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
Cause cancer Not well tested Harm animals Last forever Not “natural” Used carelessly Contaminate water Any amount is dangerous “Public Concerns” About Chemicals
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 photodecompositionHydrolysis, 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
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 herbicide to reach one-half (t1/2) of the originally applied concentration. Expressed in days, wks, months, yrs.. 1.0 lb. Ai/acre0.5 lb. Ai/acre
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
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
Dicamba: HLP = 0.48 or high Atrazine: HLP = 1.5 or moderate MSMA: HLP = 39 or low Dicamba: high high – HAZARDOUS Atrazine: high moderate – RISKY MSMA: high low - SAFE HLP/SLP Matrix ExampleLakeland Sand, low O.M. and clay, SLP = 134, High
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
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