Abstract: A control system for a tractor with a semi-mounted plough (2) hitched to it comprises a control system (13) for setting a delay between the lifting/lowering of the tractor linkage (3) and the rear of the plough (2) to cause the individual ploughshares to enter to leave work along a given line parallel to a field boundary. To accommodate field boundaries which are not perpendicular to the ploughing direction, adjustment values representative of the angle of each field boundary to the ploughing direction may be entered into a control unit (13) to cause the delay between lifting/lowering of the front and rear of the plough to be adjusted automatically as each field boundary is reached to take account of the angle of that boundary and also whether the plough is being taken out of or put into work.