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Triazine Resistance



Russell R. Hahn
Department of Agronomy
Cornell University

Herbicide resistance occurs when weeds that are normally easy to control with a herbicide or group of herbicides are no longer controlled with those same herbicides. Although herbicide resistance was documented for other herbicides such as 2,4-D and the dinitroanilines (Balan, Treflan, etc.) prior to the discovery of triazine resistance, the occurrence of triazine-resistant weeds has been of greater interest because of the importance of these herbicides to modern agriculture and because of the large number and wide distribution of triazine-resistant species.

Tolerance Versus Resistance

Triazine herbicides like atrazine, Bladex, and Princep inhibit photosynthesis -- the process by which chlorophyll-bearing plants convert water and carbon dioxide into sugar using light energy. This inhibition occurs in both susceptible and tolerant species.

Tolerant species such as corn and certain weedy annual grasses like fall panicum and crabgrass readily absorb triazine herbicides. Their tolerance to certain triazines is based on the rapid metabolism of these triazines to non-toxic substances.

Susceptible species are not capable of this rapid transformation. Triazine accumulation in these species leads to chlorosis or yellowing of leaves followed by necrosis or death of leaf tissue.

While tolerance refers to species that never were sensitive to a given herbicide, resistance refers to those weeds that survive and grow normally at herbicide rates that were originally effective in controlling a given weed species.

Nature of Resistance

Since 1970, there have been numerous reports of triazine-resistant biotypes within weed species that were originally very sensitive to triazine herbicides. In most cases, these biotypes or strains have first been identified in areas with a history of repeated triazine application. The resistant biotypes are usually not different in appearance from the susceptible strains and without exposure to triazine herbicides, would not have been recognized.

In addition, the resistant biotypes are not considered to be genetic mutations but strains which occurred in the original population in low numbers compared to the dominant susceptible strains.

It is estimated that one plant in every one million to one trillion was of the resistant strain prior to the use of triazine herbicides. However, with the continuous and often exclusive use of triazine herbicides over a number of years, the susceptible biotypes were controlled while the resistant ones increased in number until they dominated the population. From six to ten years of continuous exposure might be needed for this shift to occur.

Triazine resistance is considered absolute since extremely high use rates are not effective. In addition, there is cross-resistance among the symmetrical triazines. Resistant biotypes that developed with repeated atrazine use are not controlled with Bladex or Princep.

Widespread Distribution

Reports of triazine-resistant weeds have increased rapidly since the first report in 1970. To date, triazine resistance has been confirmed in at least 30 different weed species in the United States, Canada and Europe.

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