A steering system for a vehicle (10), the system including a steering control (21) for an operator to effect mechanical steering of the vehicle (10), the steering control (21) being mechanically connected via a mechanical steering train, to at least one steerable ground engaging wheel (14a, 14b), there being a power assistor apparatus (32, 33, 36) for power assisting the mechanical steering of the vehicle (10) when the operator operates the steering control (21), and the system including a hydraulic steering actuator (30) which is operable by an electronically controlled hydraulic steering control valve (50; 50a) to effect hydraulic power steering of the vehicle (10) independently of the operation of the steering control (21), the electronically controlled hydraulic steering control valve (50) being operable in response to signals from an electronic controller (52), the hydraulic steering actuator (30) being supplied with pressurised hydraulic fluid from a supply valve arrangement (48), there being a sensor device (70) to sense when the steering control (21) is operated and to provide input to the electronic controller (52), the controller (52) disabling the hydraulic power steering in responses.