Factors Influencing the Activity of Foliar and Soil Applied Herbicides

1. Factors Influencing the Activity of Foliar and Soil Applied Herbicides

2. Factors Influencing the Effectiveness of a Herbicide • Affect of soil characteristics on herbicide activity • Type of herbicide, plant species, climatic variation influence effectiveness of herbicide treatment • Type of herbicide treatment • Surface vs. incorporation • Depth of weed species germination • Uniformity of germination of weed seeds • Persistence of the herbicide in the rooting zone

3. Factors Influencing the Effectiveness of Foliar Applied Herbicides • Reach the plant • Be retained on the leaf • Penetrate the leaf • Move to the site of action • Remain toxic long enough to exert its action

4. Reach the plant • 1. Spray drift • 2. Volatilization • 3. "Canopy" effect

5. Be retained on the leaf • 1. Morphology of the plant • 2. Waxiness of the leaf • 3. Characteristics of the spray solution • 4. Volatility from the leaf • 5. Spray droplet size

6. Penetrate the leaf • Four factors affecting leaf penetration • 1. Remain on the surface as a crystal or liquid • 2. Enter the cuticle and remain dissolved in the nonpolar portion • 3. Enter and move in the aqueous phase along cell walls to the vascular system • 4. Enter and move directly into living cells and through them to the vascular system

7. The site of action • Movement is affected because of: Type of herbicide used • Contact herbicides • Kill only the tissues they come into contact with • Systemic herbicides • Move from the point of application to other parts of the plant

8. Herbicide movement in plant tissues: • Symplast • Comprises the sum total of living protoplasm of a plant (phloem) • Apoplast • Comprises the total nonliving cell wall continuum of the plant (xylem)

9. Plants manufacture or store carbohydrates (sugars) • "Source to Sink" concept • Source - site within the plant where sugars are manufactured • Sink - site within the plant where sugars are being used

10. A herbicide must remain toxic long enough to exert its action • May be subject to deactivation through plant metabolism

11. Factors Influencing the Effectiveness of Soil Applied Herbicides • 1. Decomposition by microorganisms • 2. Chemical decomposition • 3. Adsorption on soil colloids • 4. Leaching • 5. Volatility • 6. Photodecomposition • 7. Removal by plants by harvesting

12. 1. Decomposition by microorganisms • Algae • Fungi • Bacteria • Actinomyces

13. Factors affecting microorganisms include: • organic compounds • organic matter • organic herbicides • may increase decomposition of herbicide • microbial populations may increase and cause rapid herbicide breakdown

14. Other factors affecting microorganisms • Food supply • Temperature • Water • Oxygen • Nutrients • Soil pH

15. Other factors (con't) • Herbicides may remain toxic for extended periods of time if the soil is cold, dry, poorly aerated or other conditions are unfavorable for microorganism growth.

16. 2. Chemical decomposition • May destroy some herbicides and activate others • Hydrolysis (addition of water), oxidation (loss of electrons), isomerization (rearrangement of atoms), ionization (charge), salt formation (Mg, Ca, Na, K) • Hydrolysis and oxidation are the most important

17. 3. Adsorption on soil colloids • Soil colloids - microscopic organic and inorganic particles that have large adsorptive capacities • One cubic inch of colloidal clay has 200 to 500 square feet of adsorptive surface

18. Adsorption of a herbicide on soil colloids reduces the concentration of herbicide freely available in the soil solution. • Organic matter and clay have a larger surface area than sand.

19. 4. Leaching • Is the downward movement of a herbicide in solution through the soil.

20. Herbicide leaching is determined by: • 1. Adsorptive relationships between the herbicide and the soil, • 2. solubility of the herbicide in water, • 3. and amount of water passing downward through the soil.

21. 5. Volatility • Herbicidal loss from evaporation. • Herbicides can evaporate from the soil and be lost to the atmosphere as volatile gases. • Example: Soil fumigants such as methyl bromide or vapam.

22. 6. Photodecomposition • Degradation of herbicides by light. • Herbicide molecules absorb light energy, causing the molecule to breakdown and become deactivated.

23. 7. Removal by plants by harvesting • Plants absorb herbicides from the soil. The herbicide is then removed when the crop is harvested. • Example: Corn may be used to remove unwanted simazine and atrazine in areas where ornamental are to be grown.

24. Herbicide Concentration in the Soil • Affected by two factors: • 1. Weight of the soil. • Average 3.5 million lb/acre foot. • 3.5 lb of herbicide in the top foot of soil = 1 ppm

25. 2. Water holding capacity of the soil. • The concentration of herbicide in the soil solution depends on the amount of water available in the soil.

26. Water holding capacity of the soil (example). • Soil one = 300,000 lb of water/acre foot • Soil two = 1,500,000 lb water/acre foot • A water soluble herbicide would have a concentration (in solution) 5 times as great in soil one than in soil two.