Optical spray, or weed detector, technology has been refined over recent years to be a reliable and flexible option for use in several weed management situations.
The most common application of the technology is to treat light, patchy infestations of weeds. This often occurs following a broadacre herbicide application of pre- or post-emergent herbicide and usually requires a different mode of action or higher rate application to kill any surviving weeds and prevent seed set in the fallow.
The two systems currently available in Australia, WeedSeeker® and WeedIT®, both use infra-red reflectance units to sense the presence of green plants and can accurately deliver a set herbicide dose to those plants. Once a plant is detected, a solenoid is activated, turning on an individual nozzle and the weed is sprayed.
NSW DPI weeds technical specialist, Tony Cook says this makes the optical spray technology a highly effective way to economically treat low density weed populations.
Although the general recommendation is to apply herbicide to small weeds, the optical sensors do not recognise plants smaller than about a 50c coin diameter and in wheat stubble weeds are not reliably detected until they are about the size of the top of a beer can (5 cm across). This is why the technology is not recommended for use in high density weed situations.
“Some farmers using the optical sprayers have a dual boom spray set up with the normal boom applying a broadacre dose of glyphosate while the optical sprayers treat any larger weeds with a higher dose of glyphosate or a different mode of action herbicide,” he says.
Nufarm have recently achieved product registrations for applying several of their herbicides using the optical spot spraying technique. Nuquat® is registered to treat barnyard grass, fleabane, sowthistle, bladder ketmia, caltrop, turnip weed and Australian bindweed. Trooper® 75-D, Comet® 400, Amicide® Advance 700 and Amitrole T are registered for fleabane, sowthistle and caltrop control.
“In addition to these registrations, APVMA Permit 11163 is in place to allow users to apply a range of other herbicides using optical sprayers,” says Mr Cook. “Check the permit for application rates and use situations. However, this permit is due to expire on 28 February 2015.”
Herbicides covered by the permit include glyphosate, paraquat, paraquat + diquat, amitrole + paraquat, glufosinate, amitrole T, 2,4-D, triclopyr, fluroxypyr, quizalofop, haloxyfop, sethoxydim, clethodim, butroxydim, and fluazifop.
Mr Cook says the optical spray technology and the recent Nufarm product registrations may help extend the effectiveness of knockdown herbicides such as glyphosate, paraquat and Group A herbicides.
“Using this technology to apply an alternative herbicide group to kill weeds that have survived an application of glyphosate will help drive down the glyphosate resistant seedbank. Along with the use of other strategies that do not rely on knockdown herbicides growers have more tools in their weed management toolbox,” he says.
Resistance to paraquat and Group A herbicides may start to emerge in the near future. To prolong the life of these herbicides for use with optical spray technology may require implementation of other weed management options such as an occasional ‘patch’ cultivation to control plants that survive herbicide treatments.
For an investment in the optical spray technology to provide value, the equipment needs to be regularly used. There are some limitations that must be taken into account such as the need to travel slower (16 km/hr) than normal boom spray operating speed. The sprayers must also be correctly set up and operated according to the manufacturer’s operating guidelines.
There is also added flexibility with the optical sensors working equally well at night as in the day. This makes it possible to use paraquat-type (Group L) herbicides as the second spray in a double knock program as these products are generally more effective when applied late afternoon, in the evening or under cloudy conditions. Although these products require the use of large nozzles with low drift potential, there is a higher risk of inversion drift conditions when spraying at night.
The nozzles on an optical sprayer are set closer together and this, together with the slower operating speed and coarse droplet size, greatly reduces the risk of spray drift. However, it is not recommended to use this technology in high wind situations.
“Optical spray units can achieve chemical bill savings of 60–90 per cent compared to broadacre herbicide application,” says Mr Cook. “Herbicide volumes are generally cut by similar percentages, depending on weed density and the sprayer’s sensitivity settings.”
Optical spray technology is the only herbicide application method proven to be effective against larger, established weeds. Monitoring after an optical spray application is essential to identify and physically remove any individual survivors.
It is a useful follow up to a pre-emergent herbicide application in fallow where weeds are likely to be low density but of moderate size. The technology also has application for treating crop verges, irrigation channels, fencelines and in wide row crops (using shielded nozzles).
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