How to Tell If Hay Is Moldy: What to Look For Before You Feed It

There is a fact about moldy hay that catches most horse owners off guard. By the time you can smell it, the mold count is already at the level where researchers start flagging it as a concern.

Your nose is a useful tool, but it is a late warning system. The mold that matters most is usually growing in the middle of a bale where you cannot see it or smell it until you break the bale open.

This guide covers what mold actually looks and smells like in hay, the checks you can do in thirty seconds at the stack, when a lab test is worth the money, and why horses are more vulnerable to it than cattle or sheep.

It also covers the part most articles skip: what to do with hay that has already gone moldy, and whether soaking or steaming can rescue it.

How Can You Tell If Hay Is Moldy?

You tell if hay is moldy by breaking a bale open and checking four things: visible white, grey or black patches; a musty or sour smell; unusual heat or dampness in the bale; and a heavy dust cloud when you shake a flake.

The critical word there is open. Mold grows where moisture is trapped, and in a tightly packed bale that means the core, not the surface.

Judging a load from the outside of a stack tells you about weather exposure. It tells you almost nothing about what your horse will actually be eating.

Pull bales from different parts of the stack, especially the bottom row and anything that sat against a wall. Those are where problems concentrate.

What Does Moldy Hay Look Like?

Moldy hay shows discoloured patches in white, grey, blue-green, or black, often concentrated in clumps or along a seam where flakes press together. The affected sections usually look dull rather than the bright green of good hay.

Common hay molds include Aspergillus, Fusarium, Penicillium, and Rhizopus, and they do not all look the same. Some appear as a fine white powder that people mistake for dust; others form obvious fuzzy colonies.

Watch for clumping. Moldy flakes tend to stick together in dense mats instead of separating cleanly, because the mold growth has effectively bound the stems together.

One thing that is not a mold sign: sun-bleaching on the outside of a bale. That yellow or grey outer layer is normal weathering and the hay inside is usually fine.

Colour change deep inside the bale is a different matter. Brown, dark, or caramelized-looking hay in the core often means the bale heated after baling, which is a strong indicator of high moisture at the time it was put up.

For the broader set of purchase-stage red flags, see our guide to spotting poor-quality hay before you buy.

What Does Moldy Hay Smell Like?

Moldy hay smells musty, sour, or stale — often compared to a damp basement, old cardboard, or mildewed laundry. Good hay smells sweet, fresh, and slightly grassy.

Some heated hay develops a different smell entirely: sweet and almost tobacco-like or caramelized. That is not a good sign either. It indicates the bale went through a heating cycle, which usually means it was baled too wet.

A sharp, ammonia-like or fermented smell points to significant spoilage and the hay should not be fed.

Here is the uncomfortable part. University of Minnesota Extension notes that most people can begin to detect mold at around 500,000 colony forming units per gram — which is right at the boundary where hay stops being classified as low risk.

In other words, if you can smell it, you are not catching it early. You are catching it at the point where it has already become a question mark.

Always smell a freshly broken flake, not the outside of the bale. Cold hay in a winter barn also releases less odour, so bring a flake somewhere warmer if you are unsure.

What Else Should You Check Besides Sight and Smell?

Three more checks take almost no time and catch problems your eyes and nose miss.

Heat:

Push your hand into the centre of a bale. Mold growth generates heat, so a bale that feels warm weeks after baling is actively spoiling. This is also a fire concern — mold heating in tightly packed, poorly ventilated stacks is a recognized cause of spontaneous combustion.

Weight and dampness:

A bale that feels unusually heavy for its size may have been baled wet. Damp flakes that feel cool and clammy rather than dry and crisp are a warning.

Dust:

Shake a flake in good light and watch what comes off. A visible cloud of fine grey or white particles is largely mold spores, not field dust.

That dust matters more than most owners realize. Mold spores are what actually reach a horse's airways, and a bale can throw a heavy spore load without showing obvious visible colonies.

How Do You Test Hay for Mold?

Two different tests answer two different questions: a moisture meter tells you whether hay is at risk of moulding, and a lab mold count tells you how much mold is already there.

Moisture testing

A hay moisture probe is inexpensive and reusable. You insert it into the bale and read the moisture percentage directly.

This is most useful at buying and storage time rather than after the fact. If you are putting up your own hay or buying straight off the field, testing before stacking prevents the problem instead of diagnosing it.

Penn State Extension puts ideal hay moisture at around 15 percent, with mold risk rising above 18 percent and spontaneous combustion becoming a concern above 20 percent.

Laboratory mold counts

Most forage testing labs can run a mold and mycotoxin analysis. You collect a core sample the same way you would for a nutrient analysis and ship it in.

Reported costs generally run somewhere between $40 and $90 per sample depending on the lab and what you request, with turnaround typically around one to two weeks.

Results come back as colony forming units per gram. Every hay sample contains some mold — a zero result does not exist — so the question is always how much.

Mold count (cfu/g)	Risk classification
Under 500,000	Low risk
500,000 to 1 million	Relatively safe
1 to 2 million	Feed with caution
2 to 3 million	Observe animals closely
3 to 4 million	Dilute with other feeds
Over 5 million	Discontinue feeding

 

Classification per Mississippi State University Extension. Consult a nutritionist for thresholds specific to your species and production stage.

University of Minnesota Extension applies a tighter standard for horses specifically: hay above 1 million cfu/g should not be fed to horses because of the respiratory risk.

If your hay lands in the 500,000 to 1 million range, UMN suggests practical mitigation — pull flakes apart before feeding, feed outdoors or in a well-ventilated area, use a hay net so the horse cannot bury its nose in the pile, and wet the hay to knock down dust.

A lab test is worth the money when you are committing to a large volume, when a horse has existing respiratory disease, or when you suspect a problem but cannot confirm it by sight and smell. Our guide to reading a forage analysis explains the sampling process.

At What Moisture Does Hay Start to Mold?

Hay without a preservative generally begins to mold above roughly 15 percent moisture. Below that, there is not enough free water for fungal spores to germinate and grow.

University of Minnesota Extension publishes clear moisture guidelines for baling, originally framed around fire risk but equally useful for mold.

Moisture at baling	What happens
Under 10%	Brittle and dry; leaf shatter and nutrient loss
10–15%	Recommended range; least risk
16–20%	Could mold without preservative; slight fire risk
21–25%	Will likely mold without preservative; moderate fire risk
Over 25%	Severe heat damage; high fire risk

 

Speed matters here. Under favourable moisture conditions, spores from various mold species can germinate within 24 to 72 hours.

That is why hay baled slightly wet can look perfectly fine on the truck and be visibly mouldy a week later in your shed. The damage was already set in motion at baling.

Alberta's dry harvest conditions are a genuine advantage in this regard, which is part of why prairie-grown hay cures so well. But a wet summer or a rushed baling window can undo that in any year.

Why Is Moldy Hay Dangerous for Horses?

Two separate mechanisms cause harm: inhaled spores damage the respiratory system, and mycotoxins produced by some molds cause systemic problems.

The respiratory risk is the more common one. Mold spores in hay dust can trigger recurrent airway obstruction, also called heaves or severe equine asthma a chronic condition comparable to asthma in people.

Penn State Extension describes the typical presentation: a horse with RAO usually has a normal temperature and good appetite, but shows reduced exercise tolerance, coughing, and nasal discharge.

Laboured breathing appears during exercise and, in advanced cases, at rest.

Long-term cases develop a visible "heave line" along the flank, caused by enlargement of the abdominal muscles the horse uses to force air out.

Sensitivity varies enormously between individuals. Some horses react strongly to spore loads that others tolerate without any visible sign.

Mycotoxins are the second pathway. Documented effects associated with moldy forage include feed refusal, reduced fertility, increased water consumption, poor coat condition, lameness, impaired kidney or liver function, diarrhea, and colic in horses.

Mycotoxin problems are notoriously hard to pin down, because the signs overlap with many other nutritional and health issues. If you suspect one, that is a veterinary conversation, not a self-diagnosis.

Are cattle, sheep and goats at the same risk?

No. Horses are the most vulnerable common livestock species when it comes to moldy forage.

The reason is digestive anatomy. Cattle, sheep and goats are ruminants, and the rumen breaks down many mycotoxins before they are absorbed. Horses are hindgut fermenters, so they lack that protective first stage.

Ruminants are not immune, though. Mississippi State Extension notes they remain vulnerable during pregnancy, where moldy feed has been associated with mycotic abortion and aspergillosis.

Practical takeaway for mixed operations: hay you might accept for dry cows is not automatically acceptable for horses or for pregnant stock.

Humans are exposed too. Repeated inhalation of hay mold spores can cause a condition known as farmer's lung. Wear a proper dust mask when handling suspect bales.

Can You Make Moldy Hay Safe by Soaking or Steaming It?

Partly, and only for the dust problem. Neither soaking nor steaming removes mycotoxins, so genuinely moldy hay does not become safe hay.

Soaking is effective at reducing airborne dust. Research using wearable particulate monitors has demonstrated measurably lower dust exposure in horses fed soaked hay compared to dry hay.

Steaming goes further on the microbial side. High-temperature steaming has been shown to reduce viable mold counts, whereas soaking alone does not achieve the same reduction.

Two practical cautions apply. Soaked hay should be fed immediately, because wet hay left sitting begins growing bacteria of its own — particularly in warm weather. And soaking leaches soluble nutrients along with the dust.

If you already soak hay for other reasons, our post on what soaking does and does not change covers the nutritional trade-offs.

The honest summary: these are management tools for borderline hay and for horses with existing respiratory disease. They are not a rescue method for a bad load.

How Do You Stop Hay From Molding in Storage?

Most storage mold in Alberta traces back to three things — ground contact, trapped moisture, and no airflow.

•     Get it off the ground. Pallets, railway ties, or a gravel pad. Bare dirt wicks moisture straight into the bottom row.

•     Leave airflow between bales. Stack in an alternating pattern rather than packing everything tight.

•     Do not seal a tarp completely. A fully enclosed tarp traps condensation and creates the exact conditions you are trying to avoid. Cover the top, leave the sides breathing.

•     Do not stack too high. Tall dense stacks hold heat in the middle.

•     Watch the spring melt. A stack that looked fine in January can develop a spoiled bottom layer by April as meltwater pools underneath.

•     Inspect before feeding, not just before buying. Hay can be clean going into the shed and problematic six months later.

 

Round bales stored outdoors need particular attention, since the bottom and outer layers take the most weathering. Our comparison of bale formats and storage trade-offs covers this in more detail.

What Should You Do With Hay That Has Already Molded?

Do not feed visibly mouldy hay to horses. That is the consistent position of every extension service that publishes on this, and the potential cost of a respiratory case far exceeds the value of the bale.

For bales with isolated mouldy sections, you can sometimes discard the affected flakes and use the rest — but only if the remainder passes all four checks cleanly, and only for healthy animals with no respiratory history.

Heavily affected hay still has uses. Mouldy bales can serve as bedding in some situations, as garden mulch, or as compost material, which at least recovers some value.

Never feed it to a horse with existing respiratory disease, a pregnant mare, or a young growing animal, regardless of how mild the mold appears.

If a horse has already been eating hay you now suspect, watch for coughing, nasal discharge, reduced appetite, or signs of colic, and call your veterinarian if anything appears. Do not wait to see whether it resolves on its own.

Buying Hay You Do Not Have to Second-Guess

Most mold problems start before the hay ever reaches your property — a rushed baling window, a rained-on windrow, or a season stored on bare ground at the seller's yard.

Horse Creek Hay & Feed grows and supplies forage from Rocky View County with delivery across much of Alberta. You can see what we have available this season to get a sense of formats and types.

If you have had a bad load and want to ask real questions about how a crop was put up and stored before you commit, get in touch with our team. Ask about baling conditions, moisture at baling, and storage a seller who cannot answer those is telling you something.

Conclusion

Break the bale open. Everything useful about identifying mold happens in the core, not on the surface, and a stack can look perfect from the outside while hiding a problem.

Check for discoloured patches, a musty or sour smell, warmth deep in the bale, and a heavy dust cloud when you shake a flake. If two or more of those show up, set the bale aside.

Remember that smell is a late signal, not an early one. For high-volume purchases or a horse with any respiratory history, a lab mold count is cheap insurance compared to a veterinary bill.

Horses carry more risk here than cattle or sheep because they lack a rumen to break down mycotoxins. When you are uncertain about a load, the safe call is not to feed it — and if a horse shows respiratory signs or colic, contact your veterinarian.

Frequently Asked Questions

Can horses get sick from just a little bit of moldy hay?

Sensitivity varies widely between individuals. Some horses tolerate low spore loads with no visible effect, while horses with existing respiratory disease can react to relatively small exposures.

Because the risk is unpredictable and the signs can take time to appear, extension guidance is consistently to avoid feeding hay with visible mold rather than trying to judge an acceptable amount.

Is white powder in hay always mold?

No, but it is worth checking carefully. Fine white dust can be field dust, dried leaf fragments, or in some cases a harmless surface residue.

Mold tends to appear in defined patches, cause flakes to clump together, and come with a musty smell if the powder is accompanied by either of those, treat it as mold.

Does freezing weather kill mold in hay?

No. Cold slows fungal growth but does not eliminate mold or destroy mycotoxins already present.

Mold that developed during a wet summer remains a problem through an Alberta winter, and growth can resume when bales warm up during a Chinook or in spring.

Can I feed slightly moldy hay to cattle instead of horses?

Ruminants tolerate mycotoxins better than horses because the rumen breaks many of them down, so mildly affected hay is sometimes redirected to dry cattle.

However, pregnant animals remain at risk of mycotic abortion, and heavily mouldy hay should not be fed to any livestock.

Check with your veterinarian or a livestock nutritionist before redirecting a questionable load.

How long does hay stay good before it starts to mold?

Properly cured hay baled under 15 percent moisture and stored dry, off the ground, and with airflow can hold for a year or more without moulding.

Hay baled above about 18 percent can begin moulding within days regardless of how well you store it. Storage conditions and baling moisture matter far more than calendar age.