Abstract: A crop harvester is provided with a rotary type cutter bed in which a series of rotary cutters extend across the path of travel of the machine for rotation about individual upright axes. A hydraulic drive for the cutter bed includes a variable volume pump driven by the output shaft of an engine associated with the harvester. The operating circuit of the hydraulic drive pumps oil through the circuit at a certain rate of flow, depending upon the engine speed and the position of a swash plate within the pump. A control circuit communicating with the operating circuit is capable of sensing load in the operating circuit tending to lug down and reduce the speed of the engine, which would normally reduce the flow rate and hence the cutter speed. However, in response to detecting such increased load, the control circuit compensates by adjusting the position of the swash plate so as to correspondingly pump more oil at the lower engine speed, thus maintaining the cutter speed essentially constant.