Acorn poisoning in horses: risks, symptoms and prevention
Oak trees are a familiar sight across the British countryside and are commonly found in and around horse paddocks. While they provide valuable shade during the summer, their leaves, branches, buds and acorns can be toxic to horses if eaten.
Acorns usually present the greatest risk between September and November, when they fall from the trees in larger quantities. However, they may begin falling earlier, particularly following periods of hot, dry weather, strong winds or storms.
When grazing is limited, horses may be more likely to investigate and eat plants, leaves and acorns that they would normally avoid. It is therefore important to check paddocks regularly and take steps to reduce access to oak trees and fallen acorns.
Why are acorns toxic to horses?
Oak trees contain tannins, including tannic acid. If consumed in sufficient quantities, these substances can damage a horse’s digestive system and kidneys, potentially resulting in severe illness.
The amount required to cause poisoning varies between individual horses. Some may become unwell after eating a relatively small quantity, while others may consume more without showing immediate signs.
Acorns generally have a bitter taste, so many horses will avoid them. However, some horses appear to enjoy eating acorns and may actively seek them out, even when alternative forage is available.
Which parts of an oak tree are poisonous?
All parts of the oak tree can present a risk to horses, including:
Acorns
Young leaves
New buds
Branches and bark
Green, immature acorns, young leaves and new buds are thought to contain higher concentrations of harmful tannins and may therefore pose the greatest risk.
Acorns are often the primary concern because they fall into paddocks in large numbers and some horses find them palatable. Low-hanging branches, fallen limbs and hedge cuttings containing oak should also be removed or placed out of reach.
When is the risk of acorn poisoning highest?
Acorns most commonly fall during the autumn, usually between September and November, but the exact timing can vary according to the weather, location and individual tree.
The risk may be increased by:
An unusually large acorn crop
Acorns beginning to fall earlier than expected
Strong winds or storms bringing down acorns and branches
Hot, dry weather reducing the availability of grass
Overgrazed or sparse paddocks
Insufficient alternative forage
Horses developing a taste for acorns
Low branches allowing horses to reach young leaves and buds
Even if oak trees have not caused a problem in previous years, paddocks should still be checked. The number of acorns produced and the behaviour of individual horses can change from one season to the next.
Symptoms of acorn poisoning in horses
The signs of acorn poisoning can vary depending on the amount consumed and the individual horse’s susceptibility.
Possible symptoms include:
Colic or abdominal discomfort
Lethargy or depression
Changes in normal behaviour
Reduced movement or reluctance to move
Loss of appetite
Increased lying down
Mouth ulcers
Dehydration
Acorn husks visible in the droppings
Constipation
Dark or bloody diarrhoea
Reduced urination
Straining to urinate
Red or brown urine
Red-brown urine, reduced urination or straining may indicate kidney damage and require urgent veterinary attention.
Some of these signs can also be associated with other serious medical conditions. Do not wait until acorn husks appear in the droppings before contacting your vet if your horse is unwell.
What should you do if your horse has eaten acorns?
If you know or suspect that your horse has eaten acorns, remove them from the affected area and contact your vet for advice.
You should call your vet immediately if your horse displays:
Signs of colic
Lethargy or depression
Reduced appetite
Diarrhoea or constipation
Dehydration
Changes in urination
Dark or bloody droppings
Red or brown urine
If possible, make a note of when the horse may have eaten the acorns, approximately how many were available and when the symptoms began. Check the paddock and droppings for evidence, but do not delay calling your vet while doing so.
Do not attempt to treat the horse yourself or give medication unless instructed by a veterinary surgeon.
How is acorn poisoning treated?
There is no specific antidote for acorn poisoning. Veterinary treatment will depend on the amount believed to have been eaten, the symptoms present and whether the digestive system or kidneys have been affected.
Treatment may include intravenous fluids to correct dehydration, support the kidneys and help the body eliminate toxins. Your vet may also administer treatments intended to reduce the absorption of toxins, protect the digestive system or manage pain and colic symptoms.
Blood tests may be required to assess kidney function and the horse’s general condition. Some horses will need ongoing monitoring or hospitalisation.
The earlier treatment begins, the better the opportunity to limit further damage and support recovery.
Preventing acorn poisoning
Prevention is particularly important because there is no specific antidote. A few practical paddock-management measures can significantly reduce the risk.
Fence off oak trees
Use suitable fencing to prevent horses from accessing oak trees and the ground beneath them. Ideally, fence around the full spread of the branches rather than only surrounding the trunk, as acorns can fall across a wide area.
Check that horses cannot reach low-hanging branches, young leaves or buds from behind the fence.
Clear fallen acorns regularly
Inspect affected fields daily during the acorn season and after strong winds or storms. Remove fallen acorns, leaves and branches before turning horses out.
Depending on the size of the area, acorns can be collected using:
A rake and wheelbarrow
A tarpaulin
A nut gatherer
A paddock sweeper
A suitable outdoor vacuum
Clearing acorns can be labour-intensive, but it is safer than assuming horses will avoid them.
Provide adequate forage
The risk may increase when grazing is sparse because horses are more likely to look for alternative food.
Provide sufficient good-quality forage away from the oak trees. This can reduce the likelihood of horses investigating acorns, although it may not stop a horse that has developed a particular taste for them.
Check fields before turnout
Walk the paddock before turning horses out, paying particular attention to:
The ground beneath oak trees
Areas reached by overhanging branches
Field boundaries
Fallen or storm-damaged branches
Acorns hidden beneath long grass or leaves
Nearby trees outside the boundary that overhang the field
Remember that acorns can roll or be carried some distance from the base of a tree.
Consider using another paddock
If there are large numbers of acorns and they cannot be removed effectively, the safest option may be to keep horses out of the affected field.
Once the trees have stopped dropping and the acorns, branches and leaves have been cleared, the paddock may be suitable for use again. It should always be inspected carefully before horses return.
Managing horses that actively seek out acorns
Some horses may repeatedly seek out acorns despite having access to grass and supplementary forage. These horses may require more stringent management.
Simply providing additional hay may not be enough. Complete exclusion from the affected area is often the safest option until the acorn season has passed and the paddock has been cleared.
Make sure everyone involved in the horse’s care understands the risk and knows which areas must remain closed.
Frequently Asked Questions
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Yes. Acorns contain tannins that can damage a horse’s digestive system and kidneys when eaten in sufficient quantities. The amount required to cause illness varies between individual horses.
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Green, immature acorns are generally considered particularly harmful because they may contain higher concentrations of tannins. However, both green and fallen mature acorns should be treated as a risk and removed from areas accessible to horses.
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Yes. Acorns, young leaves, buds, branches and other parts of the oak tree can be toxic. Young leaves and new buds may be particularly harmful.
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Acorns typically fall between September and November, but they can appear earlier depending on the weather and local conditions. Strong winds and storms may cause a sudden increase in fallen acorns and branches.
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There is no single safe or toxic quantity that applies to every horse. Susceptibility varies, and some horses may become unwell after eating less than others. Contact your vet if you know or suspect that your horse has consumed acorns.
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Early signs may include lethargy, depression, reduced appetite, abdominal discomfort, increased lying down or changes in normal behaviour. Digestive and urinary symptoms may develop as the illness progresses.
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Remove the horse from the affected area and contact your vet for advice. Do not wait for symptoms to develop, particularly if you believe a significant quantity may have been eaten.
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Recovery is possible, particularly when the problem is recognised and treated promptly. However, severe poisoning can cause significant digestive and kidney damage. Early veterinary intervention offers the best chance of recovery.
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The field should not be used until the trees have stopped dropping and fallen acorns, leaves and branches have been removed. Check the area carefully before allowing horses to return.
When to contact your vet
If you suspect that your horse has eaten acorns, or they show signs of colic, diarrhoea, lethargy, dehydration or changes in urination, contact your vet immediately.
Acorn poisoning can progress rapidly. Prompt veterinary assessment and treatment provide the best chance of limiting organ damage and achieving a successful recovery.
This article provides general information and is not a substitute for individual veterinary advice. If you are concerned about your horse, please contact your vet.