Horses are herbivores with a simple stomach (nonruminant). The large intestine has an active microbial digestive ability to allow digestion of forages. However, in the horse the small intestine, which is the major site of absorption, occurs before the fermentative digestion. As a result, horses are more susceptible to mycotoxins than ruminants, since nutrient absorption occurs prior to fermentative digestion in the horse compared to ruminants in which absorption occurs after fermentative digestion.

Productive or working horses have a high energy requirement and require a higher concentrate intake, and thus would be most susceptible to problems with mycotoxin-contaminated grains. Working horses would include growing horses less than two years of age, brood mares in late gestation and early lactation, and horses at moderate or intense work levels.

Other horses, that are only lightly worked, would be more likely to be exposed to mycotoxin-contaminated hays or forages. Since moldy forages are generally less palatable than normal forage, horses fed moldy forages typically refuse feed before ingesting enough feed to cause severe intestinal tract damage. Mild colic is typically noted in such cases. Unfortunately, most molds associated with grains fed to horses do not readily affect palatability. Consequently, horses are most often exposed to the mycotoxins found in grains. Grain mycotoxins are readily absorbed and should be considered to be potentially lethal for horses.

If mycotoxin-contaminated feeds must be fed, follow these guidelines:

1. Levels in the total diet of mature, nonbreeding horses should not exceed the levels shown in Table 4.

2. Feed mycotoxin-free grains to growing horses (less than two years of age), breeding horses, and working horses.

3. Analyze the feed for mycotoxins often.

Table 4. Maximum Mycotoxin Levels for Mature, Nonbreeding
Horses

                                                     
               MYCOTOXIN                 LEVEL
                                                 
               Aflatoxin                50 ppb
               T-2 Toxin                50 ppb
               DON                     400 ppb
               Zearalenone             100 ppb
               Fumonisin             2,000 ppb
                                                         
Note: The above levels are based on field observations.
Controlled scientific studies are needed to clarify specific
mycotoxin tolerance and toxicity levels.

Heat stress, marginal nutrient plane, crowding, disease exposure,
the presence of more than one mycotoxin, and drug interactions, 
as well other factors, increase animals' susceptibility to
mycotoxins.

Thus these recommendations must be tempered with knowledge of the
animals involved.

Prepared by