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What to do if your horse has been poisoned!

Plants Poisonous to Horses




When evaluating plants poisonous to horses, it is unfortunately not as straight forward as looking at the weed and determining if it is either toxic or not.

There are many variables which affect the toxicity of a plant and the potential poisonous content, as well as numerous plant species which may or may not dangerous to your equine companion.

The poison that some plants contain can increase through growth stages or lessen with age. And some parts of plants poisonous to horses may be toxic where as other parts may not.

Plants poisonous to horses can also produce a range of results from minor irritation to the other end of the scale being death.


Did you know?.., A young horse may be more susceptible to plants poisonous to horses than a older equine. Older horses may have built up a resistance by being exposed in small quantities of a similar poison over time.



There are different types of substances which can be poisonous to horses..,

1. There are plants poisonous to horses which contain a natural sugar glucose. These are called Glycosides. Some Glycosides by themselves are not poisonous but combined with other substances become highly toxic. The result of poisoning from Glycosides may include the starvation of oxygen to the brain and other tissues. This can cause rapid respiration and convulsions in an animal.

2. Other plants poisonous to horses include a bitter tasting group that have Alkaloids. Alkaloids, when eaten, may irritate the gastrointestinal tract and cause diarrhea or colic. Severe cases may cause an adverse effect to the central nervous system and show up as blindness, convulsions and death.

3. When a plant has accumulated an amount of nitrates, it may be poisonous as it releases nitrates into the gastrointestinal track and forming a compound which releases oxygen within an animal. This poisons may cause the horse to stagger, or quickness of breath to death. Nitrate accumulation is apparently greater when nitrate fertilizers are used over when the nitrate source is an organic compound is the source. Climatic factors can also bring about different nitrate accumulation levels within a plant.

4. Molybdenum is a substance which can be found in soil. It helps bacteria and soil organisms convert nitrogen in the air to nitrogen. It is something that is needed particularly by legumes and is essential in the formation of proteins from nitrogen soil compounds. Molydenum deficiencies can be prevalent in the North Coast, however, poisoning can occur in areas where there are abnormally high quantities. Pasturing in these areas can leave an equine companion subject to emaciation, rough faded coats and scouring. It may be necessary to add copper to a diet of an animal to counter act such deficiencies. However, as with all changes and additions, consult your vet first before adding copper to your animals diet. Copper can also cause toxic effects in some instances.

5. Alkali disease or “blind staggers” can be contributed to organic levels of Selenium in plant materials. Synthetic forms of this compound are not found to be as toxic. Symptoms include the stiffness of joints and lameness (thus the name blind staggers) and loss of hair as well as hoof deformities.

6. Ergot is an alkaloid and drug (LSD) which on a medicinal level has uses from migraine headache to heart medicines to anti Parkinson treatment to therapy for cocaine addiction. Ergot is a dark mold fungus which can grow on rye after it is cut and stored. Ergot infests grasses and if eaten in high enough levels can be poisonous. Ergotoxine, is an active toxin which can cause the contraction of small blood vessels which supply the nerve vessels supplying the different parts of the body. Symptoms range from lack of appetite, stomach pain, gangrene to death dependant on the amounts consumed.

7. Affecting corn and cereals are mycotoxins. In the right environment mycotoxins can be produced by micro fungi to levels which are capable of causing disease and death to humans and animals. Mycotoxins have uses in antibiotics, growth promotants, to chemical warfare agents. Mycotoxin production can take place while a crop is still standing in a field or after it is has been stored. There are different forms of mycotoxins some which may cause swelling and irregular heat in some animals.

8. A chemical found in sweet clover is “Courmarin”. During heating or spoilage of sweet clover hay the chemical Dicoumarol may be created which lowers the clotting ability of the blood.


Did you know?.., An horse that has certain dietary deficiencies is more likely to eat toxic quantities of a poisonous plant than a well fed animal.



Other plant cautions are..,

Photosensitization: Equine can consume plants poisonous to horses which leave them sensitive to strong sunlight. This is called photosensitization. When digested these plants absorb and circulate through the blood to the skin where they are activated by the sun. Another form of these types of plants cause damage to the liver which prevents the breakdown of chlorophyll. This again is activated by the sun and may create similar symptoms such as sun burning, swellings, ulcers, gangrene or blindness. You can see these symptoms with St John’s Wort, Blue Green Algae and Buckwheat.

Blue Green Algae: Blue Green Algae in water can be a poisonous plant to horses in other ways which resemble an allergic reaction. Found generally in the warmer months in places such as slow moving water pools, symptoms can develop rapidly and can result in death. Stories of animals being found dead close to water’s edge may be the result of Blue Green Algae. This poison can cause weakness, staggering, jaundice, constipation.., or to the extreme being where the animal sinks to the ground and dies without struggle.

Mechanical Injury: This is when something may cause physical or mechanical injury to your animal. An example of this would be food which is prickly and thus becomes lodged in your horses throat. These types of injuries may cause a multitude of concerns leading to secondary more serious concerns such as infection at the injured site. A plant which may be the cause of something like this is the thistle, or other types of grasses and burrs with sharp spines that may become difficult to dislodge.




How all this relates to our horses..,

Well, this information is all well 'n good, but it is important to know how it specifically relates to the horse and what types of plants produce these types of toxins and other hazardous pet health concerns.


• A list of plants poisonous to horses.

• Horse Hay: The types of Horse Hay

• Feeding your Horse a Balanced Meal

• Article: Is your horse pasture toxic?



PLEASE NOTE: This web page is based on the research and conversations that we have had with various people and professionals on the subject of horse health and is not intended to replace veterinary care for your animals. We do not accept liability for errors or omissions. A vet, horse nutritionist or other trained professional should always be consulted with any equine concerns, or before changing any feeding or care regime.