Our horses are surrounded by toxic plants. The good news is they tend to avoid them, provided there is enough decent food around to eat. If the pickings are slim, or they aren’t careful, though, horses may consume some bad plants. So, it’s best to be aware of which plants are poisonous and remove them from pastures and hay.
The plants I’ll go over here are ones that tend to be more common in Canada, though there are some excellent guides online that include others. Many of the severely poisonous plants are ornamental, such as azaleas, rhododendron and Japanese Yew, and are less likely to be in horse pastures, so they won’t be discussed here.
Hemlock (notably water hemlock, or poison hemlock) is one of the most common plants that causes sudden death. Plants are commonly found near water, such as ponds, streams or swampy areas. The toxins in water hemlock (cicutoxin and cicutol) and poison hemlock (alkaloids such as coniine) are found mostly in the roots or seeds of these plants respectively. Consuming about 1kg of these plants can kill a horse, and while seizures and respiratory failure may be observed, most owners simply find their horses dead.