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The threat to equine

Exposure to mycotoxins in pasture, mouldy conserved forage, purchased feeds and bedding presents a real threat to the health of equine animals. This exposure can also be long-term; many horses are high value animals with a much longer life span than agricultural livestock because of their status in human society. Horses need to be athletic, fit and involvement in competitions often means a higher level of stress as a result of travelling and competing. This can have a negative effect on the immune system, which means horses can be particularly susceptible to mycotoxins.

Economic implications and diagnostic issues

The high value of horse breeding stock of both sexes means that any impact from mycotoxicoses on fertility and successful foaling is extremely costly. Any fertility or pregnancy problems should be thoroughly investigated. Unfortunately, symptoms will often be vague and vary greatly, making proper diagnosis very difficult. Careful recognition of any signs, post mortem diagnostics and thorough feed analyses are the only ways of making an accurate diagnosis of any mycotoxin-induced problem.

Mycotoxin sources

Pasture and storage forages

Horses and ponies are at risk from mycotoxins produced by fungi that live on pasture plants and in conserved forage. The threat from hay is lower than from silage, but the popularity of using small bales of high dry matter silage (haylage or baleage) has increased in recent years. Easier to make and often more palatable and of a higher nutritional value than hay, poor silage also brings with it a higher mould and associated mycotoxin threat.

Compound feed

toxins in the feed or those produced during storage. Unfortunately, many leisure horse owners do not check the quality of their feedstuffs. Additionally, feed may be left in poor storage conditions for long periods of time and this can also increase the risk of mould contamination.


In temperate regions horses are often kept indoors for part of the year. This will expose them to mycotoxins from moulds in bedding materials, such as straw, which they can also eat. In addition, other equines, such as donkeys and mules, may be fed on straw because forage more closely matches their need for a very high fibre diet.

Range of mycotoxins

Various mycotoxins cause significant health and performance issues in horses and other equine animals. These include aflatoxin, ochratoxin, specific endophyte alkaloid toxins, deoxynivalenol, zearalenone, fumonisin, ergot and T-2 toxin. The fungi Aspergillus (and Penicillium), Fusariumspp and Claviceps produce the toxins most detrimental to equines.


Fungi Mycotoxin Feed commodities affected
Aspergillus flavus
Aspergillus parasiticus
Aflatoxin Maize, hays, pasture
Aspergillus ochraceus
Aspergillus nigri
Penicillium verrucosum
Ochratoxin A Wheat, barley, maize, oats, others
Fusarium graminearum
Fusarium culmorum
Tricothecenes [Deoxynivalenol (DON)] Maize, wheat, barley
Fusarium sporotrichioides DAS and T2 Toxin Maize, wheat, barley
Fusarium graminearum Zearalenone Maize, wheat, barley, grass
Fusarium verticillioides
Fusarium proliferatum
Fumonisin Maize
Claviceps Ergotoxin Pasture grasses, hay, wheat, barley
Fusarium moniliforme Moniliforme Maize
Penicillium roqueforti PR toxin, patulin Haylage, grass, grains

Key mycotoxins: Aspergillus


Aflatoxins are of greatest concern in more tropical regions of the world where the climate is generally warm and humid – less so in colder, more temperate countries.

Aflatoxin ingestion can have severe consequences for both horses and ponies. Avoid feeding imported feeds where possible. High risk nutrition sources include by-products from third world or developing countries (e.g. palm kernel meal, copra), moist feeds (brewing by-products) and peanut hulls.

Aflatoxins are carcinogenic and cause liver damage.

Did you know?

Mature horses should not be fed feeds containing more than 20ppb Aflatoxin and breeding stock and hard working horses should be fed aflatoxin-free diets.

Key mycotoxins: Fusarium


Fumonisins are found wherever corn is grown. Horses and ponies are very sensitive to fumonisin. This toxin causes equine leukoencephalomalacia (ELEM), which essentially means the brain develops lesions or holes. This disease leads to muscle tremors, poor co-ordination, loss of the swallowing reflex and depression – rather like an equine form of Parkinson’s disease or dementia.


Tricothecenes [e.g. T-2 toxin, deoxynivalenol (DON), diaceptoxyscripenol (DAS)] are common field toxins found on harvested grain. These mycotoxins are tissue irritants.

Clinical signs of trichothecenes toxicity include loss of appetite, reduced feed intake, poor performance, colic and immune-suppression.


Zearalenone contamination of cereals (particularly maize) often occurs in warm and humid conditions and in combination with DON. However, it is more likely to occur in the field rather than during storage of the grain.

Zearalenone is an oestrogenic toxin (i.e. it mimics the action of the hormone) and therefore adversely affects reproductive function.

Key mycotoxins: Claviceps

Paspalum grass harbours the fungus Claviceps paspali, whose spores are visible on the grass seed heads as sooty, blackened spikes. This fungus can also live on a variety of other grasses, including ryegrass and bluegrass, and hay. It produces ergot toxins, which are chemically related to LSD.

Typical signs of ergot poisoning include loss of co-ordination, hallucination and dry gangrene in the body extremities. Hooves and the tail may develop major lesions and may even die off completely in severe cases.

Endophyte alkaloid toxins

Ryegrass staggers

In parts of the world where insect damage is prevalent in pastures, high endophyte grass varieties are sown. These plants live in symbiosis with the Lolitrem fungus, which releases toxins that affect gait, behavior and co-ordination causing a condition known as ‘ryegrass staggers’. If horses are removed from the contaminated pasture they usually return to normal after a week or so.

This endophyte is closely associated with leaves and proliferates in hot dry conditions and the toxins accumulate at the base of the plant. When drought periods are followed by rain, the ryegrass undergoes periods of rapid growth, spreading the toxin up the plant stem dramatically increasing the exposure to grazing equines. Consequently, ryegrass staggers is typically observed after a drought when grass is actively growing once again.

These endophytes may be passed from one generation of plants to the next via the seeds. It is therefore important that any purchased seed is endophyte-free prior to seeding any pastures for horses.

Fescue toxins

Certain fescue grasses, particularly tall varieties, contain an endophyte called Festuca arundinacea. This fungus grows between the cells inside the plant.

Pregnant mares are particularly susceptible to this toxin as it is associated with an increased risk of abortion, poor cycling and embryonic mortality – as well as ‘red bag delivery’. This is where a toughened placenta grows across the cervix making it impossible for the mare to expel the foal naturally.

Stallions exposed to fescue toxins may have reduce semen volume and fertility problems.

Did you know?

Pregnant mares should be removed from pastures contaminated with endophytes by day 300 of gestation. Two weeks prior to the expected delivery date mares can be treated with domperidone to reduce the effects of any suspected argot alkaloid toxicity.

Additional information

When harvested for hay or silage, white clover can be a source of mycotoxins if it is allowed to become damp or mouldy. Any black, slimy patches indicate that this type of mould is growing, but it may not always be visible.

White clover appears to contain two types of toxin:

  • An oestrogen analogue similar to zearalenone that mimics reproduction hormones. It can prevent mares from becoming pregnant.
  • The slaframine toxin, which causes a disease called ‘slobbers’. Affected horses drool continuously, tears run down the face and animals experience uncontrolled diarrhoea and urination.

Effective mycotoxin management is about seeing the whole challenge, from the farm to feed mill and from risk assessment to feed management. The Alltech Mycotoxin Management Team provide a number of solutions to help you mitigate the threat you could face from field or storage mycotoxins. To find out more click here.