Degradation of Herbicides Under Dry Conditions
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In years following extended drought, several agronomic issues often arise, including that of potential herbicide carryover. This article discusses herbicide degradation in soils, how it is affected by droughty conditions, and the primary pathways by which some specific herbicide chemistries degrade. This information is helpful to growers who need to diagnose herbicide carryover issues.
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Degradation of Herbicides Under Dry Conditions
- 1. Degradation of Herbicides under Dry Conditions Stephen Strachan and Kevin Hahn Herbicide Product Support and Renewal and Field Development
- 2. Introduction • Much of the Midwest encountered severe drought throughout the summer of 2012. • In years following extended drought, several agronomic issues often arise, including that of potential herbicide carryover. • Herbicides break down through microbial and/or chemical degradation in the Corn grown in 2012 exhibiting presence of soil moisture. severe drought symptoms. • When soils are very dry, herbicide breakdown via microbiological activity is diminished.
- 3. Introduction • This presentation will help growers who suspect and need to diagnose herbicide carryover issues arising from the severe drought conditions of 2012 by addressing: – herbicide degradation in soils, – how it is affected by droughty conditions, and the – primary pathways by which some specific herbicide chemistries degrade. • To understand herbicide degradation in dry soils, it is important to understand – how drought affects soil water and microbiological activity, – herbicide degradation pathways, and – the interaction between microorganisms and herbicides.
- 4. Characteristics of Water in Soil Macropore Micropores Water of Adhesion (gray shaded area around each soil Saturated Soil colloid) Macropores filled with air Soil Under Drought Conditions Micropores filled with water Water still present only in the smallest micropores and as a film Thin film of water (water of adhesion) around the soil colloids surrounds each soil colloid
- 5. Characteristics of Microorganisms in Soil Microorganisms (red dots) •Require water to survive •Reduce populations as soil water content decreases •Are relatively large (a few microns in length) and require Saturated Soil small pools of water Soil Under Drought for maximum activity Conditions
- 6. Characteristics of Herbicides in Soil Herbicides (green dots) • Exist as single molecules (a few angstroms in length) • Tend to accumulate in the film of soil water next to the soil colloids • Some percent of molecules remain in the water held Saturated Soil in micropores Soil Under Drought • Herbicide molecules Conditions move between the two water phases
- 7. Microbial Degradation of Herbicides in Moist Soil • Microbes must either ingest or be closely associated with herbicide molecules for herbicide degradation to occur • Microbes are relatively large and require ample water and space to live • Micropores filled with water best support microbial growth • Herbicide molecules move from the water film on the soil colloid into the micropores as microbes degrade molecules in the micropores Saturated Soil
- 8. Rate of Microbial Degradation of Herbicides Decreases in Drier Soils •Less available water to support microbial populations (fewer micro- organisms present to degrade herbicide molecules) •Microorganisms are about 10,000 times larger than herbicide molecules and cannot enter all locations in the film of water where the herbicide molecules are located (limited access to the herbicide molecules) Soil Under Drought Conditions
- 9. Chemical Degradation of Herbicides in Moist Soil • Chemical degradation can occur wherever water is present in the soil • Herbicide molecules associated with the thin film of water near the soil colloids and in water contained in soil micropores are susceptible to chemical degradation Saturated Soil
- 10. Chemical Degradation of Herbicides Continues in Drier Soils • Even very dry soils contain a thin film of water surrounding the soil colloids • Herbicide molecules are either attached to soil colloids or located in the thin film of water surrounding the soil colloids and are susceptible to chemical attack • As long as water is present, chemical degradation continues to occur •Rate of chemical degradation may increase because drier soils tend to have higher temperatures (rates of chemical reactions increase as the temperature increases) Soil Under Drought Conditions
- 11. Degradation Pathways of Herbicides* Typically based on chemical class (not Mode of Action) Combination of Chemical Primarily Microbial Activity and Microbial Activity References: Atrazine 1 Isoxaflutole Herbicide Handbook, Weed Flumetsulam Pyroxasulfone Science Society of Flumioxazin America, 9th Saflufenacil - Not persistent edition, 2007, and Fomesafen Simazine 2 EPA-published documents. Imidazolinones Sulfonylureas 1 Greater concern Mesotrione Chlorimuron ethyl if followed by Metolachlor metribuzin ahead Rimsulfuron of soybeans. - Safe to most crops 2 HighpH: Metribuzin Tribenuron microbial only. Sulfentrazone Thiencarbazone Low pH: chemical Flumetsulam and microbial. Flumioxazin
Notas del editor
- Much of the Midwest encountered severe drought throughout the summer of 2012. In years following extended drought, several agronomic issues often arise, including that of potential herbicide carryover. Herbicides break down through microbial and/or chemical degradation in the presence of soil moisture. When soils are very dry, herbicide breakdown via microbiological activity is diminished. This article discusses herbicide degradation in soils, how it is affected by droughty conditions, and the primary pathways by which some specific herbicide chemistries degrade. This information should be helpful to growers who suspect and need to diagnose herbicide carryover issues arising from the severe drought conditions of 2012.
- Much of the Midwest encountered severe drought throughout the summer of 2012. In years following extended drought, several agronomic issues often arise, including that of potential herbicide carryover. Herbicides break down through microbial and/or chemical degradation in the presence of soil moisture. When soils are very dry, herbicide breakdown via microbiological activity is diminished. This article discusses herbicide degradation in soils, how it is affected by droughty conditions, and the primary pathways by which some specific herbicide chemistries degrade. This information should be helpful to growers who suspect and need to diagnose herbicide carryover issues arising from the severe drought conditions of 2012.
- This table contains a few examples of how some of the more common herbicides degrade in soil. Note that the chemical class is more important than the mode of action in determining primary pathways for herbicide degradation. As an example, imidazolinone and sulfonylurea herbicides both affect the ALS binding site. However, imidazolinone herbicides degrade primarily via microbiological degradation, whereas sulfonylurea herbicides (e.g., chlorimuron ethyl, rimsulfuron, and tribenuron) degrade via both microbiological and chemical pathways.Many of the active ingredients listed in this table have been used for many years. These herbicides have been applied during drought years (e.g., 1988) and in very wet years (e.g., 1993). Product labels commonly have a “safety buffer” built into the label guidelines. If there is a concern about planting a sensitive crop into soil that was treated with a herbicide that degrades via only microbial activity, carefully check the “following crop” or “rotational crop” portion of the label guidelines, and plant the crop according to these guidelines.