Tall fescue is the most predominant grass in the U.S., covering some 36 million acres of pasture. It is a robust grass resistant to heavy grazing, drought, heat and cold, properties endowed by a synergistic relationship with endophytic fungi, Epichloë coenophiala.

This endophytic fungus, which grows within the plant, produces ergopeptine alkaloid compounds that endow the plant with natural resistance to damaging pests, heavy grazing, repeated mowing and drought, and heat and cold tolerance. When ergopeptine alkaloids are being produced in high quantities, the term “toxic tall fescue’” is used, and people might describe the pasture as “hot.” Because tall fescue that is heavily endowed with endophyte alkaloids is much more resistant to overgrazing, dry or hot conditions, it outcompetes other grazing grasses, such as bluegrass and orchardgrass, during their growing seasons.

Additionally, tall fescue can continue to grow well after warm season grasses become dormant. For example, during a mild November and December, tall fescue will continue to grow whereas bluegrass and orchardgrass will become dormant.

In mares, we recognize that ingestion of even relatively low levels of endophyte alkaloids during late gestation leads to a host of serious issues, such as premature placental separation at birth (red bag delivery, hard-to-break placental tissue) leading to fetal asphyxia at birth, poorly muscled foals, prolonged gestation, dystocia, and agalactia.

Levels above 200 parts per million (ppm) total daily intake have been a traditional benchmark for alarm for pregnant mare pastures, however, this is not an absolute as it varies depending on the analytical method, individual sensitivity and the specific ergopeptine alkaloid structures measured in the lab. This serves to highlight how little data we possess on these substances, including their effects during all stages of gestation on fetal development and of exposure to moderate levels during breeding.

Nonpregnant mares exposed to sufficient concentrations of ergopeptine alkaloids may not respond normally to seasonal photoperiod changes, leading to long spring transitional periods. These mares can experience irregular or prolonged estrus without ovulation. Increased incidence of early embryonic loss has also been reported following experimental ergopeptine alkaloid exposure. It has been reported that young stock are affected by high levels of ergopeptine alkaloids, but literature is sparce about these problems.

The limited grasp on these biological interactions is due to a multitude of factors, including that horses are not always considered an agricultural species and thus many funding sources are unavailable; there are no validated blood, urine or tissue tests to detect ergopeptine alkaloids in horses; and testing pastures have hampered research into the effects of ergopeptine alkaloid exposure in horses at any age and gestational status.

Traditionally, pasture testing has indicated that that the highest levels of ergovaline are usually in April and May. However, that does not appear to exclusively be the case anymore. Weather patterns over the last several years have supported vibrant tall fescue growth as late as November and December, during which our team has persistently identified alarmingly high levels of ergovaline in pastures. This is of tremendous importance since our traditional view was that these were ‘safe’ months for grazing pregnant mares on tall fescue pastures.

Additionally, there is a long-held view that a few very cold consecutive days and nights (between 20°F and 40°F) would stop ergovaline synthesis and lead to a rapid decline in concentrations in tested samples. However, limited anecdotal information found that after a period of more than two weeks of snow and ice cover on the ground in January 2026, ergovaline levels remained high in a pasture tested immediately after snow and ice melted.

At the University of Kentucky (UK), in collaboration with our equine industry, we are striving to address the issues and myths relating to tall fescue. Our efforts include:

• Continuing to provide accessible education on tall fescue, how to test, mitigation strategies and pasture management and refurbishment assistance. 
• Ensuring that people know that endophyte-free tall fescue is not a robust enough grass to withstand harsh conditions and is readily outcompeted just a few years after being sown.
• Novel endophyte tall fescue is a grass proven to be safe for equine stock as it is incapable of producing toxic alkaloids but is a resilient grass that persists under harsh conditions. Novel endophyte pastures do not produce toxic ergopeptine alkaloids. However, it looks identical to other tall fescue and genetic testing is required to differentiate it from endophyte infected tall fescue.
• Pasture ergovaline analysis is not just about ergovaline levels but also reflects how much other herbage is present and growing to dilute it, especially during specific months.
• Constant vigilance is needed year-round with frequent testing recommended.
• Even in pastures with a small percentage of tall fescue, levels ingested can be high if that is the only grass growing at that time, e.g. warm winter months.

The UK effort includes scientists in the Department of Veterinary Science’s Gluck Equine Research Center and Veterinary Diagnostic Lab, the UK Forage Extension Program and the U.S. Department of Agriculture Forage Animal Production Research Unit. An initial goal is to try and generate a validated method to detect ergovaline ingestion in mares as a basis for further research. We have also formed a small group of interested academic and industry partners to confidentially assess local real-time data on pasture ergovaline burdens and possible fescue-related farm problems so that we may provide better and more agile service to our community.

This is an excerpt from Equine Disease Quarterly, Vol. 35, Issue 3, funded by underwriters at Lloyd’s, London, brokers, and their Kentucky agents. It was written by Emma Adam, BVetMed, PhD, Dipl. ACIM, ACVS, of the University of Kentucky’s Gluck Equine Research Center, in Lexington, Megan Romano, DVM, Dipl. ABVT, toxicology section head at the University of Kentucky’s College of Food, Agriculture, and Environment, in Lexington, and Krista Lea, MS, agricultural extension specialist, also at the College of Food, Agriculture, and Environment.