Unlike most of the toxic plants we have featured recently, yew is not commonly found in pastures or hay but is rather an ornamental plant. For this reason, yew poisoning is uncommon – but it’s not unheard of. Yew is highly toxic to livestock and even to humans.
There are several species of trees known as yew, all falling into the genus Taxus. Colloquially, “yew” generally refers to common yew or Taxus baccata, a small evergreen tree that grows red berry-like structures. (The “berries” are technically not fruits but rather a fleshy covering over the seed called an aril.) Yew trees have great longevity and can grow for up to 600 years. All parts of the plant are toxic except for the berries. The seeds within the berries are toxic.
Yew contains several different toxic alkaloids which are collectively referred to as taxine. Taxine inhibits the sodium-potassium pumps within cardiac muscle cell membranes, causing abnormal heartbeat and ultimately cardiac failure.
The toxic dose of yew is between 0.10 and 1% of body weight for cattle and 0.05% and 0.2% of body weight for horses. For a 1,000 pound horse, that means just half a pound of yew needles can be fatal. Yew is toxic when fresh and when dried. It becomes more toxic later in the year because toxins build up during the course of the growing season.
Animals that ingest yew usually die within 2 or 3 hours and are often found dead near the yew bushes or clippings they were feeding on. If clinical signs are observed, they may include trembling, difficulty breathing, diarrhea, or convulsions. There is no specific treatment for yew poisoning, although some supportive therapies may be successful depending on the amount of yew consumed.
Yew is commonly used in landscaping because it’s easy to trim and shape. Livestock owners should know how to identify yew and, if yew is growing on the farm, take extra precautions to ensure livestock never have access to it. Yew should never be planted near fencerows, barns, or riding rings. If you have yew growing in these areas, remove it. Horse owners should be particularly cautious to avoid yew when holding or tying horses at shows or when visiting other farms.
Another common use of yew is in Christmas wreaths and decorations made from evergreen clippings. Decorations containing yew should not be hung in barns or on fences where animals might access them. If you do use decorations containing yew, be sure to dispose of them well away from animal enclosures.
Accidental exposure to yew can also occur when a well-meaning but uneducated neighbor dumps yew into a pasture. If you share a fenceline with your neighbor, communicate with them about the dangers of yew and other toxic plants that may be present in their yard clippings. It’s advisable to refrain from dumping, or allowing a neighbor to dump, any waste plant material into animal enclosures.