Environmental Fate of Herbicides Tim R. Murphy, Ph.D. The University of Georgia Crop and Soil Sciences

Public Concerns  Health  Quality of Life  Environment  Nuclear and Toxic Waste  Chemicals vs. Natural  Right-to-Know

“Public Concerns” About Chemicals  Cause cancer  Not well tested  Harm animals  Last forever  Not “natural”  Used carelessly  Contaminate water  Any amount is dangerous

Herbicide Concerns  Last forever  Contaminate water  Affect human health  Sterilize soil  Use is not needed  Kill all desirable organisms  Degrade the environment

“ For the price of a green lawn, we are poisoning our children.” Family Circle magazine, 1991

Fate of Herbicides  Original dose and ½ life  Water solubility - the extent to which a pesticide will dissolve in water  Sorption by clay colloids and organic matter Adsorption - binding of a herbicide to the surface of a soil particle. Absorption - Penetrates into plant tissue  Microbial degradation - influenced by herbicide concentration, temperature, moisture, pH, oxygen, microbial population

Fate of Herbicides  Chemical degradation and photodecomposition Hydrolysis, oxidation, reduction, and photodecomposition under field conditions  Volatilization and evaporation - Loss due to an increase in temperature, vapor pressure, and wind movement.  Plant uptake and metabolism - roots, shoots, leaves

Herbicide Fate in the Soil  Herbicide Chemical Characteristics  Soil Physical-Chemical Characteristics

Herbicide-Chemical Properties  Ionic State (cation, anion, basic or acidic)  Water Solubility  Vapor pressure  Hydrophobic/hydrophilic  Partition coefficient  Chemical, photochemical, microbial sensitivity

Soils - Solid Phase  Sand to 2.0 mm  Silt to 0.2 mm  Clay - <  Organic matter - decaying plant and and animal residue

Soils - Colloidal Phase  Consists of clay and organic matter  Huge surface area  Negatively charged  Anions (-charge) repelled  Cations (+charge) attracted  Primarily responsible for binding herbicides

Soils - Gas & Liquid Phase  Gas - oxygen, carbon dioxide, others  Liquid - water (with dissolved molecules, ions, etc.)

Soils - Living Phase  Microorganisms - bacteria, actinomycetes, fungi  Algae  Vertebrates and Invertebrates  Microorganisms degrade herbicides

Microbial Degradation  Higher with high microbial populations  May use as food source, or just degrade the herbicide  Faster under warm, moist conditions  Slower under cool, dry conditions

Herbicide Adsorption  Soil texture coarse, sandy soils have few binding sites  Permeability highly permeable soils low in CEC have few binding sites  Soil OM and clay content increase binding  Excessive moisture interferes with binding

Soil Factors Cation Exchange Capacity (CEC)  soils ability to adsorb positively charged compounds  fine-textured, high- organic matter soils have larger CEC’s than coarse, low-organic matter soils paraquat

Soil Factors Organic Matter and Texture  most important for soil applied herbicides  Indirectly influences all processes that affect herbicides!!  the greater the organic matter and clay content, the greater adsorption of herbicides

Amount of atrazine required to reduce giant foxtail growth by 50% at varying OM levels OM (%) Rate (kg/ha) Parochetti 1973

Water Movement  Surface runoff  Leaching  Capillary action

Factors That Affect Leaching

Relative Movement of Herbicides

Mobility of Preemergence Herbicides in Soil

Mobility of Postemergence Herbicides in Soil

Volatility Volatility - physical change of a liquid or solid to gas.

Volatility  Related to vapor pressure  Increases at high air temperatures  Increases under high soil moisture conditions  Higher on coarse textured, sandy soils

Preemergence Herbicide Water Solubility and Relative Volatility Herbicide Water Solubility (ppm) Vapor Pressure (mm Hg) Relative Volatility Benefin X High Pendimethalin X Low Surflan X Very Low Barricade X Very Low Dimension X Low Kerb X Moderate Bensulide X Very Low Ronstar X Very Low Aatrex X Very Low

Postemergence Herbicide Volatility

Photodecomposition Photodecomposition - Breakdown of the herbicide by sunlight (primarily UV portion).

Herbicide Persistence - Soil Usually expressed as the half-life (t1/2).

Herbicide ½ Life Amount of time it takes a herbicide to reach one-half (t1/2) of the originally applied concentration. Expressed in days, wks, months, yrs lb. Ai/acre 0.5 lb. Ai/acre

Preemergence Herbicides – Avg. t-1/2

Postemergence Herbicides – Avg. t-1/2

Herbicide Degradation

Herbicide Leaching Potential Index  HLP – developed by Warren and Weber, NCSU  Factors considered include: Binding ability Persistence (t-1/2) Application rate Amount that penetrates turf canopy and reaches soil Soil pH, O.M., type

HLP Index  Low potential for leaching - > 10.1  Moderate potential- 1.0 to 10.0  High potential- < 1.0

HLP Index – Preemergence Herbicidekg ai/haHLP Index Pendimethalin L Dithiopyr L Oxadiazon – M-L Bensulide M Atrazine M Simazine M

HLP Index – Postemergence Herbicidekg ai/haHLP Index MSMA L Glyphosate L 2,4-D H Dicamba H Imazaquin M

Soil Leaching Potential - SLP  Texture, O.M. and pH have greatest impact on herbicide leaching  Clays retard movement, sands increase  High O.M. retards, low O.M. increases  Acidic pH increases degradation  Neutral to alkaline pH decreases degradation, and can increase movement potential

SLP  S, LS, SL, L, SiL, L:10  SCL, CL, SiCL: 6  SiC, SC: 3  C or muck: 1 S= sand, L = loam, Si = silt, C = clay

SLP  Can be calculated for each soil type Based on texture and pH 0 to 91 cm Based on O.M. in upper 15 cm  High soil leaching potential: > 131  Moderate: 90 to 130  Low: < 89

Herbicide Selection with HLP/SLP Matrix Soil Leaching Potential (SLP) Rating HLP RatingHighModerateLow Groundwater Contam. Potential (GWCP) HighHazardousRiskySafe ModerateRisky Safe LowSafe

HLP/SLP Matrix Example Lakeland Sand, low O.M. and clay, SLP = 134, High  Dicamba: HLP = 0.48 or high  Atrazine: HLP = 1.5 or moderate  MSMA: HLP = 39 or low  Dicamba: high high – HAZARDOUS  Atrazine: h igh moderate – RISKY  MSMA: high low - SAFE

Best Management Practices - BMPS  Use herbicides with low HLP Indices on high SLP soils  Train employees on proper application techniques  Spot treat if possible  Follow label Be aware of any water advisory statements

BMPS (continued)  Consider mixing/loading pads, with spill containment  Do not mix or apply within 100 ft. of a well head  Prevent back-siphoning  Calibrate sprayer  Establish buffer (non-treated areas)

SU Herbicide Lateral Movement  Griffin, GA  Lateral runoff facility 5% uniform slope Wobbler irrigation heads 1.25 inches per hour Tifway bermudagrass 0.5 inch clip height

SU Herbicide Lateral Movement  Plots 25 ft. long Lower 10 ft. fall-seeded with perennial ryegrass Upper 15 ft. not overseeded  January 22, 2004, applied to dormant bermuda Monument at 0.33 and 0.56 oz./acre Manor at 0.5 oz./acre TranXit at 2.0 oz./acre Kerb at 2.0 lbs./acre

SU Herbicide Lateral Movement  Before application Applied 2.5 inches irrigation (2 hrs), 37% saturation Waited ~ 1hr, until foliage dried Applied herbicides Waited ~ 1hr, until foliage dried Applied 2.5 inches irrigation (2 hrs) Applied 0.5 inch irrigation water 24 HAT

SU Herbicide Lateral Movement  WHAT HAPPENED?? Kerb – moved 8 ft. (76% injury) Monument 0.33 oz. – 0.7 ft (3%) Monument 0.56 oz. – 3 ft. (14%) Manor 0.5 oz. – 0.7 ft. (1) TranXit 2.0 oz – 0.6 ft. (1)  NOTE: No significant injury occurred until 48 DAT

KERB Kerb Great for Poa Will move

Can SU Herbicides Move?  Yes, if: Surface film of water present at application Irrigation or rainfall immediately after application Not watering after foliage has dried Dew re-suspends and can cause tracking (prevent by watering in) Improperly adjusted irrigation heads

Atrazine Technical Facts (U.S. E.P.A)   Drinking water (U.S.A.) MCL: mg/L  2 nd most frequently detected pesticide in EPA National Survey of Drinking Water Wells DE, IL, IN, IA, KS, MI, MN, MO, NE, NY

Atrazine Technical Facts (U.S. E.P.A)  Degradation Microbial, chemical (hydrolysis at acidic pH)  Adsorption Higher in muck and clay soils, than in soils with low clay and O.M.  t-½ = 60 d

Atrazine Label Precautions  Do not apply to sands and loamy sands where water table is close to surface.  Do not mix, load within 50 ft. of wells, sinkholes, etc. (unless pad with containment is used).  Do not mix, load within 50 ft. of streams, lakes, etc.

Atrazine Label Precautions  Do not apply within 66 ft. of where surface water run-off enters streams or rivers, or within 200 ft. of lakes and reservoirs.  On highly erodible land, use a 66 ft. crop or grass buffer strip.

0 Residue Ain’t Possible!!!  1 ppm = one second in 12 days  1 ppb = one second in 32 years  1 ppt = one second in 32,000 years  1 ppq = one second in 32,000,000 years  1.0 lb. Ai/acre = 1.0 ppm in upper 3 inches

Risk Communication  “Everything is Poison. There is nothing without poisonous properties. The dose differentiates a remedy from a poison.” Philippus Aureolus Theophrastus Bombastus von Hohenheim Better known a Paracelsus

Facts  30 yrs added to lifespan in 20th century  8 yrs added since use of pesticides  only 37% of land farmed in 1950 is cultivated today Dennis Avery, Hudson Institute, Wall Street Journal, August 12, 1999  deer, turkey, geese populations increasing in GA

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