Countering imi resistance in broadleaf weeds

Resistance to mainly imidazolinones (imi) and sulfonylurea (SU) herbicides is now confirmed in populations of milk thistle, Indian hedge mustard, wild radish, prickly lettuce, long fruited turnip, bedstraw, bifora and cutleaf mignonette in the southern region.

Chris says the effects of rising resistance levels were exacerbated by seasonal conditions in 2024, where the late start to the season resulted in crops being dry sown with no pre-seeding knockdowns applied, and growers were also cautious about applying pre-emergent herbicides due to low soil moisture.

This scenario presented again to growers in the 2025 season, with arguably less soil moisture at the start of the growing season. When resistance test results came back from Plant Science Consulting in 2025, weeds such as small fruited turnip, snail medic and tares (wild vetch) were added to the growing list of resistant weeds.

“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,” he says. “In imi-tolerant crops, the application of registered imi herbicides caused yield losses because the crops were struggling to metabolise the herbicide under stressful growing seasons.”

All these conditions were outside the grower’s control.

Chris says that spray conditions during the season were also challenging, and spray efficacy was often compromised, allowing more weeds to survive herbicide applications.

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

Chris says this demonstrated how quickly herbicide-resistant weeds can take hold when seasonal conditions are unfavourable for chemical control. Taking an integrated approach that reduces the reliance on a small number of herbicides and maintains a low weed seedbank is a pre-emptive strategy.

Immediate actions during and after a difficult year centre on resistance testing and managing weedy patches with more aggressive tactics. Chris says resistance testing is the best way to determine whether the weed blowout is herbicide or weather-related. Growers will also have the information required to plan a robust herbicide and crop rotation program for the next few years.

“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 weeds like thistles that are taller than the crop and even patching out problem areas with cultivation or complete spray-out.”

The next step is to plan an integrated weed management program using a range of the WeedSmart Big 6 tactics. Chris says there is immense power in deploying a diverse crop rotation to target herbicide-resistant weeds and drive down the weed seed bank. In some situations, a short-term sacrifice with a brown manure crop may pay dividends in the longer term.

“Short, wheat-lentil rotations have allowed broadleaf weeds to build up in many southern region farming systems,” he says. “A cereal double-break tactic provides other means to control these weeds. In the future, we are also expecting lentils with metribuzin and chlorpyralid tolerance to come to market. If used strategically, these will offer growers other in-crop control options.”

Where imi herbicides are used in imi-tolerant crops, be sure to follow the stewardship program and avoid getting stuck in an imi-spiral or running into plant-back problems for the following crop in dry years. Always check the labels and be cautious around plant-back periods when planning the crop rotation.

Technologies such as green-on-green weed detection and spot spraying, and shielded sprayers can also play a part in targeting weed escapes in-crop before they set seed.

Doing everything in your power to grow a competitive crop will provide paddock-wide, season-long weed suppression and boost yield.