Variable-rate spray applications allow the machine to automatically vary the spraying rates within a paddock based on a pre-defined map or sensor. This has the ability to optimise crop protection, improve efficiency and reduce chemical usage. 

Two basic technologies for VRA are map-based and sensor-based (not covered in this chapter). Map-based VRA adjusts the application rate based on an electronic map, also called a prescription map. Using the field position from a GPS receiver and a prescription map of desired rates, the concentration of input is changed as the applicator moves through the field.

For map-based weed-control VRA systems, some form of ‘task computer’ is required to provide a signal indicating the target rate for the current location, and therefore the higher-end in-cab screens are necessary. Second, a system for physically changing the application rate to match the current prescribed rate is required. There are a number of different types of control systems on the market today that are adaptable to VRA.

There are also options to do spot spraying based on this technology (from drone or satellite image detection as described in the above section), and whilst not technically varying the rate, it acts in the same way. Essentially the prescription file has a zero rate where there are no weeds, and the desired water rate where there are weeds detected.

Flow-based control of a tank mix

The flow-based control of the spray mix is the simplest of the two types discussed here. These systems combine a flow meter, a ground-speed sensor and a controllable valve (servo valve) with an electronic controller to apply the desired rate of the tank mix. 

A microprocessor in the console uses information about sprayer width and prescribed application rate to calculate the appropriate flow rate (litres per minute) for the current ground speed. 

The servo valve is then opened or closed until the flow-meter measurement matches the calculated flow rate. If a communication link can be established between this controller and a map system, a variable-rate application can be made. 

These systems have the advantage of being reasonably simple. Consideration needs to be given to the impact that changing flow rates has on nozzle pattern (while maintaining a desired pressure range) and what the critical levels (maximum and minimum) of flow are. 

As the chemical is already in solution/mixed in the tank with the carrier (water), care needs to be taken not to exceed label rates at higher flow rates.

Chemical-injection-based control and chemical-injection control with carrier

An alternative approach to chemical application uses direct injection of the chemical into a stream of water. These systems utilise the controller and a chemical pump to manage the rate of chemical injection, rather than the flow rate of a tank mix. The flow rate of the carrier (water) is usually constant and the injection rate is varied to accommodate changes in ground speed or changes in prescribed rate.

Again, if the controller has been designed or modified to accept an external command (from a GPS signal and prescription map), the system can be used for VRA. 

Chemical injection with carrier control requires that the control system change both the chemical-injection rate and the water-carrier rate to respond to speed or application-rate changes. One control loop manages the injection pump while a second controller operates a servo valve to provide a matching flow of carrier. A perfect system of this type would deliver a mix of constant concentration as if it were coming from a pre-mixed tank. 

Once again care needs to be taken not to exceed label rates.

The VRA map is loaded into the screen in the cab, which is often also the guidance screen). This map needs to be in the right format for the screen to be able to read it and control the rates. In most cases a shapefile will work if loaded in the correct way, and the correct folder. Newer machines are now often connected to the internet, and the files can be wirelessly transferred into the screen remotely. 

The screen is connected to the controller, which actually varies the rate (depending on which system is used above). 

Some examples of variable-rate control systems from different manufacturers include the Trimble Field IQ™, Ag Leader DirectCommand™, CASE AIM Command® and John Deere GreenStar™.