Environmental Factors That Influence Herbicide Performance Dr. Eric P. Prostko Extension Weed Specialist University of Georgia Tifton

For a herbicide to work it must….. u come in contact with a plant surface (root, shoot, leaves) u remain at site long enough to penetrate or be absorbed u move to its site of action

Fate of Herbicides in the Environment

Specific Environmental Factors u soil * organic matter, texture, CEC, pH u climatic * temperature, moisture, humidity, light intensity, dew(?)

Soil Factors Organic Matter and Texture u most important for soil applied herbicides u Indirectly influences all processes that affect herbicides!! u 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 OM (%) Rate (kg/ha) Parochetti 1973

OM and texture are used to determine application rates of soil-applied herbicides.

Soil Factors Cation Exchange Capacity (CEC) u soils ability to adsorb positively charged compounds u 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 Factors Cation Exchange Capacity u influences rate of application u not found on many herbicide labels

Soil Factors pH u influences water solubility, adsorption, and hydrolysis u triazines u sulfonylureas u imidazolinones

Effect of pH on Adsorption of Atrazine pH Kd Value 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 Effects Temperature u In showy crotalaria, Blazer absorption was 4X greater at 81 0 and 95 0 than at (Wills and McWhorter, 1981) u In johnsongrass, Roundup absorption doubled as temperature was increased from 75 0 to (McWhorter et al. 1980)

Climatic Effects Moisture - PPI/PRE u activation * movement * 0.5” within 7-10 d u adsorption * availability

Effect of Rainfall Amount After Application on Herbicide Performance DualFrontier Herbicide Foxtail Control (%) 00.1"0.25"0.5"1" Simmons et al. 1997

Climatic Effects Moisture - POST u plants to develop thicker cuticles u reductions in absorption, retention, and translocation u altered metabolism Dry weather causes…..

Climatic Effects Relative Humidity u 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 Effects Relative Humidity u A higher relative humidity level ….. * extends drying period of herbicide droplets. * hydrates plant cuticles.

Climatic Effects Light Intensity u influences photosynthesis, cuticle development, stomatal openings, and photodecomposition

Climatic Effects What about dew? u studies have shown that dew can either increase or decrease foliar herbicide efficacy (Caseley 1989) u decrease - runoff and dilution u increase - cuticle hydration and uptake

Effect of Volume and Dew on Roundup Efficacy on Oats Spray Volume (GPA) Control - % 050%100% Kogan and Zuniga, 2001 Dew Level

Summary u Environmental factors cannot be controlled (except irrigation). u 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)