A center pivot irrigation machine having a swing arm for irrigating corner areas of a field. The swing arm has a steerable support tower which follows a buried cable to guide the swing arm into and out of the field corners. A double hinge connection between the main irrigation boom and the swing arm includes one hinge joint which accommodates pivoting of the swing arm and another hinge joint which controls the operation of the tower drive motors. A cam switch monitors the steering angle of the swing arm tower and turns the sprinklers of the swing tower on and off in accordance with the steering angle to achieve uniform water application. A secondary set of sprinklers on the main boom is deactivated and the entire machine is slowed when the steering angle indicates that the swing arm is covering a rapidly increasing area.