Chapter 5 Herbicide Fate in the Environment

1. Chapter 5Herbicide Fate in the Environment

2. A. What are the fates of a herbicide after leaving the nozzle Adsorbed to soil particles Photo-degradation = photodecomposition Chemical degradation � hydrolysis Lost through Wind erosion Runoff water Volatilization Vapor drift Leaching Actually hit the target (weed or soil) Taken up by plants (weeds or crops) Deposited on the crop

3. Fate of Herbicides in Soil The vast majority of herbicides applied eventually end up in the soil and their ultimate fate is determined by soil properties or characteristics. This is true for even those herbicides which are primarily used through postemergence applications

4. What are the fates of a herbicide after leaving the nozzle

5. What factors (environmental or other) affects a herbicide�s fate (doing it�s job)

6. What factors (environmental or other) affects a herbicide�s fate (doing it�s job) Environmental Wind Rainfall � activation, runoff Microbial population Humidity Soil - pH, moisture, compaction, OM, texture, fertility, slope of soil Vegetation Affects deposition to target

7. What factors (environmental or other) affects a herbicide�s fate (doing it�s job) Components of the spray Spray volume High (chemigation) - runoff Low (drift) Application pressure Higher � smaller droplet, more drift Low pressure � larger droplet, less drift Carrier Water vs. fertilizer vs. oil Herbicide formulation � liquid vs. dry Ester vs. amine formulation of 2,4-D Microencapsulated vs. non- Volatility, plant uptake, leaching, half life Incorporation � time after application- volatility, depth, runoff

8. Processes that Affect Herbicide Activity Degradation -- the destruction of the herbicide molecule into non-phytotoxic components Biological, Chemical, Photodecomposition Transfer -- the inactivation of a herbicide, but not the destruction of the herbicide Adsorption, (leaching), Volatility, & Runoff Both Degradation and Transfer processes will result in the loss of herbicide activity--that�s important!

9. Degradation vs Transfer Degradation - herbicide is gone Transfer - may still be around but may not be available for weed control

10. Degradation Process Biological decomposition or degradation soil microorganisms plants Chemical decomposition hydrolysis in the soil or even spray tank oxidation, etc. Photodecomposition breakdown by sunlight

11. Biological Degradation Algae, fungi, actinomycetes, and bacteria May use herbicide for N, C, S source Fungi--smaller number than bacteria, but larger in size Bacteria--large number, but small size Herbicides have generally not caused damage to fungi or bacteria--however, these organisms have caused herbicides to degrade

12. Factors Affecting Biodegradation Just about anything that would affect the activity of soil microorganisms Temperature (80 to 90 �F) Water (50 to 100% field capacity) Oxygen Mineral nutrient supply Soil pH (6.5 to 8) Cold and/or dry conditions are not favorable Why is this important?

13. Chemical Decomposition The breakdown of a herbicide by a chemical process or reaction in the absence of a living organism.

14. Examples of Chemical Decomposition Oxidation, reduction and hydrolysis These are heavily influenced by soil pH changes Sometimes, hydrolysis can occur while the herbicide is in the spray tank mixed with water

15. Photodecomposition Breakdown of herbicides by light Breakage of chemical bonds Incorporation of herbicides helps reduce this with soil applied herbicides Can also occur with some POST applied herbicides. Can also occur in the spray tank.

16. Transfer Processes Does not mean that the herbicide is broken down, but rather it is removed or unavailable for plant use. Text book has some of these processes under �chemical processes�, but do not confuse this with chemical �degradation�

17. Transfer Processes Adsorption-desorption Leaching--tied directly to Adsorption Strong adsorption--no leaching Weak adsorption � �may be� leached Volatility Vapor drift

18. Adsorption Soil has a net �negative charge� Positively charge ions � �cations� are adsorbed to the soil Negatively charged ions � �anions� are repelled by the soil and will sometimes (depending on solubility) leach readily. Soil texture, pH, and OM content important

19. Adsorption / Equilibrium The adsorbed herbicide (bound herbicide) is usually in an equilibrium with desorbed (unbound herbicide); however, in some cases the adsorption is irreversible and complete. Triazines--in equilibrium paraquat--complete and irreversible

20. Herbicides in the EnvironmentE. Residual vs. Persistent Residual herbicide herbicide that remains active in the soil, and continues to control weeds for a period of time after the actual application A certain amount of residual activity is usually desirable--PPI & PRE herbicides Persistent herbicide herbicide that remains active in the soil long enough to injure rotational crops the following growing season

21. Persistent herbicice Phytotoxic after effects (carryover) may cause injury to rotational crops thus restricting the planting of some potential rotational crops Herbicides which persist for 3 to 12 months provide weed control for the entire growing season and have uses in perennial crops. Longer persistence--total vegetation control. Herbicides in the EnvironmentE. Residual vs. Persistent

22. Soil Colloids that Adsorb Herbicides Organic matter--the most adsorptive 4 X more adsorptive than montmorillonite 20 X more adsorptive than kaolinite Clay--the second most adsorptive soil component. Clays do differ Montmorillonite--3 to 7 X of kaolinite Silt has low adsorptive capacity Sand has the lowest adsorptive capacity CEC--cation exchange capacity

23. Ionic Herbicides Some herbicides behave as anions, others as cations, some are acids, some are bases, and yet others are not ionic at all. Adsorption of nonionic herbicides generally increases as their water solubility decreases.

24. Leaching The downward movement of a substance with water through the soil. Capillary flow can cause upward movement Some leaching is desirable--herbicides are moved into the rooting zone Too much leaching is undesirable Herbicide dilution and poor weed control Herbicide injury to crops Ground water contamination

25. Factors and Leaching Adsorption (ionic bonding) Soil type (affects adsorption) Affects CEC of soil Impacts permeability of soil Soil organic matter (affects adsorption) Affects CEC of soil Impacts permeability of soil Herbicide solubility Atrazine = 33 mg/L Pendimethalin = 0.275 mg/L Rainfall amount Rainfall intensity Herbicide chemical properties

26. Leaching Adsorption is the most important factor influencing the leachability of herbicides in soil. �Adsorption bonds� are considered more important than water solubility in determining the leaching of herbicides. Herbicide solubility is important, but �adsorption bonds� are the most important. MSMA, paraquat Do all herbicides have the same potential to leach?

27. Herbicide Volatilization All chemicals and compounds have a vapor pressure. Water can vaporize slowly. Gasoline can easily vaporize. Herbicides can also vaporize. When a herbicide vaporizes and moves off-target, it is lost as a herbicide and is now a pollutant. It can damage non-target plants--this is not desirable.

28. Herbicides that volatilize still possess herbicide activity because the herbicide molecule is still intact--it is just in a different physical form (it is now a gas and not a liquid or solid) Sunlight will attack most herbicides which are volatile, but this does NOT result in immediate decomposition of the herbicide. Herbicide Volatilization

29. Volatility increases as temperature increases. Incorporation (mechanical or water) can reduce volatility losses. Command - clomazone H. Herbicide Volatilization

30. Herbicide Drift (Particle Drift) Drift � movement of spray droplet Affected by Droplet size (pressure, tip size, etc.) Height of release Wind velocity Humidity?

31. Do all herbicides have the same� potential to volatilize? potential to drift?

32. Drift vs. Volatilization Drift � movement of spray droplet Particle drift--the movement of small spray droplets. This type of drift can occur with any sprayed herbicide Small spray droplets can move off target by 100�s or 1000�s of feet. Volatilization � change from liquid to vapor Vapor drift--the movement of intact herbicide molecules that have changed to a gaseous state & are now herbicide vapors Herbicide vapors can move off target for miles.

33. Surface Runoff Herbicides can leave the site of application as Dissolved herbicides from soil or foliar applications Herbicides attached to soil can erode away Sedimentation Both the dissolved herbicides and those attached to soil sediments can pollute our creeks, rivers, and lakes.