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Hit your target when spraying

Perfect spray conditions are rare and there are so many factors at play that sometimes the basics get overlooked.

The three things that the spray operator can and must control are nozzle choice, boom height and ground speed of the spray rig.

Recently, there has been plenty of attention on some of the new technologies involving weed detection and artificial intelligence (AI), and companies like Goldacres are keen to deliver these to growers as they come to market, but their sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target remain central to their spray rig designs.

Goldacres sales and marketing operations manager, Stephen Richards says the technologies behind effective and reliable droplet delivery to the target are central to their spray rig designs.

“At the end of the day, if the droplets of product don’t hit the target at the required rate you might as well have left the spray rig in the shed,” he says. “The best way to ensure the correct dose is applied and avoid spray drift is to pay close attention to setting the rig up correctly and operating it well.”

In the last 20 years there has been a quiet revolution in nozzle design and much of this has been driven by the need to eliminate the risk of spray drifting downwind or being caught up in temperature inversion layers.

“Years ago the standard nozzle was the XRT-jet flat fan nozzle that operated at a pressure of 1 to 4 bar, which gave good coverage in ideal spray conditions, but also produced more fine droplets that easily drift,” says Stephen. “Modern nozzles have been designed with the emphasis on producing medium to coarse droplets and using higher water rates to achieve adequate coverage.”

The modern nozzles also have a wider pressure range of 1 to 6, or 1 to 8 bar, making the one nozzle type suitable for a variety of applications.

When considering ground speed, Stephen says the technologies behind even rate delivery through the boom have made it possible for machinery manufacturers to build sprayers that can operate at higher speeds and cover more area in a day.

“The Goldacres self-propelled sprayers have had a 3-tier nozzle system for about 20 years, where the first set of small nozzles come on when the machine is operating at 5 to 10 km/hr then the second and third sets activate when the machine is operating at higher speeds,” he says.

This ensures that the correct product rate is applied at the headlands and wherever the operator needs to slow down. Another option is the ‘pulse width modulation’ system to adjust the volume through the nozzles in response to changes in ground speed.

“Pulsing is particularly good for turn compensation with a large boom, where nozzles near the outside tip are typically moving twice as fast as nozzles near the machine,” says Stephen. “This means product would be under-applied at the tip and over-applied near the centre. Consistent under-dosing of herbicide is a particular risk in the evolution of herbicide resistance.”

As boom length increases so does the need for high tech suspension and rate compensation for variable speed and turning.

The boom height is also critical in reducing drift risk associated with the air turbulence behind the spray rig. A 20 cm change in height from the recommended 50 cm above ground to 70 cm can quadruple the quantity of air-borne droplets.

“With booms now as wide as 48 m the suspension system is more important than ever,” says Stephen. “Goldacres machines use a system that minimises yaw, roll and pitch of the boom to give a stable spray platform and optimise spray coverage in undulating or uneven paddocks.”

Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear. The large investment in spray technology can be undone if nozzle choice and maintenance is neglected.

“The jug test needs to show that each nozzle is delivering within 10 per cent of the nominated volume per minute for the specific nozzle type and size,” he says. “The cost of a new set of nozzles pales in significance against the cost of product wastage, a spray failure or the evolution of herbicide resistance on your farm.”

Before heading out to spray Stephen recommends operators check for blocked nozzles and at the start of each season, do a jug test to check for nozzle wear.

The WeedSmart Big 6 tactics that form an integrated weed management program to reduce the risk of herbicide resistance in weeds are supported by companies like Goldacres, who understand the importance of effective and safe herbicide application.

Goldacres is working with Bilberry to perfect the artificial intelligence systems required to bring green-on-green weed detection to Australian farmers. These systems, along with the optical spraying technology that has been used for spot-spraying in fallows for over 20 years, are expected to deliver more targeted herbicide use into the future.

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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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