In eastern countries where fuel is scarce the want is supplied by dried cow-dung laid up for the winter. Barley cakes were (and are) baked under hot ashes without an oven. The dung here is to be burned to ashes, and the ashes so employed.
Related Commentaries of Ezekiel 4:12
Ezekiel 4:12
12 And thou shalt eat it as barley cakes, and thou shalt bake it with dung that cometh out of man, in their sight.