Most fescue pastures are infected with the endophyte to varying degrees. Typically, infections may range from 10 to 100 percent of the fescue pasture. Problems have been reported on farms with infections across the entire range. Cutting the grass for hay does not destroy the endophyte or reduce the alkaloid content or effect. While the mechanism that causes the reproductive problems is not absolutely known, research indicates that an alkaloid resulting from the endophyte infection causes an excess production of a neuro-active chemical, dopamine.
Excess dopamine has a suppressing effect on the reproductive hormone, prolactin.
Prolactin is essential to the final stages of pregnancy and birth. Without the prolactin signal, the mare's body does not realize it is time to foal. Research so far does not indicate conclusively any other negative effects, although some evidence exists for possible harmful effects to horses other than broodmares. There have been reports of rough hair coats and slow hoof growth for growing horses on fescue, but other research suggests there is no effect. Continuing studies on the effects of fescue on horses should provide more answers.