It seems to me that the recipe was designed for a group of people that were going to be without much food and water for a long time so the had to get by on foods that were normally fed to their animals.
Take thou also unto thee wheat, and barley, and beans, and lentiles, and millet, and fitches, and put them in one vessel, and make thee bread thereof, according to the number of the days that thou shalt lie upon thy side, three hundred and ninety days shalt thou eat thereof.
And thy meat which thou shalt eat shall be by weight, twenty shekels a day: from time to time shalt thou eat it.
Thou shalt drink also water by measure, the sixth part of an hin: from time to time shalt thou drink.
And thou shalt eat it as barley cakes, and thou shalt bake it with dung that cometh out of man, in their sight.
And the LORD said, Even thus shall the children of Israel eat their defiled bread among the Gentiles, whither I will drive them.
Then said I, Ah Lord GOD! behold, my soul hath not been polluted: for from my youth up even till now have I not eaten of that which dieth of itself, or is torn in pieces; neither came there abominable flesh into my mouth.
Then he said unto me, Lo, I have given thee cow's dung for man's dung, and thou shalt prepare thy bread therewith.
Moreover he said unto me, Son of man, behold, I will break the staff of bread in Jerusalem: and they shall eat bread by weight, and with care; and they shall drink water by measure, and with astonishment:
That they may want bread and water, and be astonied one with another, and consume away for their iniquity.