herbicide persistence and carryover (a3819) - corn...

Residual herbicides are those thatcontinue to control weeds for sometime after application. The use ofresidual herbicides such as Pursuit,Spartan, and Sinbar, is fairlycommon in some horticulturalcrops. Residual herbicides extend theperiod of weed control, increasingthe efficiency of weed managementefforts. However, they may persistlonger than desired and injure orkill subsequent rotational crops.Most herbicide labels include croprotation guidelines, but rotationalrestrictions are often not listed formany horticultural crops. Herbicidepersistence, or the length of time aherbicide remains in the soil, variesgreatly with climatic conditions, soiltype, and cultural practices. It isimportant to distinguish betweenherbicide persistence and herbicideactivity. Some herbicides persist fora long time in soil but are not avail-able for plant uptake and therefore

are not active as herbicides. Giventhe difficulty in predicting herbicidepersistence, it is important to knowthe factors that lead to persistence.Incorporating these factors into cropplanning can reduce herbicideinjury risk.

The life of a herbicideResidual herbicide activity is oftendescribed in terms of the “half-life,”or the amount of time required fordissipation of one half of the origi-nal amount of applied herbicide.The half-life varies by herbicide butcan range from a few days to a fewyears. The herbicide half-life doesnot coincide very well with croprotational restrictions for severalreasons:

■ Rotational crop responsevaries greatly by herbicideand species susceptibility.Some crops tolerate a particularherbicide residue and can bereplanted soon after that herbi-cide is applied, while other cropsremain sensitive to the herbicidefor a longer time after applica-tion. Some herbicides can dissi-pate for many half-lives and stillbe injurious to certain cropspecies, while other herbicidespersist longer but are less injuri-ous to some crops. In theexample in figure 1, herbicide“A” (with a half-life of 38 weeks)persists longer in the soil thanherbicide “B” (with a half-life of5 weeks). However, potatoes aremuch more sensitive to injuryfrom herbicide “B” than herbi-cide ”A,” even after 10 half-lives.

A3819

Herbicide persistence and carryoverJed Colquhoun

herbicide B:safe to rotate topotato after 52 weeks

herbicide A:safe to rotate topotato after 23 weeks

•

2000

50

100

40 60

weeks after herbicide application

% h

erbi

cide

rem

aini

ng

Figure 1. Comparison of persistenceand carryover of two herbicides

HALF-LIFE

A

B

herbicide

5 w

eeks

38 w

eeks

■ Herbicide half-life variesgreatly with soil type, soil pH,climatic conditions, and crop-ping systems. Half-lives areoften determined in laboratoryresearch and may not reflect allfield conditions. Herbicide per-sistence is difficult to predictwith climatic variation from yearto year. In the example in figure 2,the same herbicide is applied ina wet year and a drought year.The herbicide half-life is 40weeks longer in the drought yearthan in the wet year, delayingthe earliest potential plantingdate without herbicide injury forpotato.

Where do herbicidesgo after they leavethe sprayer?Herbicide fate plays an importantrole in persistence and potential car-ryover. Herbicides are ultimatelysubjected to one of three potentialfates:

■ Herbicides are removed fromthe soil system. Herbicides canleave the area where they wereapplied through any of severalchannels. Routes include beingcarried away in surface waterrunoff, leaching out of the areain soil water, volatilization froma solid or liquid to a gas that dis-sipates in the atmosphere, anduptake by plant roots or foliage.

■ Herbicides are adsorbed tosoil particles. Adsorption isthe binding of chemicals to thesurface of solids.

■ Herbicides are degraded intocompounds that are inactiveas herbicides. Degradation isthe breakdown of a herbicide bymicrobes, sunlight, or chemicalreaction.

Herbicides in thesoil environmentHerbicides in soil typically exist inone of three phases:

■ Adsorbed to soil particles.Adsorbed herbicides are“banked.” They are not avail-able for plant uptake, degrada-tion, or movement from the soilenvironment (other than whenthe soil particle itself is moved,such as in surface water runoff).

■ In soil water solution.Herbicides in soil solution arethe most active: they are avail-able for plant uptake, degrada-tion, and movement.

■ In soil air spaces. Herbicidesin soil air are not common once aherbicide is incorporated in thesoil.

Factors that affectherbicide persistenceHerbicide persistence is directlyrelated to how quickly the productdecomposes and its availability forplant uptake. Microbes, chemicalreactions, and exposure to lightaffect decomposition, while soiladsorption and leaching in soilwater determine availability. All ofthese factors vary greatly by soiltype and pH, by climatic conditionsbetween the time of herbicide appli-cation and re-cropping, and bycropping practices. Understandingthe variables that determine persis-tence can reduce crop injury risk

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H E R B I C I D E P E R S I S T E N C E A N D C A R R Y O V E R

drought yearsafe to rotate to potato

wet yearsafe to rotate to potato

2000

50

100

40 60

weeks after herbicide application

% h

erbi

cide

rem

aini

ng

Figure 2. The effect of soil moisture on herbi-cide persistence. Herbicides persist muchlonger during dry years than during wet years.

drought

wet

safe residue level

from herbicide carryover. Table 1provides information about the rela-tive persistence and importantfactors in persistence for commonherbicides.

Factors that affectherbicide availability:■ Soil adsorption. Herbicides

that are chemically bound, oradsorbed, to solids are not avail-able for leaching, plant uptake,or microbial degradation. Suchherbicides persist until they are

released from the soil surface.Some herbicides, such asGramoxone and Diquat, bind tosoil so tightly that they persistnearly indefinitely and are nottypically available for plantuptake.

✛ Soil type is very important indetermining potential adsorp-tion. Soil organic matter and clayincrease soil adsorption becauseof their chemical reactivity andhigh number of binding sites.

✛ Soil water competes with her-bicides for binding sites. As aresult, wet soils adsorb less her-bicide than dry soils.

✛ Soil adsorption is greater inlow pH soils as there are fewerpositively charged particles tocompete for the negativelycharged binding sites.

✛ Herbicides that are highlysoluble in water do not adsorbwell to soil.

■ Herbicide leaching. Herbicideleaching in soil water can moveherbicides out of the tillage androot zone of subsequent crops.Herbicide leaching is greatest incoarse-textured soils with lowlevels of organic matter. Highlysoluble herbicides are prone toleaching.

Factors that affectherbicide degradation:■ Microbial decomposition. The

breakdown of herbicides by soilorganisms known as microbesaccounts for a large portion ofherbicide degradation in soil.Certain bacteria, fungi, and algaeuse herbicides as a food source.Microbes are herbicide-specific,and their population in the soil isrelated to the amount of herbi-cide available for consumption.Repeated use of a herbicide canlead to increased microbe popu-lations and a shorter duration ofeffective weed control.Conditions that support highmicrobe populations favor rapidherbicide degradation.

✛ Soil type is important formicrobe populations: soilorganic matter provides excel-lent habitat for microbes.

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✛ The effect of soil pH on micro-bial degradation varies bymicrobe, but the extremes in pHare typically less favorable forhigh microbe populations.

✛ Climatic conditions that favoroptimum plant growth alsofavor microbial activity.Microbes are most active whensoil moisture ranges from 50 to100% of field capacity; whenfields are flooded, microbialactivity drops due to lack ofoxygen. Similarly, microbes aremost active in warm soils (withadequate moisture); when soiltemperatures fall below about40°F, microbial activity becomesnegligible.

■ Chemical decomposition.Chemical decomposition isimportant for some herbicidessuch as those in the sulfonylureafamily (examples include Matrix,Sandea, and Accent). Mostchemical decomposition occurswhen herbicides react with soilwater (a process called hydroly-sis). The rate increases in soilswith lower pH values and atwarmer temperatures.

■ Photodecomposition.Photodecomposition occurswhen energy from the sunbreaks down a herbicide. Only alimited number of herbicides,such as Treflan and Goal, aresensitive to sunlight. To preventphotodecomposition, soil appli-cations of these herbicides areoften incorporated.

Strategies to avoidherbicide carryover■ Apply labeled rates and

follow rotational restrictions.Exceeding the rates listed on thelabel is illegal and can result inherbicide persistence. Crop rota-tional restrictions for a particularherbicide can vary by applica-tion rate and timing, geography,and soil type and pH, so be sureto read the label thoroughly. Therotational restrictions listed onthe label are based on extensivefield research. Some labels donot allow rotation to crops notlisted on the label; others includemany horticultural crops in the“all other crops” category. Beaware that persistent herbicidescan lead to illegal residue levelsin rotational crops even whenthe risk for visible crop injury isminimal. When tank-mixing her-bicides, follow the crop rotationguidelines for the more restric-tive label unless otherwisenoted.

■ Keep future cropping plans inmind when planning herbicideprograms. Avoid the use oflong-residual herbicides whenincluding sensitive crops in therotation.

■ Note climatic conditions fromthe time between herbicideapplication and the next crop.Low moisture and temperature,in particular, can slow herbicidedegradation and increase therisk for carryover.

■ Rotate herbicide mode ofaction to avoid buildup insoil. Although rare, repeateduse of the same herbicide, oreven the same herbicide family,can lead to herbicide buildup inthe soil.

■ Maintain healthy soil. Soil con-ditions that are favorable forplant growth are also favorablefor herbicide degradation.Maintain a moderate soil pH andorganic matter for optimum her-bicide degradation.

Strategies to reducecrop injury risk fromherbicide carryoverThese methods are not intended tosupersede rotational restrictions onthe pesticide label, but to reduce therisk of carryover.

■ Work the soil. Thorough tillagewill distribute residual herbicideevenly and dilute concentration,thus allowing maximum expo-sure to microorganisms and clayand organic matter that adsorbherbicides. Tillage can alsoreduce compaction and increaseaerobic microorganism activity.Tillage will not solve all poten-tial carryover issues, and in rarecases, can make the situationworse. For example, deepplowing can invert residual her-bicides, concentrating theresidue at soil depths thatremain lower in temperature.The herbicide residue can thenbe brought back to the plant rootzone with subsequent deepplowing, exposing future cropsto potential carryover. It is essen-tial to thoroughly distribute anyherbicide residue in the soil.

■ Plant more tolerant crops. Ifcrop choice is flexible, considerplanting a crop with a shorterrotational restriction in fieldswhere environmental conditionsmay have extended the length ofherbicide carryover.

■ Conduct a herbicide bioassay.With a herbicide bioassay, cropseeds are grown in pots usingsoil from the field. This simpleand economical test allowsgrowers to screen for potentialherbicide carryover. (A labora-tory analysis, by contrast, isoften very costly and the resultsare difficult to interpret in termsof rotational crop safety.)Bioassays are not fail-proof: cli-matic conditions in the field,such as available moisture, oftendiffer from plants grown indoorsin pots. Consider the following“recipe” when conducting herbi-cide bioassays.

Literature sources Devlin, D., D. Peterson, and D.

Regehr. 1992. Residual Herbicides,Degradation, and RecroppingIntervals. Kansas State UniversityExtension (Pub. C-707).Summary: Technical details ofherbicide persistence and rota-tional information for agronomiccrops.

Gunsolus, J.L. and W.S. Curran.1999. Herbicide Mode of Action andInjury Symptoms. University ofMinnesota (Pub. NCR377).Summary: Describes and showsdamage from common herbicides.

Hanson, B., T. Rauch, and D. Thill.2004. Plantback Restrictions forHerbicides Used in the DrylandWheat Production Areas of thePacific Northwest. PacificNorthwest Extension (Pub. 571).Summary: Crop rotation infor-mation for agronomic crops.

Miller, P. and P. Westra. 2004.Herbicide Behavior in Soils.Colorado State UniversityCooperative Extension Service(Pub. 0.562). Summary:Chemical characteristics of her-bicide families.

Radosevich, S.R., J.S. Holt, andC. Ghersa. 1997. Weed Ecology:Implications for Management. NewYork, New York: J. Wiley. 589 p.Summary: General herbicidecharacteristics and behavior.

Vencill, W.K., ed. 2002. HerbicideHandbook: Eighth Edition.Lawrence, Kansas: Weed ScienceSociety of America. 493 p.Summary: Reference for herbicidecharacteristics.

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How to conduct a bioassay1. Collect soil from the top 2 to 3 inches in several areas of the field

and thoroughly mix samples. Sample from areas that may havehigh residual herbicide, such as in head-row turnarounds andfield corners, and analyze these soils separately as a worst-casescenario. Representative, thorough sampling is critical to an accu-rate bioassay.

2. Fill several flower pots or similar containers with sample soil.

3. Plant the crop species that is planned for the field, or a crop thathas a long rotational restriction listed on the herbicide label. Thinplants to one per container after emergence.

4. Place pots indoors and provide uniform light and water. Uniformnatural light is better than artificial light, if possible.

5. About 2 to 3 weeks after emergence, evaluate the plants for symp-toms of damage from the suspected herbicide. For descriptionsand pictures of herbicide damage, see Herbicide Mode of Action andInjury Symptoms (NCR377) by J.L. Gunsolus and W.S. Curran.

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Table 1. Relative persistence and importance of persistence factors for each herbicide group. Consult the label for rotational restrictions before any application.

Herbicide group ACCase inhibitors (acetyl CoA carboxylase)

Herbicide families Arloxyphenoxypropionates, cyclohexanediones (Group 1)

Examples Assure II, Fusilade, Poast, Select

Relative persistence Short to moderate

General rotational risk Risk greatest for rotation to monocot (grass) crops; overall, rotational concerns are relatively low and restrictions are often 120 days or less

Persistence factor Relative importance Notes

Adsorption Low to moderate

Leaching Low

Microbial decomposition High

Chemical decomposition Low to moderate

Photodecomposition Low to high Poast decomposes on soil surface

Herbicide group ALS inhibitors (acetolactate synthase)

Herbicide families Imidazolinones (Group 2)

Examples Pursuit, Raptor

Relative persistence Moderate to long

General rotational risk Risk increases as soil pH decreases; persistence very dependent on environmental conditions after application; several vegetable crops are sensitive to low levels of imidazolinone herbicides


Adsorption Low Organic matter and clay content important Leaching Low Dependent on soil pH

Microbial decomposition Very high Poor anaerobic decomposition

Chemical decomposition Low

Photodecomposition Low

Herbicide group ALS inhibitors (acetolactate synthase)

Herbicide families Sulfonylureas (Group 2)

Examples Accent, Matrix, Sandea

Relative persistence Short to long, depending on herbicide, soil environment, and climate

General rotational risk Risk increases as soil pH increases; several vegetable crops are sensitive to low levels of residual sulfonylurea herbicides


Adsorption Low

Leaching Low to moderate

Microbial decomposition Moderate to high Important when chemical decomposition is slow

Chemical decomposition Very high Decreases as soil pH increases


7

Table 1. Relative persistence and importance of persistence factors for each herbicide group

(continued).

Herbicide group Bleaching

Herbicide families Isoxazolidinone (Group 11), triketone (Group 28)

Examples Callisto, Command


General rotational risk Selected crops quite sensitive to low levels of persistent bleaching herbicides and therefore have long rotational restrictions.


Adsorption Low

Leaching Low

Microbial decomposition High Primary degradation mechanism



Herbicide group Cell wall synthesis inhibitors

Herbicide families Nitriles (Group 20)

Examples Casoron


General rotational risk Rotational restrictions often 1 year after application


Adsorption Moderate

Leaching Moderate

Microbial decomposition High Primary decomposition mechanism



Herbicide group EPSP synthase inhibitors (Group 9)

Herbicide families Not applicable

Examples Roundup and other glyphosate herbicides

Relative persistence Moderate

General rotational risk While persistence is moderate, herbicide activity is minimal after contact with soil; minimal rotation risk


Adsorption Very high Not particularly dependent on soil type or pH

Leaching Low

Microbial decomposition High



8


(continued).

Herbicide group Lipid synthesis inhibitors

Herbicide families Thiocarbamates (Group 8)

Examples Eptam, Ro-Neet

Relative persistence Short

General rotational risk Relatively low rotation risk; residual weed control often less than 30 days


Adsorption Low to moderate Organic matter greatly increases adsorption

Leaching Moderate




Herbicide group Microtubule assembly inhibitors

Herbicide families Dinitroanilines (Group 3)

Examples Prowl, Sonalan, Treflan


General rotational risk Monocot (grass) crops typically at greater risk; some crops very sensitive to low residue levels, such as beets, and thus have fairly long rotational restrictions


Adsorption High Increases greatly with increased organic matter Leaching Low Low water solubility

Microbial decomposition Moderate to high Accounts for majority of degradation


Photodecomposition Moderate to high

Herbicide group Photosystem I inhibitors

Herbicide families Bipyridyliums (Group 22)

Examples Gramoxone, Diquat

Relative persistence Very long

General rotational risk While persistence is very long, these herbicides are bound so tightly to soil that they are not biologically active; minimal rotational risk


Adsorption Very high Extremely rapid and strong adsorption to clay

Leaching Low

Microbial decomposition Low


Photodecomposition Moderate

9


(continued).

Herbicide group Photosystem II inhibitors

Herbicide families Triazines, uracils (Group 5)

Examples Atrazine, Sencor, Sinbar


General rotational risk Rotational restrictions can be more than 2 years in some cases; rotational risk often increases at higher soil pH


Adsorption Low to high Highest at low pH (triazines) Leaching Moderate to high Greater risk at higher pH (due to less adsorption) Microbial decomposition Moderate to high

Chemical decomposition Low to moderate Fastest at low pH



Herbicide families Benzothiadiazines, nitriles (Group 6)

Examples Basagran, Buctril

Relative persistence Very short

General rotational risk Low rotational risk


Adsorption Low

Leaching Moderate

Microbial decomposition High Majority of degradation




Herbicide families Ureas (Group 7)

Examples Diuron, Lorox


General rotational risk Some crops quite sensitive to urea herbicide residues, even at low levels


Adsorption Moderate Dependent on soil type and organic matter Leaching Moderate Greatest in sandy soils




10


(continued).

Herbicide group PPO inhibitors (protoporphyrinogen oxidase )

Herbicide families Diphenylethers, triazolinone (Group 14)

Examples Cobra, Goal, Spartan

Relative persistence Short to long (Spartan)

General rotational risk Spartan rotational restrictions long for selected crops; small grain crops have longer rotation restrictions after Goal application


Adsorption Moderate to high Moderate adsorption for Spartan

Leaching Low to moderate Moderate for Spartan

Microbial decomposition Moderate to high


Photodecomposition Low to high

Herbicide group Synthetic auxins

Herbicide families Benzoic acids, phenoxyacetic acids, pyridines (Group 4)

Examples Banvel, MCPA, Stinger, Thistrol, Tordon, 2,4-D

Relative persistence Very short to short, moderate (Stinger), or long (Tordon)

General rotational risk Rotation restrictions fairly short, with the exception of Stinger and Tordon that can persist and remain active in soil; dicot (broadleaf) crops at greatest risk


Adsorption Low Maximum adsorption at low pH

Leaching Low to high

Microbial decomposition High Soil moisture and temperature important Chemical decomposition Low


Herbicide group Unknown mode of action

Herbicide families Chloroacetamides (Group 15)

Examples Dual, Harness, Intrro, Outlook

Relative persistence Short to moderate

General rotational risk Some products, such as Harness or Intrro, are very restrictive in rotation to vegetable crops


Adsorption Moderate Adsorption increases greatly with organic matter Leaching Low to moderate

Microbial decomposition High Primary decomposition mechanism




Copyright © 2006 University of Wisconsin-System Board of Regents and University of Wisconsin-Extension, CooperativeExtension.

Author: Jed Colquhoun is assistant professor of horticulture, College of Agricultural and Life Sciences, University ofWisconsin-Madison and University of Wisconsin-Extension, Cooperative Extension. Produced by Cooperative ExtensionPublishing, University of Wisconsin-Extension.

University of Wisconsin-Extension, Cooperative Extension, in cooperation with the U.S. Department of Agriculture andWisconsin counties, publishes this information to further the purpose of the May 8 and June 30, 1914 Acts of Congress; andprovides equal opportunities and affirmative action in employment and programming. If you need this information in analternative format, contact Cooperative Extension Publishing at 608-262-2655 or the UWEX Affirmative Action Office.

This publication is available from your Wisconsin county Extension office or from Cooperative Extension Publishing,To order, call toll free 877-WIS-PUBS (947-7827) or visit our web site at cecommerce.uwex.edu.

Herbicide Persistence and Carryover (A3819) I-03-2006

References to chemicals in this publication are for your convenience and are notendorsements of particular products over other similar products. You are responsi-ble for using pesticides according to the manufacturer’s current label directions.Follow directions exactly to protect people and the environment from pesticideexposure. Failure to do so violates the law.

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