Can Horses Eat Moldy Hay? Risks, Testing, and What To Do

By Dr Duncan Houston

Moldy hay is one of the most common grey-area questions in horse management.

You open a bale, see a bit of dust or smell something musty, and immediately have to decide whether to feed it, dilute it, or throw it away. The problem is that not all mold is equal, and not all horses respond the same way.

But here is the key point.

If you are unsure, guessing is where most mistakes happen.

Moldy hay is not just a quality issue. It can affect respiratory health, digestive function, toxin exposure, and overall performance, especially in sensitive horses.

Quick Answer

Moldy hay can reduce nutritional value and pose risks to horses, particularly respiratory irritation and potential mycotoxin exposure. Some mildly affected hay may be tolerated by healthy adult horses in small amounts, but visibly moldy, musty, or dusty hay should be avoided in young horses, respiratory cases, and performance horses. When in doubt, do not guess. Test or discard.

Quick Decision Guide

Hay smells musty, feels damp, or shows visible mold → do not feed until assessed

Horse has respiratory issues such as heaves or airway sensitivity → avoid all moldy hay

Light dust or minor mold with otherwise good-quality hay → may be usable in limited situations with caution

Horse refuses to eat the hay → do not force it, palatability matters

Unsure about safety or source → testing is the safest option

What This Usually Turns Out To Be

When moldy hay becomes an issue, the situation is usually one of these:

• hay was baled too wet

• rain exposure occurred before baling

• storage conditions allowed moisture back into the bale

• hay heated and developed microbial growth

• long-term storage reduced quality over time

The mistake I see most often is assuming mold is only a visual issue.

Some of the most problematic hay does not look dramatically moldy. It smells off, feels dusty, or causes subtle issues before anything obvious appears.

What Causes Mold in Hay

Mold develops when moisture and organic material combine.

The most common triggers include:

• baling hay above safe moisture levels

• rain during curing

• slow drying due to humidity or weather

• storage in poorly ventilated areas

• moisture wicking from the ground

Once moisture is present, fungi and microbes begin breaking down the hay.

At that point, the quality is already declining.

How Mold Changes Hay Quality

Mold does not just sit on hay. It consumes it.

As mold grows, it reduces:

• energy content

• digestible nutrients

• vitamin levels

• overall feeding value

In practical terms, the horse gets less usable nutrition from each kilogram of hay.

For performance horses, breeding stock, or animals needing condition, this matters more than people think.

Respiratory Risk Is the Biggest Concern

The most consistent problem with moldy hay is respiratory.

Mold spores and dust can irritate the airways and trigger inflammation.

Horses most at risk include:

• those with heaves or recurrent airway disease

• horses with inflammatory airway disease

• older horses

• horses stabled for long periods

Signs can include:

• coughing

• nasal discharge

• increased breathing effort

• reduced performance

What Vets Care About Most

If a horse’s breathing worsens after a hay change, that is a red flag.

The hay is part of the problem until proven otherwise.

Mycotoxins: The Hidden Risk

Some molds produce toxins called mycotoxins.

These are less predictable than respiratory irritation and can affect:

• the digestive system

• the nervous system

• appetite and weight

• general health

The challenge is that mycotoxins are not always visible.

Hay can look acceptable but still contain harmful compounds.

This is why testing matters when there is any doubt.

Palatability Still Matters

Horses are selective eaters.

If hay smells musty or tastes off, many horses will reduce intake or refuse it altogether.

This creates secondary problems:

• inconsistent feeding

• weight loss

• frustration and waste

If the horse does not want to eat it, there is usually a reason.

Severity Framework

When Might Moldy Hay Still Be Used?

This is where nuance matters.

Some mildly affected hay may be used in:

• healthy adult horses

• small amounts mixed with clean hay

• situations where testing confirms low risk

But it should not be fed to:

• horses with respiratory disease

• young or growing horses

• pregnant mares

• high-performance horses

• horses already showing signs of illness

Decision Checkpoint

If you would not confidently feed it to your best horse, it is probably not worth feeding at all.

Should You Test Hay?

Yes, when there is any uncertainty.

Testing can assess:

• mold levels

• nutritional value

• presence of mycotoxins

Sampling should come from multiple bales and different areas.

Testing removes guesswork.

What Not To Do

Common mistakes include:

• feeding visibly moldy hay to save cost

• assuming soaking or steaming fixes all problems

• ignoring mild respiratory signs

• feeding poor hay to sensitive horses

• relying on smell alone without considering hidden toxins

The biggest mistake is thinking “it will probably be fine.”

What Should You Do Right Now?

If you have suspect hay:

1. Check for smell, dust, and visible mold

2. Watch your horse’s intake and behavior

3. Remove hay immediately if coughing or refusal occurs

4. Test hay if there is any uncertainty

5. Replace with clean forage where possible

Simple checkpoint:

clean hay + normal breathing + good intake → acceptable

musty hay + reduced intake or coughing → stop feeding

When Is This an Emergency?

Seek veterinary attention if your horse shows:

• labored breathing

• persistent coughing

• colic signs

• neurological signs

• sudden deterioration after feeding

These are not normal responses to hay.

Prevention

Prevention starts at harvest and storage.

Key steps include:

• bale hay at the correct moisture level

• avoid rain exposure during curing

• store hay in dry, ventilated areas

• keep hay off damp ground

• rotate stock and avoid long-term degradation

Good hay is easier to maintain than bad hay is to fix.

Frequently Asked Questions

Is all moldy hay dangerous?
No, but risk varies. Some may be tolerated in limited situations, others should never be fed.

Can soaking or steaming fix moldy hay?
It may reduce dust, but it does not remove all risks, especially toxins.

What is the biggest risk with moldy hay?
Respiratory irritation is the most consistent issue.

Are mycotoxins common in hay?
Less common than in grain, but still possible and important.

Should I throw out moldy hay?
If it is clearly moldy or causing issues, yes.

Final Thoughts

Moldy hay is rarely worth the risk.

Even when it does not cause immediate illness, it reduces nutritional value, increases respiratory load, and adds uncertainty to feeding.

The safest approach is simple.

If you are unsure, test it or do not feed it.

If you want help reviewing hay quality, test results, or deciding whether a batch is safe to feed, ASK A VET™ can help you make a clear and practical decision.