There is a fungus within the fescue plant that is capable of producing a variety of ergot alkaloid type toxins, such as ergovaline. These substances are referred to as endophytes and are worse in some pastures and some years than others. There is also a genetic difference in animal responses to the alkaloids. The compounds cause a constriction of the blood flow to the outer portions of the body, which results in heat stress. The endophyte could be compared to a two-edged sword. Its presence gives the fescue plant tolerance to a variety of stressors which enables it to withstand harsh management. Without the endophyte, the stand of fescue can die out after a few years. Recently a different endophyte has been introduced into fescue. It doesn't produce the toxins, and livestock perform much better on it.

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