through hay (your horse may detect what you can't)
* Colic — recurring mild episodes that don't fully resolve
* Weight loss despite adequate feeding
* Respiratory issues — coughing, nasal discharge, heaves (from mold spores)
* Neurological signs — staggering, circling, head pressing, blindness (fumonisin/ELEM)
* Reproductive failure — early embryonic death, abortion
* Liver damage signs — photosensitization, jaundice
FAQ
Q: Some of my hay bales have a few dusty spots. Is that mold?
A: Probably. Dust in hay often IS mold — specifically mold spores. If a bale produces a visible dust cloud when you shake or break it apart, it's contaminated and should not be fed to horses. Even if the mycotoxin risk is low, inhaling mold spores damages your horse's respiratory system over time.
Q: Can I just pull out the moldy section and feed the rest of the bale?
A: No. Mycotoxins are invisible and migrate beyond the visible mold colony. If one section of a bale is visibly moldy, the toxins have likely contaminated a much larger area than what you can see. Discard the entire bale.
Alternatives
Buy hay from reputable suppliers and inspect every bale. Reject hay that smells musty, dusty, or sweet-fermented, or that shows any visible mold, discoloration, or heating. Store hay off the ground in well-ventilated, dry areas. For horses with respiratory sensitivity, consider hay steamers or soaking hay briefly before feeding. Test suspect hay or feed through a mycotoxin laboratory if you have any doubts.
Risks & Disclaimer
If your horse shows neurological signs — circling, pressing head against walls, blindness, loss of coordination — and has been eating corn-based feeds, suspect fumonisin poisoning (ELEM) and call your vet as an emergency. This disease progresses rapidly and is almost always fatal once neurological signs appear. For other mycotoxin exposures, stop feeding the suspect hay or grain immediately and call your vet.
