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Poisonous Plants

Oak and acorns contain tannins which when ingested produce toxins which can be poisonous to horses. The risk is highest in the UK at the end of long dry summers, even so most horses are sensible enough not to eat large numbers. It seems that certain individual horses are more susceptible to acorn poisoning than others, some horses can tolerate small numbers of acorns but the toxicity of the acorns can vary from year to year. Acorns and oak contain tannic acid and other tannins which are toxic when consumed in sufficient quantities. These poisons can cause damage to the gut leading to problems such as diarrhea and colic signs, they can also cause damage to the liver and kidneys. There is no specific antedote and the prognosis is poor if kidney damage develops. In rare cases acorn poisoning can be fatal.


It is sensible to prevent access to large numbers of acorns during the autumn, particularly after strong winds when large numbers may have fallen off the trees. Acorns can be swept up or electric fencing used to keep horses away from areas around oak trees with large numbers of acorns. Some people control the acorns by using a roller to push them into the ground, but it is difficult to do practically on a regular basis. Alternatively a more novel approach is to allow pigs to graze the area since they can safely eat the acorns; this has the added advantage of training your horses to accept pigs!

For further information, see our 'latest news' article on acorn poisoning HERE.

It is well known that ragwort can cause serious damage to horses’ livers. Horses usually choose not to eat ragwort when it is growing as it is said to be unpalatable but they will sometimes eat wilted ragwort in fields or in hay as it is loses its bitterness. Never pull out plants and leave them where horses can reach them because they are more likely to be eaten.  Also if the grazing is poor and there is nothing else available then they will possibly be tempted to eat ragwort as a last resort. The more ragwort they eat, the worse the damage will be.

Unfortunately the damage caused by ragwort is irreversible. The liver is very good at coping with a certain level of damage and so many horses with mild liver damage caused by ragwort appear completely healthy. However they can deteriorate very quickly and dramatically if there is any further damage. Liver damage will cause weight loss as well as loss of appetite and other vague signs but in some severe cases the liver damage can be fatal.


We advise horse owners to remove ragwort from fields that horses graze in and to be particularly careful to make sure there is no ragwort growing in fields from which hay is cut. More information about ragwort removal can be found on the BHS website.

There has been an increase in clinical cases seen at the clinic of the fatal condition atypical myopathy (otherwise known as Atypical Myoglobinuria or sycamore poisoning). We are pleased to report that progress has been made into a better understanding of the condition. A new study published in the Equine Veterinary Journal has revealed that toxins from the seeds and foliage of the tree Acer pseudoplatanus, more commonly known as the Ssycamore, is the likely cause of the condition in horses in Europe. This has been established after work in 2014 in America linked seasonal pasture myopathy (the US equivalent) to toxins from the box elder tree. Both trees produce seeds containing the agent hypoglycin A.

The new research was done by the University of Liege and involved 17 horses from Belgium, Germany and The Netherlands and is an important advance in our understanding of the condition. High concentrations of 'hypoglycin A' were found in all the horses. The pastures of 12 of the horses were visited by botanists and the sycamore was found to be present in every case. This is an exciting breakthrough, but there is still a lot to learn. There is a helpful summary of this research on the BEVA website.

Outbreaks tend to be seasonal with most cases occurring in the autumn and then in turn spring when seedlings are starting to grow. Signs of atypical myopathy include muscular weakness and stiffness, dark urine, fatigue, colic-like signs and trembling. Victims are usually kept in sparse pastures, where dead leaves are on the ground as well as the sycamore seeds (keys).

We now suggest that where horses are grazing near sycamores, it is imperative they have sufficient supplementary feed such as hay or haylage as this will minimise the risk of horses being tempted to eat seeds/foliage and will reduce the risk of over-grazing. If possible it makes sense to fence off the area of leaf fall directly under the trees during the autumn and spring when seedlings are starting to grow or where possible, move to a different field during high risk times of the year. We do not think it would be right to cut down trees as their shade can be a vital source of shelter for the rest of the year but please be aware there is a potential danger during the autumn and again in spring when seedlings (as seen below) are coming up.

There is further information on our 'latest news page. If you have any questions, please contact BELL EQUINE on 01622 813 700 and ask to speak to one of the vets.