Fescue toxicosis (toxicosis is any disease condition due to poisoning) is caused when tall fescue (Festuca arundinacaea) becomes infected with the mold Acremonium coenophialum. This mold is an endophyte (grows inside the plant rather than outside) that moves up the stem of the grass plant and into the seed as the plant grows. Therefore, you can't see it when you look at the plant with the naked eye. Other molds are transmitted from plant to plant or from the ground to the plant, but Acremonium coenophialum is the only mold of its kind transmitted through the seed to the next generation. The mold is not present in the leaf blades or roots, only in the seeds and stems.

Tall fescue is a perennial that is known for its hardiness. It grows best in cool seasons. It is the most widely grown forage grass in the United States, with some estimates saying there are more than 35 million acres of fescue pastures. Dr. Lon Lewis in his book Equine Clinical Nutrition said that, "It has been estimated that nearly 700,000 horses are maintained on tall fescue pastures in the United States."

In the late 1970s, it was discovered that much of the tall fescue grass in pastures contained the fungal endophyte that can have highly deleterious effects on the animals consuming it. Fescue toxicosis in horses can cause late-term abortions, dystocia (difficult birth), agalactia (little or no milk production), a thickened or tough placenta, and/or extended gestational period.

Lewis stated that, "It is estimated that 80% of fescue growing in the United States is infected to varying degrees, usually at levels of 70% or more of the fescue plants in a pasture. It is known that horses grazing fescue pastures are affected at infection levels as low as 25%, and levels below 7% are recommended for fescue toxicosis prevention."
The reason experts recommend clipping pastures is because horses (and cattle) prefer grazing the seedhead of fescue, which has the highest concentration of the endophyte and its mycotoxins.

Lewis also added that the causative mold Acremonium coenophialum "has been shown to produce ergot alkaloids, and extracts of infected fescue grass contain several additional ergot alkaloids." Ergot alkaloids can be produced by other plants. Ergot alkaloids in livestock can cause numerous problems, including abortion, reduced birth weight and viability, and agalactia similar to fescue toxicosis.

For more on fescue toxicosis see http://www.thehorse.com/0007/fescue_toxicosis0007.html.

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