The Collaborative International Dictionary of English v.0.48:

Red dog \Red dog\, or Red-dog flour \Red`-dog" flour\
   The lowest grade of flour in milling. It is dark and of
   little expansive power, is secured largely from the germ or
   embryo and adjacent parts, and contains a relatively high
   percentage of protein. It is chiefly useful as feed for farm
   animals.
   [Webster 1913 Suppl.]