Endophyte-Infected Fescue

Endophyte is a fungus that has a symbiotic relationship with tall fescue and sometimes perennial ryegrass, according to Chad Hall, a forage specialist with DLF International. It does not affect timothy or bluegrass.

Discovered in Kentucky in 1931, this fungus causes a stand of fescue to stay green long after everything else has gone dormant. It constricts the plant's pores so they lose less water. KY-31 fescue came to market as the answer to summer slump.

In the 1970s researchers discovered endophyte constricts the blood vessels in livestock, causing difficulty in regulating body temperature in hot or cold conditions. Horses are particularly susceptible. Working drafts should avoid feed with endophyte levels greater than 50%, because of the risk of overheating. Brood mares have no tolerance and should avoid endophyte because it can interfere with reproduction, causing mares to give little or no milk and making the placenta so tough the foal can't fight its way out.

Endophyte cannot jump from plant to plant; it cycles through the seed. If your pasture has endophyte-infected fescue, plowing will not eradicate it because the seed will still be there. Getting rid of it by shading it out is a 2-year process. Be sure to buy endophyte-free seed. Do not buy ryegrass at the garden store—every turf variety contains endophyte to help keep the lawn green. Test your pasture. If you buy hay and don't know what's in it, get it tested, too.