Managing the Effects of Fescue Endophyte in Broodmares

— Written By Paul Westfall and last updated by
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Tall Fescue is the predominant cool-season forage found in pastures across North Carolina, particularly in the counties north of Interstate 40 and in the mountains. Most of this Fescue has been established for a long time, and contains the endophyte fungus Acremonium coenephialum, sometimes referred to as toxic endophyte. Tall Fescue is a high-yielding forage that takes abuse and keeps on growing, making it a good choice as a pasture grass for livestock. However, there are also problems associated with grazing fescue that horse owners need to keep in mind. Those problems affect pregnant broodmares, foals, horses that are pastured for long periods of time on Fescue and not exercised, and – to a lesser extent – young, growing horses.

Since the major effect of Fescue toxicosis is on pregnant mares, (and since this article focuses on broodmares) let’s look at those problems. Fescue toxicity is known to cause prolonged gestation (cases up to 30 to 40 days have been documented). Prolonged gestation means that the foal is larger than usual when born, which in turn means more instances of dystocia and difficult births. Endophyte has been implicated as a potential cause of premature separation of the chorion and abortion in mares. Other problems include thickened or retained placentas, which can lead to excessive hemorrhaging in the mare, which could cause death. Uterine infections may increase, causing a delay in rebreeding. If those problems weren’t enough to deal with, the mare may have aglactia (no milk), and colostrum may be reduced or non-existent. Since colostrum carries antibodies needed by the foal to resist disease, the foal is more likely to develop an infection of some kind.

While all the mechanisms that cause the reproductive problems are not known, it is known that the ergot alkaloids produced by the toxic endophyte act as dopamine. Excess dopamine suppresses the production of the reproductive hormone prolactin. Prolactin is essential to the final stages of pregnancy and birth. Progesterone levels are also suppressed. The levels of progesterone should increase about two weeks before birth, but mares exposed to toxic endophyte have reduced progesterone levels.

Management options to avoid these problems include:

1) Having a forage test run to determine if the toxic endophyte is actually present in pastures. If it is detected, then

2) Remove pregnant mares from fields with endophyte-infected tall fescue 45 to 90 days before foaling and feed them good quality hay and grain or put them on a pasture that is not infected with toxic endophyte. This is the most conservative way of avoiding toxicity problems.

3) Manage Fescue pastures to minimize the effect of endophyte. Grazing, clipping, or mowing pastures to keep Fescue in a vegetative state helps keep the level of endophyte lower.

4) Avoid the endophyte. Use other species for grazing and hay.

5) Dilute the endophyte. Use other feeds in the diet. Grow legumes in with the Fescue. This dilutes the amount if endophyte ingested, improves the quality of the forage, and reduces the amount of nitrogen fertilizer needed for forage growth.

Tall Fescue has been and likely will continue to be the most common pasture and hay forage used in many parts of North Carolina. We should remember that not every mare displays the symptoms or problems that have been mentioned. We need to remember that mares have been grazing fescue pastures for a long time, and there have been a lot of foals born with no problems on these pastures. However, research does show that there higher instances of these problems occurring if pregnant mares are kept on endophyte infected fescue for the duration of the pregnancy. Let’s be aware of the potential for problems associated with Fescue and use one or more of the recommended strategies to minimize the risk of any of these happening.