Breaking the cycle

Group 2 herbicide resistance has spiked across the southern region — but with the right strategy, you can take control.

In 2024, a clear trend started to show up in the paddock. WeedSmart extension agronomists Chris Davey and Jana Freebairn saw a sharp rise in Group 2 resistance across broadleaf weeds in broadacre systems in South Australia and Victoria — a pattern they describe as an ‘imi spiral’.

Across the southern region, growers are now seeing resistance in a wide range of broadleaf weeds — from milk thistle and Indian hedge mustard through to wild radish, prickly lettuce, long fruited turnip, bedstraw, bifora and cutleaf mignonette — mainly to imidazolinones (imi) and sulfonylurea (SU) herbicides.

The numbers tell the story. In 2024, resistance levels across the southern region climbed sharply, particularly in Group 2 chemistries.

Chris says seasonal conditions in 2024 only added to the pressure. A late start meant many crops were dry sown without a pre-seeding knockdown, and with low soil moisture, growers were cautious about pre-emergent herbicides.

That same scenario played out again in 2025 — arguably with even less soil moisture at the start of the season. When resistance results came back from Plant Science Consulting, more weeds were added to the list, including small fruited turnips, snail medic and tares (wild vetch).

The pressure continued to build.

“In 2024, there was carryover weed pressure from sow thistle, milk thistle and marshmallow that germinated in summer, leaving growers with large weeds to control rather than small, actively growing weeds,” Chris explains. “In imi-tolerant crops, applying registered imi herbicides caused yield losses, as crops were already struggling to metabolise the herbicide under stressful growing seasons.”

Much of this was outside the grower’s control. Throughout the season, Chris says spray conditions made things harder again, with efficacy often compromised and many weeds surviving.

“Weed control was reduced due to slow translocation of imi herbicide applied around frosty periods, limited uptake by moisture-stressed weeds and windy conditions delaying spraying, meaning larger weeds were being treated,” he says.

Chris says it’s a good example of how quickly herbicide-resistant weeds can take hold when seasonal conditions aren’t favourable. It’s why an integrated approach matters — reducing reliance on a small number of herbicides and keeping the weed seedbank low to stay ahead.

After a tough season, the focus shifts to resistance testing and tackling problem areas more aggressively. Chris says testing is the best way to understand whether a weed blowout is due to resistance or weather conditions, and it gives growers the information needed to plan a strong herbicide and crop rotation program for the years ahead.

“Stopping seed set before harvest is very important as several broadleaf weeds can evade harvest weed seed control tactics,” he says. “Growers can consider double-knocking for pre-harvest desiccation, wick-wipers for taller weeds like thistles, and even patching out problem areas with cultivation or complete spray-out.”

From there, it’s about putting a solid, integrated weed management plan in place, using WeedSmart’s Big 6 tactics. Chris says there’s real power in a diverse crop rotation to target herbicide-resistant weeds and drive down the seedbank. In some cases, a short-term trade-off, like a brown manure crop, can pay off in the long run.

“Short wheat–lentil rotations have allowed broadleaf weeds to build up in many southern systems,” he says. “A cereal double-break gives growers other ways to bring those weeds back under control. Looking ahead, we’re also expecting lentils with metribuzin and chlorpyralid tolerance to come to market, which will provide additional in-crop control options when used strategically.”

Where imi herbicides are used in imi-tolerant crops, it’s important to follow stewardship guidelines — avoiding an imi spiral and reducing the risk of plant-back issues in dry seasons. Always check labels and plant-back periods when planning rotations.

Technology also has a role to play. Green-on-green weed detection and spot spraying, along with shielded sprayers, can help target escapes in-crop before they set seed.

And as always, setting up a competitive crop is one of the most effective tools available — delivering paddock-wide, season-long weed suppression while supporting yield.

Making sense of Group 2 herbicides

Sarah Wilson, senior technical services specialist with Syngenta, says Group 2 herbicides need to be used strategically.

“Problems can arise when an imi-tolerant crop is sown to avoid imi-residues from the previous crop or fallow, but then imi chemistry is also used in the crop,” she says. “This overuse of imi herbicides quickly leads to more resistant weeds surviving and spreading.”

Getting the best out of Group 2 herbicides

Getting the most out of Group 2 herbicides starts with sticking to stewardship guidelines. That means no more than two Group 2 applications in any four-year period on the same paddock and choosing the right product for the job.

“A Group 2 herbicide applied in a summer fallow counts the same as a winter crop pre-emergent application,” Sarah says. “So, no further Group 2 applications should be made in that paddock in the same year.”

For example, if you apply a sulfonylurea (e.g. Ally) in the summer fallow, followed by another Group 2 product (e.g. Logran, Paradigm or Intervix) over a cereal crop in winter, you should wait three years before using any further Group 2 chemistry in that paddock.

If you’re planting imi-tolerant varieties to get around an imi residue problem, avoid using imi chemistry over the top.

When it comes to application, timing is critical. Foliar applications are most effective on small, actively growing weeds. When plants are moisture-stressed, uptake and translocation of imi herbicides is significantly reduced. And once weeds get too large, imis won’t control them — making late applications a costly exercise.

A wide range of soil characteristics and environmental conditions affect the efficacy and behaviour of soil-applied Group 2 herbicides.

Whether applying pre- or post-emergent, consider using registered tank mixes with different modes of action to improve control and reduce resistance pressure.

Reducing crop injury and resistance risk

Microbial activity is the main way soil-applied imi herbicides break down, so soil moisture and temperature play a big role in how long they persist and when it’s safe to plant a sensitive crop.

While Clearfield and other imi-tolerant crops offer the highest level of tolerance, some non-Clearfield crops — like chickpea and field pea — also have a level of natural tolerance. Where possible, look to rotate into crops that allow the use of non–Group 2 herbicides or cultural control methods, and consider a pre-emergent from a different mode of action.

Using Group 2 herbicides less often will help slow resistance, but the real key is keeping weed numbers low. That means using a mix of WeedSmart’s Big 6 — from diverse rotations and competitive crops to effective herbicide use, stopping seed set and destroying weed seed at harvest — to keep Group 2 chemistry working for the long term.

Big thanks to Sarah Wilson from Syngenta, and Chris Davey and Cindy Benjamin from WeedSmart for sharing their insights. For more practical, cost-saving tips, head to their website.

weedsmart.org.au