Mycotoxins are naturally occurring toxic substances which are produced by moulds and fungi in your horse’s hay.

Mycotoxins can cause negative effects in horses including colic, gastrointestinal upset, weight loss, feed withdrawal, immune suppression, impaired performance or poor growth.

In serious but rare cases of mycotoxin overload, liver problems, paralysis, neurological disorders, and brain lesions can develop. [1]

Improper storage of hay or grain mixes is the leading cause of mycotoxin intoxication in horses. Your horse’s forage and feed can readily develop mould if not stored properly, particularly if exposed to moisture.

The risk of equine mycotoxin intoxication increases during the spring and summer months when the weather is damp with high humidity.

It is important to check hay and grains for any signs of mould before feeding to your horse to help minimize mycotoxin exposure. This is particularly important for horses that are immunocompromised, young foals, senior horses or horses with leaky gut syndrome.

Feeding your horse toxin binders such as yeast-derived supplements can help to lower the risk of side effects from mycotoxins. Toxin binders promote the elimination of mycotoxins from the gastrointestinal tract before they are absorbed into the bloodstream. [12]

In this article, we will discuss what mycotoxins are, how they can affect your horse and strategies to prevent and reduce the risk of mycotoxin intoxication.

Mycotoxins in Hay

Mycotoxins are toxic compounds that are natural byproducts of certain types of moulds and fungi.

They are secondary metabolites (substances formed during metabolism) of moulds that can build up to toxic levels in hay given suitable environmental conditions.

Warm, damp, and humid environments lend themselves to mould growth. Mycotoxin spore populations can develop on a variety of feedstuffs including cereal grains, dried fruits, nuts, pasture grass, hay, and straw.

Your horse is exposed to small amounts of mycotoxins in feed. But in high levels, mycotoxins are capable of causing serve disease and death in both humans and animals.

Scientists have identified over 400 different mycotoxins thus far. The most common mycotoxins posing concerns to humans and livestock include: [1][2][3][4]

  • Alfatoxins
  • Ochratoxin A
  • Patulin
  • Fusariotoxins
  • Zearalenone
  • Nivalenol/deoxynivalenol

Various factors contribute to the production of mycotoxins in feeds, including storage practices and environmental conditions which are often beyond human control. [4]

How to Identify Mouldy Hay?

Hay can go mouldy very quickly in the right environment. Studies show mould can form on forages and grains in as little as a few hours, given the presence of heat and moisture. [5]

This is why it is extremely important to check your hay before feeding. If your hay seems damp, has a smell, and has any visible signs of mould growth, it is safer for your horse not to feed it.

Some labs can test your hay to identify the presence of mycotoxins. This is recommended if you have a large amount of hay that may have gone mouldy.

Also, keep watch for early mycotoxin intoxication symptoms including low energy, loss of appetite, diarrhea, and difficulty breathing (heaves). These signs may indicate your hay has gone mouldy and should no longer be used for feed. [9][10][11]

Mould grows when hay and grain are not dried properly at harvesting or during storage. Moisture levels above 14% to 15%, heat, humidity and poor ventilation create the perfect environmental conditions for mould growth and mycotoxin production. [5]

Some species of mould commonly found in hay include: [5][8]

  • Alternaria
  • Aspergillus
  • Cladosporum
  • Fusarium
  • Mucor
  • Penicillium
  • Rhizopus

Mould growth can also lead to deterioration of hay quality as the amount of total digestible nutrients is reduced. [7] Mould growth produces heat, carbon dioxide and water which can damage hay.

It is important to note that not all moulds produce mycotoxins. However, mould can harm your horse in other ways such as leading to lung issues. [5]

Mouldy hay is also a source of spores that cause respiratory problems in horses, especially if the hay is fed in a poorly ventilated area. Mould spores can contribute to coughing, heaves (Recurrent Airway Obstruction), laboured breathing and poor exercise performance. [5][6][7][8]