Read time: 5 minutes

How can I avoid sub-lethal dosing when spraying weeds?

With Bill Gordon, Spray Application Consultant, Nufarm Australia.

There has been a distinct shift in the spectrum of weeds seen on farms across Australia toward species, such as fleabane, sowthistle, feather top Rhodes grass and skeleton weed, which have a natural tolerance of herbicides.

Bill Gordon, spray application consultant with Nufarm Australia says these ‘hard-to-kill’ species often have hairy surfaces, thick cuticles, reduced translocation due to plant stress or internal mechanisms that metabolise the product.


“The application rate and technique are critical when managing these weeds,” he says. “If the application is compromised there is a good chance that the weed will survive and go on to set seed.”

“In weeds that are generally susceptible to a herbicide mode of action we see low dose rates inadvertently applied through insufficient coverage, incorrect formulation or adjuvant choice and stressed plant condition, which all impact on the uptake of the herbicide.”

The choice of droplet size, carrier volume and sprayer setup all contribute to the delivery and effective uptake of the herbicide. If a population of susceptible weeds are exposed to long-term low dose application there is strong selection for plants that can survive and set seed, leading to increased herbicide resistance over time.

“Correct plant identification and knowing the resistance status of the population is key to choosing the most appropriate product, most effective rates and the recommended adjuvant,” says Mr Gordon. “Water is the cheapest thing that goes in the spray tank so use sufficient total application volumes to achieve the coverage required for the mode of action group, particularly in paddocks with plenty of stubble.”


Bill Gordon, Nufarm spray application consultant says herbicide application rate and technique are critical when managing the ‘hard-to-control’ weeds that are often the dominant weed species found on farms.

What are the common spray application mistakes that can contribute to herbicide resistance?
Short answer: Mismatching rates, application volumes and not rotating modes of action.

Longer answer: Application technique and the choice of product, rate, water volume and adjuvant are generally within the grower’s control. Weather conditions and the plant size and stress are harder to work around. In ideal conditions the aim is to apply the right rate of the right product to the right target to achieve plant death and prevent seed set. Herbicide resistance is known to evolve, often un-noticed, along fencelines and other green bridge situations.

How does stubble affect the dose delivered to the target weed?
Short answer: Coverage can be up to 60% less at the base of standing stubble compared to the inter-row.

Longer answer: Standing stubble can intercept many droplets before they reach the target weeds. Choose an appropriate spray quality for the target and the mode of action. Use visual indicator tools such as water sensitive paper to ensure adequate coverage is achieved where it is needed. Penetration into standing stubble can be difficult, but is easier to manage than trash on the ground.

For soil applied products, use a very coarse or larger spray quality to maximise contact with the soil. Keep in mind that if a tank mix partner is targeting emerged weeds adjust the spray quality to suit both mode of action and target types. Often the compromise is a coarse spray quality at higher application volumes.

Coverage can be up to 60% less at the base of standing stubble compared to the inter-row. Penetration into standing stubble can be difficult, but is easier to manage than trash on the ground.

How can I counteract the edge effect to apply the correct dose on paddock borders?
Short answer: Large headlands and slow down!

Longer answer: Maintain wide enough headlands to allow easy turning and entering the paddock with the full spray pressure. Using a minimum hold in the controller for non-residual products will prevent the rate dropping too low and ensure the nozzle’s spray pattern does not collapse.

For residuals, slow down and increase application volume to minimise overdosing that may impact on crop establishment, or increase plant back periods.

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