Revised December 21, 2007

*Adapted from Herbicide-resistant Weeds and Their Management, PNW 437, a Pacific Northwest Extension publication (University of Idaho, revised 2007). Authors are Carol Mallory-Smith, George R. Hyslop professor, Oregon State University; Donn Thill, professor of weed science, University of Idaho; and Don Morishita, Extension weed scientist, University of Idaho.

Herbicide resistance is the inherited ability of a plant to survive a herbicide application to which the wild-type was susceptible. Resistant plants occur naturally within a population and differ slightly in genetic makeup but remain reproductively compatible with the wild-type.

Herbicide-resistant plants are in a population in extremely small numbers. Repeatedly using one herbicide allows these few plants to survive and reproduce. The number of resistant plants then increases in the population until the herbicide no longer effectively controls the weed.

Resistant plants likely will persist in infested fields for many years, even in the absence of any additional selection with the herbicide. There is no evidence that herbicides cause the genetic mutations that lead to herbicide resistance.

Resistant plants may be resistant to other herbicides (imidazolinones as well as sulfonylureas, for example) that kill plants in the same way (same site of action or same group). This is called cross-resistance.

Weeds also can be resistant to herbicides with different sites of action (aryloxyphenoxy propionates as well as sulfonylureas, for example). In Australia, a biotype of annual ryegrass is resistant to at least five different herbicide groups. This is called multiple resistance.

Herbicide resistance is not the natural tolerance that some species have to a herbicide. For example, wheat is tolerant to Hoelon because it can rapidly deactivate it. Wild oat can only slowly deactivate Hoelon, so the herbicide can be used selectively to remove wild oat from wheat.

The first identified herbicide-resistant weed—spreading dayflower (Commelina diffusa) resistant to 2,4-D—was identified in 1957 in a sugarcane field in Hawaii. Since then, more than 200 weeds resistant to one or more herbicides have been identified worldwide. Current information on the status of herbicide-resistant weeds can be found at
Herbicide-resistant weeds are now common in the Pacific Northwest:

   • Kochia, prickly lettuce, and Russian thistle resistant to sulfonylurea herbicides (Glean, Amber, Ally, and other Group 2 inhibitors)

   • Wild oat and Italian ryegrass resistant to Hoelon and other Group 1 (ACCase) inhibitors

   • Powell amaranth resistant to triazines and other Group 5 inhibitors

   • Yellow starthistle resistant to Tordon and other Group 4 inhibitors

   • Wild oat resistant to Far-Go and Avenge

The appearance of herbicide-resistant weeds is strongly linked to repeated use of the same herbicide or herbicides with the same site of action in a monoculture cropping system (for example, wheat after wheat) or in noncrop areas (railway or road rights-of-way, etc.). To manage herbicides to delay and prevent the appearance of herbicide-resistant weeds, you must know in which chemical family a herbicide belongs and which herbicides have the same site of action.

The table below lists herbicides by group number and site of action, chemical family, common name, and trade name, and it gives examples of resistant weeds. Herbicide families that have the same site of action are the same group number.

A herbicide program to prevent resistance does not use herbicides from the same group more than once in 3 yr.

Tank-mixing herbicides is not an effective resistance management strategy. If herbicides in the tank mixture control different weed species and have different soil residual characteristics, resistant weed biotypes will continue to be selected. For example if a long-residual (Glean) and a short-residual (2,4-D) herbicide are tank-mixed, both herbicides may control emerged broadleaf weeds. However, Glean will continue to control weeds throughout the growing season and could continue to select for resistant plants. Tank-mix only when a herbicide combination is required to control the weed spectrum or will reduce herbicide use rates. Tank-mixing for other reasons is not economically or ecologically sound.
Management can prevent or delay the appearance of herbicide-resistant weeds. The following practices can be used with the information on herbicide families provided in the table to form a herbicide resistance management strategy.

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