Score the Risk & Save the Chemistry

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.