The Danger of Mycotoxins

Mycotoxins are harmful secondary compounds that specific molds and fungi produce in soils, grains, and forages when moisture levels permit.


These harmful toxins produced by molds and fungi could be lurking in your horse's feed and forage

Did you know there are toxic substances tucked away in your horse’s feed and forage? You can’t see them, and you can’t eradicate them, but chances are these compounds, called mycotoxins, are present. 

Specific molds and fungi produce mycotoxins in soils, grains, and forages when environmental conditions are favorable. Once produced, they are generally very stable and will persist for a long time during storage. Horses that consume grains and forages contaminated by mycotoxins can suffer from a variety of health issues. As horse owners we should be particularly concerned, says Max Hawkins, PhD, nutritionist with Alltech’s mycotoxin management team, because mycotoxins inhibit protein synthesis, which negatively impacts the animal’s physiology and ability to function and repair tissues. 

The molds that produce mycotoxins are visible on contaminated feed. However, the mycotoxins can still be present after the mold dies or falls off the feed. These molds can be classified as either field fungi, which grow on plants while they are still rooted in the ground, or storage fungi, which develop after plants are harvested and stored. Field fungi require high moisture conditions (20-21% moisture), while storage molds can grow at lower moisture levels (13-18%). Forage and feed producers and property managers should monitor both moisture and temperature levels carefully so they can be aware of the potential for mycotoxin-producing molds to form. Horse owners should learn to recognize the signs of mycotoxin exposure so they can have their veterinarian out to assess the horse, diagnose the problem, and start a recovery plan. 

How Harmful are They?

It is near-impossible to find grain, pasture, and hay that is completely mold- and mycotoxin-free. In a 2010 study, German researchers Liesener et al. found at least one type of mycotoxin in each of 62 samples of commercial horse feeds, with many samples having more than one. However, the levels present were well below dangerous. So instead of taking on the impossible task of eliminating mycotoxins, owners should focus their efforts on minimizing their horses’ exposure to these elements. 

Fortunately, harmful levels of some mycotoxins are rarely seen. In addition, most horses will avoid moldy (and potentially mycotoxin-containing) forages because they are not palatable. The molds and mycotoxins found in grains, however, often do not affect palatability, so nutritionists and veterinarians find these the most concerning. It is also important to realize, says Hawkins, that a feed containing low levels of several different mycotoxins might be just as detrimental to a horse’s health and performance as a single mycotoxin at a “toxic” level. 

Some problems stemming from mycotoxin consumption are acute (immediate), while others are chronic (long-term). It all depends on the type and amount of mycotoxin the horse ingests. Other factors that affect toxin response include age, workload, stress level, nutritional status, and immune status. A healthy adult show horse or broodmare that consumes a diet with low to moderate mycotoxin levels over a long time might exhibit clinical signs such as decreased athletic performance and/or breeding capability with no other observable issues. Other general signs include appetite loss, weight loss, unthriftiness, respiratory issues, increased susceptibility to infectious diseases (poor immune function), and poor growth rate. 


Mycotoxin Mold/Fungus Allowable Limit
Aflatoxin Aspergillus flavus 20 ppb (parts per billion)
Deoxynivalenol and Nivalenol (DON or Vomitoxins) Fusarium graminearum, F. colmorum 2 ppm (parts per million)
Ergot Alkaloids Neotyphodium spp .3-.5 ppm
Fumonisins Fusarium moniliforme (verticillioides) 1 ppm
Lolitrem B Neotyphodium lolii 2 ppm
Ochratoxin A Penicillium verrucosum 1 ppm (limit in swine)
Ochratoxin A Apergillus ochraceus 1 ppm (limit in swine)
Slaframine Rhizoctonia leguminicola 10 ppm
T2 and HTZ Fusarium langsethiae, F. poae, F. sporotrichioides .5 ppm
Zearalenone Fusarium spp .5 ppm

Types and Signs of Mycotoxins

Aflatoxins, produced by Aspergillus strains, thrive in hot and dry climates. Producers and property owners should monitor and test soybeans, oilseeds, and corn and other cereal grains grown in hot environments, especially if the plants are stressed during the growing season (e.g., due to drought). These crops can become contaminated both in the field and during storage. 

“The target organ of aflatoxin is the liver,” says Hawkins. “This toxin can cause ataxia (incoordination), tremors, elevated temperature, anorexia, weight loss, icterus (yellowing of eyes and skin), hemorrhages and bloody feces, and brown urine. More recently aflatoxin has been associated with recurrent airway obstruction (RAO, or heaves) when inhaled, which would be more of an issue with older and stabled horses rather than horses on pasture.” 

Ochratoxins are produced by Aspergillus and Penicillium species, mainly due to improper storage. They have been found in oats, barley, corn, and wheat grown in both temperate and tropical regions. This class of mycotoxin targets the kidneys and can cause kidney failure. Clinical signs, says Hawkins, include anorexia, poor growth rate, poor performance, immune suppression, and excessive urine output and increased water consumption due to kidney problems.

Trichothecenes, which include deoxynivalenol (DON, or vomitoxin), are field mycotoxins produced by a variety of molds, with Fusarium species being the most common. Corn and wheat products, including wheat straw, can contain DON. It appears to be produced after temperatures fluctuate between hot and cold. Horses usually refuse to consume DON--contaminated feeds and begin to lose body condition. Other signs include immune system suppression, decreased growth, poor performance, gut irritation and diarrhea, changes in blood chemistry that indicate liver damage, and colic. Hawkins calls DON a “flag” mycotoxin, meaning it usually indicates the presence of other mycotoxins.

Zearalenone is often found in plants (most commonly, small grains and corn) contaminated with DON, as they are both produced by the same mold, the Fusarium species. This mycotoxin mimics estrogens and appears to target the reproductive system in many animal species, including horses, although research in this area is limited. Hawkins says clinical signs in affected mares include an enlarged uterus, rectal and vaginal prolapse, abortion, and infertility. In stallions veterinarians have observed penis flaccidity. 

Fumonisin, produced by Fusarium moniliforme, is known to cause equine leukoencephalomalacia (ELEM, or moldy corn poisoning), which is a serious and life-threatening disease of the central nervous system. Therefore, this is one of the most-studied grain mycotoxins. Signs in equids that have consumed fumonisins include apathy, drowsiness, disorientation, walking in circles, head pressing, seizures, staggering, colic, and eventually death. “Fumonisins are generally associated with corn but can be found in other grains and forages,” Hawkins says.  

Ergot alkaloids are probably the mycotoxins with which horse owners and managers are most familiar. They are produced by the Neotyphodium species and include ergovaline, found in tall fescue, and lolitrem, found in perennial ryegrass. They are more concentrated in seed, so mowing pastures and hay fields before they go to seed can help decrease related problems. Broodmare managers and owners should be concerned about ergovaline, as it can cause problems with the mare getting in foal, along with a variety of issues in pregnant mares, including prolonged gestation, dystocia (difficult birth), thickened placenta, poor mammary development, and agalactia (lack of milk production). Ergovaline is not of as much concern in other classes of horses, although it might cause decreased performance in some animals. The lolitrem mycotoxin causes a condition called “ryegrass staggers,” in which the horse can exhibit ataxia, head shaking, and possible collapse. Horses might also be hyperexcitable to stimuli. Fortunately, these signs dissipate rapidly when horses are removed from infected pastures.

Slaframine, from the fungus Rhizoctonia leguminicola, develops on the leaves of clover and alfalfa species in wet and humid conditions and is found in both pasture and hay. The most common sign of slaframine consumption is excessive salivation, or “slobbers.” Other signs include excessive thirst, frequent urination, weight loss, and dehydration. Horses recover quickly once they stop ingesting infected forages.

Recent Research

Scientific research into mycotoxins’ effects on horses is limited, when compared to work that has been done in other livestock animals such as cattle, pigs, and poultry. So veterinarians and nutritionists make recommendations for safe limits in feeds for horses (see table above) based on experience and extrapolation from research on other species. Since this is an area of interest, however, more work is likely forthcoming. For example, Heath King, DVM, Dipl. ACT, an assistant clinical professor at the Mississippi State University College of Veterinary Medicine (, recently studied zearalenone’s effect on mare fertility. In his initial results he found no adverse effects at levels found normally in the environment, but research is ongoing. Hawkins says research on the interactions between different mycotoxins, and their effect on health, performance, and immune response is currently under way as well. “There are over 500 known mycotoxins, but that may only be the tip of the iceberg,” he says.

Some in-feed mycotoxin binders have been used with success in other species, and several feed companies now add these to their products. Many of these binders are based on yeast cell wall products that researchers have found can minimize the negative effects of DON and other Fusarium mycotoxins. Other commercially available products, sold as supplements, can be top-dressed on the horse’s normal ration. Be sure to check that manufacturers of these products are in compliance with the American Association of Feed Control Officials, as well as the National Animal Supplement Council, to verify their safety before feeding them to your horses.

How to Avoid

Minimizing mycotoxin exposure is key to avoiding the serious conditions mentioned. Here are some recommendations to reduce your horse’s risk:

  • Keep grain and hay storage areas clean, cool, and dry. Pest-proof the feed storage areas as much as possible to keep animals from chewing holes in stored feed bags, which prevents exposing feed to the elements. 
  • Feed the oldest bags first. Ideally, you want to use opened bags of feed within a few weeks in winter and within an even shorter time in summer.
  • If you store feed in bins or cans, clean them thoroughly to remove feed that gets stuck in cracks and crevices. Dispose of any grain left in the bottom of cans instead of adding more feed on top.
  • Ask if your feed manufacturer or supplier tests for mycotoxins in grains before mixing feeds, and avoid using businesses that don’t test their feeds routinely.
  • Avoid corn screenings (small parts of corn grain that routinely carry very high levels of fumonisins) completely!
  • If you have any concerns about your feeds or forages, have them tested. This could be especially important if you have a horse that shows signs of mycotoxin exposure. Hawkins says a variety of commercial labs across the country can provide this service. The biggest challenge is collecting a representative feed sample; the amount of mycotoxin that can cause problems is very small and might not be distributed uniformly.

Take-Home Message

In summary, it is important for horse owners and managers to be aware of complications associated with mycotoxin exposure. Proper management is essential to limit exposure, and recognizing signs early, when the prognosis is still positive, ensures the best outcome. And always work with reputable feed and hay dealers.

About the Author

Janice L. Holland, PhD, PAS

Janice L. Holland, PhD, PAS is an associate professor of Equine Studies at Midway College in Midway, KY. Her main academic interests are equine nutrition, pasture management, and behavior.

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