Smith' s new grazing land consists entirely of fescue grass. Half of the grass contains a species of fungus that makes cows ill when they eat it. Smith' s cows will most likely learn to graze exclusively on the grass that does not contain the fungus, since, like many other animals, cows will avoid eating a patch of grass if they sense a substance in it that has made them ill.
At least one other species of fungus is contained in the fescue grass that does not contain the fungus that makes cows ill is an assumption required by the argument.