Spray application manual
31 January 2025
Module 20: Optical spot sprayers
20.2 Systems available in Australia
Published 24 January 2025 | Last updated 20 January 2025
There are several different optical spot spray systems now available in Australia, with additional systems in various stages of development and commercialisation. The general operating principle is some form of sensing device to identify the weeds which then engages the selected nozzles only as the boom passes over the weed.
How they detect weeds
Systems work on the principle of having a sensor or camera to identify the presence of weeds in advance of the spray boom. As the boom passes over the identified weed location, a solenoid valve opens (turns on) the appropriate nozzle or nozzles to treat the individual weed. Once the boom has passed the target, the solenoid valve then closes the nozzle.
Many different brands and systems of optical sprayers are now available in Australia. Broadly then can be divided into two groups.
Sensor based detection systems measure the reflectance of particular wavelengths of light, caused by the absorption and fluorescence of red light by the chlorophyll contained in green plants. These systems are suitable for detecting green weeds in a fallow situation, commonly called green-on-brown applications. The light source, sensor and nozzle configuration can vary across different systems, so it is worthwhile talking to several different manufacturers prior to purchase to identify the features and attributes that best suit your particular situation.
Newer systems can be much lighter than original versions, resulting in less weight needing to be carried across the boom. Some common systems include WeedSeeker® and WEEDit® brands.
WEEDit® system showing green sensors
More recently, camera-based detection systems have been introduced. These systems collect images of the target area ahead of the boom many times per second and use computer algorithms to detect individual weeds.
These systems can be used for green-on-brown applications in fallow, but also have the potential to differentiate between a weed and the crop. This allows for potential use to individually target weeds in a crop, or commonly called green-on-green applications. With these systems, one camera will be positioned to scan a large area in front of the boom and will direct the operation of multiple nozzles.
To achieve individual weed recognition, these systems require a large database of images of the target weed and use computer based learning and artificial intelligence to continually refine the accuracy of weed identification.
Bilberry has been the market leader in camera-based weed detection in Australia, although several other platforms are in various stages of commercialisation.
Agrifac sprayer
For all systems, the sensor or camera must be able to sight the weed, so ensure lenses are kept as clean as possible. Positioning and angle of the sensors is very important, especially when operating under conditions where there is stubble or crop interference. Follow manufacturer advice for camera/sensor set up (height and orientation) specific to the brand of optical spray system.
Maintaining operational boom height is very important. Consistent boom height ensures correct timing of nozzle engagement, while boom height also determines wide of droplet spread on the ground and therefore correct delivery of the target application rate.
Typically, optical recognition systems work best at relatively slow application speeds, as this provides the best chance for accurate weed identification. When travel speed increases, accuracy of weed detection often decreases, especially where there is significant stubble or crop presence.
Comment on night spraying
Sensor-based systems generate their own light source and are capable of spraying at night. In fact there may be improved detection of the reflected wavelengths of light when other incidental light levels (e.g. NIR) are reduced. However, as with all spraying operations, the operator must be certain that a hazardous surface temperature inversion is NOT present to be able to safely spray at night.
Camera based systems are designed to be operated in daylight. To operate at night will require additional high intensity lighting mounted to the boom sprayer.
Calibration of the sensors
All systems will need calibration to ensure they are operating to the applicable soil and light conditions on the day of operation.
For some systems, this requires manual input from the spray operator. Other systems will automatically calibrate before starting, and during the job.
Ensure you follow the calibration advice specific to your system.
Sensor height
Maintaining a constant sensor height is very important to how optical spot sprayers perform. It is important that they are mounted on a stable platform such as a tool bar or boom with a rigid centre and excellent height control (preferably manual jockey wheels).
For sensor based systems, the intensity of the light emitted reduces rapidly with distance from the source, so the ability to detect reflected light reduces if the sensor is lifted above its optimum height. Conversely, if sensors are lowered too far, the field of view may be reduced. Follow manufacturer recommendations for the correct boom height for your system.
Optical spot sprayers
A trailing version of the WEEDit®
In recent years there has been interest in retrofitting boom sprayers with an optical system. Before considering mounting a retrofit optical spray system onto a conventional boom, it is important to consider the impact the additional weight may have on certain boom types and hydraulic fittings and if the boom design has the ability to mount the sensors at the operating height that the system requires.
Ideally, sensors would be mounted onto robust booms with rigid centres, with suitable hydraulic rams fitted to the boom sections. Auto height control may struggle to function correctly unless more robust hydraulic rams are used. Most systems retrofitted onto booms generally do not exceed a width of 24 metres.
WEEDit® units mounted onto a Hayes boom
Find out more
For more information about boom centres and stability, see Module 14: Boom stability and height control
Using optical spot spray sensors under shields
As the sensor-based systems produce their own light source, it is possible to mount sensors and nozzles under shields for specific applications, such as inter-row spraying of contact herbicides and some knockdown products. However, ensure that the minimum height requirement of the sensor can be met.
WeedSeeker® units mounted under shields for inter-row spraying
