NATURE OF TRIAZINE RESISTANCE
Russell R. Hahn
Department of Agronomy
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
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 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
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
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.
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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