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Spring Grasses and Fescue Cause Problems for Horses - Horses [Skip to Content]
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Spring Grasses and Fescue Cause Problems for Horses
by Dr. Jim Brendemuehl, Equine Extension Veterinarian, Univ. of Illinois College of Vet Med


The lush green grasses of spring 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 microbial balance in a horse's gut.

Horses are natural grazers, needing a diet high in fiber such as that found in long-stemmed, mature forage grasses. Young grasses are higher in soluble carbohydrates, and large amounts of these carbohydrates can overload the gut, disturbing the normal bacterial flora. With this high-carb intake there can be a die off of normal bacteria, which causes a release of a natural toxin called endotoxin.

Endotoxin is absorbed into the bloodstream and circulates throughout the body where it affects various blood vessels. In the horse, the vessels of the feet are particularly sensitive to these toxins, which cause profound vasoconstriction. Clinically this condition is referred to as laminitis, or inflammation of the sensitive lamina of the foot. While all horses are susceptible to laminitis from carbohydrate overloads, such as eating excessive amounts of corn or sweet feed, some individuals, such as overweight horses, horses with a previous history of laminitis, and horses with Cushing's disease are predisposed to "grass founder."

Proper forage management is the best strategy to prevent grass founder in susceptible individuals. Excessive fertilizer increases soluble carbohydrate levels in grasses and should be avoided. Also, allowing grasses to go to seed, which are especially tasty, concentrates the carbs in the seeds, which horses preferentially consume. The key is adequate supplementation of pastures with timely mowing to prevent excessive seed head formation.

If forage management is not feasible, limiting grazing of young lush grass in April and May, when the grasses are most rich in carbohydrates is advisable. While requiring additional time and effort, restricting the amount of time the horse is allowed on pasture is one option. This requires maintaining the horse in a stall or dry lot with controlled turn out on pasture. If a horse must remain on green pasture, a muzzle can limit the amount of grass consumed. 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. Fescue is quite widespread because it is disease resistant, 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 induce labor, so a pregnant mare may carry a foal for up to 13 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 induce labor and relax the birth canal muscles.

Fortunately, fescue toxicity is rapidly reversible. If the mare is removed completely from fescue pasture, she can clear the toxin within matter of days. In situations where the mares cannot be removed from the fescue pasture or where they are experiencing a prolonged pregnancy, certain pharmaceuticals are available that can help reverse the action of fescue alkaloids, allowing normal delivery.

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