Tall fescue is a hardy perennial grass that grows in a variety of soils and climates. Considering that tall fescue is resistant to drought and overgrazing, a high-quality nutrient source, and a high-yield crop, it is no wonder horse owners would want to have their horses graze these pastures. Unfortunately, not all fescue is good fescue.

Many tall fescue fields are infected with a microscopic endophyte (a fungus that lives inside a plant) that produces a cornucopia of alkaloids, which are naturally occurring nitrogen-containing organic compounds. When consumed in high enough amounts, many alkaloids have deleterious physiological effects. Thus, “fescue toxicosis” does not refer to grazing too much tall fescue per se. Instead, the toxicosis refers to grazing tall fescue that is infected with the alkaloid-producing endophytic fungus.

The problematic alkaloids that the fungus produces render tall fescue insect-resistant; however, the alkaloids also have negative effects on a variety of livestock, most notably pregnant mares.

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