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Environmental Factors That Influence Herbicide Performance

Dr. Eric P. Prostko
Extension Weed Specialist
University of Georgia
Tifton, GA

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TABLE OF CONTENTS

Environmental Factors That Influence Herbicide Performance For a herbicide to work it must….. Fate of Herbicides in the Environment Specific Environmental Factors Soil Factors: Organic Matter and Texture Amount of atrazine required to reduce the growth of giant foxtail by 50% at varying OM levels. OM and texture are used to determine application rates of soil-applied herbicides. Soil Factors: Cation Exchange Capacity (CEC) Cation Exchange Capacities of OM and Clay Soil Factors: Cation Exchange Capacity Soil Factors: pH Effect of pH on Adsorption of Atrazine The Influence of pH on the Solubility of Accent Soil pH may influence rotational restrictions because of its effects on adsorption, solubility, and degradation. Climatic Effects: Temperature Climatic Effects: Moisture - PPI/PRE Effect of Rainfall Amount After Application on Herbicide Performance Climatic Effects: Moisture - POST Climatic Effects: Relative Humidity Climatic Effects: Relative Humidity Climatic Effects: Light Intensity Climatic Effects: What about dew? Effect of Volume and Dew on Roundup Efficacy on Oats Summary UGA Extension Weed Science

SLIDE CONTENTS

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 Fate of Herbicides in the Environment 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 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 Effect of pH on Adsorption of Atrazine McGlamery and Slife, 1966 The Influence of pH on the Solubility of Accent 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 Effect of Rainfall Amount After Application on Herbicide Performance Simmons et al. 1997 Climatic EffectsMoisture - POST plants to develop thicker cuticles reductions in absorption, retention, and translocation altered metabolism Dry weather causes….. 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 Effect of Volume and Dew on Roundup Efficacy on Oats Kogan and Zuniga, 2001 Dew Level 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)