Mycotoxins in the Equine Environment

wendy pearson, horse feed, horse forage, horse pasture, herbs for horses, horse grains, mycotoxins horse, mycotoxins equine

wendy pearson, horse feed, horse forage, horse pasture, herbs for horses, horse grains, mycotoxins horse, mycotoxins equine

By Dr. Wendy Pearson, PhD (Dr. of veterinary toxicology)

Mycotoxins are compounds produced by molds and fungi, and can be found in pastures as well as on dried forages, bedding, grains, and complete feeds. Of all the agricultural species, horses are among the most sensitive to toxicosis from mycotoxins. An important reason for this is that horses typically have a much longer lifespan than any other agricultural species and exposure to mycotoxins can take place over many years.

Clinical signs of pathological exposure to mycotoxins may include reduced growth rates in foals and young horses, respiratory dysfunction, problems with fertility and reproduction, neurological and/or brain disorders, liver or kidney damage, allergic reactions, anorexia, reduced performance, and colic.

Pastures and forage

The type and magnitude of reactions to mycotoxin exposure can vary widely, depending on many factors. These include duration of exposure to the toxin (repeated exposure to low-dose toxins can result in greater sensitivity), previous toxin exposure (single bouts of toxicosis lead to intensified reactions during subsequent exposures), age, breed, workload, immune and nutritional status, and type of toxin to which the horse is being exposed. Furthermore, if the toxins are being delivered in contaminated feed, there is frequently more than one type of toxin present. This often leads to synergistic toxicities, amplifying toxic effects of individual compounds. 

Most horses spend a significant portion of their days during the growing season grazing on pastures. Certain grasses are more prone to contamination with mycotoxins, including rye-grass, fescues and white clover. Ideal conditions for the endophytes to proliferate on these grasses are periods of hot, dry weather followed by rains. Toxicosis from these endophytes may present as “ryegrass staggers,” in which horses lose coordination and balance; the condition also causes abortion, headshaking, and collapse. Other conditions include “fescue poisoning,” which presents as loss of appetite, abortion and foaling problems; and “slobbers” (excessive salivation, tears, uncontrolled diarrhea and urination) induced by exposure to slaframine from contaminated white clover.

These conditions can also result from exposure to preserved forages prepared from contaminated grasses. It has been reported that approximately 15 percent of Canadian hay is contaminated with mycotoxins. Preservation of high-moisture hay (haylage) may be an excellent strategy to reduce mycotoxin exposure, as commercial inoculants used to facilitate the ensiling process in well-made haylage create conditions that are unfavorable for mold growth and can reduce mycotoxin exposure by about 25 percent. 

Grains and Commercial Feeds

Cereal grains, especially maize (corn), can be a ready source of mycotoxin contamination. Grains can become infected with mold either in the field or during storage. Common mycotoxins found on cereal grains include aflatoxin, fumonisins, tricothecenes and zearalenone. 

Aflatoxin toxicosis can result in death, growth suppression, cancer, liver damage, and inhibition of mineral absorption including iron, phosphorus and copper. Ponies fed diets high in aflatoxin (2 ppm) show significant liver damage, whereas much lower concentrations (0.3 ppm) have reportedly caused death in horses. 

Poisoning with fumonisins results in a neurological disease called equine leukoencephalomalacia. This disease presents as loss of coordination, depression, muscle tremors, and loss of swallowing reflex. 

Tricothecene toxicosis causes clinical signs including anorexia, weight loss, immune suppression, poor performance and colic.

Zearalenone is a common culprit in horses experiencing reproductive problems.  


Bedding can be an important source of mycotoxin poisoning in horses, and straw is a particular risk because horses are often quite willing to consume it when not offered an alternate forage source throughout the day. A common practice of sprinkling water on straw bedding to reduce dust can accelerate mold growth and encourage subsequent mycotoxin formation.


Mycotoxins are an omnipresent risk throughout the equine environment. While complete eradication is an unlikely scenario, awareness of the risk and mitigation of exposure can result in a meaningful reduction in toxic signs. Tolerance limits of important mycotoxins (see table below) provide a basis for testing for mycotoxin contamination of feedstuffs and bedding, to help horse owners restrict exposure of their horses to potentially dangerous intakes.

Mycotoxin Maximum Tolerable Level (ppm)
Aflatoxin 0.02 ppm
T2 Toxin No effect on ovarian activity when fed at 1 ppm
Deoxynivalenol 2 ppm of total diet or 5 ppm in cereal feed
Zearalenone No effect on ovarian activity when fed at 1 ppm
Fumonisins Less than 5 ppm
Ergovaline 0.3 – 0.5 ppm

Reprinted with permission from

This article originally appeared in the June 2014 Issue of Canadian Horse Journal.