Score the Risk & Save the Chemistry
Scores SOA reliance
Herbicide resistance is built one repeated mode of action at a time. Rate your sites-of-action diversity, rotation, reliance, non-chemical tactics and seed-bank against the weed to get a 0–100 resistance-risk score, the dominant factor, and the single highest-leverage fix.
Rate your weed & practices
Reliance on one mode of action is the biggest driver of resistance — diversity is the cheapest insurance.
Next: Diversify sites-of-action — tank-mix or rotate 3+ effective SOAs and add a residual herbicide.
Risk model adapted from HRAC resistance risk-assessment guidelines, the WSSA Best Management Practices (Norsworthy et al. 2012) and Beckie (2006). Mode-of-action reliance carries the largest weight; the weed's intrinsic trait risk modulates the management score.
Herbicide resistance — key facts
- Biggest driver
- reliance on one site of action (SOA)
- WSSA target
- ≥ 2–3 effective SOAs per weed per season
- Severe score
- 75–100 (single SOA + monoculture + heavy seed-bank)
- Palmer amaranth
- 200,000–600,000 seeds/plant, 9 SOAs resistant
- SOA weight
- 30 of 100 management points
- Seed-bank fix
- never let escapes set seed
- Residuals
- add an SOA + cut early escapes
- Privacy
- Runs in your browser; nothing uploaded
Weed intrinsic resistance risk
| Weed | Trait risk | Seeds / plant | Breeding system | Resistance status |
|---|---|---|---|---|
| Palmer amaranth | Very high | 200,000–600,000 | Dioecious (obligate outcross) | Resistant to 9 SOAs globally |
| Waterhemp | Very high | 250,000 | Dioecious (obligate outcross) | Multiple resistance incl. HPPD, PPO |
| Italian ryegrass | Very high | 45,000 | Obligate outcrosser | Multiple resistance worldwide |
| Kochia | Very high | 30,000 | Outcrossing, tumbleweed spread | Glyphosate + dicamba + ALS resistant |
| Rigid (annual) ryegrass | Very high | 45,000 | Obligate outcrosser | World's worst resistant grass |
| Black-grass | Very high | 6,000 | Outcrossing | Multiple resistance, UK/EU |
| Horseweed (marestail) | High | 200,000 | Self + wind-dispersed | Glyphosate + ALS resistant |
| Wild oat | High | 250 | Mostly self-pollinating | ACCase + ALS resistance common |
| Common ragweed | High | 3,000 | Outcrossing | Glyphosate + PPO + ALS resistant |
| Giant foxtail | Moderate | 10,000 | Self-pollinating | ALS resistance reported |
| Common lambsquarters | Moderate | 70,000 | Self-pollinating | Triazine resistance historic |
| Velvetleaf | Moderate | 8,000 | Self-pollinating | Limited resistance cases |
Source: HRAC resistance risk-assessment guidelines, WSSA best-management practices (Norsworthy et al. 2012, Weed Science), Beckie (2006, Weed Technology) and the International Herbicide-Resistant Weed Database.
Risk factors & their weights
| Factor | Weight | Lowest-risk choice | Highest-risk choice |
|---|---|---|---|
| Herbicide sites-of-action (SOA) used per season | 30 | 4 or more effective SOAs | Single SOA only |
| Crop rotation diversity | 18 | 3+ crops, incl. a competitive break crop | Continuous monoculture |
| Reliance on herbicides as the control tactic | 22 | Integrated — herbicides + several tactics | Herbicides are the only control |
| Non-chemical tactics used | 12 | Several non-chemical tactics used | No non-chemical tactics |
| Weed seed-bank & escapes | 18 | Low seed-bank, fields kept clean | Heavy seed-bank, escapes set seed |
Management weights sum to 100; the weed's intrinsic trait risk adds up to 25 more, capped at a 100-point total.
What the resistance-risk score is
Herbicide resistance is evolution in fast-forward. Spray the same site of action on a weed population season after season and the rare survivors that carry a resistance gene reproduce, until the whole field is resistant and that chemistry is dead. The risk-score model captures the practices that speed up or slow down that process: how many effective sites of action you use, how diverse your crop rotation is, how much you rely on herbicides versus other tactics, whether you use non-chemical control, and how full your seed-bank is. Each is weighted by its real influence — SOA reliance carries the most — and combined into a 0–100 score.
On top of management, the target weed's own biology matters: a high-fecundity outcrosser like Palmer amaranth spreads resistance far faster than a self-pollinating weed, so the tool adds the weed's intrinsic trait risk to your score. The result is a dial that points the way to action — it names your weakest factor and the single highest-leverage practice to fix it, whether that is diversifying sites of action, adding a residual, or driving the seed-bank to zero. Pair it with the Herbicide Site-of-Action Rotation Planner and the Weed Seed-Bank Depletion calculator to build the plan.
How to use it — 5 steps
- 1Pick the weed
Select your target weed; its intrinsic resistance risk and biology load automatically.
- 2Rate SOA diversity
Choose how many effective sites of action you apply against it each season.
- 3Rate your system
Set crop rotation, herbicide reliance, non-chemical tactics and your seed-bank status.
- 4Read the dial
The 0–100 score, its class, and the sub-factor bars show which factor drives your risk.
- 5Apply the top fix
Act on the named highest-leverage practice for your weakest factor first.
Frequently Asked Questions
How is herbicide-resistance risk scored?+
The calculator scores five management factors — sites-of-action (SOA) diversity, crop rotation, reliance on herbicides, use of non-chemical tactics, and the weed seed-bank — each on a 0–1 risk scale and multiplied by a published-style weight. SOA reliance carries the largest weight (30 points) because mode-of-action overuse is the dominant driver of selection. The target weed's intrinsic trait risk is then added on top, giving a final 0–100 score banded as low, moderate, high or severe.
What is the single biggest driver of herbicide resistance?+
Reliance on one herbicide site of action (mode of action). Every time a weed population is sprayed with the same SOA, the rare individuals that survive pass on resistance, and the trait sweeps through the field within a few seasons. That is why SOA diversity carries the heaviest weight in the score and why diversifying — rotating and tank-mixing several effective sites of action, plus adding a residual — is the highest-leverage fix for most farms.
Why does the weed species change the score?+
Some weeds are intrinsically far higher risk. Palmer amaranth and waterhemp are dioecious obligate outcrossers producing hundreds of thousands of seeds per plant, so resistance genes spread and multiply explosively — both already show resistance to many sites of action. Velvetleaf or lambsquarters, which are self-pollinating with lower fecundity, spread resistance more slowly. The tool adds the weed's intrinsic trait risk to your management score, so the same practices score worse against a high-risk weed.
What counts as a high resistance-risk score?+
Scores above 75 are severe, 50–75 high, 25–50 moderate and under 25 low. A continuous monoculture sprayed with a single site of action, no tillage or cultural tactics and a heavy seed-bank — on a weed like Palmer amaranth — maxes the score at 100. A diverse rotation using four-plus effective sites of action, integrated tactics and a clean seed-bank on a low-risk weed can score under 10.
How many herbicide sites of action should I use?+
The WSSA best-management practices recommend applying at least two, and ideally three or more, effective sites of action against each target weed every season — through tank-mixes and sequential applications, not just across years. Using a single effective SOA scores the maximum risk on that factor; using four or more drops it to zero. Always confirm each product is actually effective on your weed, since an ineffective partner adds no resistance protection.
Does no-till increase resistance risk?+
It can, because removing tillage removes one non-chemical control tactic and increases reliance on herbicides. The calculator captures this through the non-chemical-tactics factor: a system with no tillage, cover crops, or harvest weed-seed control scores higher. No-till has real soil-health benefits, so the answer is not necessarily to till but to add other diversity — cover crops, narrow rows, harvest weed-seed control, or strategic targeted tillage of patches.
What is the highest-leverage practice to cut my risk?+
It depends on your weakest factor, which the tool identifies. For most farms it is diversifying sites of action — tank-mixing or rotating three or more effective SOAs and adding a residual. If your weakest link is the seed-bank, the top fix is to never let escapes set seed (rogue survivors, use harvest weed-seed control). The calculator names your dominant risk factor and gives the matching concrete action.
Why does the seed-bank matter so much?+
Every escaped weed that sets seed deposits thousands of seeds — including any resistant survivors — into the soil seed-bank, where they germinate for years. A heavy seed-bank both raises the number of plants exposed to selection and preserves resistant genotypes. Driving the seed-bank down by stopping escapes from seeding is one of the most powerful long-term resistance-management tactics, which is why it carries an 18-point weight.
Can I reverse herbicide resistance once it appears?+
Established resistance rarely disappears, because resistant seeds persist in the seed-bank, but you can stop it spreading and prevent new resistance. The same practices that lower the risk score — diversifying sites of action, rotating crops, adding non-chemical tactics, and emptying the seed-bank — keep the remaining effective chemistries working. Prevention is far cheaper than managing a field already lost to a resistant weed, which is the decision this tool is built to inform.
How do residual herbicides reduce resistance risk?+
Soil-applied residual herbicides control weeds before they emerge, reducing the number of plants that survive to be sprayed post-emergence and adding another site of action to the program. They are a core part of the WSSA recommendation to start clean and overlap residuals. In the score, adding residuals improves your SOA-diversity factor and lowers the seed-bank by stopping early escapes, so they move the dial on two factors at once.
Is this score a regulatory or label requirement?+
No. It is an educational planning aid based on the HRAC resistance risk-assessment guidelines, the WSSA best-management practices (Norsworthy et al. 2012) and resistance-management literature (Beckie 2006). It does not replace the product label, which is the legal authority for rates and rotation restrictions, or local resistance-management mandates. Use the score to prioritise practices, then follow label and regional guidance.