Pet Columns, Office of Public Engagement, College of Veterinary Medicine at the University of Illinois at Urbana-Champaign

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Lush Green Grass is Pretty,
But Harmful to Horses


Pet Column for the week of May 2, 2005


Office of Public Engagement
2001 S. Lincoln Ave.
Urbana, Illinois 61802
Phone: 217/333-2907
Kim Marie Labak
Information Specialist

Spring is here, and the plants are new and fresh. The pretty, lush grasses that green the countryside in April and May are young, tender, and very tasty for a horse. Unfortunately, these young grasses are also low in the fiber that horses need and are high in the soluble carbohydrates that can upset the sensitive microbiotic balance in a horse's gut.

Dr. James Brendemuehl, equine veterinarian at the University of Illinois Veterinary Teaching Hospital in Urbana, explains that since horses are natural grazers, they need a diet high in fiber such as that found in long-stemmed, mature forage grasses.

Young grasses are higher in soluble carbohydrates, and Dr. Brendemuel explains, "Large amounts of these carbohydrates can overload the gut, disturbing the balance of natural bacteria. As large numbers of natural gut bacteria die, they release a surge of a bacterial toxin called endotoxin."

Endotoxin then gets into the bloodstream, where it can travel to the vessels of the feet and induce laminitis, especially in horses that are predisposed, such as overweight horses, horses with a previous history of laminitis, and horses with Cushing's disease.

To prevent this problem, called "grass founder," Dr. Brendemuehl explains that forage management is the best strategy. "Don't overfertilize, and don't allow grasses to go to seed." Some grasses, such as southern grasses, form seed heads quickly, and horses will graze and pick for these tasty, high-carbohydrate seeds.

If forage management is not feasible, Dr. Brendemuehl suggests keeping horses from young green grasses in April and May, when the grasses are most rich in carbohydrates. It can be difficult to keep horses from eating these grasses, but confining horses to a dry lot is one option. If a horse is let out to a green pasture, a muzzle can keep it from grazing the sweet grasses.

As grasses mature and become more "stemmy," their fiber content increases and their carbohydrate content drops, thus the risk for grass founder decreases as summer approaches.

Another common problem with spring grasses is fescue toxicity. Fescue is a native grass prevalent in the central United States, and many varieties are widely grown as lawn grass and sold in garden stores. Fescue is popular because, according to Dr. Brendemuel, "It's a wonder grass--it's disease resistant, it's drought tolerant, and it resists insects and temperature extremes."

The secret behind fescue's hardiness lies in the fungus that lives inside it. Unlike the fungi that commonly grow on the outside of grains, the fescue fungus grows inside the plant and is passed on through the seeds. This fungus produces an ergot-like alkaloid, which is toxic to many animal species.

In horses, fescue toxicity can cause reproductive problems. Fescue alkaloids can delay a mare's estrus cycle. Also, they prolong pregnancy by blocking the hormones that initiate labor, so a pregnant mare may carry a fetus for up to 14 months instead of the standard 11 months. Foaling after such as prolonged pregnancy can be difficult, since the fetus is larger than normal and the mare doesn't have the hormones to relax the pelvic ligaments for delivery. Additionally, fescue can suppress prolactin, the hormone responsible for milk production.

Fortunately, fescue toxicity is reversible; if a horse is off fescue pasture, she can clear the toxin within matter of days. For mares that have had a prolonged pregnancy, certain pharmaceuticals can help reverse the action of fescue alkaloids, helping to induce labor.

For more information about grass founder or fescue toxicity, contact your local equine veterinarian.