The cows are the only animals grazing on the new land who become ill after eating the fungus contained in the fescue grass. At least one other species of fungus is contained in the fescue grass that does not contain the fungus that makes cows ill. At least some cows that have been made ill by the fungus are capable of sensing the fungus in fescue grass. The fungus that is contained in the fescue grass cannot be eradicated without destroying the grass itself.
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.
Which one of the following is an assumption required by the argument?