Molds and Mycotoxins In Horse Feed: Basic Facts

Horse owners everywhere take extreme measures to be sure their animals are safe from harm. But did you know that your horse might be ingesting toxins on a daily basis? Reports from Dairyland Laboratories and the Iowa State Veterinary Diagnostics Laboratory indicate that levels of molds and mycotoxins (toxins produced by molds and other fungi), particularly a toxin called deoxyvalenol (DON), produced by Fusarium mold, are rising in crops harvested in 2009 (a wet year with ideal conditions for mold growth). Livestock producers across the United States are taking steps to minimize feed contamination, such as drying corn, adding propionic acid (a mold inhibitor) to stored grains, and inoculating their silages and haylages. Horse owners can employ these and other methods to reduce the risk of mycotoxins.

What are mycotoxins?

Mycotoxins are harmful secondary compounds produced by molds that are found in the soil and vegetable matter including grains, forages and feed. They can be formed in the field both before and during harvest, and can continue to be formed under suboptimal storage conditions after harvest. Mycotoxins are nothing new to the poultry, swine and ruminant markets, but less is known about the adverse effects of mycotoxins in horses. These toxins can cause a wide range of clinical signs in horses, including respiratory, gastrointestinal, neurologic, and reproductive problems--even death.

How horses are exposed to mycotoxins?

Horses can be exposed to mycotoxins by eating contaminated feed ingredients such as concentrates (i.e., grains and protein supplements), whole grains, hay, and green pasture--essentially any feed ingredient. Respiratory and/or dermal (skin) entry of mycotoxins can also occur, but these routes are of less significance. The extent of mycotoxin exposure depends on how much of the contaminated ingredient is fed, and the mycotoxin concentrations present in the feed. Any feed ingredient can contain multiple mycotoxins, which will likely interact and become more toxic than they would be separately.

What are the effects of molds and mycotoxins on horses?


  • Skin allergies and inflammation
  • Respiratory blockage
  • Respiratory diseases- Chronic obstructive pulmonary disease (COPD, also called heaves)


  • Aflatoxins (predominantly found in corn, soybean meal, alfalfa pellets): ataxia (incoordination), tremors, fever, anorexia, loss of appetite, weight loss, icterus (yellowing of the eye or skin), hemorrhages, bloody feces, brown urine, and death
  • Ochratoxins (mostly found in corn, wheat, oats, rye, barley, hay/straw, grass): kidney damage
  • Deoxynivalenol (DON) and T-2 toxin (mostly found in corn, wheat, oats, rye, barley): reduced feed intake, weight loss in exercising horses, liver damage, reduced immunity
  • Fumonisins (mostly found in corn, wheat, oats, rye, barley): very toxic to horses; equine leukoencephalomalacia (commonly known as moldy corn poisoning), depression, abnormal behavior, head pressing, ataxia, staggers, stupor, lameness, seizure and death; survivors will have some degree of permanent neurological disorder
  • Zearalenone (mostly found in corn, wheat, oats, rye, barley, hay/straw, grass): vaginal prolapse, abortions, uterine and internal hemorrhage, severe flaccidity of male genitalia
  • Ergot alkaloids (mostly found in wheat, oats, rye, barley, hay/straw, grass): extended gestation length, agalactia (no milk production), red bag placentas, fetal losses, dead foals

How do we know if horses are exposed to mycotoxins?

Diagnosing mycotoxicoses is a big challenge in any species because of the lack of mycotoxin-specific clinical signs for majority of mycotoxins. Signs such as lowered feed intake, leg problems, nervous system problems, and increased mortality can also be caused by many other factors. To help confirm a diagnosis of mycotoxicosis, test suspect horse feed / ingredients for molds and mycotoxins using reliable, accredited laboratories. Sampling might not pick up a moldy area of feed (i.e., the whole batch of hay/feed might not be contaminated), but the analysis can provide valuable information.

How can you prevent mycotoxicoses in horses?

There is no magic bullet for mycotoxin prevention. Preventive programs have to be applied at different levels of crop production (field production, harvesting, and storage of feed ingredients), feed production (feed mills) and animal production (farms).

Field control of molds and mycotoxins: Agricultural practices can reduce crop contamination, but even the best management cannot totally eliminate mycotoxin contamination (Jouany, 2007). Following are some suggested preventive measures:

  • Crop rotation reduces the recontamination of the next crop with molds.
  • Soil cultivation, such as plowing or tillage, will lower the concentration of Fusarium molds in the soil.
  • Although Fusarium mold-resistant crops have been produced, none are registered as the agronomic properties were adversely affected (the crops were not as hardy, and also because the nutritional value was compromised).
  • Insect and weed control

Harvest control:

  • Harvesting crops earlier generally reduces mycotoxin concentrations.
  • Since Fusarium spores exist in the soil, higher optimal cutting height helps reduce contamination by lessening soil contact with the healthy grains.
  • Optimal fan speed of the combine harvester during harvest will eliminate damaged grains and limits risk of further contamination.

Post-harvest control: Mold growth can be controlled by storing feed materials at optimal conditions. These include:

  • Dry grains to below 13% moisture and maintain that moisture level during storage.
  • Use effective mold inhibitors if grains/concentrates are stored at more than 13% moisture.
  • Maintain optimal temperature, humidity, and ventilation for crop storage.
  • In case of hay or silage, inadequate packing, improper coverage and suboptimal face management (not keeping the silage surface smooth and packed to minimize the product's air exposure) can lead to excess aeration within the hay/silo, promoting the growth of molds and subsequent mycotoxin production.
  • Use silage inoculants in high-moisture grains and hay.

Mycotoxin control in the horse: Despite feed producers' best efforts, mycotoxins can still end up in horse feed and forages. To decrease the harmful effects of these mycotoxins in animals, organic and inorganic adsorbents or binders (which bind to the mycotoxins so the animal's system can't absorb them) can be added to feed. Following are a few points to remember about mycotoxin binders:

  • Binders are more effective if applied at the time of feed production, but they can also be added to the horse's daily feed by the owners.
  • Ideal mycotoxin binders should bind multiple mycotoxins at low dosages and should have strong peer-reviewed research support (which should be available from the manufacturers upon request).
  • It has been reported that glucomannan polymer, an organic adsorbent, prevented adverse effects of Fusarium mycotoxins (such as vomitoxin or DON).
  • Reports indicate that ergot alkaloid toxicity could be prevented in beef cattle by feeding a glucomannan polymer, which is extracted from the cell wall of yeast.

Points to Remember

  • Mycotoxins can enter horses via contaminated grains, hay, and grasses.
  • Mycotoxins are a major issue for horses and hence, extra care should be taken when choosing feed ingredients.
  • Make sure that ingredients are dried and stored properly until they are fed to horses.
  • Mycotoxin exposure can be controlled with good field, harvesting, post-harvest practices, and also by feeding mycotoxin binders or adsorbents. Use an effective mycotoxin adsorbent capable of working against multiple mycotoxins at practical dosages.



  • Jouany (2007). Animal Feed Science and Technology. 137:342-362.
  • Merrill et al., (2007). J. Anim. Sci. 85:2596-2605
  • Newman and Raymond (2005). The Mycotoxin Blue Book (Ed. Duarte Diaz). 57-76.
  • Raymond et al., (2000). J. Equine Vet. Sci. 20(10):616-621.
  • Raymond et al., (2003). J. Anim. Sci. 81:2123-2130.
  • Raymond et al. (2005). J. Anim. Sci. 83:1267-1273
  • Wright (2001). OMAFRA News Letter

About the Author

H.V.L.N. Swamy, DVM, PhD

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