Your horse might be tempted to graze the grass he finds along the trail. Is the grass safe-or toxic? Read on for descriptions of eight toxic varieties of grasses and horsetails. Photo by Kent and Charlene Krone.

You’ve removed toxic landscaping from your barn area. No Japanese yew, apple trees, or red maple. But what about poisonous grasses and horsetails your horse might be tempted to eat on the trail — or even in your pasture?

Here, we describe eight poisonous grasses and horsetails, using information excerpted from Horse Owner’s Field Guide to Toxic Plants, by Sandra M. Burger and the editors of Breakthrough Publications in consultation with Anthony P. Knight, BVSc, MS, MRCVS. Consult this guide for detailed information on more than 100 North American plants that are poisonous to horses. (Book cost is $22.50; to order, contact Breakthrough Publications, 800/276-8419; www.booksonhorses.com).

Keep in mind that prevention is the best cure, say the book’s authors. Whenever you suspect that a plant may be dangerous, it’s wise to “remove the source from the horse if not the horse from the source.” Unfortunately, for many plant poisonings, there’s no treatment. And because all horses are individuals, there’s no way to predict the course or outcome of a poisoning in your particular animal.

Therefore, the following information must not take the place of immediate veterinary attention should you suspect your horse has been poisoned. Rather, it’s meant as a reference to help you identify toxic grasses — so as to prevent plant poisonings — and to recognize symptoms of poisoning in your horse.

The reason to call your veterinarian immediately is that by the time a horse shows signs of poisoning, the condition may be life-threatening. No medications can be given safely until a diagnosis is made. Tranquilizers, pain medication, and other drugs could have harmful effects or even be toxic themselves, depending on the organs already damaged. Intensive care, including intravenous fluids and cardiac monitoring, may be needed.

While you wait for the vet: Put your horse in a quiet, deeply bedded stall with all water, hay, and feed removed; blanket your horse if he’s cold, and keep noise and traffic to a minimum.

 

Arrowgrass Triglochin maritima — Juncaginaceae Family Similar species: Triglochin palustris.

Description: Arrowgrass can be recognized by its short basal stem and clumps of grasslike leaves that are round with a flattened side. Flowers are small and greenish, produced on a tall spike. The fruits, composed of up to six capsules, enable recognition of the plant in a hayfield.

Arrowgrass has prussic acid in its leaves and can have a high cyanide content, depending on the location and conditions under which it’s grown. It’s recorded to be potentially lethal to animals at .5 percent of their body weight.

Geographic distribution: Damp, alkaline soils, shorelines, bogs, and salt marshes throughout the United States and Canada.

Signs of poisoning: Symptoms of arrowgrass poisoning are typical of cyanide poisoning: excitement, rapid respiration, weakened pulse, tachycardia, salivation, voiding of urine and feces, staggering, collapse, bright-red mucous membranes, convulsions, and death.

What to do: Administer IV solution of sodium nitrate and sodium thiosulfate. Although the plant seems to lose some of its toxicity when dried in hay, it should still be avoided.

 

Dallis Grass Paspalum dilatatum — Poaceae, Grass Family Similar species: Rye Grass; Lolium perenne (with Ergot fungus, Claviceps species).

Description: Dallis grass is a common perennial grass that may grow as tall as 40 inches under the right conditions. The blades are flat, coarse, and have pointed ends. Little hairs grow at their base, and dozens of tiny oval seeds grow up the stem.

Rye grass is a coarse green annual or perennial grass with a spiked end, similar to dallis grass, but it grows only to about 25 inches tall. Its seeds, which grow up the side of the plant, are somewhat flatter and less sparse than those of the dallis grass.

Frequently, a parasitic fungus invades the flower heads, producing “honey dew.” Insects are attracted to the secretion and help in transmitting the fungus. This fungus produces lysergic acid derivatives, ergotamine, and ergotoxine, affecting animals that ingest it. The endophytic fungus (Acremonium lolii) that invades rye grass produces a tremorgenic toxin that induces muscle tremors.

Geographic distribution: Open fields with dry, moist, or sandy soils throughout the United States. Rye grass is a frequent choice for planting in yards during winter.

Signs of poisoning: Symptoms include nervousness; trembling; staggering; abortion; convulsions; blood-vessel restriction causing nerve damage in the tail, ears, and other limbs; lameness; and gangrene. These symptoms occur within several days to several weeks of ingestion. Cattle are more commonly involved in dallis or rye grass poisoning, but other animals, including horses, may also be susceptible.

What to do: There’s no treatment except to change the animal’s diet. Always keep dallis and rye grasses mowed in your pasture, and never feed horses grass clippings. It’s also important to avoid overgrazing rye grass, as the fungus exists near the base of the plant and will be eaten if animals are grazing close to the ground.

 

Fescue Festuca species — Poaceae, Grass Family

Description: Chewings fescue, Festuca rubra, grows to three feet tall and has round, wiry leaves that grow in tufts at the base of the plant. Nematode galls are often found in the seed and have produced toxic symptoms in horses after they ingest either the seed or the grass. Coryne toxins are the toxic principle.

Tall fescue, Festuca arundinacea, is a drought-resistant, coarse perennial grass that thrives in wet areas and is often grown for forage. It has a long, flat, ribbed, dark-green blade, and can grow up to four feet. It bears many small flowers on one-foot spikes. The plant contains alkaloids, perloline, and halostachine. An endophyte fungus is known to infect fescue and is important to the development of toxicity to animals grazing the grass.

Geographic distribution: Chewings fescue grows in dry or rocky soils, on lawns throughout the United States. Tall fescue grows in wet areas throughout the United States.

Signs of poisoning: With chewings fescue poisoning, muscle trembling, ataxia, staggering and falling, abortions, and death have all been noted, with degeneration of liver and kidneys in chronic cases.

Tall fescue poisoning occurs after several days to several months of grazing endophyte-infected fescue. Symptoms may vary depending upon the time of the year: In winter, lameness, diarrhea, anorexia, rough haircoat, and possible gangrene of the tail, hooves, and ears may appear. Poor growth rates and weight loss may occur. In summer, animals may have elevated temperatures, and females have little milk for their young. Stillbirths, abortions, prolonged gestations, retained placentas, and infertility are frequent in mares. Foals may have very long hooves if they survive birth due to the prolonged gestation.

What to do: No treatment has been noted. Don’t allow animals to graze where these grasses are grown. Endophyte-free fescue should be used to reseed pastures.

 

Johnson Grass Sorghum halepense — Poaceae, Grass Family Similar species: Columbus Grass; Sorghum; Sorghum species; Sudan grass; Sorghum vulgare.

Description: Johnson grass is a coarse perennial grass with large runners (rhizomes) and topped with clusters of flowers. Sudan grass is an annual, erect plant six to eight feet high with a terminal florescence resembling corn. Sudan grass and its hybrids are often grown as a forage crop for horses and cattle. Animals consuming them in either fresh or dried form may suffer cyanide poisoning.

Both Johnson and Sudan grasses may contain hydrocyanic acid (prussic acid) and sometimes toxic levels of nitrates. Toxicity is highest in young plants and lowest when the plant is yellow, more than two feet tall, and forming fruiting heads. The levels of cyanide increase when the plant is stressed, for example during drought or frost. Leaves have a higher concentration of cyanide than stems. 

Geographic distribution: Open fields and neglected areas throughout the southern United States, and north to Iowa and New York.

Signs of poisoning: Horses may suffer from acute and chronic cyanide poisoning when eating sorghums. Symptoms of acute poisoning include excitement, rapid respiration, weakened pulse, tachycardia, salivation, voiding of urine and feces, staggering, collapse, bright-red mucous membranes, convulsions, and death.

Horses consuming sorghum hay for long periods may develop chronic cyanide poisoning that causes nerve degeneration in the hind legs, urinary tract, bladder, and rectum. Affected horses show weakness and any unsteady gait of the hind legs. They also develop urinary incontinence and an atonic rectum that becomes impacted with feces. Recovery from chronic cyanide poisoning is unlikely, as nerve degeneration is permanent. Pregnant mares may abort or give birth to deformed foals. 

What to do: Treat for cyanide poisoning (IV solution of sodium nitrite and sodium thiosulfate) or nitrate/nitrite poisoning (IV solution with 1 percent methylene blue). Avoid frost-damaged plants for animal forage. If feeding sorghum hay to horses, be sure it’s made from cyanide-free varieties of sorghum.

 

Kleingrass Panicum coloratum — Poaceae, Grass Family Similar species: Panicum vitgarum.

Description: Kleingrass is a perennial with narrow blades and looks similar to coastal grass (the hay of choice in Texas). Look for bunches of small seeds at the tops to help identify it. Kleingrass grows up to four feet tall and bears small spiklets on its tops at maturity.

The toxic principle is believed to be saponin, but it’s not found in the same quantities in all plants. Although the grass has a strange smell, it may be eaten by hungry horses when no other forage is available. Kleingrass hay is also toxic.

Geographic distribution: Texas. Kleingrass originally came from South Africa and was introduced to Texas through Texas A&M University.

Signs of poisoning: Cattle don’t seem to be affected by this grass, but other livestock and horses are very susceptible. Horses that eat kleingrass will develop liver disease and photosensitization around the coronary band.

What to do: No treatment has been noted. Horses should be removed from the kleingrass source, put on good-quality hay, and kept out of the sunlight.

 

Squirreltail Grass Hordeum jubatum — Poaceae, Grass Family Also known as: Foxtail Grass, Wild Barley.

Description: Squirreltail grass grows tall with wiry bristles and a flowering spike with tiny teeth; these teeth can penetrate flesh and hook onto it so it can’t be removed. Horses and livestock may be injured from this plant when grazing or eating poor-quality hay.

Geographic distribution: Throughout the United States and into Canada.

Signs of poisoning: The grass may pierce the skin on the animal’s ears, neck, face, or mouth, causing abscesses, ulcers, possible blindness, and the inability to eat. It may cause colic and impaction in horses.

What to do: Colic treatment or surgery and other treatment may be necessary. Consult your veterinarian. Avoid overgrazing, which allows this undesirable grass to become established.

 

Yellow Bristle Grass Setaria Lutescens — Poaceae, Grass Family Also known as: Foxtail Grass; Pigeon Grass.

Description: Yellow bristle grass doesn’t contain toxins, but it’s a poor forage for animal consumption. It has little spikes and wiry bristles with tiny barbs on the ends that cause mechanical injury to an animal’s oral tissues.

Geographic distribution: Roadsides and range areas throughout the United States and Canada.

Signs of poisoning: While being both chewed and digested, the barbed bristles cause ulcers in the mouth and digestive tract. Horses are especially susceptible to mechanical injury, because they have softer oral mucous membranes than livestock.

What to do: Oral ulcers should be explored for the embedded grass awns. The awns must be removed before healing will take place.

 

Horsetails Equisetum arvense — Equisetaceae, Equisetum Family Also known as: Foxtails; Scouring Rushes.

Description: Horsetails have windswept-looking whorls of thin, grasslike leaves at the ends of the many tall, green, segmented hollow stems, resembling a horse’s tail. They’re reported to be poisonous to all classes of livestock, although horses appear to be most susceptible.

The plant contains toxic aconitic acid, palustrine, and thiaminase. Horses have shown various degrees of poisoning after consumption, and young horses are more likely to succumb than older horses. Toxicity is higher in green plants than in aged plants.

Geographic distribution: Worldwide; in moist fields, roadsides, and drainage areas, frequently in sandy soil, but also in gravel and along waterways.

Signs of poisoning: Hay containing the horsetail plant fed for a period of two weeks has produced symptoms of ill thrift, weakness, and staggering. Sometimes trembling, muscular rigidity, diarrhea, rapid pulse, and cold extremities are also noted. Appetite generally stays the same, and coma precedes death if the animal’s food isn’t changed.

What to do: Specialized blood work can indicate thiamine deficiency, and massive doses of thiamine given early in the course of poisoning are beneficial. All sources of horsetails should be removed from the horse’s diet.