tall fescue

Tall fescue (Lolium arundinaceum) is one of the most widely grown perennial grasses in the world and covers approximately 37 million acres in the United States alone. It can be infected with an endophytic fungus (Epichloë coenophiala), which in a symbiotic relationship with the plant produces chemicals called alkaloids that confer benefits to the plant.

This tall fescue, native to Europe, was introduced into the United States in the 1800s. In 1931, E.N. Fergus, a University of Kentucky (UK) agronomist, collected tall fescue seed from the Suiter farm in Menifee County, Kentucky, on the basis of winter hardiness, persistence in high-traffic areas, and drought resistance, giving rise to the cultivar of fescue known as Kentucky 31 (KY31). However, some of the alkaloids, primarily the ergot alkaloids produced by infected plants, are detrimental to grazing animals, including horses.

Historically, the endocrine hallmark of fescue toxicosis in several animal species is a decrease in the circulating concentration of the hormone prolactin. Prolactin is secreted by the pituitary gland, and control of its secretion is complex and not completely understood. Prolactin exerts effects on a variety of systems including milk production, steroidogenesis (estrogens, progesterone, and testosterone), hair growth and shedding, libido, and synthesis of surfactant by the fetal lungs. Importantly, prolactin may also exert an effect on the fetoplacental unit by altering steroid synthesis and/or metabolism and maturation of the fetal adrenal-pituitary axis, which is necessary for parturition. One major regulator of prolactin secretion is dopamine, a hormone produced by the hypothalamus. Dopamine interacts with receptors in the pituitary gland and inhibits the secretion of prolactin.

Ergovaline is the most abundant ergot alkaloid in tall fescue. Ergovaline, and several other alkaloids from fescue, have similar chemical structures to dopamine and can bind to dopamine receptors, thereby causing a decrease in prolactin secretion, resulting in partial or complete agalactia (the inability to produce milk) in foaling mares. Additional problems associated with KY31 fescue consumption in foaling mares include altered hormone concentrations, extended gestation, thickened placenta, placental retention, dystocia (difficult birth), birth of dysmature foals, and increased foal and placental weights.

Dopamine receptors have been found in tissues other than the pituitary, including ovarian tissues and the corpus luteum, but the roles of those receptors in fescue toxicosis, if any, have not been fully elucidated. The drug domperidone is frequently used in broodmares that are exposed to KY31 fescue and prevents or reverses the adverse reactions of ergovaline. Domperidone functions by binding to dopamine receptors, but rather than suppressing prolactin production, it competes with dopamine and allows for normal prolactin secretion.

Because of the adverse health effects of common endophyte infected fescue in grazing animals, varieties of tall fescue which do not contain the fungal endophyte have been identified. Even though these endophyte-free varieties do not produce ergot alkaloids, animal performance is excellent. However, the plants do not persist well in pastures or compete well with other pasture grasses. More recently, endophyte strains that do not produce the alkaloids that are harmful to animals but still confer vigor and persistence to the plant, have been identified and inserted into tall fescue. These are called novel-endophyte varieties of fescue, and some of these are commercially marketed as “Jesup Max Q,” “Texoma Max QII,” and “Baroptima Plus E34.” More recently, “Lacefield Max QII” was released by Tim Phillips, PhD, in the UK Department of Plant and Soil Sciences.

The Alliance for Grassland Renewal is an association of seed companies, universities, and government agencies that regulate themselves by establishing certain quality control standards for novel endophyte tall fescues. For example, all seeds sold under the alliance tag must be 95% pure, have 70% live (viable) endophyte, and have independent confirmation that the fescue variety does not cause fescue toxicosis in animals and will persist well under conventional grazing conditions.

Although this article emphasizes the effects of ergot alkaloids on a dopaminergic receptor, it is important to remember that some of the alkaloids also bind to other receptor types, including adrenergic and serotonergic receptors, and thus may affect additional body systems.

 

CONTACTS:
  • Karen McDowell, PhD—kmcd@uky.edu—859/218-1104—UK Maxwell H. Gluck Equine Research Center, Lexington
  • Tim Phillips, PhD—UK Department of Plant and Soil Sciences, Lexington

This is an excerpt from Equine Disease Quarterly, funded by underwriters at Lloyd’s, London.