Herbicide Resistance

Herbicide resistance is a growing problem for Southern row crop growers because it limits their ability to make maximum use of herbicide resistance traits in genetically modified crops of cotton, corn and soybeans. Herbicide resistance is defined as the “inherited ability of a plant to survive and reproduce following exposure to a dose of herbicide normally lethal to the wild plant,” says Joe DiTomaso, weed scientist with the University of California Davis.

“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 provides the selection pressure, susceptible plants are eliminated while herbicide-resistant plants survive to reproduce without competition from susceptible plants.”

Weed resistance is not a new phenomenon, but is somewhat less known and experienced than insecticide or fungicide resistance. The first report of HR occurred in 1960 with the discovery of Triazine-resistant common groundsel. Since that time, 216 weed biotypes that cannot be controlled with recommended doses of herbicide have evolved around the world.

Factors leading to the development of herbicide resistance: Because weeds contain a tremendous amount of genetic variation that allows them to survive under a variety of environmental conditions the development of a resistant species is brought about through selection pressure imposed by the continuous use of an herbicide. Long residual pre-emergence herbicides or repeated application of postemergence herbicides will further increase selection pressure.

Factors that can lead to or accelerate the development of herbicide resistance include weed characteristics, chemical properties and cultural practices. Although few, if any, weeds developed resistance to glyphosate herbicide prior to 2000, the development of resistance in horseweed, Palmer amaranth, giant ragweed, waterhemp, Johnsongrass and Italian ryegrass to glyphosate has been problematic for many growers in the Southeast and Mid-South states.

Weed characteristics conducive to rapid development of resistance to a particular herbicide include:

• Annual growth habit.

• High seed production.

• Relatively rapid turnover of the seed bank due to high percentage of seed germination each year (i.e., little seed dormancy).

• Several reproductive generations per growing season.

• Extreme susceptibility to a particular herbicide.

• High frequency of resistant gene(s), (e.g. Lolium rigidum).

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