Fescue Toxicosis in Horses

by Kathleen Bub

Tall fescue is the most widely grown forage in the southeastern United States. Fescue toxicosis (Summer Slump) in cattle, as described in an earlier article, is the result of an endophytic fungus on tall fescue. The fungus, known as Acremonium coenophialum or Epichloe typhina, is found in all the above-ground parts of the fescue plant. A toxin produced by this endophytic relationship is absorbed into the digestive system of livestock that forage on the fescue. Unfortunately, the toxin remains active in cured hay as well. Research data from Experiment Stations in the southeast show serious production losses occurring in cattle. This is not surprising since surveys have shown that anywhere from 60 to 90 percent of tall fescue pastures are infected with the fungus.

It is now also known that fescue toxicosis is causing critical reproductive problems in pregnant mares. Mares receiving most of their daily nutritional needs from fungus infected fescue (pasture or hay) tend to be agalactic, producing little if any milk. Although their foals are usually born live, they are often weak. Most do not survive long, due to lack of food intake or absence of the immune protection normally provided by the mare’s colostrum (first-milk).

Abortion in the last quarter of gestation is also commonly seen in mares on fescue. This unfortunate occurrence is often accompanied by a retained placenta resulting in an inflammation of the uterus and difficulty in future breeding. Prolonged gestation (as much as 14 months) has also been observed as a result of fescue toxicosis. This often results in stillborn foals, or foals too large to be delivered normally. Abnormally thick placentas are also associated with this toxic condition, and even though the foal may be born live, without human assistance in breaking free, it may suffocate.

Although bovine fescue toxicosis has received a fair amount of attention, only a few controlled scientific studies have involved horses. A survey in Missouri in 1980 provided documentation to support the above described disorders. Of the 1010 mares in the survey, 26.8% grazing only fescue had reproductive problems. Only 11.5% of the mares on forage other than fescue had problems. Actual foal losses from mares maintained on fescue showed incidence of agalactia to be twelve times greater.

In a study at the University of Kentucky, a small number of pregnant mares (20) were fed fungus infected fescue. Seventy percent of their daily energy and protein needs, however, were provided by a mixed grain feed. No abnormalities in milk production or partuition were observed in these mares. This would indicate that a mare must receive more than 30% of her nutritional needs from infected fescue for reproductive problems to occur.

It is highly recommended if you have pregnant mares on fescue pasture, that they be removed 60 to 90 days before their foaling date. If this is not possible, increase their grain ration and/or feed a good quality legume hay, to provide a greater percentage of their nutritional needs. You should also monitor the mare’s udder, and anticipate trouble if it is not developing within two weeks of foaling. In this case it would be wise to have a supply of colostrum on hand for the foal. Also, try to be present at birth to assist in the foaling if necessary (i.e.: to tear a thickened placenta to prevent suffocation of the foal).