Tall fescue is a common grass species used around farms. It is a coarse clumpy grass that is very tough and excellent at erosion control. Unfortunately most old fescue stands contain a fungus called fescue endophyte fungus. This fungus produces a toxin that is harmful to animals that graze on it. Different species and breeds may vary in their sensitivity to the fungus.
Pregnant livestock are the most sensitive.
Most animals don't die but will survive very poorly on infected pastures. The biggest problem for the farmer was identifying which pastures contained fescue. It is hard but not impossible to tell the different species of grass apart. A reference book, a magnifying glass and a 15-minute lesson on grass anatomy will get you started. My recommendation was to start renovating the pastures one at a time with fungus free seed.