Hidden poisons in your paddock

Sycamore Trees: There have been increasing cases of ‘Atypical myopathy’ in horses which is caused by the ingestion of hypoglycin A in sycamore seedlings. This toxin is present in varying quantities in the leaves of the sycamore family. The quantity of seedling ingested is not directly proportional to the potential for disease. As well as this some horses appear to be affected by this toxin, while others are not. There is also a lot of evidence that if a horse has grazed on pasture with sycamore trees in the past, this does not mean the horse will not be affected in the future. Although this disease is commonly seen in the autumn, cases do also occur in the spring so therefore check your boundaries and trees and we would advise removing the horse from these areas or fencing off the areas around the trees.

Ragwort: Also known as Senecio jacobaea. This plant contains pyrrolizidine alkaloid. When metabolised this toxins affect the liver causing a range of clinical signs. Generally a large quantity of ragwort needs to be consumed over a long period of time to cause problems. However the liver has a large reserve capacity meaning that sometimes by the time clinical signs are evident the degree of liver damage is great. We therefore strongly advise all ragwort to be cleared from pasture. It is also important that ragwort is not present in hay or haylage cuts.

Foxglove (Digitalis): This plant is poisonous to ruminants and less so to horses but it is still important to be aware. Cases seen in horses tend to be associated with hay contamination. We would advise not to graze horses in proximity to foxglove and to be aware of this plant when making hay.

Rhododendrons: These plants are not usually eaten by horses but in the presence of little else to eat they will nibble of the leaves which are toxic and can cause gastrointestinal and severe cardiac problems.

Yew: These contain a toxin called taxine. All parts of this tree are poisonous however large quantities do need to be ingested to cause clinical signs.

Oak: There are reports that the tannins and gallotanins of oak trees are toxic. These exist in the buds in the spring and the acorns in the winter. Large quantities need to be ingested to cause clinical disease however it is still important to be aware of this tree in grazing areas.

Buttercups and cowslips: These plants contain a compound called protoanemonin. This causes milder clinical signs on contact. Therefore lesions can be seen around the mouth. Horses tend not to ingest these plants as they cause immediate irritation.

Bracken ferns: These contain thiaminase which is a toxin that affects the nervous system. Generally very large quantities of the plant need to be ingested to cause clinical disease. It is important to be aware that fern is not included in any conserved forage as the plant can still be toxic.

Nightshades: This group of plants is very unpalatable so are rarely ingested. However if they are eaten they can cause disease.

Potatoes: These are toxic to horses when they are green and sprouting if they are eaten as 50% of the diet. Obviously this is very rare but it is useful to be aware that horse should not be turned out on a potato field

St Johns Wort: This plant is much more toxic to livestock but horses can also be affected. Generally ingestion of this plant causes photosensitisation with lesions developing on non-pigmented skin.