Herbicide resistance is the inherited ability of a weed population to survive a dose of herbicide that would have killed a typical member of that species a generation ago. It is not a quirk of application, not a weather problem, and not a rate problem once it's established — it's a shift in the genetic makeup of the population driven by repeated selection pressure from the same mode of action.

Australia sits at the leading edge of this problem for reasons that are partly geography, partly history. The continent was settled with large-scale wheat and canola cropping on enormous weed populations. Clean-seed practices lagged. Ryegrass pastures and wheat phases alternated in the same paddock for decades. Wind-pollinated, genetically diverse weed populations were dosed with the same chemistry year after year. The outcome was predictable — and, as it turned out, world-leading: annual ryegrass (Lolium rigidum) populations in Western Australia now routinely show resistance to four, five, sometimes six herbicide mode-of-action groups in a single paddock.

This guide is written for the people who have to make decisions on the ground — broadacre grain growers and the agronomists supporting them. It's organised around three questions: what does each HRAC group actually do, which weeds are failing which chemistry right now in Australia, and how do you arrange the tools you have into a program that lasts more than one season. The chemistry knowledge you need is in Section 3. The strategy sits in Sections 6 through 10. Sections 11 through 14 are how to run it.

Weeds already cost the Australian grains industry more than $3.3 billion a year in yield loss and control expense — and that's before a single paddock goes fully resistant. When a mode of action fails on a major weed, three things happen in sequence: control costs rise, yield losses accelerate, and the management lever is gone for good.

What resistance actually costs

• Control cost inflation — Adding a second mode of action to cover a weakening first one can add $15–$30/ha per application. Across a winter-cropping program that's three or four applications per year, the number climbs fast.
• Yield loss from uncontrolled escapes — Annual ryegrass at 50 plants/m² takes 12–20% off wheat yield. At 200 plants/m² the hit is closer to 40%. Resistant patches don't stay patches — they spread.
• Foregone rotation options — Loss of selective chemistry pushes growers back to knockdown-only programs, which crowd in fallows and limit legume or canola phases that were doing double duty as nitrogen fixation and grass-breaking rotations.
• Land value impact — Paddocks with documented multi-resistance are worth measurably less in WA and southern NSW land sales. Solicitors now routinely ask for resistance test results as part of farm due diligence.
• Trajectory loss — Unlike a bad season, resistance doesn't reverse. There is no "rest the paddock and it comes back". Once a population is fixed for a resistance allele, that group is off the menu.

The multi-billion-dollar weed

Annual ryegrass sits fourth on the list of invasive species causing the most Australian agricultural loss over a 60-year period — over a billion Australian dollars in cumulative cost and counting. Wild radish, sowthistle, brome, and wild oats each carry their own line item. Feathertop Rhodes grass has moved from a northern curiosity to a southern NSW and Victorian problem in under a decade, tracking summer-fallow programs that over-relied on glyphosate.

Australia adopted the global HRAC (Herbicide Resistance Action Committee) numeric classification in 2021. The old Australian letter codes (A, B, C, D, I, L, M, N and so on) are still printed on many labels because stock turnover takes years. This table is the current Rosetta stone — every group number you'll see on a Spray Hub product page maps to a letter code and a mechanism of action. The colour strip on the left of each row is the same colour used on product badges and chem pills throughout Spray Hub and this guide.

Weeds develop resistance through two broad biological routes, and the route matters for how you manage around it. Both can be present in the same population, and both are heritable.

1. Target-site resistance (TSR)

The herbicide binds to a specific enzyme (ALS, ACCase, EPSPS, PSII, PPO, etc.). A point mutation in the gene coding for that enzyme changes the shape of the binding pocket. The herbicide no longer fits — or fits much less well — and the enzyme keeps working. The weed survives.

• Usually a single mutation — one base change at a known codon (e.g. Pro-197, Trp-574, Ile-1781) is often enough.
• Typically dominant or semi-dominant — one copy of the resistance allele is usually enough to confer resistance, which means the trait spreads rapidly through an outcrossing population like ryegrass.
• Group-specific — a Pro-197 mutation gives ALS resistance (Group 2) but leaves ACCase (Group 1) targets unaffected, and vice versa. Resistance is often detectable in fresh plant assays in a couple of weeks.
• Durable — the mutation doesn't "fade" when selection pressure is removed. Populations stay resistant even after five or six years of no selection.

2. Non-target-site resistance (NTSR / metabolic)

The enzyme is unchanged, but the plant detoxifies the herbicide before it reaches the target — usually via cytochrome P450 monooxygenases, glutathione-S-transferases (GSTs), glycosyl transferases, or active vacuolar sequestration. The herbicide enters the plant, gets converted to a non-toxic metabolite, and the plant survives.

• Polygenic — multiple genes and enzyme systems are involved. Harder to detect, harder to eliminate.
• Cross-resistance risk — the detox pathway can chew through several structurally unrelated herbicides. A population selected hard on Group 1 can show reduced sensitivity to Group 2 chemistry it's never been exposed to.
• Growing threat — the reason multi-resistance in ryegrass has accelerated. P450-mediated metabolism is the single scariest thing in the Australian resistance literature.
• Adjuvant nuance — some NTSR resistance is partly overcome by using an oil-based adjuvant with higher loading, which pushes more active across the cuticle faster than the detox system can keep up. Doesn't fix it, but extends the useful life of the chemistry.

Reduced translocation / sequestration

A third mechanism worth knowing, particularly in glyphosate-resistant populations: the weed limits movement of the herbicide around the plant, or pumps it into the vacuole where it can't act. Glyphosate-resistant ryegrass in parts of SA and Victoria shows this — stay-green symptoms on treated plants that then regrow because the herbicide never reached the meristems.

These eight species account for the majority of resistance-related program changes in Australian winter and summer cropping. Each profile below covers the biology, the confirmed-resistance status, and the current working approach. Click any thumbnail for the Wikipedia species page.

The WeedSmart Big 6 is the strategic framework the Australian herbicide resistance community has consolidated behind. It isn't new science so much as a structured way of combining six proven levers so no single one does all the work — and therefore no single one breaks. AHRI, GRDC and CropLife Australia all align their extension material with it.

The rotation rules

• No more than two consecutive applications of the same MoA group per year. Even tank-mixed with a different partner, a third pass of the same group in-season is a resistance factory.
• Rotate numbers, not brands. Three different atrazine formulations are all Group 5. Three different 2,4-D amines are all Group 4.
• Match MoA to weed spectrum. Rotating to a group the target weed isn't susceptible to doesn't count — it's selection pressure on whatever weeds the new group does control.
• Count knockdowns. Fallow glyphosate applications all select for Group 9 resistance. Double-knocks with paraquat count toward the Group 22 allocation too.
• Plan pre-em stacks at the paddock level, not the application level. Trifluralin + pyroxasulfone is a two-group stack; plan the next season's pre-em to avoid repeating both groups.

A worked 4-year example — southern NSW / Vic mixed farm

The matrix below is a template, not a prescription. Real paddock decisions flex with weed spectrum, prices, season, and registration status. The point is the structure — every cell has a HRAC number, and no number appears twice in any column.

Count the MoA groups: Groups 9, 22, 3, 15, 4, 2, 14, 13, 1, 5. Ten groups across four years. No group is used in every year. The Group 9 (glyphosate) usage is diluted across multiple partner chemistries and hit with a Group 22 double-knock every year. The Group 1 (clethodim) exposure is limited to one year. Pre-em stacks rotate across Groups 3, 13, 15 and 5.

Rotation shortcuts by region

• Northern grain belt (Qld, northern NSW): Summer fallow is the resistance hotspot. Never rely on solo glyphosate — always a double-knock or a mix with Group 14 (saflufenacil) or Group 22 (paraquat). Rotate through sorghum (Group 5 pre-em atrazine), cotton (Group 1/15), pulse (Group 1/15).
• Western Australia wheatbelt: Wild radish and ryegrass resistance is the driver. Multi-group pre-em stacks are non-negotiable. Clearfield wheat adds Group 2 options but don't over-use. HWSC adoption is highest here — chaff lining or impact mills.
• Southern NSW / Vic: Ryegrass and brome lead. The 4-year example above is the standard shape. Trifluralin + pyroxasulfone is the backbone pre-em; Sakura (prosulfocarb + S-metolachlor) rotates in.
• SA / Eyre Peninsula: Brome, ryegrass, and paraquat resistance risk. Mateno Complete adding Group 13, 15 and 27 chemistry to cereals is changing the rotation shape. Bixlozone (Overwatch) integration is accelerating.

The double-knock is a sequential application of two different mode-of-action groups, with the second hitting survivors of the first. The canonical version is glyphosate (Group 9) followed by paraquat (Group 22), and it has saved more glyphosate use than any other tactic in the Australian toolkit.

Why it works

Glyphosate is a systemic EPSPS inhibitor — slow, translocated, 5–10 days to full symptoms. Paraquat is a PSI diverter — contact, fast, burns tissue within 24 hours by generating reactive oxygen species in the chloroplast. There is essentially no cross-resistance mechanism: target-site glyphosate resistance (enzyme mutation, sequestration, translocation) doesn't help a plant against paraquat's membrane-level oxidative damage. Even a population with 100× glyphosate resistance is still killed by standard paraquat rates.

How to run it

Alternative double-knock sequences

• Glyphosate → Spray.Seed / paraquat+diquat — The most common. Handles ryegrass, sowthistle, fleabane, feathertop Rhodes, windmill grass. All SprayHub registered products work.
• Glyphosate → Sharpen (saflufenacil, Group 14) — Better on small broadleaves. Fast dessication similar to paraquat but different MoA — rotates the second-knock group.
• Glyphosate → Terrad'or (tiafenacil, Group 14) — Newer PPO partner. Works well in summer fallow where paraquat heat tolerance limits spraying windows.
• Paraquat → Glufosinate (Group 10) — Rare sequence but useful where glyphosate is known failed. Two different contact chemistries.

When NOT to double-knock

Double-knocking on weeds already dead from the first application wastes the second chemistry and wastes money. Double-knocking when the first application was compromised by rain or stressed plants doesn't address the actual problem — re-apply the first knock instead. Double-knocks are also overkill on clean fallow — reserve them for paddocks with suspected or confirmed resistance, or where the weed pressure makes insurance cheap.

Pre-emergent herbicides work by creating a soil-active zone that kills weeds as they germinate. Because weeds die before the grower sees them, pre-ems carry the heaviest load in resistance programs — they're the wall that holds the weed pressure off the crop through the most vulnerable growth stages.

Stacking two or three pre-em groups in the same application or in close sequence is the current standard for broadacre cereal production in southern Australia. No single pre-em is effective against every weed, and no single pre-em group is free of resistance pressure — the stack spreads both the weed-spectrum load and the resistance load.

The workhorses

Stack recipes by rotation

Harvest Weed Seed Control is the single most important innovation in the Australian resistance toolkit since pyroxasulfone. By capturing, concentrating or destroying weed seed in the chaff fraction at harvest, HWSC bypasses the soil seed bank and removes the feedback loop that's fuelling resistance — every resistant seed a header drops is a problem next year, and every resistant seed HWSC removes is a problem you never inherit.

Efficacy depends on the weed (seed retention height matters — wild radish 90%+, ryegrass 60–85%, brome 50–70%), the method (impact mills ≈ 95% destruction vs chaff lining ≈ 80% capture), and the setup. Annual uptake across the WA wheatbelt is now over 75% of harvested area — the tactic has moved from research to standard practice in a decade.

The five methods

Before changing a program or writing off a chemistry, confirm with a resistance test. Field observation is useful but unreliable — "didn't work like it used to" could be resistance, sub-lethal dose, timing, weather, adjuvant mis-selection, or a genuine drift of the active ingredient. A test tells you which it was.

Three Australian testing services

When to test

• Unexplained escapes after a well-timed, well-applied chemistry — especially if the escapes are clumps or strips rather than random individuals.
• Before switching to a premium alternative chemistry that costs 3–5× the old standby. Confirm the old one is actually failed.
• On a purchased or leased paddock with uncertain history — resistance test as part of farm due diligence is now standard in WA and increasing elsewhere.
• As part of a regional monitoring program — AHRI and GRDC-funded random surveys occasionally offer free or subsidised testing; sign up.

How to interpret results

Quick-Test and seed-test results are usually reported as "susceptible", "developing resistance" (10–30% survival at field rate), or "resistant" (>30% survival at field rate). "Developing resistance" is the red-flag stage — the population is still mostly controlled but the resistance allele is rising. This is when to escalate the rotation, not when to push harder on the failing chemistry. Waiting for >80% survival means you've missed the window.

These are herbicides currently registered in the Spray Hub product database — 80 products across 17 HRAC groups. Each tile links into the main app for full label text, rates, crops, weeds, adjuvant requirements, withholding periods and state restrictions.

Group 1 — ACCase (graminicides)

Group 2 — ALS (sulfonylureas, imidazolinones, triazolopyrimidines)

Group 3 — Microtubule (dinitroanilines)

Group 4 — Synthetic auxins

Group 5 — PSII inhibitors (triazines, ureas, triazinones)

Group 9 — EPSPS (glyphosate)

Group 10 — Glufosinate

Group 14 — PPO inhibitors

Group 15 — VLCFA (pre-em grass backbone)

Group 22 — PSI diverters (paraquat, diquat)

Group 27 — HPPD (resistance-breaking partner chemistry)

Group 29 — Cellulose synthesis

This is the working process for designing a program in front of a paddock map, a clipboard and a calendar. Ninety minutes of planning in autumn saves three years of regret when chemistry fails.

1. Map the problem. Walk every paddock late in the season. Flag patches of survivors on your map, note weed species, and cross-reference which chemistry each paddock received. Resistance-risk map in, paddock-by-paddock.
2. Test suspect populations. Any paddock with unexplained escapes or a flagged patch gets a sample sent to Plant Science Consulting (Quick-Test for in-season) or Charles Sturt/Adelaide (seed test end-of-season). Don't spend another $40/ha guessing.
3. Plan a 4-year MoA rotation. Block out the next four seasons on the same matrix template used in Section 7. Fallow → pre-em → early post-em → late post-em → harvest. Write a HRAC group number in every slot. No group should appear twice in the same column. Rotations at least two groups deep per paddock-season.
4. Stack pre-ems. For the upcoming winter crop, choose at least two effective pre-emergent groups. Trifluralin (3) + pyroxasulfone (15) is the baseline for ryegrass paddocks; add diflufenican (12), Sakura (15), Boxer Gold (15), or bixlozone (13) as the weed spectrum and rotation demands.
5. Spray for efficacy, not just coverage. Calibrate the boom. Target the right growth stage — 2-leaf to 4-leaf for ryegrass post-em, 6-leaf to tillering for wild oats. Use the label-recommended adjuvant (LI-700 for ALS on hard water; oil for ACCase). Don't cut rates when resistance is suspected — a sub-lethal dose is a resistance factory.
6. Double-knock the burn-down. In summer fallows and pre-sowing, plan to follow glyphosate with paraquat 5–14 days later. Even without confirmed resistance, the insurance cost is low relative to the downside. On confirmed resistance, double-knocks become non-negotiable.
7. Stop seed set on survivors. Crop-top, hay-freeze, or spray-top before seed matures on escapes. For wild radish and ryegrass specifically — which retain seed high enough for harvest capture — this is where HWSC takes over.
8. Harvest the chaff fraction. Fit chaff lining (cheapest), chaff decks, an impact mill, or at minimum narrow windrow burning. Every resistant seed captured is a problem the next season doesn't inherit.

Registration authority

The Australian Pesticides and Veterinary Medicines Authority (APVMA) registers all herbicide products sold in Australia. Every product on Spray Hub carries its APVMA registration number and links to its official label PDF via APVMA PubCRIS. Permits (e.g. minor-use permits) are listed under the Permits tab in the main app.

State chemical licensing

• NSW — Pesticides Act 1999 and regulations. Commercial applicators require a chemical user (AQF3/AQF4) training certificate and must keep records for 3 years.
• Victoria — Agricultural and Veterinary Chemicals (Control of Use) Act 1992. Agricultural chemical user permit required for certain chemicals.
• Queensland — Chemical Usage (Agricultural and Veterinary) Control Act 1988. Commercial Operator Licences (ACDC) required.
• Western Australia — Agricultural and Veterinary Chemicals (Western Australia) Act 1995. Licensing for certain S7 chemicals.
• South Australia — Agricultural and Veterinary Products (Control of Use) Act 2002. Reportable Priority Chemicals program.
• Tasmania — Agricultural and Veterinary Chemicals (Control of Use) Act 1995.

Record-keeping minimum

Most states require at minimum:

• Product name and APVMA number
• Batch number (from container label)
• Date, time, duration of application
• Rate per hectare and total volume
• Target weeds/pest/disease and growth stage
• Crop and growth stage
• Wind speed, direction, temperature, humidity
• Applicator name and equipment used
• Nozzle type, water rate, boom pressure
• Withholding periods observed

PPE and safety

Every product label specifies required PPE under "Safety Directions". At minimum for broadacre herbicide work:

• Coveralls or long trousers + long-sleeved shirt
• Chemical-resistant gloves (nitrile or rubber, not latex)
• Eye protection (safety glasses minimum, goggles for mixing)
• Enclosed footwear
• Respiratory protection for S7 chemicals (paraquat, indaziflam, etc.) during mixing — often a full-face respirator
• Tractor/rig cabin filtration meeting AS/NZS 4309 where available

Drift and off-target damage

Spray drift onto neighbouring properties is a tort (property damage + economic loss) as well as a regulatory offence. Particular attention for 2,4-D, dicamba, and other volatile phenoxy auxins near sensitive crops (cotton, grapes, horticulture, pulses). Check the state EPA for drift regulations and sensitive-crop registers before applying.

Sources & further reading

• WeedSmart — Big 6, resistance mapping, regional case studies.
• AHRI (Australian Herbicide Resistance Initiative) — random surveys, mechanism research, AHRI Insight newsletter.
• GRDC — GrowNotes, paddock practice updates, harvest weed seed control trials.
• CropLife Australia — HRAC rotation strategies, group classification updates.
• HRAC Global — global mode-of-action group reference.
• APVMA PubCRIS — registered products, active ingredients, labels.