Read time: 4 minutes

Spray small multi-resistant wild radish twice

Western Australian growers are regaining control over herbicide resistant weeds as a result of widespread adoption of integrated weed management systems.

Australian Herbicide Resistance Initiative communications manager, Peter Newman, says many crops are cleaner than ever and growers should be acknowledged for their consistent efforts over many years to implement a wide range of herbicide and non-herbicide tactics to drive down the weed seed bank on their farms.

“There was a serious problem here over the last two decades and growers have now made managing herbicide resistance in weeds a component in every farming system,” he says. “We were caught once, and had to learn the hard way. Hopefully growers in other areas will be able to take advantage of the experience and research that has helped WA growers get back in the driver’s seat with weed control.”

One piece of research that the RSCN in Geraldton, WA requested that DAFWA and then Crop Circle Consulting and Landmark conduct in 2012 and 2013 has given growers additional confidence when managing herbicide resistant wild radish.

“The objective of the 2-year study was to identify paddocks that had high populations of wild radish that was resistant to several herbicide groups,” says Mr Newman. “Although Velocity® (pyrasulfatole + bromoxynil, Group H/C) was working well in most situations, growers were aware that if there was over-reliance on the one product that soon enough that option would be gone.”

“Some growers were using two or three shots of Velocity® in a single growing season and they had little confidence in other herbicides due to their radish being multiple resistant.”

The first year trial achieved a 22 to 36 per cent yield improvement when wild radish was controlled and the researchers established that the key strategy was to spray the wild radish as early as possible while avoiding any crop damage.

“Applying Velocity at the 2-leaf stage of the wheat crop is safe for the crop and kills the majority of the small – 1 to 2 leaf stage – wild radish plants present,” he says. “When this is followed with a second spray of a herbicide from another mode of action at the crop’s 5-leaf stage, when the radish is no bigger than 4-leaf stage, the level of control is very high even in multi-resistant wild radish populations.”

Spraying small radish twice is a proven tactic to combat multi-resistant populations.

Spraying small radish twice is a proven tactic to combat multi-resistant populations.

This strategy was tested on three properties in the Geraldton area in 2013 and proved very effective. Grant Thompson from Crop Circle Consulting and Landmark led the trial work in 2013 and says the result that was the most surprising was that there were many options available for use as the second spray.

The paddocks chosen for the trials all had known resistance to a wide range of herbicides from mode of action groups B, C, F and I.

“Provided the paddock was sprayed early with a product that did not damage the crop, it didn’t make much difference what herbicide was used for the second spray,” he says. “This was a great result for growers as if gave them more opportunities to rotate modes of action across the whole rotation.”

Even multi-resistant wild radish plants succumb to a second herbicide spray provided the first spray is applied when the radish plants are small.

Even multi-resistant wild radish plants succumb to a second herbicide spray provided the first spray is applied when the radish plants are small.

“For example, growers may choose to use Jaguar (Group F/C) as the first spray in cereals and avoid using Velocity twice in the one season,” he says. “Jaguar applied at 1 L/ha when the wheat is 2-leaf and the radish is 1 to 2-leaf is giving excellent control of the first cohort of weeds in most situations.”

The trials also highlighted the importance of achieving excellent coverage with each spray. Mr Thompson stresses the importance of using the correct adjuvant, appropriate nozzles and not skimping on the water rate.

Control improves yield and minimises costs

Treating the wild radish early was very important for preserving yield. Even with one early spray, all registered treatments doubled the grain yield over the nil treatment. The second spray helps achieve optimal weed control while remaining profitable.

In the two-spray system the herbicide applications cost $19 to $29 per ha and achieved a yield benefit worth an average of $90–130/ha, and as high as $260/ha in very weedy situations.

Knowing the herbicide resistance profile of the weeds present helps growers to choose the mode of action herbicide most likely to give the best results. Herbicide resistance testing and an array of integrated weed management strategies are detailed on the Weedsmart website.

When followed with a harvest weed seed control tactic such as narrow windrow burning or chaff cart, the contribution of new seed to the weed seed bank can realistically be zero. Consistently applied over several seasons can see the weed population driven right down and, if there are no weeds, there are no herbicide resistant weeds.

Wild radish flourishing in each control patch in the trial – illustrating the multi-resistance traits in the population at this site.

Relevant links

Related Articles

View all

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

Subscribe to the WeedSmart Newsletter