In horses it causes a number of issues related to the hormonal disruptions it causes. I lost a gorgeous Appy colt in 2006 because of some hay that had a lot of fescue in it. This year, I discovered a drug called domperidone (brand name Equidone) which counteracts some, but not all, of the effects of endophyte toxicity. The result was my Symphony in Silk. The expense is $100 a month until the foal is weaned. I can't imagine the expense for a large horse! Ladyhawk weighs about 350 lbs. At 1100 or 1200, the dose is increased proportionately! OUCH!
Anyway, this lovely batch of fescue was photographed in my pasture, grown from seeds that were in that hay. So innocent looking. So dangerous for baby horses, lambs, kids . . .