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Weed application technology for the new age

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.

Weed detector (or optical spray) technology is coming to the fore as a valuable option in the war on herbicide resistant weeds. The technology is mature, reliable and supported with new herbicide product registrations.

Weed detector (or optical spray) technology is coming to the fore as a valuable option in the war on herbicide resistant weeds. The technology is mature, reliable and supported with new herbicide product registrations.

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.

Weeds that survive a pre- or post-emergent herbicide application are frequently herbicide resistant and moderate in size, making them ideal candidates for treatment with optical spray technology using a different mode of action.

Weeds that survive a pre- or post-emergent herbicide application are frequently herbicide resistant and moderate in size, making them ideal candidates for treatment with optical spray technology using a different mode of action.

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).

Watch the video below for more info!

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Never cut the herbicide application rate

Scientific studies have demonstrated that resistance can rapidly evolve in weeds subjected to low doses of herbicide. Some weeds can develop resistance within a few generations. Full rates when mixing herbicides too! When mixing herbicides it is important that each product is still applied at the full label rate to ensure high mortality. Applying different chemicals in one mix can provide an additive advantage. It is important to understand the mode of action of each herbicide on the plant when preparing a herbicide mix. This is just as important for pre-emergent grass weed mixes as it is for post-emergent mixes aimed at broadleaf weed control. ALWAYS READ THE LABEL. Surrounding weed seeds with a combination of pre-emergent herbicides with different modes of action can give a high level of control and help extend the useful life of all the chemicals used. The high level of control must be supported with additional control measures for all survivors. All products with different modes of action must be applied at full label rates for this to be an effective strategy.   Mixing two chemicals with the same mode of action can achieve some additional efficacy, however, the mix should deliver the combined full rate to ensure a lethal dose. The amount of stubble present and crop safety are all important considerations when mixing chemicals. For example, when using a tank mix of Avadex® and trifluralin to control ryegrass in wheat, the rates used will vary depending on the sowing system and level of stubble retention. Be sure to get good advice. Many herbicides on the market are a combination of two or more modes of action within the one product. These products must be applied at the full label rate to be effective. Having dual action does not negate the need to change herbicide products and rotate modes of action. Repeated use of any single strategy will reduce the effectiveness of that strategy over time.  

Spray well – correct nozzles, adjuvants and water rates

Spray application is a technical field and growers need to make sure their equipment and application techniques are spot-on. The GRDC Spray Application GrowNote provides detailed information and about 80 videos to demonstrate key skills. Prevent spray-drift The focus of spraying herbicide needs to be on doing the job right so the weeds receive the correct dose and die, and this includes reducing the air borne fraction to a bare minimum. Bill Gordon’s 10 Tips for Reducing Spray Drift Choose all products in the tank mix carefully. Understand the product mode of action and coverage requirements. Select (and check) the coarsest spray quality that will provide effective control. Expect that surface temperature inversions will form as sunset approaches and will likely persist overnight and even beyond sunrise on many occasions. DO NOT SPRAY. Use weather forecasts to inform your spray decisions. Only start spraying when the sun is about 20 degrees above the horizon and when the wind speed has been above 4–5 km/hr for more than 20–30 minutes, and clearly blowing away from any adjacent sensitive crops or areas. Set the boom height to achieve a double overlap of the spray patterns. Avoid higher spraying speeds. Leave buffers unsprayed if necessary and come back. Continue to monitor conditions, particularly wind speed, at the site during the spray operation High water rates don’t have to slow you down Some growers are concerned that increasing the water rate when applying herbicide will slow down their spray operation and cost them money. However, the biggest financial loss during spraying usually comes from a failed spray job. To keep your spray operation as time efficient as possible when using more effective and reliable application volumes, you can: Use nurse tanks around the farm to reduce the time spent travelling back to a central re-fill point. Use a larger pump, e.g. 2.5 inch, to make re-filling quicker. Pre-mix the batch while the sprayer is operating. Many mixes can be held in the mixing tank for up to 6 hours. However, wettable granules and suspension concentrates will need agitation to keep them in solution. For pre-emergent herbicides in high stubble situations, carrier volume has a large effect on the level of control achieved. Across four trial sites Dr Borger’s research demonstrated that ryegrass control with trifluralin or Sakura® increased from 53% control when the carrier volume was 30 L/ha to 78% control when the carrier volume was increased to 150 L water/ha in high Water quality and mixing order Water quality is often overlooked as a possible contributor to herbicide failure and can lead to confusion over the herbicide resistance status of weeds on a property. Water should be considered as one of the chemicals in any mix, given that water quality varies markedly depending on its source. Getting the mixing order right is essential for effective spray results. Don’t start mixing until the water quality is right Podcast – Mixing herbicides Adjuvants Sometimes adding an adjuvant is beneficial and sometimes it is detrimental; and there is an art to knowing how to best deploy these additives. When weeds are susceptible to the applied herbicides, the effectiveness of adjuvants generally goes un-noticed. Correctly applied adjuvants can reduce the impact of low level herbicide resistance by helping to maximise the amount of herbicide taken up by the plant.

Clean borders – avoid evolving resistance on the fence line

About one-quarter of glyphosate-resistant populations within broadacre cropping situations across Australia come from fencelines and other non-cropping areas of the farm. Along paddock borders, where there is no crop competition, weeds can flourish and, if not controlled, set lots of seed. The traditional approach has been to treat these weeds with glyphosate to keep borders clean but after 20-odd years this option is now failing and paddock borders are becoming a significant source of glyphosate-resistant weed seed. Weed researcher Eric Koetz said the limited options for managing weeds along irrigation infrastructure and other non-crop areas is a problem and is putting additional pressure on knock-down herbicides in irrigated systems. In some situations, cultivation can be used to kill the weeds and provide a firebreak, but on light soils this may pose an erosion risk and mowing or slashing may be safer options. Another possible tactic is to continue using herbicides but to ensure that a clean-up operation is carried out before any survivors can set seed. Some growers are choosing to increase the heat on weeds along the borders by planting the crop right to the fence and then baling the outside lap and spraying with a knockdown herbicide to kill any weeds and provide a firebreak. Another good option in some situations is to maintain a healthy border of vegetation using non-invasive grasses. In Queensland, buffel grass is a good example of a grass that can outcompete other weeds while not invading crop lands. If only herbicides are used on fencelines, resistance is inevitable. Surviving weeds on fencelines have no competition and access to plenty of soil moisture, so they set a lot of seed and resistance can easily flow into neighbouring paddocks. Other resources It’s time for a glyphosate intervention Farm hygiene cottons on – Cleave Rogan, St George What’s new in management of herbicide resistant weeds on fencelines? Keeping the farm clean – Graham Clapham, Norwin Don’t jeopardise glyphosate for clean fencelines Keeping fencelines clean Resistance risk to knock-down herbicides on irrigated cotton farms

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