By Drew Lyon, Extension Weed Scientist

I just finished reading a journal article by Hugh Beckie, Michael Ashworth, and Ken Flower, all of whom are with the Australian Herbicide Resistance Initiative at the University of Western Australia. Herbicide Resistance Management: Recent Developments and Trends provides a nice summary of how the Australian approach towards managing herbicide resistance in weeds has changed and how it continues to evolve. 

Australia has been on the forefront of herbicide resistance management for many years now because they developed some of the worst weed resistance issues in the world.

In the paper, Beckie and his co-authors discuss several developments and trends that I think Washington wheat growers should be aware of and watching for. Perhaps the most significant change in strategy in Australia has been a shift to focus on reducing the weed seed bank and maintaining low seed bank levels by whatever means possible. They call this a ‘zero tolerance policy’ or a ‘take no prisoners’ approach. 

This approach runs contrary to the approach advocated for many years — and still effectively used for insect and disease control — of using economic thresholds to determine if weed density is great enough to warrant the expense of applying an herbicide or using tillage.

The authors go on to discuss several other developments and trends in herbicide resistance management. These are:

1. Renewed efforts in herbicide discovery — the last new herbicide site of action was commercialized more than 30 years ago. Herbicide resistance, particularly to glyphosate, has incentivized companies to invest more heavily in product discovery than they have in the past decade or two. However, it is unlikely that we will ever see new herbicides coming to the market with the frequency that we did in the late 20th century.
    
2. Trait stacking in herbicide-resistant crops — releasing varieties with resistance to two or three different herbicide sites of action. They note that few weed scientists believe this is a long-term solution to herbicide resistance since it will likely just select for weeds resistant to multiple sites of action.
    
3. Increased prominence of pre-emergence herbicides — pre-emergence herbicides provide early season weed control that reduces the potential from crop yield loss, and by reducing weeds in the crop, they lessen the selection pressure for resistance evolution from any postemergence herbicide treatments. Preemergence herbicide use requires an understanding of how herbicide efficacy and longevity are affected by soil characteristics, something growers did not have to think much about with postemergence herbicides.
    
4. Plant breeders are considering weed competitiveness – I have long argued that the most effective weed control tactic is to do everything you can to make your crop as competitive as it can be. Herbicides always work better in a competitive crop. Weed scientists have long advocated for the use of weed-suppressive cultivars; however, only recently have some plant breeders begun researching and developing germplasm with enhanced weed competitiveness.
    
5. Harvest weed seed control (HWSC) is gaining momentum globally, as it's now a widely adapted weed management tool in Australia and is gaining serious attention in the U.S., Canada, and elsewhere. I have worked with Michael Walsh, University of Sydney, to look at opportunities for HWSC in the PNW. He is one of my co-authors on this Extension fact sheet on HWSC for the PNW titled, “Harvest Weed Seed Control: Applications for PNW Wheat Production Systems (pdf)” that is currently in peer review.
    
6. Site-specific weed management (SSWM) in agronomic field crops is set to take off – Currently, SSWM is mostly used in high-value, irrigated crops, but recent investments by John Deere, Bayer, and other corporations may indicate that this technology is about to arrive in agronomic crops like wheat. Real-time weed detection and control in fallow fields is already available with technologies such as WEED-It or WeedSeeker sprayers. Our research has shown these spraying systems can greatly reduce costs of herbicide use, particularly during the later stages of summer fallow when weed cover is often below 50% and patchy. This can provide an opportunity to use highly effective herbicides previously thought to be too expensive to use in fallow.

Herbicide resistance is a growing problem in the PNW that is forcing farmers to change crop rotations, often to less profitable crops, and to consider using more, rather than less, tillage. The experience in Australia over the past decade has shown that the use of effective pre-0emergence herbicides combined with agronomic practices that promote crop competition and minimize seed set or seed bank replenishment have generally resulted in sustained low weed seed bank levels and profitable grain crop production. 

These are lessons we should consider applying in the PNW. For more information on herbicide resistance, visit the Herbicide Resistance Resources page on the Wheat and Small Grains website.