Moldy hay shows up as visible patches of white, gray, green, or black fuzz on the surface or interior of bales, often accompanied by a stale, musty smell and a cloud of fine dust when you pull flakes apart. These signs can range from obvious cottony growths to subtle dustiness that you only notice after breaking a bale open, and the health consequences for both livestock and the people handling it are serious enough that learning to spot mold early matters.
What Moldy Hay Actually Looks and Smells Like
The most recognizable sign is a visible coating of fuzz or webbing on the hay stems and leaves. White or grayish-white patches are the most common, but you can also find colonies that look greenish, bluish-gray, or jet black depending on the fungal species involved. These patches tend to concentrate in the interior of the bale where moisture gets trapped, so the outside of a bale can look perfectly fine while the core is riddled with mold. Always break several bales open and inspect the center before assuming a load is clean.
The second giveaway is dust. When you peel apart a flake of moldy hay, a visible puff of fine particles rises into the air. That dust is largely made up of fungal spores, and it is distinct from the occasional bit of leaf shatter you get with any dry forage. The cloud is thicker, hangs in the air longer, and has a grayish tinge. If you sneeze or cough immediately when pulling a bale apart, that is a strong signal.
Smell is the third reliable indicator. Good hay has a sweet, grassy, slightly floral scent. Moldy hay smells musty, sour, or earthy, sometimes with an almost ammonia-like undertone when heating has been severe. If a bale smells “off” even before you see anything wrong, treat it with suspicion. Some mold growth is invisible to the naked eye, especially early on, and your nose may catch it before your eyes do.
Texture changes round out the picture. Moldy sections often feel damp, slimy, or clumped together in a way that clean hay does not. Individual stems may be discolored from their normal green or golden-brown to a dull gray or dark brown, and they tend to be more brittle once the fungal colony has consumed some of the plant tissue. A bale that feels noticeably heavier than its neighbors in the stack may have retained more moisture and is worth opening to check.
Why Hay Molds in the First Place
Mold growth on hay is driven almost entirely by moisture. Hay baled above roughly 20% moisture provides enough water for fungal spores, which are already present on virtually all field-dried forage, to germinate and colonize. The higher the moisture at baling, the worse the problem becomes. Research on alfalfa forage baled at three moisture brackets found that hay baled at 25–30% moisture and stored without any preservative suffered poor dry-matter retention and sharp increases in fungal biomass during storage.1Canadian Journal of Animal Science. Preservation of high-moisture hay in storage through the use of forage additives
Once fungi start growing inside a bale, their metabolic activity generates heat. This is why freshly stacked hay sometimes feels warm to the touch for the first few weeks. In a study of orchardgrass hay baled at different moisture levels, the highest-moisture bales reached internal temperatures up to 77°C (about 170°F), well above the range where spontaneous combustion becomes a real fire hazard.2Journal of Equine Veterinary Science. The Effect of Harvest Moisture and Bale Wrapping on Forage Quality, Temperature, and Mold in Orchardgrass Hay That extreme heating also darkens the hay, caramelizes sugars, and binds protein to fiber so that even if you could remove the mold, the nutritional value is already degraded.
Weather is the wild card. If rain falls on partially cured windrows and the farmer bales before the forage has dried back down, you get moisture pockets that become mold epicenters. Humid climates and poorly ventilated barns compound the problem. Even hay baled at acceptable moisture can pick up condensation if stored on bare ground or against an uninsulated metal wall.
Which Fungi Grow on Hay
Dozens of mold species colonize hay, and the mix depends on climate, crop type, and how wet the bale is. A survey of 55 hay lots from 20 dairy farms in southern Italy identified 33 different fungal species. The most common was Cladosporium cladosporioides, found in 84% of samples, followed by Alternaria alternata at 45% and Rhizopus stolonifer at 44%. Aspergillus flavus, the species most associated with aflatoxin contamination, showed up in about a fifth of the lots.3PubMed Central. Fungal Contamination and Aflatoxin B1 Detected in Hay for Dairy Cows in South Italy
Research focused on equine hay found a somewhat different cast of characters. Storage fungi like Eurotium amstelodami and Eurotium repens appeared across a wide range of hay conditions, while Aspergillus fumigatus dominated in the most heavily contaminated bales with the highest spore counts.4Agriculture, Ecosystems & Environment. Effect of agricultural and environmental factors on the hay characteristics involved in equine respiratory disease A. fumigatus is the species most strongly linked to respiratory disease in horses, which is why visually clean hay that still kicks up dust can be just as dangerous as hay with obvious fuzzy patches.
What this means practically is that the color or appearance of the mold does not reliably tell you which species is present. White fuzz could be Rhizopus or an Aspergillus species. Green patches could be Cladosporium or Penicillium. The only way to identify the exact fungus is laboratory culture, which most farm operations will never do. For everyday purposes, any visible mold on hay should be treated as potentially harmful regardless of its color.
Risks to Horses
Horses are especially vulnerable to moldy hay because they eat with their noses buried in it, inhaling spores with every breath. Equine asthma, a condition that used to be called recurrent airway obstruction or “heaves,” is one of the most common consequences. A review of environmental risk factors for equine asthma identified Aspergillus fumigatus from moldy hay as a key contributor to worsening symptoms, alongside other fungal species found in poor-quality forage.5PubMed Central. The Most Common Environmental Risk Factors for Equine Asthma—A Narrative Review Affected horses develop chronic coughing, nasal discharge, exercise intolerance, and in severe cases a visible “heave line” along the abdomen from labored breathing.
The condition is not always reversible. Horses with longstanding exposure to dusty or moldy hay can develop permanent airway remodeling that limits their performance even after the offending forage is removed. For horse owners, switching to low-dust forage is the single most effective management change, and it should happen before clinical signs appear, not after.
Risks to Cattle and Other Livestock
Cattle face a different profile of mold-related problems. Mycotoxins, the chemical byproducts that certain fungi produce, are the bigger concern in beef and dairy operations. Aflatoxin B1, which can be produced by Aspergillus flavus growing on hay, is both a potent liver toxin and a carcinogen that passes into milk.3PubMed Central. Fungal Contamination and Aflatoxin B1 Detected in Hay for Dairy Cows in South Italy Regulatory limits on aflatoxin in milk are strict, so a contaminated hay supply can create both a health problem and a financial one.
Some mold species pose more exotic risks. Mortierella wolfii, a fungus that can grow in poorly stored hay and silage, has been documented to cause abortion in cattle followed by severe systemic infection. A case report described a four-year-old cow that developed fatal brain inflammation and uterine infection with widespread blood vessel damage after aborting.6PubMed Central. Systemic infection with Mortierella wolfii following abortion in a cow Cases like this are uncommon, but they illustrate that moldy forage is not just a quality issue; it can be lethal.
Animals also show behavioral responses to moldy feed. Dairy calves given a choice between clean and moldy alfalfa hay consistently avoided the moldier option. Their forage intake dropped sharply as fungal contamination increased.7PubMed. Effect of fungal biomass in moldy alfalfa hay on preference by dairy calves with no previous exposure to moldy feeds Interestingly, when moldy hay was offered as the sole feed with no alternative, intake was driven more by the hay’s remaining nutritional characteristics than by the mold itself, meaning animals will eat moldy hay when they have no other option.8Animal Feed Science and Technology. Establishing a feed value for moulded hay That is worth knowing: just because your animals are eating it does not mean it is safe.
Risks to People Handling Moldy Hay
Farmers and barn workers face two well-documented conditions from breathing in moldy hay spores. The more serious is farmer’s lung disease, a form of hypersensitivity pneumonitis caused by repeated exposure. It is essentially an immune overreaction in the lungs triggered by inhaling mold spores, and it can progress from acute episodes of fever, cough, and shortness of breath to permanent lung scarring if exposure continues over years.9PubMed Central. Farmer’s Lung Disease
The second condition is organic dust toxic syndrome (ODTS), which looks similar in the moment but works differently. ODTS is not an allergic reaction; it is a direct inflammatory response to an extremely high dose of spores, usually from a single exposure event such as cleaning out a grain bin or moving heavily molded hay. A study of 80 farmers with ODTS found that the material triggering the episode was almost always described as extremely moldy, and symptoms resolved within 24 hours in nearly half of cases and within a week in 95%.10PubMed Central. Organic dust toxic syndrome among farmers The distinction matters because farmer’s lung requires ongoing exposure and can cause lasting damage, while ODTS is typically a one-off reaction that resolves on its own.
How much exposure is too much? Measurements on farms where ODTS occurred found average spore concentrations about a hundred times higher than on reference farms, with an estimated dose of around 20 billion spores associated with an episode.11PubMed. Exposure to microorganisms associated with allergic alveolitis and febrile reactions to mold dust in farmers You cannot count spores in the air by eye, but the practical takeaway is that if a bale releases a thick visible dust cloud, you should not be breathing it in without a respirator. A simple N95 mask provides meaningful protection for occasional handling; anyone working daily around marginal hay should use at least a half-face respirator with P100 filters.
Steaming and Soaking as Pre-Feeding Treatments
If you have hay that is slightly dusty but not visibly rotten, steaming and soaking are the two most common ways to reduce spore counts before feeding, especially to horses. Of the two, high-temperature steaming is consistently more effective. One study found that steaming eliminated typical mold colonies entirely, dropping counts to zero, while soaking left measurable colonies behind.12PubMed Central. Feed Intake Parameters of Horses Fed Soaked or Steamed Hay and Hygienic Quality of Hay Stored following Treatment
Research comparing the bacterial profiles of hay under different pre-feeding treatments reinforced this conclusion. High-temperature steaming reduced bacteria known to cause respiratory and dental disease, without wiping out overall bacterial diversity the way soaking did. Soaking actually increased gram-negative bacteria, the kind more likely to trigger inflammatory responses, while reducing diversity. The researchers concluded that steaming was the most suitable pre-feeding treatment for horse health overall.13PLOS ONE. The haybiome: Characterising the viable bacterial community profile of four different hays for horses following different pre-feeding regimens
A few caveats. Commercial hay steamers are an investment, and steamed hay should be fed within a few hours because the warm, moist environment that kills spores also creates ideal conditions for rapid regrowth if the hay sits around. Soaking is cheaper and still reduces respirable dust, but it leaches water-soluble nutrients and creates a nutrient-rich runoff that needs disposal. Neither method rescues hay that is heavily molded or has been through significant heating; those bales should be discarded.
Preventing Mold During Baling and Storage
The most reliable prevention is baling at the right moisture. For conventional dry hay, that means getting moisture below 20% before the baler runs, and ideally below 18% for large square or round bales that are slower to shed residual heat. Baling above 25% without a preservative is essentially guaranteeing mold growth and nutrient loss.1Canadian Journal of Animal Science. Preservation of high-moisture hay in storage through the use of forage additives
Chemical preservatives, most commonly propionic acid or buffered organic acid blends, buy some margin when weather forces you to bale at higher-than-ideal moisture. A meta-analysis across multiple studies found that propionic acid reduced visible mold by roughly 45 percentage points compared with untreated high-moisture hay, making it the most effective single-acid treatment. Other acids helped but to a lesser degree.14Journal of Animal Science. Meta-analysis of the effects of chemical and microbial preservatives on hay spoilage during storage That said, preservatives are not magic. A study on propionic acid applied to alfalfa found that while it reduced heating and dry-matter loss during the first month, after six months the high-moisture treated hay still showed much more mold and fiber degradation than hay that had simply been baled dry.15Applied Engineering in Agriculture. Preservation of Alfalfa Hay with Propionic Acid The acid slows the problem; it does not eliminate it.
Even research on newer preservatives tells a similar story. A study in northwestern China found that a preservative applied to low-moisture bales kept temperatures and microbial counts at acceptable levels, but when the same preservative was applied to high-moisture bales, severe mildew still developed in the bale cores despite inhibiting surface microbial growth.16Grassland Research. Optimization of moisture content and application rate of hay preservative during high‐moisture alfalfa hay baling in northwestern China The lesson is consistent: preservatives help at the margins but are not a substitute for proper drying.
Storage conditions matter as well. Stack bales off the ground on pallets or gravel to prevent moisture wicking from soil. Leave space between rows and walls for airflow. In humid climates, a well-ventilated barn beats outdoor storage under tarps, which can trap condensation. Monitor bale temperatures for the first few weeks after stacking; a probe thermometer inserted into the center of a bale is inexpensive insurance. If internal temperatures exceed about 55°C (130°F), move the bale away from the stack and monitor closely for fire risk.
Wrapped Bales and Baleage
Wrapped forages, sometimes called baleage or haylage, follow different rules than dry hay. Because these bales are sealed in plastic and preserved through fermentation rather than drying, the microbiological risks shift. The fermentation process lowers pH and creates an anaerobic environment that discourages typical hay molds. But if the wrap is punctured by rodents, machinery, or rough handling, air gets in and spoilage organisms move fast.
A review of wrapped forage for horses highlighted that maintaining high hygienic quality is critical, in part because pathogens like Listeria monocytogenes can grow in improperly fermented bales.17Grass and Forage Science. Silage and haylage for horses The same review noted that once a wrapped bale is opened, aerobic deterioration begins immediately, and small horse operations that take several days to finish a bale face a real window where mold and yeast can proliferate. In practice, this means you should use opened baleage within two to three days in warm weather and inspect each feeding for any off-smell or visible surface growth. Bales with damaged wrap should be checked thoroughly before feeding and discarded if there is any sign of spoilage inside the compromised area.
When Animals Eat Moldy Hay Anyway
One common misconception is that if animals eat the hay willingly, it must be fine. Research shows animals do prefer clean hay when given the choice, but they will eat moldy forage when no alternative is available, and their intake under those conditions tracks the hay’s remaining nutrient content rather than its mold level.8Animal Feed Science and Technology. Establishing a feed value for moulded hay In other words, a hungry animal does not refuse moldy hay just because it is moldy. The health damage from mycotoxins and spore inhalation accumulates whether or not the animal appears to mind the feed.
If you discover that livestock have already been eating moldy hay, removing the source is the first step. Watch for signs of respiratory distress (coughing, nasal discharge, labored breathing), reduced milk production in dairy animals, or unexplained weight loss. Horses with a history of airway sensitivity may need veterinary evaluation after even short-term exposure. For cattle, mycotoxin exposure can affect liver function and suppress immune response in ways that do not produce obvious symptoms immediately but increase susceptibility to other infections over the following weeks.