The walking mills resembled fat metal serpents, rather larger than those Chinese paper dragons animated by files of men in procession. Sensory robot devices in their noses informed them that the waiting wheat had reached ripe perfection. As they advanced, their heads swung lazily from side to side, very much like snakes, gobbling the yellow grain. In their throats, it was threshed, the chaff bundled and burped aside for pickup by the crawl trucks of a chemical corporation, the kernels quick-dried and blown along into the mighty chests of the machines. There the tireless mills ground the kernels to flour, which was instantly sifted, the bran being packaged and dropped like the chaff for pickup.
One of the best short stories by Fritz Reuter Leiber a prolific American writer of fantasy, horror and science fiction. “Bread Overhead”, written in 1958, appears in a 1964 collection “A Pail of Air”.