Moldy Hay and Horses: When to Throw It Away & What Happens If Your Horse Eats It

Last updated: July 7, 2026

By: Miles HenryFact Checked

You open a bale of hay you paid good money for and find mold. Do you throw away the stack, remove the bad flakes, or risk feeding it to your horse? For horse owners, this is a common and expensive problem. After years of buying hay for my horses, I’ve learned that mold doesn’t mean all the hay ruined — but it does mean you need to inspect carefully before feeding.

Signs Your Hay Has Mold

Good hay has a fresh, slightly sweet smell and a green to light golden color. It feels lightweight and pulls apart cleanly. Moldy hay looks, smells, and feels different. Knowing the signs before feeding can prevent a bad outcome.

I Bought Hay in Bulk and Found Mold — Is It All Ruined?

This is the real question for racehorse owners and trainers who buy large quantities. Finding mold in one bale does not mean every bale in the stack is bad, but it is a signal to inspect everything before feeding.

Moldy Hay for Horses: Health Risks

Mold in hay creates two distinct problems: airborne spores that enter the respiratory system when horses eat, and mycotoxins — chemical compounds produced by certain molds — that affect the digestive system, immune function, and in pregnant mares, the pregnancy itself.

Common molds found in hay and their primary health effects in horses

Mold type	Primary concern	Notes
Aspergillus	Mycotoxin production; respiratory disease	Can produce aflatoxins; particularly dangerous
Fusarium	Mycotoxin production; colic	Associated with reproductive problems in mares
Penicillium	Mycotoxins; digestive disruption	Common in improperly stored hay
Alternaria	Respiratory irritation	Produces airborne spores that trigger heaves
Cladosporium	Respiratory irritation	One of the most common hay molds
Mucor / Rhizopus	Digestive issues; respiratory	Fast-growing; often found in wet hay

The most serious respiratory consequence is heaves (recurrent airway obstruction), a chronic condition that permanently reduces a horse’s lung capacity and performance. For racehorses and performance horses, even mild respiratory irritation from mold exposure can measurably affect breathing under exercise. The University of Wisconsin Extension notes that moldy hay is one of the most common environmental triggers for heaves in stabled horses.

How Dangerous Is Moldy Hay?

The danger depends on three factors: the type of mold present, the amount consumed, and the individual horse’s health and sensitivity. Some horses eat small amounts of lightly moldy hay with no obvious ill effects. Others develop colic, breathing problems, or worse from the same exposure. Pregnant mares face the highest risk — mycotoxins from certain molds can cause a mare to abort, and the foal is at risk even if the mare shows no symptoms herself.

In severe cases, moldy hay can contribute to life-threatening colic, serious respiratory disease, or toxic exposure that requires emergency veterinary care. The risk is not uniform or predictable, which is the core reason feeding moldy hay is not worth the gamble — you cannot know in advance how a specific horse will respond to a specific mold load.

What to Do If Your Horse Ate Moldy Hay

Act quickly. I have seen how fast a horse can get access to questionable hay if a bale is set aside but not clearly marked or moved away from the feeding area. The steps below apply whether the amount was small or substantial — the difference is in what the vet recommends based on how much was consumed.

Time matters. If too much time passes after a large ingestion, the toxins absorb into the bloodstream and become harder to treat. When in doubt, call the vet before symptoms appear rather than after. For colic symptoms specifically, see our guide on what to do when your horse colics.

How Mold Gets Into Hay

Mold grows when hay retains or absorbs moisture. Most problems start at one of three points: harvest, storage, or feeding.

At harvest, hay must be cut, dried, baled, and stored in dry conditions. Properly baled hay has a moisture content between 15 and 20 percent. If the grass is cut and then rained on before baling, or if it is baled too early before fully drying, the excess moisture creates conditions for mold to develop. Haymakers use a “fluffer” to turn cut grass and speed drying; if they have to fluff twice after rain, the risk of field mold increases.

In storage, dry hay absorbs moisture from humid air if ventilation is poor. A barn that holds heat and humidity will develop mold in hay that was fine when it arrived. Ground contact wicks moisture from concrete or dirt floors directly into bales. And bales stored against walls can absorb moisture from condensation on cold surfaces.

How to Store Hay to Prevent Mold

For more on barn design and storage planning for small farms, that article covers layout decisions that affect hay storage quality. A well-designed barn with good ventilation reduces hay loss significantly.

Are Round Bales Safe for Horses?

Round bales are not inherently dangerous for horses — the problem is how they are typically used. Most round bales are set in open pastures where they are exposed to weather, ground moisture, and slow consumption over days or weeks. That combination increases the likelihood of mold developing in the outer layers and, over time, throughout the bale.

If you use round bales, keep them covered and off the ground. A well-maintained round bale stored under cover in a ventilated structure and fed out within a reasonable time is manageable. The same bale dropped in a wet paddock and left for two weeks is a mold problem waiting to happen. Horses will often eat the outer moldy layer because they have no alternative, which is exactly the scenario you want to prevent.

FAQs: Can Horses Eat Moldy Hay?

For more on horse nutrition and hay, see our complete horse hay owner’s guide and our article on why horses cannot eat fresh-cut grass — the reasons overlap with the mold risk in improperly handled forage.