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`HERBICIDE INFORMATION
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`
`Factors Affecting Herbicide Performance
`January 05, 2018 / Celestine Duncan
`
`Section 3 of the Understanding Herbicides
`Series
`
`Herbicides must overcome biological and
`environmental barriers to control a target plant.
`In this article, learn the general processes by
`which herbicides control invasive plants and
`environmental factors that can in;uence
`herbicide performance.
`
`TEST YOUR
`KNOWLEDGE
`
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`
`Page 1 of 9
`
`SYNGENTA EXHIBIT 1012
`Syngenta v. FMC, PGR2020-00028
`
`

`

`Herbicide Uptake, Translocation, and Mode of Action
`
`To be effective, systemic herbicides must: 1) adequately contact plants; 2) be absorbed by
`plants; 3) move within the plants to the site of action without being deactivated; and 4) reach
`toxic levels at the site of action. Herbicides can enter a plant through roots, shoots, stems,
`foliage, or buds. The primary site of entry is specific for each herbicide and may be influenced
`by the method of application. Some herbicides can enter a plant through multiple sites such as
`leaves and roots (e.g., Milestone® and Transline® specialty herbicides).
`
`
`
`HERBICIDE UPTAKE (ABSORPTION)
`
`Foliar Applied Herbicides
`
`Leaves are the primary sites of entry for
`most foliar-applied herbicides. To enter the
`leaf, herbicides must first penetrate the waxy
`cuticle (primary barrier to absorption) or
`other specialized cells on the leaf surface.
`The cuticle is a layer of overlapping waxy
`platelets. Not all leaf components absorb
`herbicide; guard cells around stomates, cells
`surrounding hairs, and cells overlying veins
`are primarily responsible for herbicide
`absorption. Once inside the leaves, systemic
`herbicides move within the plant either cell to cell, or in phloem.
`
`DIAGRAM ADAPTED FROM WEEDCENTER.ORG, MONTANA STATE
`UNIVERSITY
`
`Soil Applied Herbicides
`
`Soil-applied herbicides can be absorbed (taken up) by seeds, roots, and shoots. Herbicides
`
`Page 2 of 9
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`

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`are absorbed into plant roots in the same way as plant nutrients and water, and are moved
`through the plant initially in the xylem.
`
`HERBICIDE TRANSLOCATION (MOVEMENT IN PLANT)
`
`Translocation is the process of moving an herbicide from the site of uptake to the site of
`action where it kills the plant. Some herbicides (e.g., glyphosate, and Milestone, Transline,
`Opensight®specialty herbicides) are mobile within the plant (systemic), whereas others are
`immobile (contact) (e.g. diquat). Movement within the plant is primarily by xylem and phloem
`vascular systems.
`
`The xylem is a non-living, hollow, thick-walled, pipe-like conduit that moves water and
`nutrients from the roots up to all plant parts. Translocation in the xylem is the primary means
`of moving root-absorbed herbicides to above-ground plant parts. The rate of movement of
`herbicides in the xylem is determined by soil moisture and the transpiration demands of the
`plant.
`
`The phloem is comprised of living cells that carry organic nutrients, particularly sucrose
`(sugar) to all parts of the plant. Translocation in the phloem is the primary means of moving
`foliar-absorbed herbicides to newly forming roots and shoots.
`
`The figure at left shows the site of
`uptake (absorption) and movement
`(translocation) of some common
`herbicides. Foliar applied, contact
`herbicides (diquat) do not
`translocate within the plant,
`compared to foliar applied
`herbicides such as glyphosate
`(Accord® XRT II herbicide) that
`translocate in the phloem, killing
`plant shoots and roots. Commonly
`used range and pasture herbicides
`
`IMAGE CREDIT: FABIAN MENALLED, MONTANA STATE UNIVERSITY.
`
`Page 3 of 9
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`

`

`such as aminopyralid (Milestone), clopyralid (Transline), and others are absorbed by leaves and
`roots and are translocated throughout the plant in both phloem and xylem. Uptake of
`tebutiuron (Spike® 20P) is by roots with transport in the xylem.
`
`
`
`SITE OF ACTION / MODE OF ACTION
`
`Following absorption and translocation to the site of action, an herbicide must remain active
`and at toxic levels to kill the target plant. Mode-of-action is the manner in which herbicides
`affect plants at the tissue or cellular level. Herbicides with the same mode-of- action may have
`similar translocation patterns and produce similar injury symptoms. For example Milestone
`and Transline herbicides are both auxin growth regulators and plant injury symptoms include
`twisting, bending of leaves and stems, and swelling of roots.
`
`Knowing and understanding each herbicide’s mode
`of action is an important step in selecting the proper
`herbicide for the target plant, diagnosing herbicide
`injury, and designing a successful weed
`management program. Over-reliance on a single
`herbicide active ingredient or mode of action places
`heavy selection pressure on a weed population and
`may eventually select for resistant individuals.
`Detailed information regarding herbicide mode of
`action is available at
`http://www.extension.purdue.edu/extmedia/ws/ws-
`23-w.html.
`
`A plant must be actively growing when an herbicide
`is applied in order for the chemical to upset or stop
`certain plant processes. Therefore, herbicides are
`often applied during the spring and early summer. Some herbicides such as Milestone are
`effective when applied in the fall to perennial weeds such as Russian knapweed (Acroptilon
`
`PLANT GROWTH REGULATOR TYPE HERBICIDES
`TRANSLOCATE IN XYLEM AND PHLOEM, AND
`ACCUMULATE AT GROWING POINTS. PLANT INJURY
`SYMPTOMS INCLUDE TWISTING, BENDING OF LEAVES
`AND STEMS, AND SWELLING OF ROOTS.
`
`Page 4 of 9
`
`

`

`repens) or Canada thistle (Cirsium arvense). With fall applications, uptake is mainly by roots
`rather than foliage since underground roots and crowns are still actively growing and
`producing root buds for the following spring.
`
`
`
`Environmental Impacts on Foliar Herbicide Performance
`
`Environmental conditions before, during, and after application influence herbicide coverage
`on the plant or soil surface, and absorption/ translocation within the plant. This can ultimately
`affect the level of control achieved from an herbicide.
`
`TEMPERATURE, MOISTURE, HUMIDITY
`
`Prolonged exposure to low moisture and humidity, and high temperatures reduces herbicide
`penetration by increasing the thickness of the leaf cuticle and closing stomata. Late season
`application to invasive plants may also be less effective since mature leaves tend to have
`thicker cuticles that are more resistant to herbicide penetration. Addition of a surfactant may
`aid movement of the herbicide through plant cuticles and especially under stress conditions.
`
`Both high and low temperature extremes slow plant metabolism and can reduce the
`effectiveness of herbicides. The best temperatures to kill weeds are between 65 and 85
`degrees F. High temperatures, low humidity, and wind can dry spray droplets before the
`chemical is absorbed. Under these adverse conditions, the herbicide may either vaporize (if
`the herbicide is volatile) or crystallize on the leaf surface where it may then be removed by
`wind or rain, or degraded by sunlight.
`
`SUNLIGHT
`
`Plant response to foliar applied herbicides is usually more rapid on sunny days since light
`enhances photosynthesis. When photosynthesis is reduced other metabolic pathways in a
`plant slow or stop completely reducing both translocation and action of herbicides.
`
`Some herbicides may photodegrade in sunlight if they remain on the leaf surface for extended
`
`Page 5 of 9
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`

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`periods. Photodegradation (breakdown by sunlight) on the leaf surface is negligible for some
`herbicides including Transline and Milestone.
`
`RAIN FOLLOWING APPLICATION
`
`Rain following application of herbicides can influence control of the target plant. Some
`herbicides that are absorbed rapidly, such as 2,4-D Ester, may be “rainfast” in as little as 15
`minutes after application. However, some herbicides require as long as eight hours free of rain
`following application for best performance, so rain too soon after application can cause
`variable results. Be sure to read the herbicide label to determine the “rainfast (rain-free)”
`period for the herbicide you are applying.
`
`Table 1: The approximate number of hours required between herbicide application and
`rainfall (rainfast) for some commonly used natural area herbicides.
`
`FOLIAR CANOPY FACTORS
`
`Foliage area determines the amount of herbicide intercepted by a plant. Weeds with small
`leaves may intercept an inadequate amount of herbicide to obtain control. Also, plants with
`hairy leaves, such as common mullein, do not absorb foliar herbicides readily because hair
`holds spray droplets away from the leaf surface. An excessive amount of old growth from the
`previous growing season may also intercept herbicides, preventing contact with new growth.
`
`Page 6 of 9
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`

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`Herbicides such as Milestone and Transline that are taken up by both roots, stems and leaves
`can perform better than herbicides that are mainly taken up by foliage such as 2,4-D.
`Surfactants can be added to the spray solution that may increase penetration of hairy or waxy
`leaf surfaces.
`
`
`
`EVALUATING EFFECTIVENESS OF HERBICIDE TREATMENTS
`
`How long does it take to determine if an herbicide treatment was effective? Plants treated with
`herbicides during the spring and summer may show symptoms of herbicide activity within a
`few days following treatment depending on the herbicide applied. Milestone and Transline are
`growth regulator type herbicides. Injury symptoms will usually occur within several days
`during optimum growing conditions. Control results are most evident 6 to 8 weeks following
`application. Herbicide treatments made in late summer or fall should be evaluated the
`following spring. It may take days for some herbicides to be translocated throughout the plant,
`so cultivation or digging should be delayed for 10 days to 2 weeks to increase control.
`
`
`
`>> Additional information on herbicide absorption and translocation
`
`Published April, 2014; Updated June 2019
`
`®™Trademarks of Dow AgroSciences, DuPont or Pioneer and their affiliated companies or
`respective owners.
`
`Page 7 of 9
`
`

`

`State restrictions on the sale and use of Accord XRT II, Garlon 4 Ultra, Opensight and Transline
`apply. Consult the label before purchase or use for full details.
`
`Milestone and Vastlan are not registered for sale or use in all states. Contact your state
`pesticide regulatory agency to determine if a product is registered for sale or use in your state.
`
`Opensight and Milestone: When treating areas in and around roadside or utility rights-of-way
`that are or will be grazed, hayed or planted to forage, important label precautions apply
`regarding harvesting hay from treated sites, using manure from animals grazing on treated
`areas or rotating the treated area to sensitive crops. See the product label for details.
`
`Tordon 22K is a federally restricted use pesticide. Always read and follow label directions.
`©2019 Corteva
`
`Active ingredients for herbicide products mentioned in this article: Accord® XRT II
`(glyphosate), Vastlan® and Garlon® 4 Ultra (triclopyr), Milestone® (aminopyralid), Opensight
`(aminopyralid and metsulfuron), Accord® XRT II (glyphosate), Tordon® 22K (picloram),
`Transline® (clopyralid).
`
` Education/Outreach
`
` PART 2. HERBICIDE PROPERTIES ...
`
`Share
`Share
` EFFECTIVE HERBICIDE USE STARTS WITH ... 
`
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`
`ABOUT TECHLINE INVASIVE PLANT NEWS
`
`TechLine Online is a digital publication featuring new, innovative, and proven science-based information for invasive plant
`managers.
`For more information, email editor@techlinenews.com
`
`Page 8 of 9
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`

`

`© TechLine Invasive Plant News 2018.
`
`
`
`Page 9 of 9
`
`

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