Read time: 5 minutes

Are we gaining ground against herbicide resistant weeds?

with Dr Ken Flower, Director, AHRI

There’s no doubt that weed management is a numbers game, first and foremost. Overlooking this fact is the underlying cause of herbicide resistance.

Dr Ken Flower, director of the Australian Herbicide Resistance Initiative, says random weed surveys carried out in Australia since 2003 show an upward trajectory in the percentage of samples that register as resistant or developing resistance to a suite of common herbicides.

GRDC invests in a national herbicide resistance survey led by Dr John Broster at CSU with teams from DAF Qld, Adelaide University and UWA.

“In Western Australia, for example, in 2020, random surveys conducted by AHRI’s Dr Mechelle Owen found over 70 per cent of wild radish samples had some level of resistance to diflufenican and 2,4-D,” Ken says. “In annual ryegrass, over 90 per cent of samples were resistant or developing resistance to diclofop. Between 2015 and 2020, glyphosate resistance in annual ryegrass has jumped from 1 per cent to about 12 per cent.”

The jump in glyphosate resistance is likely due to increased applications of glyphosate in herbicide-tolerant canola hybrids and in summer fallow spraying programs.

“Herbicides are by far the most efficient weed control tactic to apply over large tracts of land, and, understandably, growers rely heavily on herbicides. Unfortunately, this is how herbicide resistance gains a foothold,” he says. “Rather than just looking for new broadacre herbicides or technological fixes, we need to get better at locating and mapping the weeds that escape the main control tactics. Treat weed patches in the current crop to stop seed set and strategically in future crops.”

Herbicides work more effectively when crop competition does the heavy lifting to suppress weed germination and seed set.

The WeedSmart Big 6 is built on robust and diverse crop rotations where each crop provides opportunities to drive down the weed seed bank. Monitoring and managing individual weeds or small patches is essential in an effective control strategy.

How can we get a better bang for our herbicide buck?

In brief: Less herbicide overall, with more-targeted application. Unfortunately, 30 or 40 years of integrated weed management research has not reduced the reliance on herbicides in Australia (or elsewhere).

The details: Herbicides are the most heavily used agrochemicals in Australian agriculture, with over six applications per paddock per year, on average, in WA broadacre cropping systems.

Worldwide there is increasing pressure to reduce the quantity of agrochemicals applied. Some countries are banning certain products, and the European (Union) Green Deal Farm to Fork Strategy has set a target to halve agrochemical use by 2030.

Similarly, the WA Agricultural Research Collaboration has an aspirational target for the state’s grain farmers to produce an average crop of 25 million tonnes per annum by 2035, using 50 per cent less fertiliser, crop protection and fuel inputs compared to 2022 inputs.

Precision agriculture in low weed environments may go some way to achieving such targets. Efficient scouting and identification of individual weeds and weedy patches for targeted control with herbicide or non-herbicide tools will be the basis for success, but it is easier said than done.

What is the ultimate challenge in precision weed control?

In brief: Weeds that mimic the crop are the ultimate challenge for in-crop weed detection.

The details: Green-on-green optical spraying is revolutionary technology and a valuable addition to the weed management toolbox. Like all other tools, it has limitations and will apply selection pressure to favour weeds that look most like the crop or can ‘hide’ beneath the crop canopy and still set seed.

We see this phenomenon in hand-weeded rice paddies overseas, where barnyard grass can proliferate because it is difficult for workers to distinguish between the crop and weed.

Green-on-green weed detection requires the support of other tactics to reduce selection pressure on weeds.

Another way to reduce herbicide reliance is to harness some of the new molecular technologies and genome mapping of our main weeds. Who knows, it may even be possible to reverse herbicide resistance.

What should be my weed control focus for this season?

In brief: Use whatever means you have available to map weedy areas and treat them differently to the rest of the field.

The details: Mapping weed patches over time helps you gauge the success of tactics you implement. Consider using cultural controls such as very high seeding rates to increase crop competition, even at the expense of increased screenings for a small portion of the paddock. Inter-row cultivation or crop topping may be other options to consider.

Running down the seedbank in targeted areas needs to factor in potential seed dormancy, so be prepared to treat these patches for several years in a row.

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