The present disclosure relates to a wood chipper that has a feed roller driven by a hydraulic motor. Hydraulic fluid under pressure is provided to the motor through a two-stage pump, providing separate flows of fluid under pressure. The flows of the pump stages are combined for normal operation of the chipper. A bypass that will bypass the flow from one pump stage when the pressure of the fluid under pressure driving the motor exceeds a pre-selected level from increased load on the feed roller is provided. When the bypass is open the feed roller is slowed to reduce the feed rate of material into the chipper, and when the feed roller loads reduce, the flows will again be combined to speed up the feed roller.