`California Exotic Pest Plant Council
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`1996 Symposium Proceedings
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`Why Herbicides Are Selective
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`David W. Cudney
`Department of Botany and Plant Sciences
`University of California, Riverside, CA 92521-0124
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`Selective herbicides have been used extensively since the introduction of 2,4-D in the late '40s. They have
`been one of the miracles of modem agriculture, releasing thousands of people from the drudgery of hand
`weeding. A selective herbicide is one that kills or retards the growth of an unwanted plant or "weed" while
`causing little or no injury to desirable species. 2,4-D used in turf will kill many of the broadleaf weeds that
`infest turf while not significantly injuring the turfgrass. But selectivity is a fickle, dynamic process. Excessive
`rates of 2,4-D applied to stressed turfgrass may injure the turf.
`Selectivity has always depended on proper herbicide application. Normally herbicides work selectively
`within a given rate of application. Too little herbicide and no weed control, too much and crop injury may
`occur. But selectivity is more complex than this. It is a dynamic process that involves the interaction of the
`plant, the herbicide, and the environment.
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`The Plant
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`Factors that involve plant response include: genetic inheritance, age, growth rate, morphology, physiology,
`and biochemistry. The genetic make-up of a plant determines how that plant responds to herbicides and its
`environment. The age of the plant often determines how well an herbicide works, older plants are generally
`much more difficult to control than seedlings. Preemergence herbicides often work only on plants during the
`germination process and will have little effect on older plants. Plants which are growing rapidly are usually
`more susceptible to herbicides.
`The morphology of a plant can help to determine its susceptibility to herbicides. Annual weeds in a deep-
`rooted crop can be controlled because the herbicide is concentrated in the first inch of soil where the weeds
`and weed seeds are. Weeds with exposed growing points may be killed by contact sprays, while grasses with
`protected growing points may be burned back, but escape permanent injury. Certain leaf properties can allow
`better spray retention and thus better kill (broadleaf species vs. grasses or hairy vs. smooth leaves). Sprays
`tend to be retained on pigweed and mustard leaves and bounce off of onion or grass species.
`The physiology of a plant can determine how much of an herbicide will be absorbed onto the plant and the
`speed with which it is transported to its site of action. Plants with thick waxy cuticles or hairy leaf surfaces
`may not absorb sufficient herbicide to be injured. Wetting agents in herbicide formulations are used to combat
`these leaf characteristics and increase absorption. The transport rate of herbicides in plants varies. Usually
`susceptible plants transport herbicide more readily than resistant ones. Some plants can adsorb herbicides
`along the transport pathway, preventing them from reaching their site of action.
`Biochemical reactions also account for selectivity. Most herbicides have a biochemical reaction within
`susceptible plants which accounts for their herbicidal activity. They may bind to critical enzymes within
`susceptible plants and block important metabolic processes (glyphosate), they may block photosynthesis
`(diuron) or respiration, or they may affect cell division (trifluralin). Herbicides may be absorbed as relatively
`innocuous chemicals (2,4-DB) and activated to deadly compounds (2,4-D) within susceptible plants. Other
`herbicides (atrazine) may be detoxified within some plants (com) while killing weeds which fail to metabolize
`the herbicide.
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`Herbicides are quite specific in their structures as to whether or not herbicidal activity is possible. Slight
`changes in conformation or structure will alter herbicidal activity. Trifluralin and benefin differ in only a
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`The Herbicide
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`SYNGENTA EXHIBIT 1011
`Syngenta v. FMC, PGR2020-00028
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`California Exotic Pest Plant Council
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`1996 Symposium Proceedings
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`methyl group moved from one side of the molecule to the other, yet trifluralin is about twice as active as
`benefin. Esters of phenoxy (MCPP etc.) acids are usually much more active than are amines. The manner of
`formulation of an herbicide can affect its selectivity. The most extreme case of this might be granular
`formulations which bounce off desirable plants to reach the soil where they then limit germinating weeds.
`Other substances known as adjuvants or surfactants are often added to improve the application properties of a
`liquid formulation and increase activity.
`The manner in which an herbicide is applied can affect its selectivity. When a broad-spectrum
`postemergence herbicide like glyphosate is applied as a shielded, directed, or wicked application within a
`susceptible crop, susceptible foliage is avoided and selectivity is achieved with this normally non-selective
`herbicide.
`Herbicides can be grouped into families based on the type of action that they have within affected plants
`(their mode of action). Herbicides which affect similar sites or processes within affected plants produce
`similar injury symptoms. The herbicide families listed below are grouped on the basis of how they affect
`plants:
`1. The Growth Regulator Herbicides (2,4-D, MCPP, dicamba, and triclopyr). These are mostly foliar applied
`herbicides which are systemic and translocate in both the xylem and phloem of the plant. They mimic
`natural plant auxins, causing abnormal growth and disruption of the conductive tissues of the plant. The
`injury from this family of herbicides consists of twisted, malformed leaves and stems.
`2. The inhibitors of amino acid synthesis (glyphosate, halosulfuron, imazethapyr, and sulfometuron). Both
`foliar and soil applied herbicides are in this family. Glyphosate translocates in the phloem with
`photosynthate produced in the leaves. Others in this family move readily after root or foliar absorption.
`These herbicides inhibit certain enzymes critical to the production of amino acids. Amino acids are the
`building blocks of proteins. Once protein production stops, growth stops. Symptoms are stunting and
`symptoms associated with lack of critical proteins.
`3. Cell membrane disrupters - with soil activity (oxyfluorfen, lactofen, and acifluorfen). Soil and foliar
`applied with limited movement in soil. These herbicides enter the plant through leaves, stems, and roots,
`but are limited in their movement once they enter the plant. Membrane damage is due to lipid
`peroxidation. Symptoms are necrosis of leaves and stem.
`4. Lipid biosynthesis inhibitors (diclofop, fluazifop, sethoxydim, and clethodim). Foliar applied Diclofop has
`both soil and foliar activity. Herbicides in this family move in both the xylem and phloem of the plant and
`inhibit enzymes critical in the production of lipids. Lipids are necessary to form plant membranes which
`are essential to growth and metabolic processes. Symptoms include stunting and death of tissue within the
`growing points of plants.
`5. Pigment inhibitors (norflurazon, fluridone, and amitrol). Soil applied and move in the xylem except
`amitrol, which moves in both phloem and xylem. These herbicides inhibit carotinoid biosyntehsis, leaving
`chlorophyll unprotected from photooxidation. This results in foliage which lacks color. Symptoms
`include albino or bleached appearance of foliage.
`6. Growth inhibitors of shoots (thiocarbamate herbicides including: EPTC, cycloate, pebulate, and
`molinate). Soil applied and somewhat volatile, requiring incorporation. Enter the plant through the roots
`and translocated through the xylem with the transpiration stream to the growing points in the shoot. Mode
`of action is unclear, but affects developing leaves in growing points of susceptible plants. Symptoms
`include stunting and distortion of seedling leaves.
`7. Herbicides which disrupt cell division (trifluralin, DCPA, dithiopyr, oryzalin, pronamide, pendimethalin,
`and napropamide). All are soil applied, with limited movement in the soil. Absorbed through roots or
`emerging shoot tips. Once absorption takes place, movement is limited (site of action is near the site of
`absorption). These herbicides inhibit cell division or mitosis, except pronamide and napropamide which
`stop cell division before mitosis. Symptoms include stunting and swollen root tips.
`8. Cell membrane disrupters - no soil activity (paraquat, diquat, glufosinate, acids, oils, soaps). These
`herbicides are foliar applied with no soil activity. They enter the plant through the leaves and stems and
`do not move significantly within the plant once absorbed. These herbicides either act directly on cell
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`California Exotic Pest Plant Council
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`1996 Symposium Proceedings
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`membranes (acids, soaps. oils) or react with a plant process to form destructive compounds which result in
`membrane damage. Symptoms include rapid necrosis of the leaves and stem.
`Inhibitors of photosynthesis (atrazine, simazine, metribuzin, cyanazine, prometryn, diuron, linuron,
`tebuthiuron, and bromacil). These are soil applied herbicides, however, all except simazine also have
`foliar activity. They move readily in the plant in the xylem with the transpiration stream where they
`concentrate in the leaves at the site of photosynthesis. Once there they block the electron transport system
`of photosynthesis, causing a build up of destructive high energy products which destroy chlorophyll and
`ultimately the leaf tissues. Symptoms include chlorotic (yellowed ) leaves which become necrotic.
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`The Environment
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`9.
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`There are many ways that the environment interacts with herbicide selectivity. The soil determines how much of
`soil applied herbicides are available for activity. Sandy soils, with low organic content, are much more active and
`conversely less selective than clay soils with high organic content at a given rate of herbicide application. Irrigation
`or rainfall amount and timing influence the depth to which herbicides may move in the soil and plant growth and
`stress, all of which can increase or decrease herbicide selectivity. Temperature affects the rate of herbicide transport,
`the rate of biochemical reactions, plant growth, plant stress, and ultimately herbicide selectivity. Wind, relative
`humidity, insects, plant pathogens, and nutritional status also affect plant growth and stress which can increase or
`decrease herbicide selectivity.
`Herbicide selectivity is truly a dynamic process. It involves complex interactions between the plant, the
`herbicide and the environment.
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`
`
`Ashton, Floyd M. and W. A. Harvey. 1976. Selective chemical weed control. University of California bulletin 1919. 17 pages.
`Warren, G. F., F D. Hess, et al. 1995 Herbicide action short course at Purdue. Syllabus. 787 pages.
`Tickes, Barry, D. W. Cudney, and C. L. Elmore. 1996. Herbicide injury symptoms. University of Arizona Bulletin 195021.
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`Suggested Reading
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