This common pasture grass is good for grazing but bad for broodmares

If you look out over your pasture and see bunch grass with course-textured, flat, and ribbed-surface leaf blades, you are probably looking at tall fescue. Tall fescue, a productive, well-adapted, and persistent cool-season grass is one of the most abundant and heavily utilized forages in the United States. This grass occupies more than 10% of the U.S. land area—­approximately 37 million acres—with an estimated 700,000 horses grazing or fed tall fescue.

From a historical perspective, says Kyle McLeod, PhD, associate ruminant nutrition professor at the University of Kentucky, in Lexington, tall fescue (Festuca arundinacea Schreb.) was thought to have been introduced into the United States as a contaminant in the meadow fescue seeds imported from England until the late 1880s. It was recognized for its ability to thrive and was cited for its superior growth; tolerance of extreme conditions, disease, and insects; and ability to withstand heavy hoof traffic and grazing. By the 1940s two cultivars were released: Alta and Kentucky-31 (KY-31). Alta was selected for its winter hardiness, persistence, and ability to remain green even during drought conditions. This cultivar was planted throughout the Pacific Northwest and intermountain regions of the western U.S. The Kentucky-31 cultivar was noted for its adaptability to a wide range of soil types and ability to provide grazing throughout much of the year. The southern U.S. experienced a wide planting of KY-31 for forage, soil conservation, and roadside coverage. To date, tall fescue ranges from Florida to Canada.

Because of its nutrient composition and agronomic traits, tall fescue is the forage base of most livestock enterprises, particularly beef cattle. Jim Henning, PhD, extension forage professor at the University of Kentucky, says well-managed fescue produces a high-quality forage with crude protein (CP) and digestible energy (DE) concentrations from vegetative (the period of growth between germination and flowering) to boot (the reproductive stage when the seedhead is enclosed within the sheath of the flag leaf) to mature stages of growth, ranging from 11% to 16% CP and approximately 60 to 68% DE (mature to vegetative).

Tall Fescue’s Side Effects

Despite all its positive traits, tall fescue is not without shortcomings. By the 1950s, says McLeod, fescue had gained a reputation for causing poor performance in livestock—primarily cattle but also small ruminants and horses—­consuming the grass. Cattle often developed a chronic, unthrifty condition, especially during the summer. Some occasionally developed lameness and lost portions of their feet and tails during fall and winter. Other perceived side effects included failure to shed winter coats, and, thus, heat intolerance, and reduced conception rates. Mares on tall fescue appeared to have higher foal mortality and agalactia (absence of milk production).

Scientists began studying the cause of these signs, says McLeod, and by the mid-1970s USDA researchers discovered an endophytic fungus that infects the fescue plant. “Endo” (within) plus “phyte” (plant) means a plant that lives within another plant. In this case the host is the fescue plant, and the toxic endophyte is a fungus identified as Epichloe coenophialum. Two characteristics of the endophyte have great practical importance. First, the organism does not affect either the growth or the appearance of the grass, and it requires a laboratory analysis to detect its presence. Second, it is transmitted solely by seed. So, the endophyte is beneficial to the plant but toxic to grazing livestock.

Krista Lea, MS, horse pasture evaluation program coordinator in the Department of Plant and Soil Sciences at the University of Kentucky, says the toxic endophyte produces several ergot (fungus) alkaloids (nitrogen-containing metabolites of the plant), including ergovaline, ergotamine, ergocristine, and lysergic acid, with ergovaline being the most prevalent (84-97% of the total ergot alkaloids produced). Although all classes of horses can experience endophyte-related issues, pregnant mares develop the most pronounced problems.

Lea says pregnant mares grazing endophyte-infected fescue might carry their foals several weeks past their due date, resulting in dystocia (difficult birth) because of the increased size of the foal. Thickened and/or retained placentas are common for mares grazing endophyte-­infected fescue. Frequently, the foal arrives normally but is encased in a tough and thickened chorioallantois (membrane surrounding the foal in the placenta), which he cannot break through. Consequently, he might suffocate unless someone cuts open the chorioallantois immediately. Premature placental separations (commonly referred to as red bag deliveries) are also common in mares grazing toxic endophyte-infected fescue.

Further, says Bob Coleman, PhD, PAS, associate equine extension professor at the University of Kentucky, mares grazing endophyte-infected fescue produce reduced amounts of or no milk and colostrum (antibody-rich first milk). Colostrum might contain lower concentrations of the antibody immunoglobulin G (IgG) and, in some cases, IgG absorption is lower in foals born to mares grazing toxic endophyte-infected fescue.

Scientists don’t know the exact mechanisms that cause these reproductive problems in mares; however, they do know the ergot alkaloids are agonistic to ­dopamine (D2) receptors, meaning they cause excess dopamine production. Research conducted in 2000 by now-retired Marc Freeman, PhD, and colleagues in the Department of Biological Sciences at Florida State University, in Tallahassee, revealed a significant reduction in circulating and releasable prolactin (a hormone essential to the final stages of birth) from the anterior pituitary gland and agalactia in mares grazing endophyte-infected fescue. Another contributing factor to prolonged gestation is suppression of the hormone progesterone. Progesterone levels should increase about two weeks before parturition (foaling), but mares consuming endophyte-infected fescue have reduced progesterone levels.

Grazing endophyte-infected tall fescue does not appear to have as much impact on other classes of horses, says Coleman. In one study conducted at Auburn University, researchers looked at the growth and development of yearlings grazing endophyte-infected fescue pasture. In another study a team at the University of Georgia considered the growth and development of yearlings eating endophyte-infected tall fescue hay. Neither research group found significant differences on average daily gains or wither heights of horses consuming fescue compared to controls.

A team from Missouri State University evaluated performance horses fed endophyte-infected fescue seed mixed in their grain ration and found no significant impact of the infected fescue on the parameters measured. Lea says researchers at the University of Kentucky have tested ergot alkaloids’ vasoactivity (ability to cause blood vessels to contract or dilate). While they documented vasoconstriction, with ergovaline being the most vasoactive ergot alkaloid, the horses did not appear to be outwardly affected.

Measuring Endophyte Levels

So, what can breeders do from a management perspective to avoid tall fescue toxicity in broodmares? Lea and Coleman agree that pasture management is the ideal option.

The first step is to know for certain the pasture is infected and at what level. For specific sampling procedures, costs, and shipping requirements, contact your local county Cooperative Extension office. However, here are some general sample collection guidelines:

When to sample. Samples must be collected when the plant has been actively growing for at least a month; this provides the best opportunity to find any endophyte present.

Collection. Gather tiller (stem) samples of the plant that are at least 1/8-inch thick. Cut with a razor or sharp knife at the soil surface, avoiding stems that have seedheads. Take at least 10 to 20 random tillers per five acres to get a representative sample of the pasture.

Storage. Place samples with a cold pack in a sturdy, plastic-lined box to take to a county extension office or overnight express to a testing laboratory. Refrigerate samples to ensure sample quality.

Results. The report you receive will indicate the percentage of submitted tillers that were infected with the endophyte. Some U.S. laboratories also determine ergovaline concentrations.

Lea says ergovaline concentrations vary seasonally and closely follow the tall fescue’s cool-season growth curve, with spikes occurring in spring and fall. However, some farms do not test for ergovaline concentrations. Based on data collected over the past 15 years through the University of Kentucky’s Horse Pasture Evaluation program, Lea’s team has developed a relative (ergovaline was not tested or tested outside of the normal months) risk scale to help breeders manage late-term mares’ grazing. Here is an excerpt from the table:

Because the ergovaline produced by the endophyte is what causes problems in broodmares, knowing how much ergovaline the mare is ingesting will provide you with more detailed information to guide your management practices. Researchers have shown that signs of fescue toxicity appear in pregnant mares consuming fescue testing greater than 300 parts per billion (ppb) of ergovaline. However, most extension publications suggest using 200 ppb as the threshold value. The University of Kentucky team has established risk levels for late-term pregnant mares based on ergovaline concentrations in the total diet (see table below).

Most pastures are not 100% tall fescue, says Lea. Pastures contain other grasses and legumes mares prefer to eat, thus diluting the actual amount of ergovaline in the pasture. You can calculate the amount of ergovaline in the pasture based on the estimated percentage of other grasses or legumes in the field, using this formula:

% tall fescue
_________________
(% tall fescue + % grass A + % grass B) x ergovaline (ppb)
=
ergovaline in available forage

From this calculation you can determine your mare’s risk level for grazing a pasture.

Management Strategies

Remove pregnant mares from any pasture (or hay) containing endophyte-infected tall fescue 60 to 90 days pre-foaling, says Coleman. You might move mares to a drylot area where you can meet their nutrient requirements with hay and concentrate or to a pasture with forage species other than endophyte-infected tall fescue. Agronomists and researchers consider this the most conservative way to avoid toxicity problems.

For mares in the moderate to high-risk category, you can administer domperidone, a drug that stimulates normal prolactin and progesterone production, to avoid agalactia and dystocia. Domperidone should be administered daily for 30 days prior to foaling. Grazing and/or mowing the pasture will keep the fescue plants young and in the vegetative state. Coleman stresses the importance of not overgrazing the pasture, because the endophyte is in the basal part of the plant in addition to the seedheads. Don’t let the horses graze the grass below 3 inches, says Lea.

You can also dilute endophyte-infected tall fescue in a pasture by incorporating other grasses and legumes. Because tall fescue is not the most palatable grass, mares will choose not to eat it if something more desirable is available. With this strategy you will also benefit from improved pasture quality and production.

Although an expensive option, the final management strategy might be to kill infected stands and replant. Lea says there are essentially three types of tall fescue:

1. The toxic endophyte, naturally occurring Kentucky-31. This is a very hardy and persistent grass and is the one causing the toxicity problems—it’s the type you are trying to get rid of in your pastures.
2. Endophyte-free tall fescue (e.g., Teton II, Select, Tower, Bronson). This grass does not contain an endophyte, so it is safe for horses to graze. Its downside is it’s not very hardy and cannot tolerate heavy grazing. If you plant this tall fescue, be ready to reseed repeatedly.
3. Novel endophyte tall fescue (e.g., Jesup MaxQ, Tower Protek, Kora Protek, BarOptima PLUS E34, Estancia ArkShield, Martin 2 Protek, Lacefield MaxQII). The endophyte contained within this plant still provides it with hardiness but does not produce the toxic compounds; therefore, horses can graze this fescue safely. Consult your local herbicide dealer to determine the best option for renovating your pasture.

Take-Home Message

Because tall fescue is a popular and well-established grass across pastures in the U.S., eliminating the risk of toxicosis in broodmares is nearly impossible. The only way to avoid fescue toxicosis is to understand the nature of the plants in your pasture. Know when ergovaline levels will be high, and choose the most beneficial management options to reduce the risk to broodmares