All parts of the Yew Tree are extremely poisonous, with the ability to stop the heart abruptly, resulting in sudden death. An exception to this is the flesh of the berry* of the yew tree, which is sweet and non-poisonous, despite its warning bright red colour. The seed coating is also poisonous.
Examples of the toxins are the alkaloid: Taxine and the cardiac glycoside: Taxiphyllin.
The Yew is a popular ornamental tree and is a common subject for topiary, resulting in abundant small clippings, which must be disposed of very carefully.
The reason a special page is devoted to this subject is the special danger that yew presents. Horses and other herbivores are unlikely to eat the shoots of the living tree, unless desperate for browsing and roughage (fibre). However, lawn or grass cuttings, from areas in which yew trees grow (e.g. gardens and churchyards) may contain small pieces of yew, which are still deadly dangerous. Grass cuttings are often thrown over fences for grazing animals, by gardeners or by well-meaning neighbours. This is the most common means of yew poisoning in horses and farm animals.
* technically not a berry but an ‘aril'.