Mycotoxins are naturally occurring toxic substances which are produced by moulds and fungi in your horse’s hay.

Mycotoxins can cause negative effects in horses including colic, gastrointestinal upset, weight loss, feed withdrawal, immune suppression, impaired performance or poor growth.

In serious but rare cases of mycotoxin overload, liver problems, paralysis, neurological disorders, and brain lesions can develop. [1]

Improper storage of hay or grain mixes is the leading cause of mycotoxin intoxication in horses. Your horse’s forage and feed can readily develop mould if not stored properly, particularly if exposed to moisture.

The risk of equine mycotoxin intoxication increases during the spring and summer months when the weather is damp with high humidity.

It is important to check hay and grains for any signs of mould before feeding to your horse to help minimize mycotoxin exposure. This is particularly important for horses that are immunocompromised, young foals, senior horses or horses with leaky gut syndrome.

Feeding your horse toxin binders such as yeast-derived supplements can help to lower the risk of side effects from mycotoxins. Toxin binders promote the elimination of mycotoxins from the gastrointestinal tract before they are absorbed into the bloodstream. [12]

In this article, we will discuss what mycotoxins are, how they can affect your horse and strategies to prevent and reduce the risk of mycotoxin intoxication.

Mycotoxins in Hay

Mycotoxins are toxic compounds that are natural byproducts of certain types of moulds and fungi.

They are secondary metabolites (substances formed during metabolism) of moulds that can build up to toxic levels in hay given suitable environmental conditions.

Warm, damp, and humid environments lend themselves to mould growth. Mycotoxin spore populations can develop on a variety of feedstuffs including cereal grains, dried fruits, nuts, pasture grass, hay, and straw.

Your horse is exposed to small amounts of mycotoxins in feed. But in high levels, mycotoxins are capable of causing serve disease and death in both humans and animals.

Scientists have identified over 400 different mycotoxins thus far. The most common mycotoxins posing concerns to humans and livestock include: [1][2][3][4]

  • Alfatoxins
  • Ochratoxin A
  • Patulin
  • Fusariotoxins
  • Zearalenone
  • Nivalenol/deoxynivalenol

Various factors contribute to the production of mycotoxins in feeds, including storage practices and environmental conditions which are often beyond human control. [4]

How to Identify Mouldy Hay?

Hay can go mouldy very quickly in the right environment. Studies show mould can form on forages and grains in as little as a few hours, given the presence of heat and moisture. [5]

This is why it is extremely important to check your hay before feeding. If your hay seems damp, has a smell, and has any visible signs of mould growth, it is safer for your horse not to feed it.

Some labs can test your hay to identify the presence of mycotoxins. This is recommended if you have a large amount of hay that may have gone mouldy.

Also, keep watch for early mycotoxin intoxication symptoms including low energy, loss of appetite, diarrhea, and difficulty breathing (heaves). These signs may indicate your hay has gone mouldy and should no longer be used for feed. [9][10][11]

Mould grows when hay and grain are not dried properly at harvesting or during storage. Moisture levels above 14% to 15%, heat, humidity and poor ventilation create the perfect environmental conditions for mould growth and mycotoxin production. [5]

Some species of mould commonly found in hay include: [5][8]

  • Alternaria
  • Aspergillus
  • Cladosporum
  • Fusarium
  • Mucor
  • Penicillium
  • Rhizopus

Mould growth can also lead to deterioration of hay quality as the amount of total digestible nutrients is reduced. [7] Mould growth produces heat, carbon dioxide and water which can damage hay.

It is important to note that not all moulds produce mycotoxins. However, mould can harm your horse in other ways such as leading to lung issues. [5]

Mouldy hay is also a source of spores that cause respiratory problems in horses, especially if the hay is fed in a poorly ventilated area. Mould spores can contribute to coughing, heaves (Recurrent Airway Obstruction), laboured breathing and poor exercise performance. [5][6][7][8]

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Symptoms of Mycotoxin Ingestion

Mycotoxin intoxication presents with vague symptoms that are also associated with other equine ailments. It may not be apparent at first when your horse has encountered mycotoxins in its diet.

Usually, a diagnosis is made by a veterinarian after ruling out several other causes for symptomatic behaviour.

If you suspect your horse has ingested mouldy hay, it is important to watch for mycotoxin symptoms which include: [9][10][11]

  • Reduction of feed intake
  • Anorexia (loss of appetite)
  • Diarrhea
  • Signs of intestinal irritation
  • Lethargy
  • Airway obstruction/difficulty breathing
  • Coughing
  • Colic
  • Paralysis
  • Deterioration of organ function

Treatment of Mycotoxin Intoxication

The primary treatment for mycotoxin ingestion is to stop feeding the contaminated forage or grains. Immediately eliminating the source of your horse’s mycotoxin exposure can help to lower the risk of developing potentially fatal health conditions.

Consult with your veterinarian to determine an appropriate treatment regimen. Ask about liver support supplements like milk thistle that may help with recovery.

Preventing Mould Growth

If your horse has been exposed to mycotoxins, it’s a good time to evaluate your current feed storage practices and develop a plan to reduce the risk of mould growth in the future.

Store hay away from your horse in a dry, temperature-controlled environment if possible. Minimize exposure to moisture and ensure proper ventilation so that dust particles and mould spores do not accumulate at high levels.

The following practices can also help to prevent mould growth in your horse’s:

  • Dry hay thoroughly before storing
  • Create space around bales to encourage ventilation
  • Allow space above the hay for moisture to evaporate
  • Stack bales in alternate directions
  • Reduce stack sizes if possible
  • Consider feeding hay cubes if mould growth is an ongoing problem
  • Soak dusty hay to reduce respiratory issues
  • If you steam your hay, feed it shortly after steaming it to reduce the opportunity for mould to grow
  • Consider using hay desiccants such as potassium or sodium carbonate
  • Consider using hay preservatives such as propionic and acetic acid
  • Get a hay test done if you are concerned about possible mycotoxin accumulation

How Mycotoxins Affect Health

There are a wide variety of moulds, fungi, and mycotoxins that can be present in your horse’s hay and grains. These have detrimental effects on your horse’s health when consumed at high levels by interfering with several important processes in the body.

Fusariotoxins from Fusarium fungi


These mycotoxins are most likely to be at high levels on corn and to a lesser extent in wheat. [20]

Of the three main types of fumonisin toxins (FB1, FB2, FB3), FB1 is the most common and most well-studied. It has been shown to interfere with the synthesis of sphingolipids. These are important fats that are part of cell membranes. Horses are more sensitive to this toxin than other livestock animals.

High levels of FB1 in the diet can cause a specific syndrome called equine leukoencephalomalacia (ELEM) which involves sudden onset of abnormal movements such as poor coordination, head pressing and circling. Liver damage can also occur in some cases, resulting in loss of appetite and jaundice.

Feeds with levels over 10 ppm of FB1 should not be fed to horses. [20]


These metabolites produced by Fusarium fungi are typically found on cereal grains. They are most likely to exist at high levels when there are long periods of cold, wet weather during harvest season.

The most common issue with trichothecene ingestion is a decrease in feed intake.

There are several types of Trichothecenes mycotoxins that have different toxicity effects on horses: [20]

Deoxynivalenol (DON): Horses are more resistant to this toxin than cattle and don’t seem to show adverse effects even with severely contaminated grains. However, when grains are contaminated with DON along with other mycotoxins, adverse effects can be seen. This commonly includes decreased feed intake.

T-2 Fusarium mycotoxin, nivalenol, and diacetoxyscirpenol (DAS): Less is known about how these mycotoxins affect horses, but they are less common than DON.

Grains harvested during cold, wet weather should be assessed for trichothecene contamination as this can result in reduced food intake and nutrient deficiencies in horses.

Zearalenone (ZEN)

This type of mycotoxin produced by Fusarium fungi can be found on grains, hay and sorghum. Conditions of high humidity and low temperature are most likely to result in zearalenone contamination.

High levels of ZEN can decrease feed intake and affect reproductive organs by binding to estrogen receptors. In mares, ZEN contamination of feed can lead to abortions and changes in uterine and vaginal structure. Males can also be affected and show flaccidity of the genitals. ZEN levels of approximately 2.7 ppm in the diet were shown to elicit these effects.

Aflatoxins from Aspergillus fungi

Aflatoxin contamination is more common during conditions that induce plant stress late in the growing season, such as drought or insect infestation. Cereal grains including corn and oats are most at risk of aflatoxin contamination. [21]

The most common and well-studied form is aflatoxin B1. The US Food and Drug administration has established 0.02 ppm as the safe limit of aflatoxin B1 in feeds.[21]

Depending on how long horses are exposed to contaminated feed, levels between 0.5 and 1 ppm can result in liver damage and clinical signs of aflatoxicosis. Horses consuming contaminated feeds might have low appetite, depression, fever, tremors and coughing. Levels above 2 ppm are typically fatal for horses.

Some research also suggests that inhaling aflatoxins can affect lung health in horses and contribute to chronic obstructive pulmonary disease (COPD). [21]

Toxin Binders for Mycotoxins

Toxin Binders are natural substances that bind to toxins in the horse’s gastrointestinal tract and help to eliminate them from the body before they are absorbed.

While there is still limited research in this area, preliminary evidence suggests a benefit to incorporating toxin binding supplements into your horse’s diet.

Adding toxin binders to your horse’s feeding program can help reduce the adverse effects of mycotoxin ingestion. These supplements could reduce the chances of your horse becoming severely ill if he or she has ingested large quantities of mouldy hay.

Note that toxin binders should not be used as a strategy to counteract the negative effects of feeding mouldy hay. If hay appears mouldy, you should never feed it to your horse.

However, these supplements may help to mitigate the effects of unintended exposure to mycotoxins and other harmful substances found in your horse’s feed.

A variety of mycotoxin binders are available such as clay minerals, yeast cell wall extract, organic polymers, activated carbon and aluminosilicate. These digestive health supplements all aim to bind and remove mycotoxins from the gastrointestinal tract before they have the chance to enter the bloodstream. [12]

Several studies show that horses consuming mycotoxin binders have reduced amounts of mycotoxins present in their gastrointestinal tract and experience reduced effects of mycotoxin intoxication. [13][14][15][19]

Commonly Used Toxin Binders

Yeast Cell Wall Extract

Yeast is commonly supplemented in the equine diet as a probiotic to support hindgut function. Yeast cultures can promote nutrient absorption from the diet and improve feed efficiency.

Yeast can also be used as a toxin binder and to support immune function. The cell wall of yeast has high binding potential.

Supplements like Bio-Mos® from Alltech are composed of Saccharomyces cerevisiae yeast and can help maintain gastrointestinal health and stimulate natural immune defences.

Bentonite Clay

Bentonite clay (Montmorillonite) is a popular natural digestive supplement given as a toxin binder to horses. It is a highly absorbent aluminum phyllosilicate clay that has been used as a traditional digestive remedy in humans for centuries.

Bentonite clay work by binding to heavy metals and toxins in the gut to decrease their absorption. While it effectively binds toxins, it can also bind to important nutrients in the diet and inhibit their absorption.

There is limited research data available to assess the efficacy or safety of bentonite clay. It is not recommended for long-term use due to the risk of potential secondary deficiencies.

Optimum Digestive Health

Mad Barn’s Optimum Digestive Health (ODH) is formulated as a comprehensive digestive supplement that target many aspects of gut health in one product.

Optimum Digestive Health contains prebiotics, probiotics, yeast, and digestive enzymes that promote gut health and digestive function.

ODH also contains nucleotides and toxin binders which can support your horse’s immune function and may help to limit the harmful effects of mycotoxin exposure in hay.

Optimum Digestive Health

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  • Prebiotics, probiotics & enzymes
  • Support hindgut development
  • Combats harmful toxins in feed
  • Complete GI tract coverage


Mould growth on hay is a common occurrence in damp and humid conditions. Mould can produce mycotoxins which can cause serious health complications in horses.

Early signs of mycotoxin ingestion include lethargy, loss of appetite, diarrhea and digestive discomfort. Over-exposure to mycotoxins can cause colic, paralysis, and potentially fatal neurological conditions.

Never feed hay to your horse if it has gone mouldy. Consider adding a toxin binder to your horse’s diet to help promote immune and gut health.

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