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

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

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 - 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

Water Movement

Surface runoff

Leaching

Capillary action

Factors That Affect LeachingIncrease Decrease

Coarse soils Fine Soils

Low O.M. High O.M.

Water soluble Water insoluble

Non-binding Readily bind

High rainfall Normal rainfall

Relative Movement of Herbicides

Soil Type

Herbicide Silt loam ½ Loam + ½ Sand

Trifluralin 2 5

Simazine 14 27

Bromacil 55 88

Dicamba 91 95

Mobility of Preemergence Herbicides in Soil

None to slight

Low

Moderate

High

DNA’s Dimension

Ronstar Betasan Pennant Devrinol Rubigan

Aatrex Princep Sencor Prograss

Kerb

Mobility of Postemergence Herbicides in Soil

None to slight

Low

Moderate

High

Diquat Roundup MSMA

Buctril Acclaim Vantage Fusilade TranXit Revolver Certainty

Image Manage Corsair Metsulfuron Monument Katana

MCPP 2,4-D Vanquish Basagran Triclopyr Clopyralid Finale

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

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 herbicideto reach one-half (t1/2) of the originallyapplied concentration. Expressed in days, wks,months, yrs..

1.0 lb. Ai/acre 0.5 lb. Ai/acre

Preemergence Herbicides – Avg. t-1/2

Herbicide

Soil Persistence t-1/2

Benefin Prodiamine Oryzalin Trifluralin Pendimethalin Dithiopyr Oxadiazon

40 d 120 d

20 - 128 d 45 d 44 d 17 d 60 d

Postemergence Herbicides – Avg. t-1/2

Herbicide

Soil Persistence t-1/2

MSMA Bentazon Imazaquin Glyphosate Glufosinate

180 d 20 d 90 d 47 d 7 d

Herbicide Degradation

0.0

0.2

0.4

0.6

0.8

1.0

lbs.

/acr

e

Herbicide Concentration

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

Herbicide kg ai/ha HLP Index

Pendimethalin 3.4 65 - L

Dithiopyr 0.6 143 - L

Oxadiazon 3.4 9.2 – M-L

Bensulide 11.0 1.0 - M

Atrazine 2.2 1.5 - M

Simazine 2.2 1.9 - M

HLP Index – Postemergence

Herbicide kg ai/ha HLP Index

MSMA 3.4 39 - L

Glyphosate 4.5 112 - L

2,4-D 0.8 0.4 - H

Dicamba 0.6 0.48 - H

Imazaquin 0.6 3.6 - 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 Rating High Moderate Low

Groundwater Contam. Potential (GWCP)

High Hazardous Risky Safe

Moderate Risky Risky Safe

Low Safe Safe Safe

HLP/SLP Matrix ExampleLakeland 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: highhigh – HAZARDOUS

Atrazine: highmoderate – RISKY

MSMA: highlow - 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)

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 1493-1541 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

Thank you very much!!

georgiaturf.com