How to read efficacy ratings
FieldMarshal compares weed-guide ratings and label claims on one percent control scale. Each rating becomes a range, such as 80–90%, drawn from the guide or assigned by FieldMarshal.
The guide's scale comes first
We use a guide's own percentages wherever it defines them. Sections without a scale use the guide's scale from elsewhere; a section with its own scale keeps it. For example, we use North Dakota's main scale for its fall tables. [1]
Only guides without a percent scale for their grades use these FieldMarshal defaults:
| Guide grade | Default percent control |
|---|---|
| E | Excellent: 90–100% |
| G | Good: 80–90% |
| F | Fair: 70–80% |
| P | Poor: 0–70% |
Our Excellent and Good defaults start at the same percentages as North Dakota's grades. [1] We chose Fair's 70% floor to match the EPA's historical control benchmark and use Poor for the values below it. [2]
The letters you see in FieldMarshal summarize the resulting range. They can differ from the guide's letters when its scale differs from ours. North Dakota calls 65–80% Fair; our Fair starts at 70%, so an ND F shows as P-F. [1] Tap a rating to see its percent range and source; we use that range to rank products.
How we compare label claims
Ranges for label wording
We assign "control" 70–100% and both "partial control" and "suppression" 60–80%. These ranges let us compare claims; they are neither product measurements nor guarantees.
The United States Environmental Protection Agency's 1982 Pesticide Assessment Guidelines, Subdivision G, §94-3(a)(1), gives a usual control benchmark of at least 70% reduction in weed stand or vigor. We use that historical benchmark for comparison, though it is not a universal definition for every label. [2]
Partial control and suppression overlap in meaning. Kentucky extension describes partial control as stunting or suppressing growth, so we give both claims the same range. [3]
Canada's Pest Management Regulatory Agency gives a general minimum of 80% for control and 60% for suppression. [4] The Canadian Weed Science Society places suppression at 60–79%. [5] These numbers support our comparison range, but its 80% ceiling is our choice.
We found no US agency standard defining percentages for partial control or suppression in the guidance we reviewed. The EPA's herbicide guidance discusses both without assigning numerical bands; its later general guidance and Label Review Manual give no such bands either. [2] [6] [7]
How we rank products
For each weed, we choose the first available source in this order:
- Your state weed guide. We use the Pacific Northwest handbook for Oregon, Washington and Idaho.
- WinField ratings.
- The product label's claim.
In states without a guide, we use WinField and then labels. We do not substitute a neighboring state's guide.
For each of your weeds, we take the lowest control the source supports and add those numbers across your weeds. The higher total ranks first because it gives us the strongest combined minimum across the weeds you want to control. If those totals tie, the higher top-end total wins; if both tie, more guide ratings put a product ahead.
For example, in an Iowa soybean comparison, Surveil has waterhemp at 81–100% and giant ragweed at 91–100%; Fierce XLT has waterhemp at 91–100% and giant ragweed at 81–90%. Both low-end totals are 172 (81 + 91). Surveil ranks first because its high-end total is 200 (100 + 100), compared with Fierce XLT's 190 (100 + 90).
No data is different from no control
N means the source reports no control, which we count as 0%. No data gets no percent range because control is unknown. A guide's N can mean either: North Dakota uses N for no data in its fall tables, so we leave those ratings without a range. [1]
Sources
- 2026 North Dakota Weed Control Guide. North Dakota State University Extension. PDF pp. 7 and 14: fall-table N = No data; main rating scale. Our copy; original; retrieved 2026-10-01.
- Pesticide Assessment Guidelines, Subdivision G: Product Performance (EPA 540/9-82-026). US EPA. §94-3(a)(1), printed p. 174 (PDF p. 183). Our copy; original; retrieved 2026-10-01.
- HO-85: Honeyvine Milkweed Control in Tree Fruits, Small Fruits, and Grapes. University of Kentucky Cooperative Extension. p. 2, Table 1 and notes. Our copy; original; retrieved 2026-10-01.
- PMRA Guidance Document, Value Guidelines for New Plant Protection Products and Label Amendments (27 July 2023). Health Canada, Pest Management Regulatory Agency. §2.2.4.1, Table 1, printed pp. 9-10 (PDF pp. 13-14). Our copy; original; retrieved 2026-10-01.
- Description of 0-100 Rating Scale for Herbicide Efficacy and Crop Phytotoxicity (browser print-to-PDF). Canadian Weed Science Society / Société canadienne de malherbologie. Tables 1 and 2. Our copy; original; retrieved 2026-10-01.
- Product Performance Test Guidelines OPPTS 810.1000: Overview, Definitions, and General Considerations (March 1998). US EPA. §§(a)(2), (b)(2), (d)(1), (d)(3). Our copy; original; retrieved 2026-10-01.
- Label Review Manual (December 2024 compilation). US EPA. Chapter 12, pp. 12-2 and 12-6 (PDF pp. 196, 200). Our copy; original; retrieved 2026-10-01.