CWSS-SCM Rating Scale

Description of 0-100 Rating Scale for Herbicide Efficacy and Crop Phytotoxicity.

Canadian Weed Science Society / Société canadienne de malherbologie (CWSS-SCM)

Updated: January 15, 2018

The 0-100 scale is used to rate both weed control efficacy and crop tolerance, where 0 = no plant phytotoxicity, and 100 = complete plant death.

A) Evaluation of Herbicide Efficacy

The assessment of herbicide action of a product is based on the comparison of the treated plot with an untreated control plot. The aim is to assess as accurately as possible the decrease in growth (density, biomass, height, etc.) per weed species as compared to the control. The decrease in growth is attributed to the action of the product. This reduction can be expressed by means of a linear scale (Table 1). The assessment is subjective and it is important that this assessment does not measure actual weed densities, weed biomass, height, etc..

Table 1: CWSS-SCM rating scale for herbicide efficacy.

Activity Range Description of Control Suggested Interval size
91-100% Very Good to Excellent 2%
80-90% Good to Very Good 5%
60-79% Suppression 5%
< 60% Poor 10%

Without an exact count or weight, there are limits to the accuracy of assessment even for the practiced eye. It has therefore been found useful to aim for a differentiation of approximately 2% exactitude in the very good to excellent action range, but below that to estimate for an accuracy of no more than 5% or 10 %. For herbicide registration, the Pest Management Regulatory Agency (PMRA) definition of herbicide performance claims are presented in Table 2.

Table 2: PMRA Summary of performance claims for herbicides (Pest Management Regulatory Agency, 2016).

Label Claim Performance standards
Control A consistent level of weed management, as defined by commercial standards and expectations in the market. In general, weed control ratings would be at least 80%.
Suppression A consistent level of weed management that is less than full control, as defined by commercial standards and expectations in the market. In general, weed control ratings would be at least 60%.
Partial Suppression A level of weed management that is less than suppression, as defined by the commercial standards and expectations in the market. This label claim will generally be considered only for non-conventional herbicides. In general, weed control ratings would range between 30% and 60%
Top Growth Control For perennial weeds where consistent reduction of top growth has been demonstrated in the year of treatment.

B) Evaluation of Crop Tolerance (Phytotoxicity)

The same basic principals apply. The evaluation should again be done with a comparison to the untreated control. It is much easier to assess crop tolerance in weed-free plots since the results aren’t confounded from weed competition in the untreated controls. If crop tolerance is to be rated in a weed control efficacy trial, then one should include a hand weeded control plot in the study.

Table 3: CWSS-SCM rating scale for crop tolerance.

Phytotoxicity Range Assessment of Injury1 Suggested Interval size
0-9% Slight Discoloration and/or Stunting 2%
10% Just acceptable
11-30% Not Acceptable 5%
> 30% Severe 10%

110% or less is considered acceptable injury (Pest Management Regulatory Agency, 2016).

Initial Damage of up to 10% will generally be outgrown and will disappear over time. The impact of these low levels of injury will not be reflected in yield losses.

Crop injury from herbicide application can manifest itself in many different ways such as chlorosis, necrosis, stunting, height reduction, biomass reduction, and mortality. The person rating the injury may want to rate the injury as general phytoxicity, or rate specific symptoms (eg. % chlorosis, % height reduction, etc.).

How to Cite Us:

Canadian Weed Science Society. 2018. Description of 0-100 rating scale for herbicide efficacy and crop phytotoxicity [Online] Available: [2018 Jan. 151]

1 Insert the date you accessed this site.


Pest Management Regulatory Agency. 2016. Value guidelines for new plant protection products and label amendments. [Online] Available: PMRA Value Guidelines for New Plant Protection Products and Label Amendments [2018 Jan 15].