Mold and Mycotoxins in Horse Hay

What Causes Mold to Grow on Hay?

Much hay has been rained on or left lying in the field for prolonged periods of time due to excessively wet and humid conditions. The long drying periods with high humidity can allow mold to grow on the hay in the field.

Rain and poor drying weather has caused some hay to be baled wetter than desired. With high humidity, normal drying in storage may not occur and hay can retain elevated levels of moisture allowing mold growth. Mold and bacteria will grow on hay (without preservative added) at moisture levels above 14% to 15%. The mold growth produces heat, carbon dioxide and water, which further damages the hay. Moldy hay can result in dry matter and nutrient loss and produce spores and dust. Drying of stored hay is enhanced by increasing ventilation, creating air spaces between bales, reducing stack size, and stacking in alternating directions. Since moisture tends to move up and out the top of a stack of bales, ample headspace should be provided above a stack in a barn, allowing moisture to evaporate.

Molds commonly found in hay include Alternaria, Aspergillus, Cladosporum, Fusarium, Mucor, Penicillium, and Rhizopus. These molds can produce spores that cause respiratory problems, especially in horses and, under some conditions, will produce mycotoxins.

Horses are particularly sensitive to dust from mold spores and can develop a respiratory disease like asthma in humans called Recurrent Airway Obstruction (RAO), commonly referred to as heaves. A horse with RAO will have a normal temperature and a good appetite, but will often have decreased exercise tolerance, coughing and nasal discharge. Labored breathing occurs during exercise and, in some cases, while at rest. Hypertrophy of the abdominal oblique muscle used for expiration creates the characteristic 'heave line' seen on horses with RAO. Some horses are highly allergic to certain mold spores while others seem to be minimally affected. Strategies to reduce dust exposure are as follows: