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 some cows that have been made ill by the fungus are capable of sensing the fungus in fescue grass is an assumption required by the argument.