Endophyte-Infected Tall Fescue and its Effect on Broodmares

Photo: Anne M. Eberhardt/The Horse

Tall fescue is a ubiquitous forage grass that became popular because of its drought tolerance, hardiness, and good forage performance. However, in the 1970s scientists discovered a high percentage of tall fescue harbors an endophyte that produces a toxic alkaloid with the ability to cause disorders in ruminants and horses that consume large amounts.

Garry Lacefield, PhD, UK plant and soil sciences extension forage specialist refers to the Mid-South region, which includes Kentucky, as the "tall fescue belt" due to tall fescue quantity in the area.

Tall fescue covers an estimated 35 million acres in the United States, including pastures, high-traffic foot paths, golf courses, and backyards. A majority is endophyte-infected (EI). An endophyte is a fungus that lives symbiotically within the host plant and is not visible to the eye. The endophyte produces toxic alkaloids that, when eaten, cause different disorders including tall fescue toxicosis.

Tall fescue toxicosis causes problems in broodmares during the last third of gestation. Thus, understanding and managing tall fescue is important, particularly as the foaling season approaches.

According to "Understanding Entophyte-Infected Tall Fescue and Its Effect on Broodmares," tall fescue toxicity in broodmares can manifest as follows:

  • Poor animal performance (rough coat, low grade fever)
  • Longer pregnancy (as long as 13-14 months)
  • Agalactia (poor milk let-down)
  • Dystocia (difficulty foaling)
  • Thickened placenta ("red bag" foal)
  • Reduced breeding efficiency following parturition (difficulty in rebreeding after foaling)
  • Foals born weak or dead

If you suspect toxicosis, consult with your veterinarian immediately.

"It's especially important for anyone with broodmares to find out if there is tall fescue in their fields," said Bob Coleman, PhD, University of Kentucky (UK) horse extension specialist. "It's easy to assume you have it, but you truly need to know. And think about all sources of it--grass bedding and hay can be full of tall fescue."

The endophyte in tall fescue is only transmitted by seed, so regular mowing before seed heads develop is a way to limit spread of EI tall fescue. Scientists have developed novel endophyte-free tall fescues, but EI tall fescue is still prevalent. Eradication is another option.

"Over the years we have learned how to manage it, but the concern is still there," said Bill Witt, PhD, a professor in UK's department of plant and soil sciences. Witt has been involved in research and eradication programs for EI tall fescue on some of Kentucky's major Thoroughbred farms. His research results show that fall is the best time to remove tall fescue from pastures.

"Killing tall fescue with herbicide in September or October and then overseeding with bluegrass will result in fewer weed-prone areas," Witt said. (See the UK publication "Establishing Horse Pastures" for more information.)

Another way to limit toxicity is to plant legumes and other grasses, a strategy that dilutes EI fescue's presence by limiting its ability to spread and by increasing the number of alternative grasses and legumes for animals to eat. "Horses won't eat tall fescue first," Lacefield said.

According to Lacefield, adding bluegrass, ryegrass, and clover can be effective means of EI fescue dilution. However, broodmares should be removed from EI-infected pastures toward the end of gestation, as dilution of the toxic feed source is not effective at this point.

UK's Horse Pasture Evaluation Program, a fee-based service conducted by forage experts, is a way for Kentucky horse owners to understand the composition of their fields and what forage is available for their horses. The program aims to analyze pasture management practices, reduce feed cost with improved pasture, and assess tall fescue toxicity risks for broodmares. The service provides a comprehensive evaluation of up to six paddocks or 80 acres on a farm and includes:

  • An assessment of pasture species composition;
  • The percentage of tall fescue plants infected with fungal endophyte;
  • The concentration of ergovaline (toxin) in tall fescue at the date of sampling;
  • An estimate of ergovaline intake for horses on the pasture; and
  • Additional management tools such as parasite management, soil maps, satellite maps, carrying capacity, and an action log for record-keeping.

"Tall fescue, like any plant, has its advantages and disadvantages," Lacefield said. "The key for broodmare owners is to understand the disadvantages well in advance of the start of foaling season."

Additional Resources:

  • "Tall Fescue Endophyte Concepts," by DM Ball, GD Lacefield, CS Loveland, SP Schmidt, and WC Young III. 2003. Oregon Tall Fescue Commission Special Publication 1-03.
  • "Understanding Endophyte-Infected Tall Fescue and Its Effect on Broodmares," by Robert J. Coleman, Jimmy C. Henning, Laurie M. Lawrence, and Garry D. Lacefield, University of Kentucky Cooperative Extension Service.
  • "Tall Fescue Toxicity for Horses: Literature Review and Kentucky's Successful Pasture Evaluation Program," by S. Ray Smith, Laura Schwer, and Thomas C. Keene, University of Kentucky Plant and Soil Sciences.
  • University of Kentucky Horse Pasture Evaluation Program

Karin Pekarchik is an editorial officer in UK's Agricultural Communications Services.

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