A spray control method employs a spray vehicle including a material tank, a pump communicating with the tank, and nozzles of a spray boom communicating with the pump. A GNSS receiver mounted on the vehicle and interfaced to a controller tracks its position in relation to stored position coordinates of field boundaries separating spray zones from spray exclusion zones. The tank is activated and deactivated by the controller to retain spray of the material within the spray zones and to prevent spray of the material in the exclusion zones, by processing an offset of the spray nozzles from the receiver, the spray range of the nozzles, spray turn-on and turn-off lag times, and the velocity of the spray vehicle, all in relation to the field boundaries. An alternative embodiment individually controls spray from the nozzles by using associated valves interfaced to the controller.