There are numerous plants in the UK that are poisonous to horses. Generally horses will avoid these unless food is scarce or the plants are incorporated into hay.
It is very important that we identify and remove these plants from our fields so that they don’t pose a risk to grazing horses.
Common plants and trees that are poisonous to horses in the UK
Ragwort (Senecio jacobea) is a weed found in pasture throughout the UK which contains poisonous substances (toxins). These toxins (pyrrolizidine alkaloids) cause damage to the liver of a number of animals including horses and donkeys.
Most animals tend to avoid eating Ragwort as it is not very palatable, although if food is scarce or there are a large number of Ragwort plants present within a pasture, horses may be forced to eat it. Furthermore, the toxin is very stable and remains toxic even when the dried plant is incorporated into hay.
What are the signs and symptoms of ragwort poisoning?
There are two types of Ragwort poisoning – acute (immediate) and chronic (long-term). The acute form is rarely seen as such large quantities of Ragwort need to be eaten.
However, the first sign is often sudden death of the horse.
Chronic poisoning is the most common form and the signs of poisoning are not usually seen for months. The toxins gradually accumulate in the horse's liver causing damage to the liver cells. The cells become replaced with scar tissue, leading to the liver shrinking in size and functional capacity.
The liver has large functional reserves and so it is only once these reserves have been exhausted that signs of poisoning can develop. At this stage the signs often come on suddenly, although in some horses and ponies, mild illness can precede more severe symptoms.
Signs of chronic disease include loss of appetite, depression, diarrhoea, weight loss, sensitivity to sunlight and jaundice (the mucus membranes or the whites of the eyes look yellow).
Neurological signs, such as weakness, circling or head pressing, can also be seen in cases of ragwort poisoning. This is because the liver is responsible for filtering the blood of many potentially harmful substances, such as ammonia, and when it is not functioning properly these build up within the blood stream and can affect the brain.
How can I prevent ragwort poisoning?
Ragwort is a biennial plant, which in its first year forms flat rosettes. In the second year it becomes much taller and produces yellow flowers. The only reliable method of prevention is to remove the weed from pasture.
The plants should be removed, including the roots and disposed of away from livestock. It is important to ensure that animals have no access whatsoever to any plants as even dried they are still poisonous.
The poison can also be absorbed through the skin of humans so it is important that impervious gloves are worn. Plants on adjacent land should be removed to avoid the spreading of seed back into your paddocks.
Always ensure that there is adequate grazing or alternative food sources such as hay, so that your horse or pony is not tempted to eat any ragwort that may have been missed.
Sprays are available for the control of ragwort and advice can be sought from your local farm merchant on the appropriate one for you.
There is a DEFRA Code of Practice to prevent the spread of ragwort. Those who disregard the need for the weed's control can face prosecution by the government (Ragwort Control Act 2003).
Horses are unlikely to eat fresh Foxgloves as they are unpalatable but unfortunately they are more palatable in hay where they may be eaten. Only a small quantity (about 100g) needs to be eaten to prove fatal within a few hours.
Buttercups need to be eaten in very large quantities to pose a threat to horse health. It is very unlikely that horses will ingest a large amount of buttercups as the toxin is bitter tasting and can cause mouth ulcers. However, poisoning can occur in overgrazed pastures where there are little to no other plants for horses to consume. Symptoms of toxicity include excess salivation, diarrhoea and colic.
The dried plants in hay are not poisonous.
Buttercups, however, can cause irritation of the skin they are in contact with (contact dermatitis), particularly of the lower limbs and muzzles of horses.
If there are large amounts of buttercups in your field it is advisable to seek professional advice on spraying to remove them.
All parts of the yew tree are poisonous and unfortunately the leaves are toxic when dried. Poisoning usually occurs when horses ingest discarded cuttings that are dropped or blown into their field.
Be vigilant for this as poisoning can therefore still occur even if the tree is fenced off. Yew is so toxic that it can take as little as a few mouthfuls to cause death.
Oak poisoning usually occurs due to horses eating fallen acorns in the autumn; however the leaves, stems and oak blossoms are also toxic to horses.
Whilst eating a few acorns is harmless, they can become addictive and horses will then actively seek them out. Acorns are toxic as they contain tannins which can cause kidney failure, colic and death. Symptoms of acorn toxicity include inappetence, depression, colic, diarrhoea and blood in the urine.
Acorns must be collected up or the trees fenced off to prevent the acorns being eaten.
Rhododendrons are highly poisonous trees and shrubs which contain toxins known as cardiac glycosides. Horses are unlikely to eat Rhododendrons unless the grazing is very sparse, but if ingested symptoms include diarrhoea, hyper-salivation, collapse and death.
General Pasture Management
Described above are some of the most common poisonous plants found in the UK; however there may be some others that are a threat to your horse. It is essential that you regularly check your horse’s grazing and remove any potentially poisonous plants and burn them away from your pasture.
Remember do not leave any pulled up plants where your horse could get access to them as many of them will remain toxic when they are dried and are potentially more palatable than the fresh plant.
Make sure you are able to identify poisonous plants and if you are unsure about any plants in or around your horse’s field then obtain expert advice.