Do Not Feed Horses Moldy Hay

Table 1. Feeding Risksa at Various Mold Spore Counts
Mold Spore count per gram	Feeding Risk and Cautions
Under 500,000	Relatively low Risk
½ to 1 million	Relatively Safe
1 to 2 million	Feed with Caution
2 to 3 million	Closely observe animals and performance
3 to 5 million	Dilute with other feeds
Over 5 million	Discontinue feeding
a Risks refer primarily to effect of mold per se without regard to possible mycotoxin content. Dust may also reduce feed consumption. Data from Richard S. Adams, Kenneth B. Kephart, Virginia A. Ishler, Lawrence J. Hutchinson, and Gregory W. Roth. 1993. Mold and mycotoxin problems in livestock feeding. The Pennsylvania State University.

Sometimes mold spores are counted on moldy feeds to obtain an indication of the extent of molding and relative risks in feeding them. Table 1 contains classification of risks at various mold spore counts. While most molds do not produce mycotoxins, the presence of mold indicates the possibility of mycotoxin presence and animals being fed moldy hay should be watched carefully for mycotoxin symptoms.

Mycotoxins effects on animals:

1.    Intake reduction or feed refusal;

2.    Reduced nutrient absorption and impaired metabolism, including altered digestion and microbial growth, diarrhea, intestinal irritation, reduced production, lower fertility, abortions, lethargy, and increased morbidity;

3.    Alterations in the endocrine and exocrine systems;

4.    Suppression of the immune system which predisposes horses to many diseases. A suppressed immune system may also cause lack of response to medications and failure of vaccine programs;

5.    Cellular death causing organ damage.

If you have mold in hay, watch for the symptoms mentioned above. If hay is dusty, take care in feeding to sensitive animals and those, especially, in areas with poor ventilation. If hay is moldy, the recommendation is to not feed it to horses at all. If symptoms of mycotoxin poisoning are observed (which can occur from mold not visible), check with a nutritionist to make sure the ration is properly balanced and with a veterinarian to eliminate other disease/health problems. Quick test kits (ELISA kits) are available to determine presence of a limited number mycotoxins but they can give false positives. Some forage testing laboratories will provide other mycotoxin tests. Often, the best strategy is to remove a suspected mycotoxin-contaminated feedstuff from the diet and see if symptoms disappear. If mycotoxins are present, the feedstuff can often be fed at a diluted rate and/or with approved feed additives.

In summary:

§  Do not feed moldy hay to horses.

§  Most moldy hay problems are due to mold spores which can produce respiratory disease in horses.

§  Many of the commonly diagnosed mycotoxins from molds are produced in the field when harvest is delayed.

§  If a mycotoxin problem is suspected, a comprehensive review of animal nutrition and health is essential – i.e. problems blamed on mycotoxins may be other disorders or nutritional issues. Diagnosing a mycotoxin problem is difficult and often involves the elimination of other possible factors.

§  The physical dust problem associated with moldy forage can be reduced by feeding in a well ventilated area, mixing with a high moisture feed or wetting the hay, but these will not reduce mycotoxins if present.