Herbicide Site-of-Action & Rotate Groups, Outrun Resistance

A herbicide's site of action (also called mode of action) is the specific biochemical target it attacks in the plant — for example EPSPS for glyphosate (Group 9) or ALS/AHAS for the sulfonylureas (Group 2). Weeds develop resistance to a site of action, not to a brand, so spraying the same group season after season selects for survivors that carry resistance to that target. Rotating to a different site of action each season means a weed that survived one group is still killed by the next, which is the single most effective way to slow resistance.

HRAC is the Herbicide Resistance Action Committee, and its Global Classification assigns every herbicide a group number based on its site of action. The 2020 revision harmonised the old letter codes and regional numbers into a single 1-to-N numbering shared worldwide. This planner carries fifteen of the major groups — including Group 1 (ACCase), Group 2 (ALS), Group 4 (synthetic auxins), Group 5 (PSII), Group 9 (EPSPS / glyphosate), Group 14 (PPO), Group 15 (VLCFA) and Group 27 (HPPD) — with an intrinsic resistance-risk rating for each.

It starts at zero and adds penalties: 14 points for every back-to-back repeat of the same group, 18 points for each distinct group your target weed already resists, and a weighting for solo use of intrinsically high-risk groups. It then adds the weed's own documented resistance pressure, log-scaled from its number of resistant biotypes, and subtracts a diversity bonus of 9 points for each distinct site of action beyond the first. The result is clamped to 0–100, where under 30 is good, 30–59 is moderate and 60 or more is high risk.

The rotation track draws one tile per season, coloured by its HRAC group. When a season repeats the group used immediately before it, the tile is outlined in red and resistant-weed sprouts appear beneath it — and the longer the same group runs unbroken, the more sprouts spread. It is a visual stand-in for the real biology: each repeat of a site of action multiplies the selection pressure and lets a resistant population build.

Each weed in the database carries the list of HRAC groups it is documented to have evolved resistance to. If your rotation applies one of those groups, the planner flags it: that pass may simply fail on the resistant population, wasting the spray and the trip. For example, Palmer amaranth widely resists Groups 2, 5, 9, 14 and 27, so building a program around glyphosate (Group 9) alone against Palmer amaranth is a known dead end.

Ten of the world's most resistance-prone weeds: Palmer amaranth, waterhemp, annual ryegrass, black-grass, horseweed (marestail), kochia, goosegrass, wild oat, junglerice and the common/giant ragweed complex. Each carries its approximate number of documented resistant biotypes and the HRAC groups it most often resists, drawn from the international resistant-weed database.

Rotation helps, but the strongest programs also use multiple effective sites of action in the same pass — a tank-mix where two different groups both control the target weed — plus residual herbicides layered through the season, full label rates, and timely application to small weeds. Resistance evolves fastest under a single, repeated, sub-lethal selection pressure, so diversity in both rotation and mixture is what keeps a group working.

As a rule of thumb, aim for at least three distinct sites of action across a typical crop-rotation cycle and never apply the same group in consecutive seasons against a high-pressure weed. The planner rewards each distinct group and penalises consecutive repeats precisely to push you toward that diversity; the more independent ways you have to kill the weed, the slower any one of them is lost.

When the planner detects a problem it suggests a swap-in: the lowest intrinsic-risk HRAC group that is not already in your plan and that your target weed is not documented to resist. Swapping a repeated season to that group breaks the stacked site of action and adds diversity. It is a starting suggestion — always confirm the group is registered, effective and rotation-legal for your crop.

No. 'Low risk' describes the intrinsic likelihood that repeated solo use selects resistance — groups like the VLCFA inhibitors (Group 15) or cellulose-synthesis inhibitors (Group 29) have historically been slower to fail. But any site of action used alone and repeatedly is under selection pressure, and low-risk groups have eventually produced resistant populations too. Rotate them as well; the rating only tells you where the pressure is highest.

Yes, where it still works — but it is the textbook high-risk case. Glyphosate's intensive solo use across glyphosate-tolerant crops selected resistance in dozens of weeds, including Palmer amaranth, waterhemp, horseweed and annual ryegrass. Used inside a diverse rotation, mixed with a second effective site of action and backed by residuals, it remains a valuable tool; used alone year after year, it is the fastest route to a resistant field.

Treat them as planning figures. The risk score, group ratings and resistance lists are reconciled from the HRAC Global Classification and the international resistant-weed database, but local resistance status, crop labels, rotation restrictions and plant-back intervals all matter and vary by region. Use this planner to design and compare rotations, then confirm the specific products and groups with your agronomist and the current labels.