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How can studying resistance genetics in weeds help farmers?

with Dr Qin Yu, Research Associate Professor, Australian Herbicide Resistance Initiative.

The old saying ‘know thy enemy’ certainly applies to managing herbicide resistance. Dr Qin Yu’s research into the biochemical and molecular processes plants use to evade destruction is essential for the development of effective strategies to manage resistance.

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Working with 33 populations of annual ryegrass that had survived treatment with Hoegrass® (diclofop), Dr Yu and her team identified the resistance mechanisms being used. Their findings were both alarming and illuminating—91% of the resistant populations studied possessed target site mutations, 80% possessed metabolic mutations, meaning 70% were using both types of mechanisms to survive herbicide treatment.

“Target site mechanisms are relatively easy to identify and study,” says Dr Yu. “We now have a good understanding of these mechanisms, which can lead to high levels of resistance. The resistant plants can usually be removed using alternative herbicides of the same mode of action.”

“Non-target site mechanisms are more difficult to study and harder to manage,” she says. “This type of resistance is related to the metabolism of the herbicide within the plant, making it more complex and unpredictable. This is harder to manage in the field.”

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How can target site and non-target site mechanisms occur in the same individual plant?

Short answer: Through gene sharing.

Longer answer: Some weed species have a very high capacity to respond and adapt to environmental conditions, including herbicide applications. Cross-pollinator weeds such as annual ryegrass readily exchange genetic material, with each new generation having the capacity to accumulate resistance genes from different individuals.

What is the value of theoretic science such as resistance genetic studies?

Short answer: Better information for better decisions.

Longer answer: Advances in diagnostic technology to determine what mechanisms are present in a resistant weed population can identify effective management strategies. Finding ways to capitalise on the ‘fitness penalty’ that may come with herbicide resistance, such as increasing crop competition or using other non-chemical control tools, is an important outcome of this research.

What can resistance genetic studies tell us about using selective herbicides?

Short answer: Resistance can occur within just a few years if low rates are used or survivors are ignored.

Longer answer: Selective herbicides work by taking advantage of a crop’s greater ability to metabolise a herbicide compared to the target weed’s lesser ability to metabolise the herbicide. Studies of annual ryegrass and wild oats have proven that both weeds possess the genetic capacity to breakdown selective herbicides if these herbicides are applied at sub-lethal rates. Resistant populations can be resistant to other selective herbicides, even ones that are not yet on the market. Applying full label rates and removing any survivors is essential to preserving these chemicals for use in-crop.

What are some ways to improve herbicide efficacy in resistant populations?

Short answer: Ambient temperature makes a difference to the efficacy of some herbicides and the resistance level.

Longer answer: Some resistance mechanisms are temperature-dependent. Understanding these mechanisms can help researchers optimise the way growers use herbicides that the weeds are resistant to. For example, glyphosate should be applied during relatively cool (but not warm) temperatures for better control of resistant winter-grown ryegrass, and applied at warm (but not hot) temperatures to improve control of resistant summer-grown barnyard grass.

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