Which plants are most toxic to horses? In this series of posts, I continue to describe the most common plants that are toxic to horses. Here we look at the mighty Oak.


Oak (Quercus spp.) is a tree, native to Britain, and commonly found in woodland, parkland, and near pastures. Oak is also widespread across North America. The oak may live for hundreds of years. Trees often have their lower branches lopped, which reduces the life of the tree. In the normal cycle, the Oak puts out lower branches which then support the tree in later life. Without these lower limbs, the tree can end up splitting. Farmers often cut the lower branches so they can plough around the tree. This may plough more of the field. But it compacts the soil around the surface roots and also does the tree no favours.

Many Oaks in Britain are now over 500 years old. Many have seen history roll by, since the time of the Wars of the Roses, and even before. In her book “Wilding“, Isabella Tree describes how Ted Green (custodian of the Oaks in Royal Windsor Park) talked of the life history of the tree. “The Oak grows for the first 300 years. Rests for another 300 years. And then spends the last 300 gracefully declining”. 900 years. It really makes one think! Back in the 17th Century, the Herbalist Culpeper wrote of the Oak.

What the herbalist says:

The herbalist Culpeper tells us that: This tree grows to a vast height, spreading out into innumerable and irregular branches. The leaves are oblong, obtuse, deeply sinulated, and of a dark green. The flowers are both barren and fertile on the same tree ; the former are collected into loose catkins ; the latter are seat­ ed in the buds, and both sorts are small and inconsiderable, The seed is oval-formed, of a leather-like coat, which ap­pears as if rasped at the base, and is fixed to a short cup. It is too common to require a particular spec1fi­cation of the place of its growth. The flowers appear in April, and the acorns are ripe in October and November. Jupiter owns the tree. The leaves and bark, and the acorn cups, bind and dry much. The inner bark and the thin skin that covers the acorn, are used to stay the spitting of blood, and the flux.

The active agents:

The poisonous agents in Oak are the tannins, and these can be extremely toxic. Nonetheless, information on the toxicity of oak appears confusing. On the one hand, some horses have apparently died from Oak poisoning. On the other, there exist reports of horses eating acorns with little or no ill effect. In my own experience, I have seen certain horses of a herd enjoying fallen acorns. Others will graze around them. No member of that herd suffered any consequence in the three years I managed them.

Herbal Medicine

In herbal medicine, Quercus has been used as an astringent and an antiseptic.

Poisoning with Oak?

Cynthia Gaskill, American Board of Veterinary Toxicology, has written about the Oak. She explains that acorn poisoning has been well documented in cattle. Very few cases have been confirmed in horses (in the US). Symptoms of acorn toxicity include depression and loss of appetite. There may be digestive-tract issues: colic, gastric upset and diarrhoea (often bloody). Kidney and possibly liver damage are possible. The more acute the symptoms, the higher the likelihood of death. Horses that experience a gradual onset of symptoms are more likely to survive.

In November 2013, a large number of horses apparently died in the New Forest, in the UK. An article in Horse and Hound states that 31 ponies roaming on the New Forest died, along with 12 cattle. In a normal year, many fewer animals are affected each autumn. 2013 was what is known as a “mast” year, when oaks and apple trees produced plenty of fruit. As such, extreme prudence is necessary.

As with many toxic plants, horses are more likely to eat them if they do not have enough grass or hay. Imagine several horses are being kept underfed. An oak tree in the paddock drops a large number of acorns. It is possible that some of the horses might now eat a large number of acorns to supplement their food.

It is important to note that the amount of Tannins varies during the year and season. As such it is difficult to predict how toxic the leaves/acorns might be. If you have an oak in your pasture, the best strategy would be to fence it off. This ensures that it will be out of grazing range for the herd.


I hope this article has been helpful. Other articles about plants toxic to horses appear on my Professional Facebook Page. If there is a plant that you would like me to write about, please get in touch via Contacts.

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Isabella Tree: Wilding – The return of nature to a British Farm. Picador, 2018.
Keith Allison: A guide to Plants Poisonous to Horses. J.A. Allen. Revised edition, 2011.
John Gerarde: The Herball or General Historie of Plantes, 1636.
Culpeper: Index Of Herbs By Nich. Culpeper, Gent. Student in Physick and Astrologie, 1616, 1654
Cynthia Gaskill: Are acorns poisonous to horses?
Horse and Hound: A huge rise in equine deaths due to acorns.

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