1. Common poisonous ornamental plants
2. Very harmful pasture weeds
3. Less dangerous pasture weeds
The leaves of these plants are highly toxic. (Incidently, so is the nectar from the flowers - making honey made from these flowers toxic as well.) As little as 3 ml nectar/kg body weight or 0.2% of the body weight of the leaves may be toxic or lethal. For a 1000 lb horse, that's less than 1 lb of leaves. For a 300 lb mini, that's less than 1/3 of a pound of leaves. So, it doesn't take long for a "munching" horse to take in a toxic dose. Signs of toxicity include: salivation and a burning sensation in the mouth, (vomiting in ruminants), colic, diarrhea, muscular weakness and impaired vision. A slow heart rate, low blood pressure, and arrhythmia are potential cardiac effects. Difficulty breathing, depression, and lying prostrate can be seen. Death can occur within 1-2 days. Treatment for ingestion includes giving activated charcoal (to absorb the toxin) and supportive care. If you suspect your horse, donkey, sheep or goat has gotten into this plant, call your veterinarian immediately.
breathing, and collapse are sometimes seen in mild cases. Death is often attributed to severe cardiac arrhythmia.
Again, if you have concerns that your horse may have eaten this plant, you should call your veterinarian immediately. The best bet is to not have this shrub planted anywhere near where herbivores are kept.
The leaves of the red maple are toxic to horses, and the toxic principle is most concentrated when they are wilted. The toxin slowly degrades as the leaves dry. This is why it is important to check your fields after storms to make sure no branches have fallen into the pasture. Fresh leaves are apparently not toxic (so if your horse takes a bite of one on a trail ride, don't panic!) A horse must ingest 0.3% of their body weight in wilted leaves for toxic levels to be reached - that translates to 3 lbs for a 1000lb horse.
There is no antidote to red maple toxicity - the only treatment is supportive care, including blood transfusions and intravenous fluid therapy.
The best prevention is to remove red maples from horse properties, especially from the paddocks. Fencing around these trees is often not sufficient, since the wind will blow leaves or branches onto grazing areas, allowing horses to ingest them.
Stay tuned for the next installment of our poison plant series next week!
For more information on poisonous plants, check out Texas A&M Extension's list of sites.