Response: Moldy hay is bad (and possibly deadly) for any horse and should not be fed. Horses ingesting moldy hay are at higher risk of respiratory disease and colic. If only 2 horses are eating off the round-bale, it would be best if it was covered by a feeder or placed inside a shed or lean-to to help reduce the negative effects of weather. Ideally, the round-bale would be consumed quickly enough so it would not mold during adverse weather.
Encourage the barn manager or owner to remove the moldy hay and take steps to avoid mold formation in the future. Because the horse has PSSM, work with a nutritionist to ensure his diet (hay, grain, and treats) is at or less than 10% non-structural carbohydrates (NSC). Research from the University of Minnesota has shown that horses with PSSM respond better to diets lower in NSC. This, along with regular exercise (as long as its approved by your veterinarian), should help reduce the number and severity of his PSSM attacks.
By: Krishona Martinson, PhD, University of Minnesota