Tall fescue is one of the most widely grown pasture grasses in the U.S., over a million acres grown in North Georgia alone. A bunch grass, it is the most heat tolerant of all the cool season grasses. However, that heat tolerance comes with a price, an ergot alkaloid producing endophyte that can have negative effects on certain animals.

The toxic endophyte (E+) in tall fescue is Epicloe coenophialum, (formerly Neotyphodium coenophialum and Acremonium coenophialum) and it grows within the plant in a symbiotic relationship. The plant benefits from increased drought and heat tolerance, improved seed germination and
production, seedling vigor, tiller growth rate, improved mineral uptake, and insect and disease resistance. In return, the E+ receives nutrients, protection within the plant and dissemination through the seed.

These challenges to animal health are commonly referred to as “fescue toxicosis.” When compared to cattle, broodmares are more sensitive to the alkaloids in tall fescue infected with E+. Horses can exhibit prolonged gestation, increased incidence of foal and mare mortality from dystocia (difficulty foaling), thickened placentas that tend to separate from the uterine wall prematurely (aka red bagging), agalactia (lack of lactation) in post-partum mares and less vigorous and immune-challenged foals.

There are a variety of methods to deal with the problem of fescue toxicosis in horses. One is to eliminate the toxic tall fescue by killing the stand and replacing it with an alternative forage. Endophyte-free tall fescue cultivars are available but don’t always persist well in the south, nor do they have the drought and pest resistance of the toxic tall fescue varieties. Another alternative is to establish winter annual grazing or a novel endophyte-infected tall fescue. Novel endophyte tall fescues contain a naturally occurring endophyte that does not produce the toxic alkaloids. Novel endophyte tall fescues have all the positive agronomic aspects of the toxic tall fescue (persistence, drought tolerance, and pest resistance). For more on novel endophyte tall fescues, see the Extension bulletin entitled, “Novel Endophyte Infected Tall Fescue”. However, destroying and replacing a toxic tall fescue pasture or hayfield is inherently risky and quite expensive. The significant cost premium for the seed of the novel endophyte tall fescue varieties makes the renovation even more expensive.

Another option would be to manage the toxicosis by removing late gestation mares from infected tall fescue pastures the last trimester. Veterinarians recommend removal of mares from infected tall fescue pastures 45 to 90 days prior to the expected foaling date. This will reduce the risk but does not necessarily eliminate it completely. Since the toxins are sometimes stored in fat cells, some mares can show signs of fescue toxicosis even with long-term removal from infected tall fescue. A third possible method would be to dilute the toxins with either concentrates or other forage types to reduce the ingestion of E+. While there are no beneficial effects of feeding grain to gravid mares grazing fescue, yearling horses can have slightly higher average daily gains. Most tall fescue pastures and hayfields are infected with the E+ that produces toxic alkaloids. These alkaloids are highly toxic to horses, resulting in prolonged gestation, dystocia, agalactia, thickened placentas, and the increased risk of mortality of both mare and foal. Horses are much more sensitive to E+ fescue than other livestock. However, the toxic effects of the endophyte can be successfully managed by eliminating the grazing or feeding of toxic tall fescue.

Brenda Jackson, Murray County Extension Coordinator and Agriculture & Natural Resource Agent