Most horses won't eat poisonous plants because of the smell and bitterness of the plant, but if there is nothing else to eat out of desperation they will. Some plants will make the horse sick, while others will cause sudden death.
There are some plants that increase in toxicity because of drought or a hard freeze. Keep in mind that some plants will poison your horse over time because of the accumulation of low levels of toxins being deposited into their system which destroys the liver.
If the plant does not kill your horse it could cause other things such as diarrhea, colic, anemia, or neurological problems.
Red maple is a tree that contains green leaves during the spring and summer, but during the fall the leaves turn to a bright red. The bark of the tree is smooth and gray in color with branches that are reddish-brown. It is mostly found in the Eastern United States, but it can be found most everywhere.
The fallen and wilted leaves are what causes the problem it only takes one to two pounds to be fatal. When ingested it takes oxygen out of the bloodstream and signs can appear within a few hours or as long as four to five days later. Signs include loss of appetite, increased respiration, rapid heartbeat, dark red-brown or black urine and pale yellowish gums.
Milkweed is a plant with milky sap and has pods that contain silky, hair-encased seeds. It is found in dry, open areas, in pastures, along the roadside and around woods. Milkweed when ingested causes loss of appetite, drooling, constipation, difficulty in breathing, staggering, weak and rapid pulse, convulsions and most likely result in a coma and death within hours.
Oleander also known as Rose laurel is an evergreen shrub that is used in landscaping. The leaves are thick and leathery and it has large clusters of red, pink or white flowers. This plant is mostly found in hot climates. Leaves that are dry will remain toxic. When ingested this plant disrupts the heartbeat, causing a heart attack. It only takes 30 to 40 leaves for it to be fatal. Signs include difficulty breathing, colic, irregular heart rate and tremors.
Nightshade Family which includes tomatoes, potatoes, horse nettle, bittersweet, black nightshade and some species of ground-cherry. All of these contain solanine and affect the gastrointestinal tract and the central nervous system. The green berries have the highest toxicity. The results are fatal to a horse that ingests between one to ten pounds of the plant. Symptoms include loss of appetite, diarrhea, sudden depression, loss of muscle coordination and convulsions.
Tansy ragwort is a weed with lots of stems that produces clusters of daisy-like yellow flowers. There are 70 species that grow through out the United States. When ingested this plant over time will inhibit cell division in the liver and it is irreversible. Most of the time you will not notice anything until signs of liver failure start to appear. These signs are loss of appetite, weight loss, loss of coordination, jaundice, photo sensitization and depression. Consumption is between 50 to 150 pounds.
Find out what poisonous plants are common in your area then walk your pasture and make sure you don't have those plants in your pasture. Be sure to check along the fence line and also a few feet beyond the fence line since horses will reach through the fence to eat.
Contact your local agricultural extension office for help. Make sure you have good quality forage in the pasture and don't let it get overgrazed otherwise your horse may eat these plants. If you suspect your horse has eaten a poisonous plant contact your veterinarian immediately.