Fescue Grass Toxicity in Broodmares

Fescue Grass Toxicity in Broodmares

Photo: Anne M. Eberhardt

The spring rains have helped pastures with fescue grass turn green. But during foaling season, these lush fescue pastures can create problems for pregnant mares.

Here, David Anderson, DVM, MS, Dipl. ACVS, head of Large Animal Clinical Sciences at University of Tennessee (UT) College of Veterinary Medicine, and Carla Sommardahl, DVM, PhD, Dipl. ACVIM, clinical associate professor and section head of Large Animal Medicine at UT talk about fescue grass toxicity and management options.

Tall fescue is a grass which grows on over 35 million acres of land in the United States. As many as 700,000 horses graze fescue pastures or are fed fescue hay each year. Many of these pastures contain fescue that is infected with an endophytic fungus that is toxic to horses. When the horse ingests the grass or hay made from these pastures, it is steadily ingesting alkaloids produced by the fungus.

Owners might not realize is that there are some significant health risks associated with broodmares and young horses eating endophyte fungus-infected (EI) tall fescue pasture or hay. Both the mare and the foal can be affected by these toxic alkaloids produced by the fungus. Some health problems associated with fescue toxicity include:

  • Prolonged gestation in mares;
  • Dystocia (foaling difficulty);
  • Decreased to no colostrums or milk production; and
  • Decreased growth rates in weanlings and yearlings in some studies, if forage is not supplemented with grain.

Some of these problems can be minimized with careful management of horses and pastures. Follow these management tips from the American Association of Equine Practitioners to reduce the risks of health problems caused by EI tall fescue:

  • Have your pasture tested to determine the level of infection;
  • Mow fields prior to the development of seed heads, which contain the highest levels of toxins in the plant;
  • Remove horses from EI fescue pastures in conditions of extreme heat and drought;
  • Remove broodmares from EI fescue pastures 30 days prior to breeding and a minimum of 60 to 90 days prior to foaling;
  • Keep accurate records of breeding and anticipated foaling dates;
  • Notify your veterinarian for initiation of drug therapy if your mare has been grazing EI fescue prior to foaling;
  • Monitor the mare closely during late pregnancy;
  • Contact your veterinarian if impending signs of birth, including udder development, relaxation of vulva, and muscles around the tailhead fail to develop within the expected timeframe;
  • Attend the birth. If mare fails to show signs of normal birth progression, contact your veterinarian immediately; and
  • Keep mares and foals off EI fescue until after weaning to prevent poor milk production.

Converting EI fescue to endophyte-free pasture is a challenging task. Changing to endophyte-free fescue might not be the most practical idea in some cases, because the endophyte increases the forage's hardiness and promotes growth on marginal soils. If replanting a pasture, it is extremely important that all infected plants and seeds be destroyed prior to sowing.

Another option is to diversify a pasture with other forages. This helps reduce fescue toxicity by diluting the concentration of infected fescue. Discuss the best methods for eliminating stands of infected fescue with an agronomist, toxicologist or your county extension agent.

A video with Anderson and Sommardahl on fescue toxicity is available online

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