Provided are methods and systems of propagating a microorganism on a stillage composition. The methods involve growing microorganisms in a propagation medium formed from a polysaccharide-containing stillage composition with the majority of the of the polysaccharides in the propagation medium coming from the stillage composition. The propagation medium also includes cellulases and/or amylases to form monosaccharides from the polysaccharides. A first cell mass is grown in the propagation medium to form a second cell mass which is greater than the first cell mass.