Managing Pastures to Avoid Tall Fescue Toxicity

Anyone who breeds horses knows that endophyte-infested tall fescue can cause problems in pregnant mares. In this article from the University of Kentucky College of Agriculture you will learn more about tall fescue toxicity and how to manage it.

Anyone who breeds or raises horses knows that endophyte-infected tall fescue can cause problems in pregnant mares. In this article from the University of Kentucky College of Agriculture you will learn more about tall fescue toxicity and how to manage it in your horse pastures. Contributing to this article are Ray Smith, PhD, Laura Schwer, and Tom Keene, who are researchers in UK’s Plant and Soil Sciences Department.

Broodmare operations in the Southeastern United States are interested in managing tall fescue toxicity because of the complications it can cause in pregnancies.

The University of Kentucky Horse Pasture Evaluation Program has evaluated more than 13,500 acres of Central Kentucky horse pastures over the past five years. In doing so, researchers involved in the evaluation have learned a tremendous amount about managing tall fescue toxicity. 

Ray Smith, PhD, pasture evaluation program director, said program evaluations have shown tall fescue makes up approximately 20% of the pasture composition on horse farms in Central Kentucky. (The remaining composition is 30% Kentucky bluegrass, 11% orchardgrass, 9% clover, 20% weeds and 10% bare soil.) 

Tall fescue toxicity symptoms in pregnant mares include increased gestation length, agalactia (lack of or reduced milk production), foal and mare mortality, tough and thickened placentas, retained placentas, weak and immature foals, reduced serum prolactin levels and reduced progesterone levels. Other symptoms include abortions, decreased conception, early embryonic mortality and dystocia.

Studies found a general lack of elevated body temperatures, unlike fescue toxicity in cattle, but some studies reported increased sweating in pregnant mares. Since horses possess more sweat glands than cattle, evaporative cooling from sweating more freely regulates body temperature.

Research has shown toxicity symptoms appear in pregnant mares at ergovaline levels greater than 300 parts per billion (ppb). However, most extension publications suggest a more conservative level of 150 to 200 ppb. During the last trimester of pregnancy, scientists generally recommend managers remove mares from endophyte-infected pastures to avoid serious complications. Fortunately, fescue toxicity in other classes of horses (such as geldings and stallions) has been minimal. 

Producers can adopt pasture management practices to reduce tall fescue toxicity complications. Those practices include removing endophyte-infected tall fescue, planting endophyte-free or novel endophyte-infected tall fescue seed, diluting endophyte-infected tall fescue pastures, strategic mowing, and grazing tall fescue containing pastures only in the winter.

Endophyte-free tall fescue pastures will not cause toxicity, but often do not persist well in the southeastern United States. They do show good persistence in cooler temperature locations, like the Pacific Northwest and the Midwest. As a result, researchers have developed novel endophyte varieties that contain non-ergot alkaloid-producing endophytes. The novel endophytes provide stress tolerance and improved plant survival without producing the harmful ergot alkaloids. Novel endophyte varieties such as MaxQ (Pennington Seed, Madison, Georgia) have been researched and patented and are available for purchase. 

Herbicides are also commercially available to remove tall fescue in pastures. For instance, imazapic is a herbicide which kills tall fescue, but does not harm Kentucky bluegrass or orchardgrass. Another option is to dilute concentrations of toxic tall fescue in pastures by overseeding other grasses and legumes. Since horses do not prefer tall fescue, having other grasses available significantly reduces the chances for toxicity.

Ergovaline concentrations are the highest within the seedheads of the endophyte-infected tall fescue. Therefore, strategic mowing of the infected pastures to prevent seed development can reduce the risk of a spike in toxicity levels. The primary toxin in tall fescue, ergovaline, dissipates from the plant after several winter freezes.

Fescue containing pastures are safest during the months of December, January, February and March (Figure 1). On Thoroughbred farms, broodmares are usually in their last trimester during the winter months, therefore the risk for toxicity is much lower. Mares due in April and May should never be on pastures containing tall fescue because the extremely high ergovaline levels result in a high risk for toxicity. Generally, ergovaline drops again in the summer months, but this varies from year to year (Figure 2).

“With budgets tight, several horse farm managers have told me that they’ve reduced the cost of buying straw for bedding by simply harvesting over-mature grass pastures for hay,” Smith said. “On the surface it makes a lot of sense to use this stemmy hay for bedding, but be cautious using this bedding for pregnant mares during the last trimester.”

Smith said it is not uncommon for horses to eat some of their bedding, especially hay (even over-mature hay). If the harvested fields contained significant amounts of tall fescue in the seedhead stage, the bedding will likely contain toxic levels of ergovaline.

Primer on Tall Fescue Toxicity

Since the fungus that causes tall fescue toxicity only lives inside the plant, scientists refer to it as an endophytic fungus. It grows between the plant’s cells and benefits the plant by providing increased drought tolerance, insect resistance, long-term survival and other attributes. The fungus also produces toxic ergot alkaloids, such as ergovaline, which leads to detrimental symptoms in most livestock, especially in pregnant mares during the last trimester. The levels of ergovaline are highest in the stem and seedheads and lowest in the leaves.






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