How do we define herbicide resistance, susceptibility, and tolerance?

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Herbicide resistance is the naturally occurring, inheritable ability of an individual plant or population to survive a herbicide application that would kill a normal plant or population of the same species. These surviving plants can mature and produce seed, and the resistance trait is passed on to the next generation. Herbicide susceptibility means the weed or crop are killed by the normal use rate of the herbicide; as an example, the foxtails and other annual grasses are susceptible to the Group 15 herbicides such as s-metolachlor. In contrast, herbicide tolerance is the inherent ability of an entire plant species to survive and reproduce after herbicide treatment at a normal use rate. This term is generally used to describe both weedy plants and crops that were never susceptible to certain herbicides. For example, many broadleaf weeds are susceptible to the group 4 synthetic auxin herbicides, while grassy weeds like giant foxtail and johnsongrass are naturally tolerant. Most grassy weeds are susceptible to the group 1 grass-specific herbicides, while all broadleaf or dicot weeds are tolerant to those herbicides.

Wild oats in wheat in Washington. (Photo credit: Henry Wetzel, Washington State University)
Kochia in a wheat fallow field in Kansas. (Photo credit: Vipan Kumar, Cornell University)
Italian ryegrass in wheat in soybeans in Virginia. (Photo credit: Claudio Rubione, GROW)
Palmer amaranth escapes in a row in Maryland. (Picture credit: Ben Beale, University of Maryland Extension)
Horseweed in soybeans in Delaware. (Photo credit: William Curran, Penn State University)
Waterhemp in soybeans in Illinois. (Photo credit: Aaron Hager, University Illinois)
Giant ragweed in soybeans in Wisconsin. (Photo credit: Rodrigo Werle, University Wisconsin)
Johnsongrass in Texas. (Photo credit: Claudio Rubione, GROW)

Figure 1. Examples of key herbicide-resistant weed species across the U.S.

Resistance in a plant may be naturally occurring or it may be introduced through genetic engineering or other techniques. By definition, crops that have been developed to withstand herbicides that previously would kill them (e.g. the Roundup Ready trait) are herbicide-resistant crops. Herbicide-tolerant crops have the natural, inherent ability to survive the herbicide.
The level of herbicide resistance in weeds can vary due to weed biology as well as the resistance mechanism. A weed with low-level resistance might only survive the labeled herbicide rate, while some species with high-level resistance might survive a dose 100X the labeled rate or greater. A population of common ragweed in Delaware is resistant to the group 2 herbicide cloransulam (FirstRate) and can survive at least a 25X dose (Figure 2). Some of these differences are due to the type of resistance that evolved, which can include target-site and non-target-site. Read more about these resistance types under Section 8.

Figure 2. Two populations of common ragweed sprayed with three rates of cloransulam (FirstRate). The top row is the resistant population and the bottom row is the susceptible population. From left to right are untreated plants, and those treated with cloransulam at 1X, 5X, and 25X a normal field use rate. (Photo credit: Mark VanGessel, University of Delaware)