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

2. 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

3. Fate of Herbicides in the Environment

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

5. 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

6. Amount of atrazine required to reduce the growth of giant foxtail by 50% at varying OM levels. 0 0.5 1 1.5 2 2.5 3 3.5 4 4.5 5 0.81.93.96.41118 OM (%) Rate (kg/ha) Parochetti 1973

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

8. 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

9. Cation Exchange Capacities of OM and Clay Bailey and White 1964

10. Soil Factors Cation Exchange Capacity u influences rate of application u not found on many herbicide labels

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

12. Effect of pH on Adsorption of Atrazine 0 2 4 6 8 10 12 14 3.94.75.368 pH Kd Value McGlamery and Slife, 1966

13. The Influence of pH on the Solubility of Accent

14. Soil pH may influence rotational restrictions because of its effects on adsorption, solubility, and degradation.

15. Climatic Effects Temperature u In showy crotalaria, Blazer absorption was 4X greater at 81 0 and 95 0 than at 64 0. (Wills and McWhorter, 1981) u In johnsongrass, Roundup absorption doubled as temperature was increased from 75 0 to 95 0. (McWhorter et al. 1980)

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

17. Effect of Rainfall Amount After Application on Herbicide Performance 0 20 40 60 80 100 DualFrontier Herbicide Foxtail Control (%) 00.1"0.25"0.5"1" Simmons et al. 1997

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

19. 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)

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

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

22. 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

23. Effect of Volume and Dew on Roundup Efficacy on Oats 0 20 40 60 80 100 163248 Spray Volume (GPA) Control - % 050%100% Kogan and Zuniga, 2001 Dew Level

24. 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.

25. University of Georgia Extension Weed Science (gaweed.com)