Beer residue is a byproduct of the brewing process. Beer begins with a mash of barley (and sometimes other grains) and hot water, which after an hour or so of enzymatic activity, converts the grain’s starch into sugar and is then drained and rinsed to extract that sugar. That’s what the brewer wants: sugar, which is the starting place for fermentation. Left behind are the starchy endosperm, residual protein and whatever residual sugars the brewer couldn’t rinse away. These protein- and fiber-rich leftovers are excellent feed for everything from cows to chickens.