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ALSase Inhibitors:


Developed by American Cyanamid, coincided with development of sulfonylurea herbicides. Share many characteristics with that family.

Members include imazaquin (Scepter), imazethapyr (Pursuit; 1984), imazapyr (Arsenal; non-selective), imazamethabenz

Imazaquin is used for broad spectrum weed control (cocklebur, pigweed spp., Pennsylvania smartweed) in soybeans. It can be applied preemergence, preplant incorporated or postemergence. Soil applications require rainfall or irrigation to move the herbicides into the soil water for plant uptake.

Herbicides in this family have exhibited physical incompatibility with 2,4-D and MCPA when tank-mixed.

The imidazolinones were the second family introduced that have high specific activity: low (grams) active ingredient application rates per acre.


Despite many similarities between the imidazolinones and the sulfonylureas, structurally they are quite different.

Physiology and Metabolism of the Imidazolinones

Mode of Imidazolinone Action

The imidazolines act by inhibiting branched chain amino acid (valine, isoleucine, leucine) biosynthesis. Specifically, they inhibit the catalytic action of acetolactate synthase (ALSase), also known as acetohydroxyacid synthase (AHASase; the name prefered by American Cyanamid). Refer to sulfonylurea mode of action discussion.

Mode of Imidazolinone Lethality

Death results from starvation for needed proteins in growth. Refer to sulfonylurea mode of action discussion.

Uptake and Movement of Imidazolinones in plants

Imidazolinones are readily absorbed by roots and shoots.

They translocate both apoplastically in the xylem and symplastically in the phloem.

They readily move to the plant meristem where they exert their toxic action, growth stopping fairly quickly.

Basis of Selectivity between Plant Species

In general, imidazolinone herbicides are more active against dicot species relative to monocot species.

Imazethapyr selectivity is not based on differences in uptake or translocation, nor is it based on differences in sensitivity of the ALSase target in plant species. It is based on metabolic detoxification differences between species.

Herbicide Resistance.
-Most weed species found resistant to sulfonylurea herbicides have been shown to also be resistant to imidazolinone herbicides.
-Past work by Molecular Genetics Co., in Minnesota, found imazaquin resistant corn cell lines that were subsequently grown to whole plants. These resistant lines were given to Pioneer Seed Co. which has now developing a high-yielding line of corn seed resistant to imazaquin. Garst introduced an imidazolinone resistant corn hybrid in 1991. Many feel this is the solution to imazaquin carryover problems

Fate of the Imidazolinones in the Environment


There is incomplete information on the fate of many imidazolinone herbicides in the soil.

The persistence of them in the soil varies, but many of the more commonly used members are persistent in the soil and can persist until subsequent crops in a rotation are grown, causing injury.

Imazaquin persistence in the soil
-Probably a function of hydrolysis in the sunlight: Photohydrolysis. This mechanism is not well characterized.
-Soil pH does not appear to play the dominant role in degradation that it does with the sulfonylureas.
-Imazaquin persists longer under droughty conditions, with cold weather, and in the absence of tillage.
-Some breakdown of imazaquin in aerobic soils is microbial, little breakdown occurs under anaerobic conditions.
-Imazaquin will persist for long periods of time in the soil because photohydrolysis degradation requires the presence of both light, herbicide and moisture. Typically, imazaquin is carried into the soil by rainfall where light is not present. If conditions are dry this will not occur, light will be present but moisture will not be present.

The summer of 1988 in Iowa created conditions that led to extensive imazaquin carryover in the 1989 maize crop. Imazaquin was widely used in the state, including in the Des Moines lobe glacial soil region typified by high pH soils originating from calcareous parent materials. Application of imazaquin in soybeans was preceded, and followed, in most areas by very dry weather. This also resulted in soybean injury in many, if not most, fields. Take a look at a case study from near Madrid, Boone Co., Iowa for how those injury symptoms looked that year. When imazaquin was first registered for commercial use, it was not registered for weed control in Iowa. Later, the label was rewritten to include Iowa. Soybean plants were subsequently injured, probably due to unfavorable plant growth and detoxification metabolism necessary for crop safety. Many corn growers were concerned with the effect these potential imazaquin residues might have on their 1989 corn crop. Moldboard plowing was extensively used in many fields in the fall of 1988 in an attempt to dilute the imazaquin residues in the soil profile and thereby avoid crop injury in 1989, partially solving the immediate problem but causing extensive winter wind erosion with this tillage.


Imazaquin is not lost from the soil due to volatility. It can drift off-target and cause injury to susceptible species such as corn, cotton or vegetable crops.


Imazaquin solubility in water is 60-120 ppm at 25oC.

Animal Toxicity

ALSase does not occur in animals which rely on plants for these essential amino acids. As such, the primary toxicity of these herbicides is plant specific. Toxicity is similar to that of the sulfonylureas

Plant Injury Symptomology of the Imidazolines in Plants

Symptoms often develop slowly.

Plants often emerge and symptoms develop after that time.

Susceptible plants stop growing due to meristem inhibition. Meristems can become necrotic at this point. This inhibition of apical dominance can lead to bud proliferation lower on the stem of some species (e.g. morningglory, Imopoea spp.).

Soybeans can exhibit internode shortening under stressful growth conditions such as excessive rainfall or cool temperatures.

Young leaves become deformed, chlorotic (sometimes interveinal) and purple, anthocyanin, discolorations can occur.

Most of these symptoms are held in common with sulfonylurea herbicides.

Chlorimuron-ethyl injury can sometimes be differentiated from that caused by imazaquin by better cocklebur and foxtail spp. control with imazaquin, and better velvetleaf control with chlorimuron-ethyl.


Chemical Weed Tactics | ALSase Family
Sulfonylureas | Imidazolinones | Triazolopyrimidines