For some time now pregnant mares have had an increasing frequency of foaling problems when fed tall fescue hay or pasture their last trimester of pregnancy and has been labelled fescue toxicity. The problems seen have been abortion, prolonged gestation, agalactia (failure to produce milk), and weak newborn foals. The fungus Acremonimum coenophialum, which grows inside the tissues of fescue grass, was identified as the culprit and produces a toxin that effects the normal hormone levels of the mare. This fungus is called an "endophyte" because it lives in the grass tissues and therefore can not be seen. Most fescue is infected but to varying degrees. This article deals with prevention and treatment of this growing problem including pasture management, the use of endophyte free fescues, prevention and treatment of lactation failure.

The fungus Acremonimum coenophialum grows within the tissues of tall fescue that release toxins that cause the diseases associated with its grazing by pregnant mares. Because the endophyte confers some amount of resistance to insects and drought it easily establishes itself in stands of fescue and spreads through both tillers and seeding though plant to plant transmission is limited.

Acremonimum coenophialum produces both loline and ergopeptine classes of alkaloids. The primary ergopeptine in fescue is ergovaline though also present is ergosine, ergonine, and lysergic acid amine (structurally similar to LSD). These toxins interfere with the production of prolactin in mares grazing the infected fescue. This interference causes problems with lactation in the mare and normal development of the fetus.

The lolines are saturated pyrrolizidine alkaloids with mild vasoconstrictive properties. These alkaloids can constrict blood vessels, reduce blood flow to the extremities, and may be responsible for an increase in body temperature.