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Evaluating and Managing the Risk of Tall Fescue Toxicosis to Broodmares

Tall fescue grass can contain toxins that cause reproductive problems in late-term broodmares. Learn about the risks of grazing pregnant mares on tall fescue and what you can do to reduce the risk of health problems.

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Updated:

June 25, 2024

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Photo credit: Laura Kenny

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Certain varieties of tall fescue (a cool-season grass) are infected with a fungal organism called an endophyte. This type of fescue is known as E+ tall fescue. Compounds produced by the endophyte make the tall fescue extremely hardy, giving it excellent resistance to drought, pests, and grazing. However, these endophytes also produce toxins called ergot alkaloids. The most prevalent ergot alkaloid in tall fescue, ergovaline, is known to cause reproductive problems in late-term pregnant mares, known as fescue toxicosis. Signs include prolonged gestation, dystocia, thickened/retained placenta, poor or no milk production, red bag deliveries, and dysmature foals. It may also affect conception and early pregnancy including mare cycling, pregnancy rates, and early embryonic death. It affects ruminants in different ways, but this article will focus on equines.

Research is not conclusive on how much E+ tall fescue affects performance horses. Some studies have shown no effect on performance and recovery, and others have shown minor changes in heart rate and respiratory rate after exercise. At this time, there are no widespread recommendations to avoid E+ tall fescue for performance horses.

Pennsylvania is not part of the "fescue belt," an area in the upper Southeast and Midwest United States. In that area, E+ tall fescue is extremely common due to environmental pressures (hotter weather), and fescue toxicosis is a common concern. While the problem is not as common here in PA, it can occur in any location containing E+ tall fescue.

Although broodmare fescue toxicosis may not be a common issue in Pennsylvania, any farm owner who plans on breeding horses and grazing them on pasture should know whether their fields contain large amounts of tall fescue. It can become a problem even after years of successful foaling if mares consume enough of it. This article will describe how to evaluate and manage the risk of tall fescue toxicosis in broodmares.

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Part 1: Endophytes and Ergovaline in Pasture and Hay

Endophytes in Pasture

The endophytes in tall fescue are completely contained within the plant; they cannot spread from plant to plant. They are concentrated at the base of the stem and in the seed heads. You cannot tell visually whether a plant is infected. Tall fescue is not always a palatable grass species for horses, so they may avoid it if there is a good mix of other forage plants in the pasture. However, if pastures are overgrazed or there are no other forages available, horses may be forced to graze the tall fescue.

Figure 1. During the heat and drought of summer, tall fescue may be the only green grass available to graze. Photo credit: Laura Kenny

There is a naturally occurring variety of E+ tall fescue called Kentucky 31 (no relation to Kentucky bluegrass) that grows throughout Pennsylvania. It was discovered in 1931 and widely planted in the 1940s before the negative effects of endophytes and the toxins they produce were noticed. Now, it grows naturally and can outcompete other grasses due to its hardy attributes. If your pastures contain tall fescue and you did not plant it, chances are good that it's Kentucky 31. Even if you did plant tall fescue, many of the commercially available varieties in seed mixes are still E+.

There are two major considerations when determining the risk level of a pasture that has tall fescue:

The infection rate: what proportion of the pasture forage is tall fescue and how many of those tall fescue plants are E+?
The concentration of ergovaline in the E+ plants.

Laboratories can analyze both of these items, discussed in detail later in this article.

Factors affecting ergovaline levels in pasture

The level of ergovaline in E+ tall fescue is not consistent. It fluctuates with the seasons and from year to year. During winter in Pennsylvania, ergovaline levels are lowest. When the grasses start to green up in the spring, ergovaline levels will begin to rise. They are highest in late spring and early summer, particularly in the seed heads. Levels can remain high through the fall; ergovaline concentrations rise during times of active grass growth. Nitrogen fertilization can also increase ergovaline levels in tall fescue.

Ergovaline content varies by location in the tall fescue plant. Ergovaline levels are highest in the seedheads and the lowest 3 inches of stem.

Endophytes in Hay

Since ergovaline is highest in the tall fescue seed heads, tall fescue hay harvested after heading may contain high levels of ergovaline. However, ergovaline levels can drop by 30 to 60% soon after harvest, so the levels in hay will be lower than the levels in fresh forage. Ammoniation of hay further reduces ergovaline levels. However, these levels could still be over the safe threshold for broodmares consuming pure tall fescue hay exclusively. While ergovaline levels are known to be lower in fescue hay compared to fresh pasture, the hay should still be considered in cases of fescue toxicosis.

Part 2: Evaluating Risk Level Through Fescue Analysis

Sampling and Choosing an Analysis

The first step is to determine how much of the pasture in question is composed of tall fescue. It is crucial to be able to accurately identify tall fescue. It is easier to identify grasses when they are in the reproductive stage with seed heads visible (see Figure 2). Farm managers can contact their local Extension office or another agricultural professional to determine the percentage of tall fescue in a pasture.

Figure 2: Tall fescue can be identified from several attributes. It grows in a clump or bunch. The seed heads first appear as a tightly packed, spiky collection of seeds. Later in its growth, the seed heads spread out as they flower. The leaves are rigid with jagged edges and deep veins in the upper surface. Photos by Laura Kenny

Once tall fescue is confirmed to be present, managers can choose from two types of laboratory analysis. Samples should be collected in the summer/fall when endophyte levels are highest.

Endophyte analysis. Send a certain number of tall fescue tillers (stems) to the lab and they will determine what percentage of the tillers are infected with endophytes.
Ergot alkaloid analysis. Send a sample of tall fescue pasture (similar to what a grazing horse would eat). The lab will analyze the concentration of ergovaline and perhaps other ergot alkaloids in that sample, typically reported in parts per billion (ppb).

Each lab has its own specific instructions for how to sample the pasture. In general, it requires cutting a specific number of tall fescue plants randomly throughout the pasture. Make sure to call the laboratory before you begin sampling to confirm the availability of the test, the fees, and ensure that you sample correctly and send the sample in the appropriate manner. Some laboratories require the samples to be mailed on ice, and some do not. Make sure to use the same lab each time you test if you wish to compare results over time.

Many laboratories can also analyze hay samples for ergovaline concentration. For a list of some laboratories that can perform this testing, see Tall Fescue: Potential Problem for Pregnant Broodmares.

Interpreting the Results

Percent tall fescue: This result can be estimated without sending samples to a laboratory. The harmful effects of ergovaline can be diluted if late-term broodmares are grazing other plants in the pasture. Mares may not graze the tall fescue at all if there are plenty of other options. Equine researchers at the University of Kentucky have suggested that if a pasture is less than 10% tall fescue, it is considered low-risk for reproductive problems. At 10-25% tall fescue, risk is slightly higher, especially when seed heads are present and during the last 60-90 days of gestation. At 25-50% tall fescue, risk becomes significant, and at 50-100%, risk becomes high.

Endophyte analysis: This test tells you what percent of tall fescue samples are infected. Combine this percentage with the proportion of tall fescue in the field to calculate how much of the field is E+ tall fescue. For example, if a field is 50% tall fescue and 30% of the tillers are infected, then 15% of the field is E+ (0.50 * 0.30 = 0.15). This gives you an idea of the potential for ergovaline accumulation in that field.

Ergovaline levels: Results will indicate ergovaline levels (generally in parts per billion), but this result is only part of the risk factor. The true risk is based on the amount of this toxin in the horse's total diet. Therefore, it will be necessary to calculate what proportion of the horse’s diet is composed of tall fescue before interpreting the numbers. For a horse on pasture only, this can be calculated by multiplying the percent E+ tall fescue with the ppb ergovaline from the lab (example: 15% E+ tall fescue * 300 ppb ergovaline = 45 ppb ergvaline in the diet). For a horse that also consumes hay and grain, those feeds must also be included in the calculation for ergovaline in the total diet, as they will dilute the effect of the toxin.

There is currently no scientifically established threshold for a safe level of ergovaline in a late-term broodmare's diet. Some sources state that no amount is safe. However, an equine research team at the University of Kentucky concluded that an ergovaline level of less than 200 ppb in the total diet is low-risk for pregnant mares. Moderate risk for late-term pregnant mares is 201-500 ppb, and 500+ ppb is unsafe for these mares.

Keep in mind that ergovaline levels in tall fescue fluctuate over time; therefore this test is a snapshot of the current state of the pasture. If a manager is testing to diagnose foaling problems, the ergovaline levels at the time of testing may not reflect what the levels were 30 days prior to foaling. If a manager is testing to preemptively analyze the risk of a pasture, it is recommended to test during the late spring or early summer when levels are highest. Alternatively, test pastures during the time of year when late-term mares will be grazing there.

Part 3: Managing Risk of Fescue Toxicosis

Options for High-Risk Pastures

For farms that graze late-term mares on high-risk pastures, there are several ways to reduce the risk of fescue toxicosis, depending on your individual situation. Confer with your veterinarian when deciding which to choose.

Remove pregnant mares from the E+ pastures 30 to 90 days before foaling, when ergovaline causes the most harm. This requires an alternate turnout option with low E+ tall fescue risk. This option can be as simple as rearranging the farm’s turnout plan so that pregnant mares are in a different pasture during late pregnancy.
Eliminate tall fescue from the pastures. This is a very difficult task since E+ tall fescue is so hardy. It takes careful planning and time—potentially years. It may be warranted when all fields are highly infected with E+ tall fescue and there are no alternate turnouts. Work with your local Extension agent to develop a plan that works for your operation.
If you absolutely cannot remove mares or reestablish the pasture, you can carefully manage the pastures.
1. Improve the pasture by overseeding with other forage species. Mares should preferentially graze the other species, which would dilute the total amount of ergovaline in their diet. Feeding high-quality hay can also help to dilute the ergovaline in the diet. Pasture improvement requires soil testing, weed control, overseeding, and a long rest to establish new seedlings. See Basic Pasture Management for the Equine Owner and Pasture Seeding Timeline for more information on improving pastures.
2. Strategic mowing and grazing. Ergovaline levels are highest in the seed heads and lower stem of the tall fescue plant. Graze before seedheads appear. Mow the seed heads off tall fescue plants before grazing broodmares and do not allow horses to graze pastures below 3 inches in height.
3. Suppression of seedheads with pasture herbicides. There are some herbicide ingredients, such as metsulfuron and imazipic, which can target tall fescue but not Kentucky bluegrass and orchardgrass. When applied before heading, these herbicides can suppress seedhead development in fescue. Work with a local Extension agent to discuss this option and only use herbicides labeled for use on forages.

Figure 3. Setting your mower high can remove seed heads without sacrificing too much nutritious leaf area. Photo credit: Laura Kenny

Work with your veterinarian to determine the best course of action for your situation. A drug called domperidone can be prescribed to mares on E+ tall fescue pastures for 10-30 days before foaling, which effectively prevents the signs of fescue toxicosis in a majority of mares.

Selecting Grass Seed

Endophyte-free and novel-endophyte tall fescue

Plant breeders have developed varieties of tall fescue that are safe for broodmares. Endophyte-free (E-) tall fescue has no endophytes at all, which, unfortunately, makes the grass less grazing tolerant than E+ varieties. Novel-endophyte tall fescue varieties have a different endophyte strain which produces the plant-protective compounds but not the toxic compounds such as ergovaline. These varieties are economically important in southern states with hotter climates where tall fescue, a cool-season grass, may not survive without endophytes.

However, despite the safety of novel-endophyte tall fescue, it may be wise to choose other grasses such as orchardgrass and brome when choosing pasture seed on a Pennsylvania breeding farm. Why? Simply put, if any E+ tall fescue reemerges in the pasture, managers will not be able to tell the difference between the E+ and E- or novel tall fescue.

E+ tall fescue can reappear in a newly seeded pasture if seed is reintroduced through contaminated seed mixes, hay fed in pastures, mowing equipment, spread from nearby fields or fencelines, or even through manure. Additionally, the original stand may not have been fully eliminated during the renovation or there may be tall fescue seeds from the original stand buried in the soil. Since it is impossible to tell the difference between E+ and E- or novel tall fescue visually, managers will not be able to differentiate between the new variety and the E+ variety. By avoiding tall fescue altogether, if a manager identifies tall fescue in a broodmare pasture, it becomes an immediate red flag.

Related grasses: Meadow fescue, perennial ryegrass, and festulolium

Meadow fescue is a common grass included in Pennsylvania seed mixes. It is related to tall fescue and may contain endophytes. However, these endophytes only produce the plant-protective compounds and not the harmful ergot alkaloids.

Perennial ryegrass can also be infected with endophytes which produce toxins including lolitrem B and ergovaline. The ergovaline concentration tends to be much lower in E+ perennial ryegrass, but can still exceed the 200ppb threshold for mares. High levels of lolitrem B can cause a condition called ryegrass staggers in horses, which is rarely seen in the US but common in Australia and New Zealand.

Festulolium is a hybrid of tall or meadow fescue and perennial or annual ryegrass. It combines the hardiness traits of tall fescue with the higher palatability and nutritive value of ryegrass. Its physical appearance varies based on the cross- some look like ryegrass and some look like fescue, so identification can be difficult. It is available with and without endophytes, but none of the varieties appear to produce ergovaline.

Take-Home Messages

While Pennsylvania is not located in the fescue belt, fescue toxicosis is still a possibility in late-term broodmares consuming large quantities of E+ tall fescue.
Ergovaline levels in E+ tall fescue are not consistent throughout the year or between years. However, ergovaline levels are always highest in seed heads and the lower 3" of stem.
Ergovaline levels decrease 30-60% within 30 days after E+ tall fescue is cut for hay.
Useful tests to determine risk include percent tall fescue in the pasture, percent infected tall fescue, and ergovaline levels in tall fescue. Both fresh pasture and hay can be tested.
Remove broodmares from high-risk pastures 90 days before foaling. If this is not possible, work with your veterinarian to use preventative drugs and manage pastures to reduce risk.
Eliminating E+ tall fescue from pastures is a big project that can take a year or more.
Even though novel-endophyte tall fescue is safe for broodmares, consider planting other forage grasses in late-term broodmare pastures to make it easier to spot E+ tall fescue that reappears.

Reviewed by Dr. Jennine Lection, DVM