Herbicide resistance: causes, testing, and integrated weed management

Australia has the highest rate of herbicide-resistant weed species in the world. Ryegrass (Lolium rigidum) resistant to multiple herbicide modes of action (MOA) is present in virtually every cropping district. Brome grasses, wild oats, and wild radish have also developed significant resistance populations. Managing this pressure is now a central constraint in winter crop production.

How resistance develops

Resistance develops through natural selection. Any herbicide application that kills 99 out of 100 weed plants leaves 1 in 100 that were naturally able to survive. Those survivors reproduce, passing on resistance traits to progeny. With ryegrass capable of producing 10,000 seeds per plant, even a very small surviving population can dominate a paddock within three to five seasons.

Two distinct mechanisms cause resistance:

Target-site resistance (TSR): A mutation in the herbicide's target enzyme means the chemical can no longer bind and inhibit it. ACCase inhibitors (Group A: fops and dims) and ALS inhibitors (Group B: sulfonylureas) are most commonly affected.

Enhanced metabolism resistance (EMR): The plant detoxifies the herbicide before it can cause damage. EMR is more dangerous because it can confer resistance to multiple chemistries simultaneously, including ones the population has never been exposed to. Glyphosate (Group M) and some Group A products are increasingly affected.

Testing for resistance

If you suspect a herbicide has stopped working, do not assume product quality or application error before testing. GRDC-supported resistance testing services (WeedSmart, state department labs) offer pot bioassay testing and molecular diagnostics.

Bioassay: Plants are grown from seeds collected from surviving weeds and treated with standard herbicide rates. Comparison to a known susceptible population confirms resistance and gives a rough frequency estimate.

Molecular testing: PCR-based tests identify specific TSR mutations in ryegrass and other species. Faster than bioassay but does not detect all resistance mechanisms, particularly EMR.

Testing costs typically range from $200-$500 per paddock. Given the management implications, this is almost always justified when herbicide efficacy is uncertain.

Integrated weed management (IWM) tactics

No single tactic controls resistant weeds. An effective IWM program uses combinations that attack the weed lifecycle at multiple points:

Delay seed set. Chaff carts, chaff decks, windrow burning, and narrow windrow burning all reduce the weed seed bank returned to the soil at harvest. Research by GRDC shows windrow burning alone can reduce ryegrass seed return by more than 95%.

Harvest weed seed control (HWSC). Integrated systems that collect, destroy, or concentrate weed seeds at harvest include chaff carts, the Integrated Harrington Seed Destructor (iHSD), and narrow windrow burning. A three to five year HWSC program can substantially deplete weed seed banks.

Crop competition. High seeding rates, narrow row spacings, competitive crop varieties, and early sowing to maximise crop establishment all increase the crop's competitive advantage over emerging weeds.

Rotation and break crops. Summer crops allow the use of different herbicide MOAs and different competitive dynamics. A break in the cereal sequence disrupts the weed lifecycle and gives different herbicide windows.

Pre-emergent herbicides. Soil-active pre-emergents (Group J, K, L, and N products) act before weed emergence and can significantly reduce post-emergent pressure in resistant populations. Efficacy depends on adequate soil moisture incorporation after application.

WeedSmart research across Australian cropping regions consistently shows that IWM programs combining HWSC with pre-emergent chemistry and break crops can reduce resistant ryegrass populations by 70 to 90% within five years, compared to programs relying on post-emergent chemistry alone.

Mode of action rotation principles

Use different MOAs sequentially and avoid repeated use of the same group within and across seasons. Maintain a written record of the MOAs used in each paddock each year. The GRDC MOA guide provides a herbicide classification for all registered products, using the Herbicide Resistance Action Committee (HRAC) system.

Key rotation rules:

• Never use the same MOA twice in one season on the same paddock
• Avoid using only Groups A and B in cereal-on-cereal programs
• Rotate between pre-emergent chemistry classes (not just between active ingredients within the same class)
• Use knockdown chemistry (Group M) sparingly and in combination where possible