Spent grain is the name given to material left after the grain is fermented and the alcoholic solution is drawn off. It is used as a cattle feed because of its protein content. The value to the farmer is not great, so smaller breweries are pleased to give the material away for free if the farmer will truck it.
Spent grain is normally wet, with 80% to 85% moisture content. In this state it is heavy to haul, and it is likely to drip while being transported. Furthermore, it will go sour in a few days.
The practice is to run spent grain through a press. Typically a screw press will reduce the moisture content into the range of 61% to 70%. At this point the weight will have been reduced by a third to a half, and the “shelf life” will have been extended.