Poisonous plants adversely affect the health of a horse when eaten. Not all plants are poisonous to horses at all times. Whether or not a plant is poisionous can depend on the growth stage of the plant, the nature of the poison and the time it takes to exert its effect on the animal.
Almost anything can be poisonous if eaten to extremes. For example, excess consumption of wheat will result in a horse being unwell, but wheat itself is not a poison. Likewise, excess amounts of a herb such as St John’s Wort can cause photosensitisation, but at a more moderate intake it is fine.
In the UK, by far the most common form of poisoning is caused by the ingestion of ragwort, but there are plenty of others which can cause problems if the right conditions prevail.
Poisoning can take many forms in a horse, from ill thrift and photosensitisation to disease or disruption of the function of key body systems. The most extreme result is, of course, death.
There are seven different broad types of poison — alkaloids (as found in ragwort, yew, hemlock), glycosides, nitrates, photosensitisers, saponins and complex proteins. Within each category are many different active compounds.
Horses are most likely to consume poisonous plants if they have nothing else to eat as a result of a combination of poor grazing with a heavy infestation of such plants, or due to their undetected presence in hay or haylage.
NB: The list below includes many common poisonous plants, but it is not exhaustive
Common poisonous plants for horses
Every horse owner’s summer nightmare is digging this ever-persistent yellow flowered plant out of their horse’s paddocks.
One of the biggest problems with ragwort is that it is still just as toxic when it has been dried, so it is a massive threat when it is found in hay.
The symptoms of poisoning can take many weeks to show. The signs include cardiac problems, gastric problems, nervous problems, chronic liver damage. When ragwort has been injested over prolonged periods the damage can result in liver failure and death.
Oak leaves and acorns
Although most horse owners are aware of the dangers of acorns, but it should be remembered that the leaves of the oak tree are also poisonous to the horse. Acorns are more dangerous when green.
If eaten, leaves and the acorns can result in depression, colic, feaces problems including bloody diarrhoea, liver and kidney damage plus urinary problems.
Most commonly seen in cemeteries, this tree is deadly to horses. As little as 6-8 ounces of the fresh plant can kill an adult horse within five minutes. Some cases have been found dead with the remains of leaves still in their mouths.
The whole plant is toxic and poisoning typically occurs when clippings from these plants are tossed into grazing fences by people who do not understand the potential harm. Horses should not be allowed to grass anywhere near this plant.
Horsetail / Mare’s tails
This plant typically grows among crops and in pastures. Most poisoning occurs when the mare’s tails or horsetail is cut and incorporated into hay which is then fed to horses and other livestock.
When eaten it can cause gastric problems such as diarrhoea or constipation and problems with the horse’s central nervous system.
Horses may exhibit signs of depression as well as weakness, muscles tremors and a lack of co-ordination.
A common sight in the summer months, these plants contain varying amounts of a poisonous oil called ranunculin.
When it is crushed or chewed, it forms a toxic blistering agent.
Buttercup poisoning is rare, but it can cause excessive salivation, gastric problems such as mild colic and diarrhoea.
Plants from the nightshade family can be found in hedgerows, on stone walls or creeping up post and rail fencing and certainly live up to their “deadly” name.
If eaten by horses they can cause excitability followed by depression, decreased heart and respiratory rate, gastric problems such as colic, co-ordination problems and muscle weakness and convulsions.
If injested in large amounts nightshade can cause death from cardiac arrest.
This common plant is found in damp or marshy areas and along canals, streams and brooks and is extremely dangerous to horses.
It is often mistaken for members of the parsnip or parsley families. If eaten it can cause permanent paralysis and death within hours of ingestion.
Hemlock: Salivation, muscle tremors, paralysis and death. Found in hedgerows and around ponds
Bracken: Weight loss, weakness and lack or co-ordination. Found in fields and in contaminated hay
Also known as Autumn Crocus, this plant has crocus-like flowers. It contains the poison colchichine, which if eaten can cause gastric problems, ataxia and death.