Oleander---(Nerium oleander)-- Extremely poisonous, even humans have been poisoned when using twigs as skewers. A single leaf is fatal to a human, 40-60 will kill a horse. Usually they won`t eat this, but often it is in trimmings, and is eaten along with everything else. Burning isn`t recommended either, the smoke can also be bad.
Bracken Fern--(Pteridium aquilinum)--Poisoning is usually in the fall, when nothing else is left in the pasture. Whether green or dry, all parts are poisonous. Most will not eat it, but some become accustomed to it, and will seek it out. If it is in hay, and makes up to 20% of it or more, you will see symptoms in a month. He must eat it from 30-60 days to see real symptoms. It causes thyamine deficiency and central nervous system imbalances.
Horsetail--(Equisetum species)--All species are poisonous, and can cause poisoning when large amounts of it are cut in meadow hay.
Castor Bean--(Ricinus communis)--The source of castor oil is toxic. The oil is not toxic because the toxin is not soluble in oil. The seed is poisonous, the rest of the plant is relatively non-toxic. It should neverbe around animals, as many will be curious and nibble. Even humans can bepoisoned. As little as 1/5 ounce of seeds can kill a horse. More commonly,about 150 beans will take out a 1000# horse. Feed grains can be contaminated with them as well. It causes severe irritation and inflammation to the intestinal tract.
Red Maple Leaf-- Bark, wilted leaves will cause severe and rapid hemolytic anemia. Signs come 3-4 days afterward.
Yellow Star Thistle--(Centaurea solstitialis) and Russian Knapweed--(closely related species)--They are distantly related to the sunflower family. It takes 30-60 to show up, but it hits instantly. When the horse can`t eat and is unable to also drink, call a vet ASAP. Caught early, and aggressively treated, they may live.
Lupine--There are many species that are non-toxic, but it is very difficult to tell them apart from the very toxic ones. The toxic lupine is poisonous at all stages, but worst as young plants and at seed stage. Once again, if there are quantities in hay, the poisoning can be slow. Will kick the liver, and cause gastrointestinal problems.
Locoweed--(Astraglus & Oxytropis)--There are 300 hundred species, only about 20 are poisonous. AS with lupine, even a botanist would have a hard time telling the good from the bad. They are poisonous at all stages, at all times of the year, even after being cut and dried. All parts are toxic. Once again, signs come in at around 6 weeks.
Timber Milk Vetch--(Astraglus)--Poisoning comes quickly in 2-7 days, and it causes a roaring sound, which is advent to death from affixiation.
Some plants that cause selenium poisoning are--
Nicotine -- (Solanaceae)--
Potato family plants are potentially poisonous, as well as wild tobaccos--(Nicotiana).
Cirrhosis of the Liver can be caused by the following plants--
Fiddleneck, Tarweed, Fireweed, Yellow Burr Weed, Buckthorn--(Amsinckia), Ragwort, Groundsel, Stinking Willie--(Senecio), (Senecio jacobaea), Rattle Weed, Rattle Box, Wild Pea--(Crotalaria spectabilis),(Crotalaria sagittalis)--Related to the pea family.
Ground Ivy--(Glechoma hederaceae)--Always a problem no matter what stage, even dried and cured.
Poison Hemlock--(Cituta)--All parts are poisonous, especially from spring to time it flowers. Horrible for horses, humans, cows, and sheep. 4-5 # fresh leaves have killed horses. Signs occur within hours of ingestion.
Western Water Hemlock--Sometimes confused with poison hemlock, it is the most poisonous plant growing in the US. It may also be called poison parsnip,snakeroot,false parsley,. Tiny amount of the toxic alcohol is needed to poison. It is wetland plant. Horses show signs within 15 minutes to 6 hours, and usually die within an hour of shown symptoms.
Forage Poisoning--Caused by damaged corn and moldy corn. Acts like sleeping sickness.
Larkspur--(Delphinium)--Garden delphiniums are just as bad. 2nd only to locoweed in livestock death.
Marijuana--(Cannabis)--Death occurs within 15-30 minutes of ingestion. Hemp does grow wild, so beware.
Sorghum & Sudan Grass Toxicity-- Show 2 clear kinds of poisoning, prussic acid (cyanide), and the infection of the urinary bladder that travels up to the kidneys. It is not seen with hay made from these plants, but beware of it in pasture. Johnson Weed has also done this.
Bermuda Grass Tremors--Ergot is a fungus that grows in certain grasses, such as this. It is also seen on rye and dallis grass. More common in late summer, fall, and winter.
Chokecherry--(Prunus)--The leaves cause cyanide poisoning, and are more toxic when wilted, dry, frozen, etc. Death can occur within minutes to hours after ingestion.
Other Misc. Poisonous Plants--
Common garden flowers like Iris, Foxglove.
Black Locust-bark and all.
Overripe Crimson Clover
Large Quantities of Apples, Chives, Onions, Wild Onions can cause toxicity and colic.
Mechanical Injurious Plants--
Longbearded grasses, Barley, Foxtail, Cheat grass(Bromus tectorum), Yellow Bristle grass(Setaria glauca, S. lutescens). These all can/will cause ulcers and sores within the mouth and throat.
There are more, but this is the main list.