Toxic forages such as fescues are the most common causes of poisoning in horses. Toxins are produced in tall fescue forage or in the seeds contaminated with an endophytic fungus. With the arrival of spring and horses being turned out to pasture, horse owners should be proactive in making sure their pastures are safe.
Tall fescue grass
Toxic forages such as fescues are the most common causes of poisoning in horses.
© 2018 by EquiMed
The alkaloid-producing fungus proliferates most during the lush stages of growth during springtime and also when autumn rains follow a dry summer and closely associates with the growing flower and seed head. It is through contamination of the seed that infection by fescue toxins are spread.
The consequences of fescue toxicity in horses are almost exclusively related to poor reproduction. Mares that consume large amounts of endophyte-infested tall fescue can suffer from a number of reproductive problems, and the range of fetal deaths in the last third of pregnancy for herds affected by toxicity varies from 0 to 100%.
Although most horses can eat fescue pasture grass without any problems, horse owners should not allow pregnant mares to graze in pastures that contain fescue. An endophyte in fescue affects a number of hormones in the mare's system and has the overall effect of decreasing milk production, prolonging pregnancy and/or causing a thickened placenta that may result in a difficult birth, an abortion or a stillborn foal.
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 agronomist, collected tall fescue seed from the Suiter farm in Menifee County, KY, 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.
Ergovaline is the most abundant ergot alkaloid in tall fescue. Ergovaline, and several other alkaoids 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 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, birth of dysmature foals, and increased foal and placental weights.
Although the effects of ergot alkaloids on a dopaminergic receptor are often the focus of discussion, 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 in the mare.
Horse owners who are replanting pastures or purchasing hay for their horses should be aware of the Alliance for Grassland Renewal which is an association of seed companies, universities, and government agencies that regulate themselves by establishing certain quality control standards for novel endophyte tall fescues.
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.