Myco is the Greek term for fungus, while toxins are poisonous substances. Mycotoxins are toxic compounds produced by certain fungi and moulds, and they can occur in pasture, hay and other feeds.
Some fungi live inside plants as endophytes. Perennial ryegrass, for example, can contain endophytes that produce compounds including lolitrem B and ergovaline. Lolitrem B is strongly associated with ryegrass staggers, while ergovaline can affect circulation, temperature regulation and reproductive function.
Mycotoxins aren't always obvious. A horse can be grazing apparently healthy pasture without there being any obvious sign that fungal toxins are present.
Where do mycotoxins come from?
Fungal growth is favoured by certain environmental conditions, particularly warmth and moisture. Mycotoxins can occur in pasture as well as stored feeds and forage.
In Australia and New Zealand, some of the pasture plants of particular interest include:
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Perennial ryegrass
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Paspalum
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Clover
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Couch or Bermuda grass
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Tall fescue
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Phalaris
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Kikuyu
The presence and concentration of mycotoxins can vary considerably depending on the plant, fungal species, growing conditions, stage of growth and environmental conditions.
Perennial ryegrass
Perennial ryegrass (Lolium perenne) can become infected with fungal endophytes. Some endophyte strains produce alkaloids that are toxic to grazing animals.
The most important toxin associated with ryegrass staggers is lolitrem B. Horses exposed to sufficient levels can develop tremors, incoordination and an unsteady gait. Research in horses has demonstrated action-related tremors and vestibular ataxia following exposure to lolitrem B.
Ergovaline is another endophyte-associated compound. It can cause vasoconstriction and interfere with temperature regulation. In pregnant mares, significant exposure can contribute to prolonged gestation, dystocia, reduced milk production and foal losses.
Importantly, not all perennial ryegrass carries the same endophytes. The type of endophyte and the alkaloids it produces can vary between cultivars and growing conditions.
Paspalum
Paspalum can become particularly dominant in Australian and New Zealand horse paddocks during summer and autumn when other grasses have browned off.
Paspalum can develop an ergot fungus on its seed heads. These infected seed heads are associated with paspalum staggers, a neurological condition caused by tremorgenic compounds produced by the ergot.
Heavy grazing of infected pasture can also increase exposure, particularly when horses are forced to graze very close to the ground.
Why are mycotoxins important for horses?
Horses spend a large proportion of their lives eating forage. That means pasture quality matters enormously.
Mycotoxins can affect the nervous system, circulation, temperature regulation, digestion and reproductive function depending on the particular toxin and level of exposure.
Some horses can appear particularly sensitive to changes in pasture. Behavioural changes may be among the first things an owner notices, while other horses may show more obvious neurological or physical signs.
This is one reason we believe it's important to look at the whole diet and pasture, rather than treating a behavioural or physical problem in isolation.
Signs that a horse may be affected
The signs associated with mycotoxins can vary considerably. Some can also overlap with mineral imbalances and other pasture-related problems.
Behavioural changes
A horse may become:
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Tetchy or unusually reactive when touched
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Stiff or tense
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Girthy or difficult when saddling
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Crabby when ridden
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Impulsive or wanting to run off
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Touchy around the ears or difficult to bridle
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Unusually aggressive towards people or other horses
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Excessively herd-bound
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Prone to sudden bucking or bolting
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Hypersensitive or excessively spooky
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Difficult to catch
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Extremely sensitive to noise or confinement
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‘Spaced out’, wired or seemingly not quite themselves
Coordination and movement
More obvious neurological signs may include:
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Stumbling
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A heavy forehand
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Standing base-wide
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Difficulty backing up
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Difficulty walking downhill
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Uncoordinated movement
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Staggering
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Hindquarter weakness
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Dragging the back feet
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Reluctance to canter
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Tremors or other abnormal movements
These signs are particularly relevant when considering ryegrass or paspalum staggers. Horses affected by lolitrem B can develop tremors and ataxia, with signs becoming more obvious during movement or activity.
Heat stress and sweating
Some horses may also show:
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Excessive sweating
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Unusual sweating patterns
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Heat intolerance
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Agitation
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Running around the paddock
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Fence walking
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Difficulty settling in warm weather
Ergovaline can interfere with blood flow and temperature regulation, making heat stress a recognised feature of endophyte toxicosis.
Head flicking and sensitivity
Head flicking, nose rubbing and unusual sensitivity can sometimes occur alongside other pasture-related signs.
If these signs appear seasonally or alongside changes in pasture, it's worth looking closely at what the horse is eating.
Reproductive problems
Mycotoxins and endophyte-associated toxins can be particularly important for breeding mares.
Significant ergovaline exposure has been associated with:
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Prolonged gestation
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Dystocia
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Reduced milk production
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Poor mammary development
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Retained placenta
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Foal weakness and loss
These effects are particularly associated with exposure during late pregnancy.
Ill-thrift
A horse affected by poor-quality or problematic forage may also show more general signs such as:
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A dull or rough coat
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Difficulty maintaining weight
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Poor topline
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A bloated or ‘potty’ belly
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Changes in manure
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Scouring or diarrhoea
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Dull or glazed eyes
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Low energy
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Lethargy
Not every sign above means mycotoxins are involved. The important thing is to look at the pattern, particularly when symptoms coincide with particular pasture conditions or seasonal changes.
Mycotoxins and mineral imbalances
This is where things can become complicated.
Pasture-related problems don't always fit neatly into one category. A horse may be dealing with fungal toxins, mineral imbalances or several dietary factors at the same time.
Rapid pasture growth, weather changes and differences in grass species can alter the mineral balance of the diet. Potassium, magnesium, calcium and sodium are particularly important considerations.
Some mycotoxins can also interfere with nutrient utilisation.
For this reason, when a horse is showing signs of being ‘grass affected’, we recommend looking at both feed quality and mineral balance, rather than assuming there's only one cause.
For more information on the mineral side of pasture, read our article on Horse Pasture and Mineral Balance: Magnesium, Potassium and Nitrate.
What can you do?
The first step is to look closely at the horse's environment and diet.
Look at the pasture
Learn to recognise the grasses growing in your paddocks. Pay particular attention to perennial ryegrass, paspalum and other grasses known to have associations with fungal toxins.
Avoid allowing horses to graze problematic pasture right down to the base where possible, particularly when conditions favour fungal growth or when infected seed heads are present.
Good pasture management is an important part of reducing exposure.
Look at the hay and feed
Mycotoxins aren't restricted to pasture. Moulds can develop in hay, grain and other stored feeds, particularly when moisture and storage conditions allow fungal growth.
Always pay attention to the quality, smell and storage of forage and feed.
Consider a toxin binder
A toxin binder is designed to bind certain toxins within the digestive tract so they can pass out with the manure rather than being absorbed.
Greenpet's Calm Healthy Horses ToxDefy contains a yeast cell wall extract designed to bind mycotoxins without affecting essential nutrients. It's particularly useful to consider when horses are grazing pasture where fungal toxins may be a concern or during warm, humid conditions when fungal growth is more likely.
ToxDefy is available from Greenpet:
A toxin binder isn't a substitute for addressing the underlying pasture or feed problem. If the horse continues to consume a significant source of toxins, the exposure continues.
Don't forget the bigger picture
Mycotoxins are only one piece of the pasture puzzle.
The type of grass, stage of growth, weather, soil, mineral balance, grazing pressure and quality of the forage all influence what your horse is consuming.
This is why we encourage horse owners to become familiar with their pasture and to look at changes in their horse alongside changes in the grass.
If your horse becomes noticeably different at particular times of year, after changes in weather or when certain grasses dominate the paddock, pay attention to that pattern.
It may be telling you something important about the diet.
For more information about pasture grasses, read Horse Pasture: Which Grasses Are Best for Your Horse?
For pasture management, read Horse Pasture Management: Choosing, Growing and Managing Grass.
For mineral balance, read Horse Pasture and Mineral Balance: Magnesium, Potassium and Nitrate.
Need help with your horse?
If you're unsure what's contributing to your horse's symptoms, complete our complimentary Horse Questionnaire and tell us about your horse, their pasture, diet and the problems you're seeing.