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Environmental Factors That Influence Herbicide Performance. Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton. For a herbicide to work it must…. come in contact with a plant surface (root, shoot, leaves)
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Environmental Factors That Influence Herbicide Performance Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton
For a herbicide to work it must….. • come in contact with a plant surface (root, shoot, leaves) • remain at site long enough to penetrate or be absorbed • move to its site of action
Specific Environmental Factors • soil * organic matter, texture, CEC, pH • climatic * temperature, moisture, humidity, light intensity, dew(?)
Soil FactorsOrganic 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
5 4.5 4 3.5 3 Rate (kg/ha) 2.5 2 1.5 1 0.5 0 0.8 1.9 3.9 6.4 11 18 OM (%) Amount of atrazine required to reduce the growth of giant foxtail by 50% at varying OM levels. Parochetti 1973
OM and texture are used to determine application rates of soil-applied herbicides.
Soil FactorsCation 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
Cation Exchange Capacities of OM and Clay Bailey and White 1964
Soil FactorsCation Exchange Capacity • influences rate of application • not found on many herbicide labels
Soil FactorspH • influences water solubility, adsorption, and hydrolysis • triazines • sulfonylureas • imidazolinones
14 12 10 8 Kd Value 6 4 2 0 3.9 4.7 5.3 6 8 pH Effect of pH on Adsorption of Atrazine McGlamery and Slife, 1966
Soil pH may influence rotational restrictions because of its effects on adsorption, solubility, and degradation.
Climatic EffectsTemperature • In showy crotalaria, Blazer absorption was 4X greater at 810 and 950 than at 640.(Wills and McWhorter, 1981) • In johnsongrass, Roundup absorption doubled as temperature was increased from 750 to 950.(McWhorter et al. 1980)
Climatic EffectsMoisture - PPI/PRE • activation * movement * 0.5” within 7-10 d • adsorption * availability
0 0.1" 0.25" 0.5" 1" 100 80 60 Foxtail Control (%) 40 20 0 Dual Frontier Herbicide Effect of Rainfall Amount After Application on Herbicide Performance Simmons et al. 1997
Climatic EffectsMoisture - POST Dry weather causes….. • plants to develop thicker cuticles • reductions in absorption, retention, and translocation • altered metabolism
Climatic EffectsRelative Humidity • Liberty treatment at 95% RH resulted in complete plant death in contrast to only a 30% inhibition in growth at 40% RH(Anderson et al. 1993)
Climatic EffectsRelative Humidity • A higher relative humidity level ….. * extends drying period of herbicide droplets. * hydrates plant cuticles.
Climatic EffectsLight Intensity • influences photosynthesis, cuticle development, stomatal openings, and photodecomposition
Climatic EffectsWhat about dew? • studies have shown that dew can either increase or decrease foliar herbicide efficacy (Caseley 1989) • decrease - runoff and dilution • increase - cuticle hydration and uptake
0 50% 100% 100 80 60 Control - % 40 20 0 16 32 48 Spray Volume (GPA) Effect of Volume and Dew on Roundup Efficacy on Oats Dew Level Kogan and Zuniga, 2001
Summary • Environmental factors cannot be controlled (except irrigation). • Need to understand environmental effects and make applications when conditions are favorable for optimum performance if possible.
University of Georgia Extension Weed Science (gaweed.com)