In many parts of the world horse pastures contain a fair percentage of fescue, a hardy perennial grass that thrives despite heavy hoof traffic, intense grazing, and adverse growing conditions. Unfortunately, there's a drawback: 75% of all fescue is infected with the endophyte Acremonium coenophialum.

An endophyte is a fungus that grows inside another plant without detriment to the host plant. In some instances the fungus actually benefits the host plant. Such is the case with A. coenophialum. The fungus produces alkaloids, which help protect the plant against some natural hindrances such as insects and nematodes (roundworms). Because of their increased durability, the plants become more tolerant to marginal soils and suboptimal growing conditions.

Endophyte-infected tall fescue causes legions of problems, collectively called fescue toxicosis, in broodmares, including prolonged gestation (as long as 13 to 14 months), foaling difficulties, thickened placentas (including red bag deliveries, where the placenta detaches and comes ahead of the foal), and a decrease or complete absence of milk upon delivery. The ill effects of tall fescue consumption can continue beyond foaling, as affected mares might be hard to get back in foal, leaving breeders with a smaller foal crop the following year.

Removing mares from endophyte-infected tall fescue 90 days before foaling has been an effective management technique. However, grazing in not the only way mares can come in contact with fescue, as hay and bedding might also contain the forage