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Solubility key to pre-emergent options

Choosing and applying the right pre-emergent herbicide can be difficult, particularly if herbicide resistance is becoming a challenge in a no-till system.

Dr Chris Preston, University of Adelaide (UA) associate professor weed management says the choice is simplified when the chemistry of the available products is understood.

“Most growers are aware of the need to incorporate trifluralin into the soil within 24 hours of application and that this chemical does adhere to stubble, which can render it ineffective if insufficient chemical reaches the soil,” he says.

Another major concern is the increasing populations of weeds that are resistant to trifluralin.

“Because of these two problems we’ve done a lot of work with industry to bring new products to the market,” he says. “Boxer Gold® and Sakura® are now available and it is important to understand the differences between how these new products work and how trifluralin works.”

The main difference between Boxer Gold® and Sakura® and trifluralin is their respective levels of solubility.

Dr Preston says trifluralin has no water solubility to speak of, which means that it won’t move from where it is applied unless there is soil movement or exceptionally heavy rain.

Wheat is not very tolerant of trifluralin but this herbicide can still be used safely in a wheat crop provided adequate chemical-to-soil contact is achieved and the chemical is not applied to soil that will come in contact with the emerging wheat seedlings.

This means planting at the correct depth and making sure that trifluralin-treated soil does not end up above the wheat seed.

Generally disc seeders displace too little soil from the seed row to make trifluralin a safe option for use.

The amount of stubble also needs to be considered because trifluralin will stick to stubble and be rendered ineffective. Dr Preston says using higher rates and bigger droplets can help get the chemical through heavier stubble and onto the soil but if stubble is matted on the ground the trifluralin will not get through to the soil and will not work.

However, if trifluralin-resistant ryegrass is present a grower will need to look at other pre-emergent herbicide options.

“Where ryegrass populations are not big or overly resistant, we’ve found that a mixture of trifluralin and Avadex® can achieve a reasonable level of weed control, particularly in areas where wireweed is a problem,” he says. “But, if the population is large or there is significant resistance to trifluralin then this strategy will not work. This has been demonstrated repeatedly in our trial work.”

This is where the new products, Boxer Gold® and Sakura®, have their place but the different chemistry needs to be understood. Dr Preston says that these new products are more soluble in water than trifluralin, Boxer Gold® more so than Sakura®.


“One of the difficulties with the new chemistry is the management of the herbicide down the seed row,” says Dr Preston. “Boxer Gold® only needs about 5–10 mm of rain to wash it in and activate it, giving really good early weed control. But too much rain after application can wash the herbicide into the seed row and cause crop damage. We have found that wheat is more susceptible to damage than barley.”

He says knife-point application of Boxer Gold® works better than using a disc seeder to apply the herbicide.

“This chemical degrades fairly quickly in Australian soils, generally within a few weeks, so in longer season areas or in years with rainfall through late winter and into early spring it is likely that later flushes of ryegrass will escape,” says Dr Preston.

“The location of the crop and the level of competition needs to be considered as we have seen situations where Boxer Gold® has fallen away very badly at the end of the season, but it does provide very good early control.”

The other new product on the market, Sakura®, is also water soluble, requiring about 10–15 mm to activate it in the soil. Dr Preston says this characteristics means problems can arise if the soil is dry on the surface but there is moist soil underneath.

“What can happen is a small amount of rain might fall that is sufficient to germinate the weeds but not enough to activate the herbicide and the weeds can grow through it,” he explains. “Sakura® is also harder to get through a heavy stubble than Boxer Gold®.”

The advantage with Sakura® lies in its residual action that will continue to control ryegrass late in the season. Dr Preston says sometimes, if there is a dry start to the season after early rain, there is an escape of early ryegrass but these plants become stunted as the season progresses because Sakura® is still present and working on the weeds’ roots.

Registrations and rotations need to be carefully considered as Sakura® is only registered for use in wheat and triticale crops (not durum) and may affect following crops such as oats and durum.

Dr Preston says another important finding of the Grains Research and Development (GRDC) supported research work was that, as with trifluralin, adding Avadex® to Boxer Gold® and Sakura® often gives better control than using these herbicides on their own.

“We have seen this added efficacy consistently with Avadex®.” he says.

Growers can use the GDRC weeds app to identify weeds, and download fact sheets here.

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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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