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.