David Brunton, PhD student at The University of Adelaide’s Weed Science Research Group

I’m off a family farm in South West Victoria; it’s a mixed farming enterprise comprising of cropping (mixed cereal and pulses), fine wool and prime lambs. I undertook my undergraduate degree at the University of Tasmania in Hobart on a full scholarship, it was a fantastic opportunity to be exposure to such diverse agricultural production; and during my honours year I investigated the impact of herbicide residues in poppies. My passion for research into herbicide resistance continued to grow and after finishing university I started a job with SARDI working in cereal, pulse and canola herbicide tolerance. I am currently undertaking a PhD and my research is aimed at providing a better understanding of resistance development in L. rigidum to pre-emergent herbicides in the Group J (thiocarbamates) family. This also  includes management strategies for grower implementation. I was fortunate to make the discovery and publish a scientific paper ‘Resistance to multiple PRE herbicides in a field-evolved rigid ryegrass (Lolium rigidum) population’. Being supervised by three highly respected experts in this field Chris Preston, Gerjeet Gill and Peter Boutsalis at the University of Adelaide’s Weed Science Research Group, has been an incredible experience and privilege.

In laymen's terms, what did your research find?

A population of ryegrass from a paddock on the Eyre Peninsula, South Australia is confirmed resistant to all of the pre-emergent herbicides, Avadex, Arcade, Trifluralin, Propyzamide, Sakura, EPTC and Thiobencarb. This study documented the first case of field-evolved resistance to thiocarbamate herbicides in ryegrass. 
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​In non laymen's terms: A population of rigid ryegrass (Lolium rigidum Gaudin) from a field on the Eyre Peninsula, South Australia, was suspected of resistance to thiocarbamate herbicides. Dose–response studies were conducted on this population (EP162) and two susceptible populations (SLR4 and VLR1). The resistant population exhibited cross-resistance to triallate, prosulfocarb, EPTC, and thiobencarb with higher LD50 to triallate (14.9-fold), prosulfocarb (9.4-fold), EPTC (9.7-fold), and thiobencarb (13.6-fold) compared with the susceptible populations SLR4 and VLR1. The resistant population also displayed resistance to trifluralin, pyroxasulfone, and propyzamide. The LD50 of the resistant population was higher for trifluralin (13.8-fold), pyroxasulfone (8.1-fold), and propyzamide (2.7-fold) compared with the susceptible populations. This study documents the first case of field-evolved resistance to thiocarbamate herbicides in L. rigidum.

Associate Professor Chris Preston, Geoff Philips, Dr Nick Berry, Associate Professor Gurjeet Gill, PhD Candidate David Brunton, Graduate engineer Keagan Grant

What is the problem with the herbicides, why are weeds becoming resistant so quickly?

We’ve spent the past 20-30 years using herbicides to control weeds and it's going to take considerably longer to undo. We know ryegrass is exceptional at evolving resistance to herbicides, but to do it in such a short space of time - wow. We have selected through herbicides, weed species that have the capacity to survive these applications. Widespread resistance to post-emergent group A and B and pre-emergents herbicides such as trifluralin is well known due to repeat use over many years. However, of greatest concern is resistance evolution to newer chemistries such as Boxer Gold, Arcade and more recently Sakura. Adoption of non-herbicidal weed control strategies remains low. Herbicides still remain the most cost effective weed control method and as a result resistance continues to evolve.   ​

What is driving the rise in Herbicide Resistance we’re seeing?

History of herbicide use plays a major role but a reluctance by industry to readily adopt non-herbicidal weed control tactics has been a major driver of resistance evolution. Currently it seems when one herbicide fails, we go to the next and new herbicide modes of action are becoming less common. We’re not the only ones to have found herbicide resistance, in northern NSW and Queensland there have been new cases of glyphosate resistance in sowthistle, fleabane, feathertop rhodes grass and ryegrass; it seems to be a daily occurrence now. Getting industry excited about new methods of weed control is important. It will be necessary to incorporate non-herbicidal methods in combination with herbicides. Doing so will aid in prolonging the use of this precious resource. ​

What does your research mean for Australian Farmers in practical terms?

Herbicide resistance is a major threat to cropping systems worldwide. Many herbicides previously used for weed control, have failed or are failing to provide adequate control of major grass and broadleaf weeds. Annual ryegrass is one of these problematic weed species, which has developed resistance to a number of pre and post-emergent herbicides, across multiple herbicide groups (modes of action). The current story of herbicide rotation has clearly been demonstrated as a non-sustainable long-term tactic. Incorporating non-herbicidal methods of weed control in combination with strategic herbicide use is fundamentally the way forward.

• HWSC: every paddock every year. All crop types (pulses, canola and cereals) provide an effective opportunity to implement harvest weed seed control (HWSC) tactics. However each situation needs to be carefully thought out and changes to harvest method may need to be considered. Back home, we found narrow windrow burning to be a very effective strategy to manage ryegrass in lupins. It allowed us the chance to capture seed from late germinating individuals and put these into a narrow chaff line which would later be burnt. Previously this ryegrass seed would have been spread back onto the paddock. On our farm we have demonstrated that the implementation of HWSC when used in combined with herbicides has allowed us to better manage problematic weeds such as ryegrass and reduce the weed seed bank.
  HWSC methods include: Narrow Windrow Burning, Chaff Cart, Chaff Tramlining or Chaff Lining, Impact mill technology
• Impact Mill Technology: A capital investment in weed control equipment is big, but what equipment isn’t? Investment in mechanical weed control methods in combination with herbicides will have a longer term impact than herbicides alone. A combination of these practices over multiple years will reduce the amount of viable seed returning to the seed bank. A mature ryegrass plant can produce many thousands of seeds and combined with seed longevity of 1-4 years (depending on conditions) requires several years of management to reduce the seed bank. Think of what you will do in 5-10 years if we continue to rely on herbicides alone and are no longer able to control problematic weeds using herbicides due to resistance.
• Testing for Herbicide Resistance: in SA  Peter Boutsalis, Plant Science Consulting, resistance testing both seed and plant test (QuickTest) http://www.plantscienceconsulting.com.au/
• For your available herbicide modes of action, it puts emphasis on Canola and pulse crops where you still have available chemistry.
• Windrow to ensure weed seed capture before seed set in combination with HWSC measures
• Crop Competition​

Can you explain the testing process you are doing for Seed Terminator at Weed Science Research Group of the University of Adelaide?

It is really exciting to be working with Nick Berry as at the end of the day mechanically killing weed seeds is fundamentally important to incorporating non-herbicidal forms of weed control into our current farming systems. The Weed Science Research Group has been testing new mill outputs allowing for rapid and concise dissemination of outcomes. We take the test samples provided, spread them on to trays in a growing medium, after four weeks an emerging weed count is performed and these results are documented and provided to Seed Terminator. These outcomes have allowed for further development and overall improvement of this technology. This technology is easily adapted to commonly used harvesters in existing farming systems, I'm excited to be a part of solutions coming out of this research. 

"Different responses of 2 ryegrass biotypes to pre-emergent herbicide treatment.  Top to bottom: Untreated, Avadex Xtra®, Arcade®, Boxer Gold® and Sakura®."
Image taken from The University of Adelaide Weed Science Group, Current PhD Projects, David Brunton 27/9/2018 
​https://agwine.adelaide.edu.au/research/farming-systems/weed-science/research-projects/

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