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Equines and Toxic Weeds


Horses are exposed to toxic plants and weeds in many ways. Some plants horses will avoid when grazing, but in hay and grass clippings horses will eat them. Some plants are so toxic that the horse can not recover. To avoid unseen toxic plants in pastures try to not overgraze your pastures.


When the grass gets short the horse tends to eat more weeds. Many weeds grow better when the pasture is stressed or sparse. If your pastures get sparse, rotate them especially in dry weather, and reseed. Avoid swamps and woods for pastures. Walk your pastures and look for weeds and cut weeds around the fence line. Remove branches blown down from storms.


The plants that will kill a horse out-right and can not be eaten in any amount include: Japanese Yew, water hemlock, ground hemlock, foxglove, oleander, and lily of the valley. Less than 1 once can kill an 1100# horse.


Horses do not mind their taste and the toxic property slows conduction of the heart and they collapse and die. Milk weeds also contain cardiac glycosides and should not be fed to horses. Water hemlock has a delicate white umbrella flower and salivation, excitation, and colic may precede death. Rhododendron and box wood are highly toxic plants common in our yards.  Larkspur, Monkshood, and Avocado leaves must also be avoided.


There are several varieties of nightshade in the U.S. of which the berries are the most toxic part. Symptoms include progressive central nervous system deterioration with drowsiness, weakness, paralysis, and colic.  


Less toxic but still important toxic plants include rhubarb, cherries, onions, apples, plum, and tomatoes. In large quantities these can cause colic. Many fruits concentrate cyanide in their seeds and leaves. Cyanide causes respiratory difficulty and other problems.  Bracken fern can get into the hay and produce thiamine deficiency. Acute poisonings will cause fatal internal hemorrhages. Low levels will cause weakness, and incoordination.


Cocklebur is a common weed. Horses will eat the younger seedling plants. Poor quality hay can contain this and cause liver damage, muscle weakness, colic, and potentially convulsions, and can be fatal. Poison sumac and oak should also be watched for.


Hounds tongue, groundsel, rattle box, heliotropun, and fiddle neck contain pyrrolizidine alkaloids. These plants cause chronic progressive damage to the liver. Symptoms include depression, anorexia, jaundice, diarrhea, and photosensitivity. These horses look like they have sunburn. Buckwheat and St. John’s wort also can cause primary photosensitizations.  Horse tail, Lupines, mountain laurel, Azaleas, Jimsonweed, morning glory, pigweed, blue green algae, yellow star thistle, Russian knapweed, white snake root, and rayless goldenrod are all toxic and must be avoided.


Fescue is a common forage grass. It is prone to a fungus or endophyte that can prolong gestations, cause abortions, thickened placentas, and agalactia (lack of milk) in pregnant mares. You can plant endophyte free fescue grass. Always ask about this when buying hay.  In wet humid conditions, or under tree canopies clover may cause liver damage with secondary photosensitivity. This may be due to a mold growing on the clover. Another fungal toxin on alfalfa and clover is Rhizoctonia leguminicola called slaframine that causes profuse salivation. In high contamination weight loss, diarrhea, and abortion may occur.


Oak trees are common in our area. The toxin in these trees is tannins. Tannins cause damage to the lining of the intestinal tract and can produce bloody diarrhea, constipation, laminitis, and colic. They can also damage the liver and kidneys. The Spring buds and Fall acorns concentrate the tannins. Red maple trees can cause lethal hemolytic anemia and horses show difficulty breathing, jaundice, cyanosis, and brownish red urine. Horses deteriorate very rapidly and may die in 12-48 hours. Leaves in the Fall that get into the hay and storms that knock down leaves lead to poisonings. Turning horses out into woods with new maple growth can be very dangerous. Black walnut trees are a problem when they get into the shavings we use as bedding. Contact with the feet can cause acute laminitis, inflammation, and edema of the legs. Be very careful that it is not in your shavings.


Hoary Alyssum is a ubiquitous weed in Minnesota. This light green plant with small white flowers and at later stages small green seeds on the stems is toxic in pastures and hay. Swelling of the legs is the most common sign, but fever, lameness, colic, and diarrhea can occur.


The Astragalus genus such as Milk Vetches produces neurotoxins that act on the respiratory and central nervous system causing incoordination and weakness.  Grasses such as foxtail, Seteria, wheat, and rye have awns that can become embedded in the cheek and tongue creating ulcers and salivation. Keep burdock and sandburs out of the tail, forelock, and mane to avoid eye injuries.


Late cuttings of hay can contain Blister beetles. These insects have a toxin called cantheridae that is quite toxic to horses. If adding supplements, such as selenium, do not exceed 5ppm of total diet.  Chronic high levels can cause foot problems. 25ppm selenium can cause acute toxicity with pulmonary edema and sudden death.


Remember, avoiding toxic plants in our pastures and hay is the most important aspect of dealing with them. Treatment is often too late to save them.


Jeff S. Johnson DVM