Almost anything can be poisonous if used to the extreme. For example, excess consumption of wheat will cause a horse great ill, but wheat itself is not a poison. Likewise, excess amounts of a herb such as St John’s Wort can cause photosensitisation, but at more regular intakes 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 likely to consume poisonous plants if they have nothing else to eat as a result of a combination of drought or poor grazing and a heavy infestation of such plants, or due to their undetected presence in hay or haylage.

Common poisonous plants

Ragwort: Liver damage and death. Found in fields and in contaminated hay

Oak leaves and acorns: Dark diarrhoea and depression. Fields containing or overhung by oak trees. Only poisonous when green

Yew: Rapid death. Found in hedgerows

Foxglove: Diarrhoea and rapid death. Found in hedgerows

Horse or mares tail: Weight loss, weakness and lackor co-ordination. Found in fields

Bracken: Weight loss, weakness and lack or co-ordination. Found in fields and in contaminated hay

Buttercup: Excess salivation, colic and diarrhoea. Intake from contaminated pasture, but not from hay

Hemlock: Salivation, muscle tremors, paralysis and death. Found in hedgerows and around ponds