Herbicide Families

Chapter 10

 

GLYPHOSATE AND GLUFOSINATE

 

Effects of Glyphosate on Corn and Cotton Pollen and Effects of Simulated Glyphosate Drift on Cotton and Tobacco 

 

Glyphosate is formulated as an isopropylamine salt of N-(phosphonomethyl)-glycine, a diammonium salt of glyphosate, or a trimethylsulfonium salt of glyphosate.  Glufosinate is the sulfonium salt of glyphosate, and it is formulated as this salt.  Following foliar absorption, the salts of these herbicides ionize, and the anion (common to both glyphosate and glufosinate) is the herbicidally active portion.  The chemical structures of glyphosate and glufosinate are:

  


Glyphosate


Glufosinate

 

Principal Use:

 

  Glyphosate is principally used in the nonselective POST control of annual, biennial, and perennial grass and broadleaf weeds, woody brush and trees, aquatic weeds, and for selective POST weed control in glyphosate tolerant soybean, cotton and corn.  Tolerant varieties in wheat and some trees are also being developed.

  

Glufosinate is used as a nonselective, POST herbicide for control of annual and perennial grasses and broadleaf weeds in non-bearing groves, orchards, and vineyards, and in noncrop areas.  Glufosinate tolerant corn and soybeans have been developed, and research is underway with cotton and rice.

  

Mode-of-Action:

 

Glyphosate inhibits the shikimic acid pathway by inhibiting 5-enolpyruvylshikimate-3-phosphate (EPSP) synthase.  It also inhibits the synthesis of  -aminolevulinic acid (ALA), thereby blocking porphyrin ring synthesis and, subsequently, all compounds normally containing porphyrin rings such as chlorophyll, cytochromes, and peroxidases.

 

Glufosinate inhibits gluatmine synthetase, which causes a rapid buildup of ammonia in cells. 

  

Translocation:

 

Following foliar application, glyphosate is readily absorbed and translocated throughout above-ground foliage and underground plant parts.  Glufosinate is rapidly absorbed by the foliage.  Unlike glyphosate, only very limited if any translocation occurs with glufosinate.

  

 

Selectivity:

 

Both herbicides are nonselective and are used postemergence.  They are strongly adsorbed to soil colloids, and have little or no soil activity.  Selectivity is achieved by use of directed or shielded sprays to avoid contact with the foliage and green stems or bark of desired plants, and by applications made prior to crop planting or emergence.

  

Characteristics:

 

1.         These herbicides are essentially nonvolatile and photostable.

 

2.         Glyphosate is generally nonpersistent and in soil has a half-life less than 60 days.

  

3.         Glyphosate does not leach in soil nor is it herbicidally active due to strong adsorption to soil colloids.  Therefore, even though the salts of glyphosate are very water soluble, they generally do not leach.

  

4.         In soil, microbial degradation is the principal degradation pathway.

 

 5.        Plants do not readily metabolize glyphosate.

 

 6.        It has a mammalian acute oral LD50 of greater than 4000 mg/kg.

 

 7.        Next to flea collars for pets and Raid bug spray, the American public is probably more familiar with Roundup than any other pesticide.

   

8.         Since glyphosate is applied POST and has no soil activity, foliar absorption is required for activity.  Rapidly growing, non-stressed plants of most species are most sensitive to glyphosate and show symptoms of injury within 3 to 10 days after application.  Severe environmental conditions, such as temperature extremes, or drought will both slow and reduce the effectiveness of glyphosate.  Glyphosate does not appear to be influenced by drought-stress on grasses as much as the POST graminicides.

 

 

9.         The uptake, translocation, and activity of glyphosate can be affected by the addition of other herbicides.  One should be careful in picking tank mix partners, especially the use of contact herbicides that result in rapid disruption of membranes.

 

10.       Glyphosate activity can be greatly reduced when the water carrier contains high levels of salts or iron.

  

11.       The use of spray adjuvants with glyphosate has been extensively researched.  The use of adjuvants may enhance the activity of glyphosate under some conditions.  The use of ammonium sulfate as an additive increases activity in some species and is permitted on the Roundup label.  Reducing carrier volumes, which results in more concentrated spray droplets, also appears to enhance activity.

  

Information of Practical Importance:

 

 Glyphosate is the active ingredient in several water soluble formulations: Ranger, Rodeo, Roundup, Roundup Ultra, Glyphomax, Touchdown, and numerous other products.

   

Glufosinate is the active ingredient in a water-soluble liquid formulation called Liberty.

  

Spray solutions that contain glyphosate or glufosinate react with galvanized or unlined steel containers or spray tanks and produce hydrogen gas, which is highly combustible.  If ignited by open flame, spark, welder's torch, lighted cigarette, or other ignition source, this gas can flash or explode and cause serious personal injury.  Do not mix or apply these spray solutions in galvanized steel or unlined steel (except stainless steel) containers or spray tanks.  Spray solutions of glyphosate or sulfosate should be mixed, stored, and applied only in stainless steel, aluminum, fiberglass, or plastic (or plastic-lined steel) containers.

  

Ranger is a formulation of glyphosate used for POST control of annual and perennial weeds, such as quackgrass, and in annual cropping systems, such as pastures and sods.

   

It needs to be applied before crop emergence.

  

 Rodeo is a glyphosate formulation for POST control of aquatic weeds that are above the water surface.  There is no restriction on the use of the treated water for irrigation, recreation, or domestic purposes.

   

It will not control submerged plants or have a majority of their foliage underground.  Applications to flowing water must be made while traveling upstream (in a direction opposite to the flow of water) to prevent the herbicide from becoming too concentrated.  When applying Rodeo along banksides, do not allow the application to overlap more than 1 ft into open water.

  

Rodeo contains no surfactant and a suitable nonionic surfactant must be added to the aqueous spray mixture prior to use.

 

Roundup is applied POST to control a variety of annual and perennial weeds, both grasses and broadleafs.

  

It contains no surfactant and one must be added to get suitable control.

  

Roundup may be used with special application equipment to prevent the herbicide from contacting weed foliage.

  

Formulation does significantly affect activity, and sometimes selectivity of glyphosate.  Polado (a glyphosate formulation) is used as a plant growth regulator to enhance sugar content in sugarcane.  Quotamaker was another formulation that was used as a yield enhancer/growth regulator in peanuts (very briefly).  It was a dismal failure within one year of commercialization.

 

Liberty is applied POST for the control of a variety of annual and perennial grass and broadleaf weeds.  A nonionic surfactant should be added to achieve satisfactory control.

  

HERBICIDE RESISTANCE: Weeds resistant to glufosinate have not been reported yet.  There are reports of goosegrass in Malaysia and horseweed in Delaware with resistance to glyphosate.

 

Growth Regulator-Type Herbicides:

  

This class of herbicides is used on more land area worldwide than any other herbicide group.  Some are used extensively on the three major leading world crops (wheat, rice, and corn) and there is substantial use on brush, rangeland, turf, and other grass crops.  Historically, 2,4-D and MCPA are important because they provided the stimulus for the development of the agricultural chemical industry.

  

The growth regulator-type (hormone-type) herbicides are characterized by similarities in use, mode-of-action, translocation, and selectivity.  Herbicides in this category are grouped for ease of discussion into two herbicide families based on chemical structure, and then there are two herbicides that do not fit into families based on structure.

  

Characteristics of Growth Regulator-Type Herbicides:

  

The principal use of the growth regulator-type herbicides is the postemergence control of annual and perennial broadleaf weeds and woody shrubs.  There is one herbicide (quinclorac) which controls certain grass and broadleaf weeds only in rice.

   

Mode-of-Action:

  

The primary mode of action of the growth regulator-type herbicides is not known.  They have auxin-like properties, and their mode of action involves interference with nucleic acid metabolism and disruption of normal transport systems in plants through induced massive cell proliferation.

  

These compounds affect plant growth in a similar way and appear to act on the same site as the natural plant auxin IAA (indole acetic acid).  However, all of these herbicides are much more active than IAA, and persist in the plant longer.

  

Translocation:

 

The growth regulator-type herbicides translocate throughout the plant in the phloem following absorption.  They tend to accumulate in regions of high metabolic activity such as the meristems.

 

Selectivity:

 

These herbicides selectivity control broadleaf weeds in grass crops, rangeland and established pastures, and in non-croplands. As previously mentioned, quinclorac is an exception.

  

These herbicides produce profound effects upon the growth and structure of plants, symptoms include malformed leaves, epinastic bending and swelling of stems, deformed roots, and tissue decay. Treatment causes parenchyma cells to divide - producing callus tissue, excessive vascular tissue in young leaves, plugging of the phloem, and root inhibition.  Meristematic tissues are more affected than mature tissue.  Cambium, endodermis, pericyle, and phloem parenchyma are particularly sensitive.

  

Selectivity to growth regulator herbicides does not seem to be a result of a single factor, but it is determined by the sum of many plant reactions to the herbicide.  Some factors that may contribute to selectivity may include one or more of the following:

   

1.         The arrangement of vascular tissue in scattered bundles surrounded by sclerenchyma tissue in grasses may prevent destruction of the phloem by disorganized growth caused by growth regulator herbicides.

   

2.         Aryl hydroxylation of 2,4-D is a major pathway for 2,4-D metabolism.  Aryl hydroxylation of 2,4-D results in the loss of auxin activity.  Conjugation of 2,4-D with amino acids is reported in many species.  However, many of these amino acid conjugates are biologically active and may not represent a major mechanism of detoxification.  Metabolism reactions serve to reduce the amount of herbicide within the plant.

   

3.         Some plants release herbicides through the root system.

  

 Herbicide resistance:

 

Biotypes of weeds resistant to the phenoxy-carboxylic herbicides have only recently started to appear.  The broad spectrum mode-of-action of the phenoxies selects against the development of resistance.  However weed species that have developed resistance to one or more of these herbicides include wild mustard, yellow starthistle, and chickweed.  Additionally, there are frequent reports on the varying tolerance of biotypes within a species.

  

 

Information of Practical Importance

  

Phenoxy-Carboxylic Herbicides:

   

Members:

  

2,4-D - numerous available             

Dichlorprop  - 2,4-DP

2,4-DB - Butyrac, Butoxone                        

Mecoprop  - MCPP, MCPB, MCPA

 

 These herbicides have molecular structures characterized by (1) a benzene ring; (2) an oxygen atom substituted for one hydrogen bonded to the ring, (3) a carboxyl group bonded indirectly to the oxygen atom, separated from the oxygen atom by an aliphatic chain of one or more carbon atoms; and (4) various substituents on the ring.

 


2,4-D


2,4-DB

(Butyrac, Butoxone)


MCPB


DICHLOROPROP


MECOPROP

 

Members of the phenoxy herbicides are distinguished from one another by the length of the aliphatic chain of carbon atoms separating the carboxyl radical from the oxygen atom bonded to the ring, by the substituents and their location on the ring, or both.

 

Dichlorprop (2,4-DP) and mecoprop (MCPP) are racemic mixtures of their respective optical isomers.  The herbicidally active enantiomer of dichlorprop is the dextro (+) form, and it has been named dichlorprop-P.  Its herbicidal characteristics are the same as for dichlorprop, but only half as much herbicide is required to produce the same effect.  Mecoprop is only available as a racemic mixture.

 

The phenoxy-carboxylic herbicides are usually formulated as salts and esters of their respective parent acids.  The chemical structures of the salts and esters of other members of this herbicide family are similar to those of 2,4-D.  In practice, the acid form of the phenoxies is rarely used.  An exception is Weedone 638 in which the active ingredients are a mixture of the 2,4-D acid and the butoxyethyl ester of 2,4-D.  Weedone 638 is more effective than 2,4-D amines in controlling hard-to-kill perennial weeds such as field bindweed, Russian knapweed, Canada thistle, leafy spurge, cattails, and tules.

   

Salt formulations:

   

The common salt forms of the phenoxies are the dimethylamine and sodium salts.  Of these, the dimethylamine form is the more common. 

 

 Amine salts are ammonia-alcohol derivatives of the parent phenoxy-carboxylic herbicides.  The ammonia-alcohol group is cationic, and it replaces the hydrogen ion on the acidic carboxyl group of the parent acid molecule.  On the amine ion, suitable alcohols replace one or more of the hydrogens of ammonia (NH3).  For example, a substitution of one of the hydrogens with ethanol results in ethanolamine; a substitution of all three hydrogen atoms of ammonia with three ethanol groups results in triethanolamine.  Methyl and ethyl radicals also may be substituted for hydrogens of ammonia.

The amine salts of the phenoxy-carboxylic herbicides readily dissolve in water, forming true solutions.  They, like other salt forms, disassociate in water; in hard water, the calcium and magnesium ions present may form insoluble Salts with the anionic portion of these herbicides.  Such insoluble salts will precipitate, resulting in a reduction of herbicide in the spray solution and plugging of screens and nozzle orifices of the spray equipment.  The amine salts are less affected by hard water than are the sodium and ammonium salts.

  

In aqueous mixtures with liquid fertilizers, cations of the fertilizer may replace the cationic portion of the salt forms of herbicides, resulting in the formation of insoluble precipitates.

 

The salts of the phenoxy-carboxylic herbicides may be readily washed from the plant foliage by water.  There is not serious loss of phytotoxicity, however, if the salt form of the herbicides remains in contact with the foliage for at least 6 hours.  After being washed from the leaves by rainfall, the herbicides may still enter the plant via root absorption.

  

* Under normal conditions of plant growth, the salt forms of the phenoxy-carboxylic herbicides are nonvolatile.  However, disposal of their containers by burning has resulted in the vaporization of chemical residues, with subsequent plant injury caused by vapor drift.

  

Ester formulations:

  

Esters of the phenoxy-carboxylic herbicides are soluble in oils and insoluble in water.  For practical use, they are commonly formulated as emulsifiable concentrates for application in either oil or water carriers.

  

Esters of the phenoxy-carboxylic herbicides are formed by the reaction of their acid form with an alcohol.  The reaction replaces the hydroxyl (OH) of the carboxyl group with the respective alcohol.  The hydroxyl group of the carboxyl combines with the hydrogen of the hydroxyl group on the alcohol to form water, and the alcohol is bonded to the carbon of the carboxyl group by its oxygen atom - the oxygen linkage characteristic of an ester.  The resulting ester molecule is nonionic, does not disassociate in water, and thus does not react with Ca2+ and Mg2+ in hard water to form insoluble precipitates.

 

A particular ester of the phenoxy-carboxylic herbicides is identified by the alcohol reacting to replace the hydroxyl of the carboxyl group of the parent acid herbicide.  For example, the methyl ester is formed when the alcohol is methanol, and the isopropyl ester is formed from isopropanol.  In some cases, two or more alcohols, connected by an ether (-C-O-C-) linkage react with the parent acid herbicide to form long-chain esters such as butoxyethyl ester.  The long-chain structure of butoxyethyl ester is  -O-CH2-CH2-O-CH2-CH2-CH2-CH3.

  

*The short-chain esters (methyl, ethyl, isopropyl, butyl) of the phenoxy-carboxylic herbicides are highly volatile; under ordinary temperatures of plant growth, their vapors represent a real hazard to susceptible plants.  The long-chain esters are much less volatile than the short-chain esters.  It is important to emphasize, however, that the long-chain esters are less volatile, not nonvolatile, and that their use does present a potential hazard from subsequent vaporization and vapor drift. *In general, the short-chain esters are commonly referred to as high-volatile esters, whereas long-chain esters are referred to as low-volatile esters.

  

1.         Applied in their salt or ester form, the phenoxy-carboxylic herbicides are converted within the plant to their respective acid forms, and it is in this form that they are ultimately phytotoxic.

  

*2.       Most broadleaf crops are susceptible to the phenoxy-carboxylic herbicides.

  

3.         These herbicides are readily degraded in plants, except for mecoprop, which is degraded more slowly.

 

*4.       The butanoic acid herbicides 2,4-DB and MCPB are in themselves nonherbicidal.  They are converted to their respective acetic acid form by beta-oxidation within susceptible plants (but not in tolerant plants), and it is in the acetic acid form that they are phytotoxic.  Plants incapable of rapid beta-oxidation are not harmed by the butanoic acid herbicides, thereby providing a means of selectivity among broadleaf species.  Peanuts are tolerant to POST applications of 2,4-DB because of this feature (or lack thereof).

  

*5.       Avoid spray drift of these herbicides, as light to severe injury will occur to most broadleaf plants.  Cotton, grapes, and tomatoes are extremely sensitive to these herbicides.

  

            Flat dose response symptoms often develop at concentrations well below the lethal dose.  This type of response creates a potential problem with spray drift to susceptible plants.

  

*6.       The ester formulations are more effective than their corresponding salt formulations for the control of perennial weeds.

 

7.         The salt forms of the phenoxies are readily leached in coarse-textured soils and less so in fine-textured soils.

  

8.         In general, the phenoxies persist 1-4 weeks in warm, moist, soil.  Longer persistence usually occurs under cool, dry conditions.  Their decomposition in soils is primarily under microbial attack.

  

 

                                                        PYRIDINE HERBICIDES

 

Pyridine herbicides are of the growth regulator type.  Their chemical structures all contain the pyridine ring.  This ring structure is similar to benzene, except that a nitrogen atom replaces a carbon atom in the 1-position of the ring, resulting in a ring called pyridine.  Individual pyridine herbicides differ in the substituents bonded to the pyridine ring.  The chemical structure of the pyridine ring is:

 


(Clopyralid)

 

                                               

 Members:

 

clopyralid  - Stinger                           triclopyr  - Garlon, others

picloram  - Tordon

  

Clopyralid is the active ingredient in a liquid formulation (Stinger, Reclaim, Transline) and is formulated as monoethanolamine salt.

  

1.         Clopyralid is used for selective POST control of broadleaf weeds in sugar beets, field corn, small grains (wheat, barley, and oats), fallow cropland, rangeland and permanent grass pastures, Christmas tress, and non-crop areas to control broadleaf weeds and woody brush species.

  

2.         It is highly effective for honey mesquite control either alone or in combination with picloram.  Applied alone it can kill about 80% of the foliar treated honey mesquite trees.

 

3.         Reclaim is a specialty product used to control mesquite, acacias, and other woody brush in rangelands and pastures only in New Mexico, Oklahoma, and Texas.  Reclaim is a specialty product for broadleaf weed control in industrial sites, rights-of-way, and other non-crop areas.

  

Picloram is the active ingredient in a liquid formulation call Tordon 22K and it is formulated as the potassium salt of picloram.  Tordon is a Restricted Use Pesticide due to its possible phytotoxicity to susceptible non-target plants.

  

1.         It is used in areas west of the Mississippi River to control broadleaf weeds and woody plants on rangeland and permanent grass pastures, fallow cropland, small grains, flax on grain-land (land not flooded or sub-irrigated and not rotated to broadleaf crops), non-cropland, and on Conservation Reserve Program (CRP) areas.

  

2.         Do not apply Tordon in residential areas or near desire ornamental trees or shrubs, as the root systems of desired plants may absorb the herbicide following its leaching in soil or excretions from roots of nearby treated trees.

  

3.         Precautionary Statement: Picloram can move through the soil, and, under certain conditions, it may contaminate groundwater used for irrigation and drinking purposes.  Users are advised not to use picloram where soils have a rapid to very rapid permeability throughout the profile (such as loamy sand to sand); where the water table of an underlying aquifer is shallow, or where soils contain sinkholes or limestone bedrock, severely fractured surfaces, and substrates that would allow direct introduction into an aquifer.

  

Triclopyr is the active ingredient in several liquid formulations including Garlon 3A, Redeem, Remedy, Grandstand, and Pathfinder.

  

1.         These formulations have different registrations but include some of the following: weed control in non-crop areas, rangelands, pastures, and as basal treatments to stumps for woody plant control.

  

Dicamba is a substituted benzoic acid herbicide.  Its structure is:

 


DICAMBA

(Banvel)

 

1.         Dicamba is the active ingredient in a water-soluble formulation called Banvel.

 

2.         It is used for POST selective control of annual and perennial broadleaf weeds in asparagus, corn (field, pop, silage), sorghum, small grains (wheat, barley, oats), turfgrass, pastures, and non-crop areas.  It does have some activity on some seedling grasses.

 

3.         Banvel selectivity appears to be caused by the ability of tolerant plants to form non-phytotoxic conjugates more quickly than susceptible species and to tolerate higher concentrations of either herbicide within the plant.

 

4.         Banvel is readily leached in soils.

 

5.         It may be excreted, or leaked, from the roots of plants following foliar or root absorption, and this exudate may be reabsorbed by roots of the same plant or by those of nearby plants in sufficient amounts to induce a plant growth regulator or herbicidal effect.

 

6.         The soil persistence of Banvel is influenced by the rate of leaching and by conditions amenable to its rapid decomposition in soil.  It may be leached below the zone of its activity in humid regions within a period of 3-12 weeks.  Applied at 0.5 lb ai/acre, dicamba may persist in a highly active state for more than 3 months.

 

7.         Herbicide resistance has developed in some wild mustard biotypes in the western prairie provinces of Canada.

 

 

Quinclorac is another growth regulator-type herbicide.  Its chemical structure is:

 


   

1.         Quinclorac is the active ingredient in a wettable powder formulation called Facet.

  

2.         Facet is used only in rice to control certain grass and broadleaf weeds.  It can be applied PRE, delayed PRE, and early POST.

  

3.         It is absorbed by the roots, coleoptiles, and leaves of grasses.

 

4.         It controls barnyardgrass, broadleaf signalgrass, junglerice, large crabgrass, eclipta, and sesbania.

  

5.         In case of crop failure, rice may be immediately replanted. Do not plant any other crop for a period of 10 months after application.  Do not plant carrots and tomatoes within 2 years following application.

 

 

ACETOLACTATE SYNTHASE (ALS) INHIBITING HERBICIDES

ALSO CALLED ACETOHYDROXY ACID SYNTHASE (AHAS) INHIBITORS

 

There are four herbicide families with this mode-of-action where the ALS enzyme is inhibited.  These herbicides are all extremely active at very low use rates with use rates well below 0.1 lb ai/acre.  Herbicide families with this mode-of-action include the imidazolinones, sulfonylureas, triazolepyrimidines, and the pyrimidyl-oxy-benzoates.

  

IMIDAZOLINONE (IMIS) HERBIDES

 

 The imidazolinone herbicides have as their common nucleus the imidazolinone ring with methyl and isopropyl radicals bonded to the 4-position of the ring and an oxygen double-bonded to the 5-position.

  

The imidazolinone herbicides differ in the ring structure substitutions at the 2-position of the imidazolinone ring.  These ring structures may be benzene, pyridine, or quinoline, with various substituents on their rings.

  

Characteristics:

  

The principal use of the imidazolinone herbicides is for control of grass and broadleaf weeds in certain crops, warm-season turfgrasses, and for general weed and brush control in non-crop areas.  Additionally, several of these herbicides control yellow and purple nutsedge.

  

Mode-of-Action:

  

The primary mode-of-action of the imidazolinone herbicides in susceptible plants is the inhibition of the enzyme ALS.  This enzyme is involved in the first step in the biosynthesis of the branch-chained amino acids; valine, leucine, and isoleucine.  Its inhibition deprives the plants of these essential amino acids, interfering with DNA synthesis, and cell growth.  This pathway is unique to plants.  Consequently, these herbicides have extremely low mammalian toxicity.

  

Translocation:

  

The imidazolinone herbicides are absorbed by roots and foliage and translocated throughout the plant (both symplastically and apoplastically), accumulating in the meristems such as apical buds at root and shoot tips and axillary buds in leaf axils.

   

Selectivity:

  

The basis for selectivity is rapid metabolic degradation in tolerant plants and the inability of susceptible plants to do the same.

  

Plant Tolerance:

  

Imazaquin is reported to have a half-life of 3, 12, and 30 days in tolerant soybeans, moderately tolerant velvetleaf, and susceptible cocklebur, respectively.  Increased tolerance of velvetleaf at the four-leaf growth stage, as compared to the cotyledonary stage, is attributed to a 72% decrease in imazaquin absorption and a 62% decrease in its half-life.

   

Susceptible grass species such as wild oat quickly metabolized the methyl ester of imazethabenz to its phytotoxic acid form, while tolerant grass species such as corn and wheat did so much more slowly.

  

Plant tolerance to the imidazolinone herbicides is, in some cases, due to the presence of an altered form of the enzyme ALS. In imidazolinone-susceptible corn, the ALS enzyme present was 3000 times more sensitive to inhibition by imidazolinone herbicides than was the ALS enzyme in tolerant corn.

  

Common Characteristics:

  

1.                  The imidazolinone herbicides are translocated into, and kill underground storage organs of susceptible perennial plants.

 

2.                  Susceptible plants stop growth soon after herbicide absorption and are no longer competitive with the crop.  Symptoms may not appear for 2 weeks or more after treatment, and complete kill may not occur for several more weeks.

 

3.                  Applied POST, it essential to add a nonionic surfactant or crop oil concentrate, and, if needed, an anti-foaming agent and/or drift retardant to spray solutions.

 

4.                  These herbicides are adsorbed to soil colloids and do not leach in soils.  Little leaching has been documented under field conditions even though some laboratory studies have shown moderate mobility.

 

5.                  The soil persistence may vary from a few months to 2 years or more, depending on the herbicide and application rate.  Soil persistence is a real potential problem for rotational crops.  The degree of sensitivity of the rotational crop is of primary importance.  Greater adsorption, and therefore persistence, occurs with decreasing soil pH and increasing soil organic matter.

 

6.                  Crops tolerant to one or more of the imidazolinone herbicides include small grains (wheat, barley), choen, edible beans, peanuts, safflower, soybean, sunflower, and certain warm-season turfgrasses.  Some crops that have resistant or tolerant cultivars include corn and canola.  Efforts are also directed towards developing tolerant cotton cultivars.

 

7.                  Spray drift of the imidazolinones also poses problems to sensitive crops located nearby.

 

8.                  Dissipation in soil is by microbial degradation.  Under drier conditions, more herbicide is bound to clay and organic matter and less is available for degradation or plant uptake.

 

9.                  Imidazolinone herbicides will degrade in light but little photolysis has been observed on soil or plant foliage under field conditions.  However, there are numerous reports in the literature where imazaquin applied PPI was more injurious to cotton grown in rotation than imazaquin applied PRE.  Imazaquin applied PPI would not be subjected to photodegradation whereas a PRE application, especially if rainfall or irrigation was not forthcoming, would be subjected to photodegradation.

  

                                             Information of Practical Importance

 

Imazamethabenz: is the active ingredient in an EC formulation called Assert.

 

1.                  Assert is a POST herbicide in wheat (including durum), barley, and sunflower to selectively control wild oats, roughstalk bluegrass, interrupted windgrass, and certain annual broadleaf weeds.

 

2.                  Assert can be applied to wheat and barley from the two-leaf stage to just before jointing.  Do not apply Assert when freezing temperatures are forecast.

 

3.                  Always use a nonionic surfactant.

 

4.                  Do not plant sugar beets for at least 20 months after application. 

 

 Imazapyr is the active ingredient in an aqueous formulation called Arsenal or Chopper.  The formulation contains a surfactant, and it is not necessary to add another adjuvant.

  

1.         Arsenal is used for nonselective control of annual and perennial grass and broadleaf weeds in non-cropland areas.

  

2.         Arsenal is most effective when applied POST to weed foliage.  However, the herbicide also provides residual control of newly germinating weeds in many instances.

  

3.         Arsenal translocates and kills vegetative organs in susceptible perennial weed species.

 

4.         Arsenal can be used under asphalt, pond liners, and paved areas in industrial sites, or in areas where the roots of desired plants will not encroach.

  

5.         Chopper is used for nonselective POST control of broadleaf weeds and woody brush in non-crop areas.  It can be used as a cut stump, tree injection, frill or girdle treatment, or as a low-volume basal treatment.

  

Imazaquin is the active ingredient in a water dispersible granular formulation called Scepter 70DG and in an EC formulation called Scepter.

  

1.         Scepter is used only in soybeans and can be applied PPI, PRE, or POST to control broadleaf weeds and some grass weeds.

  

2.         Scepter controls more weeds PPI or PRE than when applied POST.  POST it primarily will control pigweed species and common cocklebur.

  

3.         There are significant rotational crop restrictions following the use of Scepter.  Cotton in particular is sensitive to Scepter.

  

4.         Image is another imazaquin formulation registered for POST control of grass and broadleaf weeds in some warm-season turfgrasses including bermudagrass, centipedegrass, St. Augustinegrass, and zoysiagrass.

 

5.         Image is effective for garlic, wild onion, purple nutsedge, and yellow nutsedge.

  

6.         Do not apply Image to golf course greens, aprons, or similar use areas that are closely mowed.  Imazaquin can be quite injurious to azaleas and numerous other woody ornamentals.

   

Imazethapyr is the active ingredient in a water-soluble liquid concentrate formulation called Pursuit.

  

1.                  Pursuit can be applied PPI, PRE, EPOST, and POST in peanuts and soybeans for broadleaf weed control.  Pursuit also has activity on small seedling grasses including seedling johnsongrass, and yellow and purple nutsedge.

 

2.                  Always use a nonionic surfactant or crop oil concentrate.

 

3.                  Pursuit is a very good EPOST treatment for velvetleaf control and is the most widely used broadleaf herbicide in U.S. soybeans.

 

4.                  Corn and canola varieties with tolerance and/or resistance to Pursuit have been developed.

 

5.                  It was the only imidazolinone herbicide with registration on peanut.  Pursuit does not control sicklepod, Florida beggarweed, common lambsquarters, or common ragweed.  It was the first herbicide to provide POST control of purple nutsedge.

 

6.                  It is the most widely used residual herbicide for broadleaf weed control in U. S. soybeans.  It has better POST activity on velvetleaf and annual grasses than Scepter.  Additionally, corn is more tolerant to Pursuit and thus is less likely to be injured in rotation, especially following a dry year.

 

  

Imazapic

  

Imazipic is the active ingredient in Cadre, an experimental herbicide under development for registration in peanut and for use in industrial and rights-of-way weed control.

  

1.         Field research has indicated variable soybean tolerance to Cadre, some of which may be attributed to a differential varietal sensitivity.  The environmental conditions at the time of application may also influence sensitivity.

  

2.         It is a very very good herbicide for yellow and purple nutsedge, pigweeds, and sicklepod control.  It provides outstanding control of numerous broadleaf and small grasses including rhizome johnsongrass.  It has problems with tropic croton, common lambsquarters, common ragweed, and showy crotalaria.

  

3.         Cadre will have significant rotational restrictions concerning cotton and at least some vegetables.

  

4.         Cadre is a far better POST herbicide for southern weeds than Pursuit.

  

SULFONYLUREA (SU) HERBICIDES

  

The sulfonylurea herbicide family was first commercialized in 1982.  It is a herbicide family with high levels of activity at low application rates.  By changing structure slightly, selectivity can be changed from one crop to another.  Examples of crops which are presently tolerant to one or more sulfonylurea herbicides include wheat and other small grains, soybeans, rice, rapeseed, corn, potatoes, peanuts, sugar beets, turf, and conifers.

  

 The sulfonylurea herbicides differ in one or more substituents bonded to the sulfonylurea nucleus.

  

Characteristics:

  

In general, the sulfonylurea herbicides are used to control broadleaf weeds.  Several of the sulfonylurea herbicides also control annual and perennial grasses in certain crops including corn.  Additionally, they are used extensively for nonselective control of annual and perennial grasses and broadleaf weeds in non-crop and industrial areas.  Weed control can be achieved with soil and/or POST applications.

 

Mode-of-Action:

 

The sulfonylurea herbicides inhibit the ALS (also called AHAS) enzyme.  ALS is an enzyme used in the first step towards biosynthesis of the branched chain amino acids (valine, leucine, and isoleucine).

  

Translocation:

  

The sulfonylurea herbicides are readily absorbed by both roots and shoots of plants and are translocated in the phloem and xylem to meristematic regions of the plants.

  

Selectivity:

  

Plant selectivity results from rapid metabolic detoxification in tolerant plants and the lack of this feature in sensitive plants.

  

Chlorsulfuron-resistant grasses, such as annual bluegrass, barley, wheat, and wild oat metabolize more than 90% of the chlorsulfuron in their leaves, while chlorsulfuron-sensitive broadleaf plants, such as cotton, mustard, soybean, and sugar beet metabolize less than 20% of the chlorsulfuron in their leaves.

  

Weed resistance to the sulfonylurea herbicides is associated with a genetically altered form of the enzyme ALS.  A single nucleotide change in the gene coding for the ALS enzyme is responsible for the observed resistant plants; the altered ALS molecule is less sensitive to the herbicidal inhibition.

 

 

Following repeated use of the sulfonylurea herbicides (or herbicides with a similar mode-of-action), resistant biotypes of certain broadleaf weeds has evolved.  Examples are kochia, prickly lettuce, and Russian thistle.

  

When such biotypes are present in areas about to be treated with ALS-inhibiting herbicides, the herbicides should be applied in a tank mix with other broadleaf herbicides that will control the resistant species with a different mode-of-action, or treated with these herbicides in a separate application.

  

Common characteristics:

  

1.         Crop tolerance to sulfonylurea (SU) herbicides results from the rapid metabolic detoxification of the herbicides in these plants.

  

2.         The SU herbicides rapidly inhibit the growth of susceptible plants, but symptoms of dying may not be apparent until 1-3 weeks after application.

  

3.         In areas with a soil pH greater than 7.0 and with prolonged periods of low soil temperature and low annual rainfall, the SU herbicides can remain active in the soil for 2-4 years or more and they can thereby injure subsequent, nontolerant rotational crops.  In general, the SU herbicides should not be applied to soils with a pH of 7.9, or greater, or crop injury may occur.

  

Exceptional characteristics:

  

1.         Bensulfuron (Londax) is the only SU herbicide registered for use in rice, and it is used only in rice.

  

2.         Chlorimuron (Classic) is the only SU herbicide registered for use in peanuts, but it is also used in soybeans and non-crop areas.  Most other crops are very sensitive to chlorimuron.  The peanut registration is a very unusual registration; chlorimuron can only be applied 60 days after peanut emergence.

  

3.         Metsulfuron (Ally), tribenuron (Express) and chlorsulfuron (Glean) are used in wheat and barley, Glean may also be used in oats.

  

4.         Metsulfuron is the active ingredient in Ally and Escort.

  

5.         Nicosulfuron (Accent) and primisulfuron (Beacon) are used only in field corn to selectively control annual and perennial grasses POST.  These were landmark registrations of exceptional impact.

  

6.         Thifensulfuron (Pinnacle in soybeans, Harmony or the Harmony part in Harmony Extra) poses the least hazard to rotational crops.

  

7.         SU herbicides can be mobile in the soil, depending somewhat on their chemical structure.  These herbicides move more readily in high pH soils than at lower pHs.

  

8.         In general, all SU herbicides degrade in the soil by both chemical hydrolysis and microbial breakdown.  Both microbial breakdown and chemical hydrolysis occur in acid soils.  In high pH soils, chemical hydrolysis is drastically reduced and degradation is primarily by microbes.

  

Precautions:

  

1.         These herbicides are extremely biologically active on sensitive species. 

 

2.         Common use rates can be routinely below 50 g ai/ac.  Care must be taken to reduce drift, thoroughly clean spray equipment after use and before use in sensitive crops.  Ammonia at 5% of the spray tank volume will inactivate these herbicides almost immediately.  Be sure to thoroughly clean the sprayer (tops of spray tank, nozzle screens, by pass agitation system, etc).  Read the label very carefully!

  

3.         Carefully read the label for registered tank mixes, adjuvant uses, rotational crop restrictions, and use of insecticides in cropping systems that use or may potentially use SU herbicides.

  

 

INFORMATION OF PRACTICAL IMPORTANCE

 

 

Bensulfuron is the active ingredient in a dry flowable formulation called Londax.

  

1.         It is used for selective PRE and early POST control of broadleaf weeds and sedges in rice.  Grass weeds may be suppressed.

  

Chlorimuron is the active ingredient in a water dispersible granular formulation called Classic.

  

1.         Classic is used for selective PRE and POST control of broadleaf weeds and yellow nutsedge in soybeans and peanuts.  In peanuts, it is registered for POST use only, applied no sooner than 60 days after peanut emergence.

  

2.         Classic is the POST herbicide of choice for sicklepod control.  In peanuts, it is applied only in Georgia, Alabama, and Florida for Florida beggarweed control.  It is also very good on morningglories, small ragweed, small bristly starbur, common cocklebur, and fairly good control of small pigweed species in soybeans.  It has virtually no grass activity and is weak on tropic croton, common lambsquarters, and prickly sida.

  

Chlorsulfuron is the active ingredient in a dry-flowable formulation called Glean and another formulation for industrial use is called Telar (non-cropland weed control).

  

1.         It is used for selective PRE and POST control of broadleaf weeds on land primarily dedicated to long-term production of wheat, barley, and oats.  Be very careful of your rotational crops.  Certain weedy grasses, such as Italian ryegrass and Setaria spp. are partially controlled.                       

 

2.         It is only used west of the Mississippi River.

  

3.         There have been numerous cases of weed biotypes developing resistance very rapidly (less than 3 years) to this herbicide.  Consider very carefully the sustainability of your weed management program for your cropping system if you are using this herbicide.

   

Metsulfuron is the active ingredient in several dry flowable formulations (Ally, Escort).  It is similar to chlorsulfuron in general use pattern.

  

1.         It is used for POST control of broadleaf weeds in wheat (including durum), barley, and on land in the Conservation Reserve Program.  The margin of selectivity, especially in barley, may be narrower than chlorsulfuron.  It may be slightly more active on many broadleaf weeds than chlorsulfuron.

  

2.         Ally does not control wild oats or other grasses.

  

3.         Escort is used for PRE and POST control of annual and perennial broadleaf weeds and woody brush in conifer plantations and non-crop areas.

  

4.         There are some indications that metsulfuron may be slightly less persistent than chlorsulfuron in the soil.

 

 Nicosulfuron is the active ingredient in a water dispersible formulation called Accent.

  

1.         Accent is used POST in corn for the control of annual and perennial grasses including shattercane and johnsongrass.  This registration and the primisulfuron (Beacon) registration were the first herbicides to provide selective POST control of grassy weeds in corn.  Accent and Beacon cannot be used in sorghum. 

  

2.         Accent is the POST herbicide of choice for annual grass control.  Both Accent and Beacon are good on johnsongrass and shattercane.  Beacon is a better broadleaf herbicide.

  

3.         Do not apply Accent or Beacon in a tank mix with Basagran or significant corn injury may occur.

  

4.         Read the label very carefully concerning the use of organophosate insecticides (Counter) in corn and possible interactions with Accent and Beacon.

  

Primisulfuron is the active ingredient in a water dispersible granule formulation called Beacon.

 

1.         Beacon can be used POST in field and popcorn.

 

2.         Many of the things listed under Accent (nicosulfuron) apply to primisulfuron.  Accent is a better annual grass herbicide while Beacon is a better broadleaf herbicide.

  

Sulfometuron is the active ingredient in a water dispersible formulation called (Oust).

 

1.         Oust is used for PRE and POST control of annual and perennial grass and broadleaf weeds in non-crop areas and industrial sites.  It can be used in forest site preparation and release of several types of pines (loblolly, slash, longleaf, jack, and Virginia) and in dormant white pine, white spruce, and Douglas fir.  To avoid injury to the trees, do not use a surfactant in the spray mixture.  It has virtually no selectivity in grass-crops and has a wider range of activity than chlorsulfuron.

 

2.         Oust is absorbed by both roots and foliage.

 

Thifensulfuron is the active ingredient in a dry flowable formulation called Pinnacle.

  

1.         Pinnacle is used POST for selective broadleaf weed control in soybeans.

  

2.         The only instances that I would use Pinnacle in soybeans would be for pigweeds and common lambsquarters control.  Pinnacle is also a good treatment for velvetleaf.  However, I would use Basagran or Pursuit for velvetleaf control, unless I had lambsquarters or pigweeds.  (With these three weeds, if only two were present, how would you decide on whether to use Pinnacle, Basagran, or Pursuit)?

  

3.         Pinnacle is a fairly harsh treatment on soybeans.  DuPont developed STS soybeans (sulfonylurea tolerant soybeans) so that an application of Pinnacle + Classic would not be so injurious.

  

4.         Most crops can be planted 45 days after Pinnacle application; it has a very short persistence in the soil.

  

5.         Pinnacle tank mixed with Classic provides an extremely broad spectrum of activity on numerous annual broadleaf weeds.  Since both herbicides are ALS inhibitors, careful consideration of the potential for herbicide resistance development should be considered.

  

6.         Thifensulfuron was the active ingredient in a wheat herbicide called Harmony.  Harmony is no longer available (see information below about Harmony Extra).  Thifensulfuron is a very good wild garlic herbicide.  Wild garlic is a major weed problem for wheat producers.

  

Triasulfuron is the active ingredient in a water dispersible formulation called Amber.  It is similar to Glean and Ally in selectivity and soil persistence.  It is somewhat more effective against vetch and perhaps bedstraw.

  

1.         Amber is used PRE and POST in winter wheat and spring wheat.  Amber applied PRE controls Italian ryegrass and suppresses downy brome and cheat.  Applied POST it controls many broadleaf weeds.

  

2.         Adsorption of Amber to soil colloids is relatively low, and leaching can occur.

   

Tribenuron is the active ingredient in a dry flowable formulation called Express.

 

1.         Express is used for selective POST control of annual broadleaf weeds in wheat and barley.

  

2.         A commercial prepackage of tribenuron (Express) and thifensulfuron (Harmony) is marketed and used extensively in wheat east of the Mississippi River.  This formulation is called Harmony Extra.  Harmony Extra is very good for controlling wild garlic and numerous broadleaf weeds.  The Harmony Extra formulation provides a broader spectrum of control than either Harmony or Express alone.  The inclusion of Express with Harmony prevents soybean producers from using Harmony for broadleaf weed control in soybeans for $1/acre or less.

  

3.         It is very short lived in the soil.

 

Ethametsulfuron (Muster) is selective in rapeseed.  It is sold in Canada, but it is not registered in the U.S.

  

Rimsulfuron (Titus, DPXE9636) is a new SU herbicide registered for PRE and POST control of grass and broadleaf weeds in corn and potatoes (I am not sure if it is registered yet).

  

Triflusulfuron (Peak, CGA152005) is registered for use in corn for broadleaf weed control.  It is very effective for common ragweed and does not control sicklepod.  It is prepackaged with Beacon and this mixture is called Exceed.

  

Herbicide Resistance: At least 14 weed species have developed resistance to one or more the herbicides in the sulfonylurea herbicide family.

  

 

PYRIMIDYL-OXY-BENZOATES

  

The pyridmidyl-oxy-benzoates (POB) are being developed by Kumiai chemical industry.  The POBs have the same mode-of-action as the imidazolinone and sulfonylurea herbicides (ALS inhibition).  These herbicides will translocate in the xylem and the phloem.  The first registration of a POB occurred this fall (KIH 2031).  This herbicide has the common name of pyrithiobac sodium and will be marketed under the tradename of Staple.  It will be marketed and distributed in the U.S. by DuPont.

 

 

PYRITHIOBAC

 

 


Pyrithiobac sodium is the active ingredient in Staple.

  

1.                  Staple is of importance because it will provide POST control of certain broadleaf weeds in cotton.  It is the only ALS inhibiting herbicide to be registered for direct use in cotton.

 

2.                  Staple should be applied with a nonionic surfactant.  It will primarily be used as a POST herbicide.  Later the registration may be expanded to include soil applications.

 

3.                  Staple provides excellent POST control of pigweed species including Palmer amaranth, morningglories with the exception of tall morningglory which it will not control, small prickly sida, small cocklebur, small bristly starbur, spurred anoda, and velvetleaf.  It will not control sicklepod, common ragweed, or common lambsquarters.  It has some activity on small seedling grasses and some activity on nutsedge species.

 

4.                  Staple will have some significant carryover concerns, especially concerning corn, grain sorghum, and undoubtedly certain vegetables.

 

5.                  The 1995 growing season has seen two landmark registrations in cotton including Buctril for BXN cotton (first transgenic cotton and herbicide registration) and Staple (first selective broadleaf herbicide for non-transgenic cotton).

 

6.                  Staple likely will not contaminate groundwater and has a very low mammalian toxicity.

   

Herbicide resistance: Although a member of this herbicide family only received registration in October of 1995, resistant weed biotypes already have occurred.  This cross resistance is the result of this herbicide family having the same mode-of-action as the imidazolinone and sulfonylurea herbicide families.