Did you know that 2025 is a mast year?
That means oak trees and other species, such as beech and hazel, are producing a bumper crop of acorns and nuts. Mast years occur roughly every five to ten years and are nature’s clever way of ensuring new trees survive. By producing more seeds than wildlife can eat, trees guarantee that some will escape predation and grow into the next generation of oaks.
While this is great news for squirrels and jays, it’s bad news for horses.
Why 2025’s mast year matters
Thanks to the long, hot summer, perfect pollination weather, this year’s acorn crop is exceptionally heavy. Across the UK, equine vets and hospitals are already reporting a sharp rise in cases of acorn toxicity.
Acorn poisoning in horses is usually uncommon, largely because acorns are bitter and most horses don’t eat enough to cause harm. However, during mast years, with so many acorns on the ground, even the most selective grazers may be tempted and the results can be fatal.
Why acorns are toxic to horses
Acorns and oak leaves contain tannins, which can damage the gut and kidneys. Horses are particularly sensitive, although not all individuals are affected in the same way. Historically, in the New Forest, pigs were turned out to eat fallen acorns, helping protect the ponies that grazed beneath the trees.
This year’s hot, dry weather has created ideal conditions for acorn production, and vets believe it may also influence tannin concentration – or even the mechanical damage acorns cause in the gut. Several equine hospitals are now studying whether this year’s spike in cases is linked to increased tannin levels or other factors.
Signs of acorn toxicity
Look out for:
- Depression or dullness
- Loss of appetite
- Colic
- Diarrhoea (sometimes with blood)
- Acorn husks in droppings
- Signs of kidney damage in severe cases
If you suspect your horse has eaten acorns or shows any of these signs, call your vet immediately. Early veterinary intervention can be life-saving.
What owners can do
- Avoid grazing horses in fields with oak trees, if possible
- Provide plenty of alternative forage and hay to reduce acorn scavenging
- Clear fallen acorns regularly from paddocks
- Monitor closely for any signs of illness
