Definition of Herbicide Resistance

Joe DiTomaso, UCCE Weed Specialist has stated, "Herbicide resistance is the inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild type. In contrast, tolerance can be defined as the inherent ability of a plant to survive a herbicide treatment at a normal use rate. In a plant, resistance may be naturally occurring or induced by such techniques as genetic engineering. Resistance may occur in plants by random and infrequent mutations; no evidence has been presented to demonstrate herbicide-induced mutation. Through selection, where the herbicide is the selection pressure, susceptible plants are killed while herbicide resistant plants survive to reproduce without competition from susceptible plants. Thus, the appearance of herbicide resistance in a field is an example of rapid weed evolution."

Status of Herbicide Resistance

Herbicide resistance is not a new phenomenon, and in the past has occurred less frequently than insecticide or fungicide resistance. The first report of herbicide resistance was in a Washington tree nursery in 1970, when common groundsel could not be controlled with simazine, a triazine compound. Since that time, 216 weed biotypes around the world have developed resistance to herbicides.

In California, herbicide resistance today is most widespread among aquatic weeds in rice production (Table 1). Many of these weed species have been selected for resistance to the sulfonylurea herbicide, bensulfuron (Londax). It was just recently reported at the California Weed Society meeting last month that horseweed (marestail), Conyza canadensis, has shown resistance to glyphosate. Locally, some observations are indicating that in one area of the Valley, littleseed canary grass is showing a high tolerance to sethoxydim (Poast).

Table 1. Specific instances of herbicide-resistant weeds in California and situations of occurrence (for additional information or updates see: University of California Research and Information Center web site at www.wric.ucdavis.edu).

The appearance of herbicide resistance in plants is increasing at an exponential rate, mirroring the trends previously seen with insecticide and fungicides in recent years. Despite these examples, there are still relatively few reports in California compared to the rest of the United States and the world. The diversity of California's agriculture allows growers to rotate to many different crops, which reduces the selection pressure of the herbicide on target weeds. An exception to this is in the rice growing areas where a single crop is grown, and dependence on one herbicide has lead to resistant weed species.

Another area of concern is the genetically engineered crops that are resistant to certain herbicides, like the various "Roundup Ready" crops that are on the market. Farmers who grow these crops will tend to rely more heavily on a single herbicide. Such a strategy will likely select for weed biotypes that are resistant to that herbicide or mode of action. Integrating other herbicides into the system along with mechanical weed control will help delay the selection process of resistance.

Resistance Management Approaches

The first step to preventing herbicide resistance is early detection. Scout fields and be on the lookout for patterns that would indicate resistance. Whole fields infested with weeds or strips of weeds do not typically indicate resistance. A common pattern of weed resistance is patches of dense populations with lower population areas radiating out from the central patch and escapes scattered in no particular pattern throughout the field.

Rotate Crops

The potential for herbicide resistance should receive serious and thoughtful attention. Crop rotation is one of the best tools for preventing resistance. Rotation to different crops allows the grower to use a greater range of both chemical and non-chemical control methods. As weed management systems change with new herbicides, and herbicide resistant crops are introduced, resistance management must be an integral part of the production system. If selection pressure is maintained through the continuous use of the same herbicide on the same crop, or even on different crops, weed selection is likely to occur and a new resistant biotype will eventually emerge.

Condensed from "Herbicide Resistance: Definition and Management Strategies". For more information, see website www.wric.ucdavis.edu.

Full Disclaimers

Issued in furtherance of Cooperative Extension work, acts of May 8 and June 30, 1914, in cooperation with the U.S. Department of Agriculture, James A. Christenson, Director Cooperative Extension, College of Agriculture and Life Sciences, The University of Arizona.

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