Tall Fescue Testing: Understanding the Numbers

Property managers often test their pastures to evaluate the risk of fescue toxicity; however, how you test can produce drastically different results.

Photo: University of Kentucky's College of Agriculture, Food, and Environment

Most horse breeders are aware of endophyte-infected tall fescue's toxic effects on late-term pregnant mares. Prolonged gestation; difficulty foaling; thick, retained placentas; and depressed milk production are a few of the more common clinical signs seen in mares grazing infected tall fescue.

Property managers often test their pastures to evaluate the risk of fescue toxicity; however, testing isn’t all that simple, and how you test can produce drastically different results. Here are some things to remember when testing horse pastures for infected tall fescue.

Understanding Tall Fescue Infection

A few key concepts about infected tall fescue:

  • Tall fescue itself is actually nutritious forage that is not harmful to horses.
  • Plants are either infected with an endophyte or they are not; there is no in between. An infected plant will always be infected, and a non-infected plant will always be non-infected. Infected and non-infected plants can be found next to each other and their status will not change.
  • Non-infected plants will always produce non-infected seed; however, infected plants can produce infected or non-infected seed. Plants that grow from infected seed are also infected.
  • You cannot determine if a plant is infected or non-infected by looking at it in the field, and neither can your horse, cow, sheep, or neighbor.
  • The endophyte itself is not harmful, but it produces many compounds, some of which are toxic to livestock.
  • The compounds that horse owners are concerned with are called ergot alkaloids. There are many ergot alkaloids, but the most prevalent is ergovaline (representing 84-97% of all ergot alkaloids present). Therefore, this is the one that most laboratories focus on.
  • Ergovaline can “transform” into its isomer--or chemically identical with a different structure, ergovalinine and back again in a process known as isomerization when in solution in the laboratory.
  • New novel endophyte tall fescue varieties have been developed that produce little or no ergovaline.

Endophyte Infection

As previously stated, a plant's infection status will not change. However, the percentage of infected plants across a pasture can change over time. Pastures that have not been managed to reduce infected tall fescue will generally show 75% or more infection rates (meaning 75% of the plants are infected and 25% are not) in areas where infected tall fescue is common, such as the southeastern United States.

Most laboratories that perform endophyte testing request that property managers submit 30-60 individual tillers or grass shoots from a pasture. Each tiller will be tested using immunoblot and will be determined to be infected or not. Your results will be expressed as a percentage of tillers submitted that are infected with the endophyte. But we must consider what else is in the pasture (and what isn’t) to interpret these numbers.

For example, if pasture No. 1 contained 80% desirable forages (such as bluegrass and orchardgrass) and only 10% tall fescue that was 100% infected, we would call this a low-risk pasture because horses will likely be consuming mostly bluegrass. However, if pasture No. 2 is mostly bare, with only 10% desirable forages and 10 % tall fescue that is 100% infected, as much as half of the horses’ diets contain infected tall fescue. That would be considered a high-risk pasture. Finally, pasture No. 3 contains 70% desirable forages and 20% tall fescue, but only half of the tall fescue is infected. In that instance only about 10% of the total forage in the pasture is infected, so we would call this a medium- to low-risk pasture.

Pasture Desirable Forages (%) Tall Fescue (%) Endophyte infection rate (%) Weeds and Bare Soil (%) Overall risk to mares
1 80 10 100 10 Low
2 10 10 100 80 High
3 70 20 50 10 Medium

Here are a few things to remember when sampling and testing pasture for endophyte presence:

  • Test results are only as good as the samples taken. Select tillers at random from throughout the field, not just in one location. For details, check out “Sampling for Tall Fescue Endophyte in Pasture or Hay Stands.”
  • Keep tillers cool--preferably in a cooler or freezer--and transport to the testing facility as soon as possible.
  • Endophyte testing should be performed when the plant is growing vigorously. Testing in the winter or early spring can result in false low endophyte infection results and should be avoided.
  • Be sure you are sampling tall fescue only and that you are cutting at the soil surface, as most of the endophyte is found at the base of the plant. For assistance in plant identification, see the “Forage Identification and Use Guide.”

Ergot Alkaloid Analysis

Sample analysis for ergot alkaloids can be complex and confusing. However, these results are often used to make cost-effective management decisions; therefore, it is important to make sure they are done correctly, from sampling to interpretation. While a pasture's endophyte status will change very little over time, ergot alkaloids will change daily depending on many variables. Thus, it is important to consider the alkaloid concentrations a snapshot of what is happening in the pasture at a specific time. Keep in mind that alkaloid production follows plant growth, and levels are expected to be highest in the spring and fall when tall fescue is growing most vigorously. Ergot alkaloid concentrations are generally very low in winter.

Similar to endophyte testing, take tall fescue samples from throughout the field. But rather than cutting at the soil surface, clip to get a representative sample of what your horse will be eating. Collect material at the grazing height of animals--generally 2-3 inches above the soil surface--and keep samples on ice. Researchers have indicated that sample handling is crucial. Ideally, you should take samples to the laboratory the day of cutting; if this is not possible, store the sample in a freezer until it can be transported. If testing dry stored hay, these samples can be left at room temperature. Many ergot alkaloids have been shown to be stable in hay after the curing process but very volatile in fresh tall fescue plants. For more information on sampling pastures, see “Tall Fescue Sampling for Ergovaline.”

Why Results Don’t Always Match

  • Test offered Some laboratories test for ergovaline only while others test for a number of other ergot alkaloids. Tall fescue infected with the endophyte Neotyphodium coenophialum will contain almost exclusively ergovaline and ergovalinine. However, other fungi, such as Claviceps spp., can infect many grains and grasses, including tall fescue, and produce other ergot alkaloids such as ergosine, ergotamine, ergocornine, and others. Each alkaloid, including ergovaline, affects animals to a different degree and scientists have observed an additive affect when multiple alkaloids are present. For those farms located in a region where other fungal infections are not common (such as Central Kentucky), ergovaline analysis is adequate; farms located in regions where other fungal infections are common (such as the northern great plains) should seek labs that perform a full alkaloid panel, which will likely cost more. Testing facilities generally offer the test that is most useful for their area, so begin by contacting a laboratory near you.
  • Sample handling How samples are handled in the lab is just as important as how they are handled in the field. Fresh plant samples are the most sensitive to heat, air, and time. Ask each lab how it stores samples to minimize effects. Best results are always obtained with less storage time.
  • Analysis method There is no standard analysis method that each lab uses. Method details will vary from lab to lab and technician to technician performing the analysis. Unfortunately, there no real way to avoid this, so accurate interpretation is key in this instance (see point 5).
  • Reporting results This is where many people can get frustrated. Each lab will report results in slightly different ways. Some report ergovaline only, while others report ergovaline and ergovalinine. Some report all ergot alkaloids individually or summed. Results can be reported in many different units such as parts per million (ppm), parts per billion (ppb), micrograms per kilogram (ug/kg), or nanograms per gram (ng/g). Samples can be analyzed on a dry matter or as fed basis. If the report is not clear, ask the lab exactly what it is reporting.
  • Interpretation Now that you have an answer, what does it mean? That depends on who you ask. Find a lab you are comfortable working with, and send samples exclusively to them. This will reduce inter-lab differences. Additionally, each lab will have different threshold values to compare to determine if the level in your pasture or hay puts your horses at risk. Differences in threshold values across labs exist for the same reasons that analysis results differ. Compare your analysis results only to threshold values published by the testing laboratory. If you have additional questions, those too should be directed to the testing facility that did the work, because only they are familiar with how your samples were handled, analyzed, and reported.

Dilution Effect

Keep in mind that ergot alkaloid results are only part of the final equation. Cattle farmers in fescue-prone areas have long lived by the adage that “the solution to the pollution is dilution.” They will often plant a mixture of grasses and clover in pastures or feed other forage supplements to dilute the effects of tall fescue toxicity. The same principle can be applied to horses. Maintaining a good mix of desirable forages in your pasture will greatly dilute tall fescue toxicity.


Tall fescue toxicity is a complex issue on breeding farms, and hard, fast answers are not always realistic. Work with a reputable, accredited laboratory in your area to perform the most beneficial tests for your situation. Discuss results with the laboratory toxicologist and your veterinarian to understand the meaning and limits of these values. Finally, keep up with new tall fescue toxicity research. Hundreds of scientists across the world are working to better understand this complex challenge, all with the goal of giving you the most accurate, consistent, and concise information possible.

Testing Laboratories

The following is a list of a few laboratories that offer endophyte and/or ergot alkaloid analysis:

Agrinostics Ltd. Co.
P.O. Box 882
Watkinsville, GA 30677

Iowa State University Veterinary Diagnostic Laboratory
1600 South 16th
Ames, IA 50011

Oregon State University Endophyte Service Laboratory
139 Oak Creek Building
Corvallis, OR 97331

University of Missouri Veterinary Medical Diagnostic Laboratory
P.O. Box 6023
Columbia, MO 65205-6023

University of Kentucky Regulatory Services
103 Regulatory Services Building
Lexington, KY 40546-0275

University of Kentucky Veterinary Diagnostic Laboratory
1490 Bull Lea Rd.
Lexington, KY 40511

Krista Lea, MS, UK’s Department of Plant and Soil Sciences; Cynthia Gaskill, PhD, DVM, UK Veterinary Diagnostic Laboratory; and Ray Smith, PhD, UK Department of Plant and Soil Sciences provided this information.

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