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.

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