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Fate of Soil-Applied Herbicides (05/26/16)

Fate of Soil-Applied Herbicides

Question: What happens to my soil-applied herbicides if there is no rain to activate them for several days after application? Are they gone?

Answer: Most soil-applied herbicides are stable under prolonged periods without moisture to activate them. Most soil-applied herbicides are not affected by breakdown from the UV spectrum of sunlight and do not volatilize from the soil surface. There are exceptions. All Group 8 herbicides (Ro-Neet, Eptam, and Far-Go) are volatile and require immediate incorporation after application through tillage. All Group 3 herbicides (Treflan, Sonalan, and Prowl) are volatile and susceptible to degradation from UV light. Prowl is much less volatile and affected by UV light which allows PRE labeling in sunflower. However activity of Prowl is greater when incorporated. The requirement to incorporate Treflan and Sonalan soon after application reduces risk of volatility and deactivation from sunlight. Recently, s-metolachlor (Dual Magnum, Dual II Magnum) has been found to be susceptible to breakdown from sunlight. However, other similar herbicides like Outlook, Surpass, Harness, and Warrant are not broken down by sunlight.

The good news: With the exception of those herbicides listed above, soil-applied herbicides can remain on the soil for an extended period of time. The ‘activity ‘clock starts when sufficient rain (or irrigation) moves the herbicide into water phase in the soil profile and it is moved into the germination zone of weed seed, providing PRE weed control on germinating weeds at that time. Generally soil-applied herbicides require 0.5 to 1 inch of water for activation depending on the herbicide. For example, a rain event of 0.5 inch may activate Outlook but Dual and Zidua products may require 0.75 to 1 inch of rain. Researchers have seen enhanced activity from Zidua after more than one rain event.

The bad news: The problem with lack of activating moisture soon after application is that weeds may emerge and generally soil-applied herbicides will not kill weeds once they emerge. A few soil-applied herbicides including Zidua (pyroxasulfone), Spartan/Authority (sulfentrazone), Valor (flumioxazin), and Sharpen (saflufenacil) have shown a phenomenon some call ‘reach-back’ where they may kill small emerged weeds (0.5 inch tall) after an activating rain.

More good news: If weeds that emerge through the PRE herbicides are killed following an activating rain, then the residue of the PRE herbicide will prevent labeled weeds from emerging until the herbicides are degraded through microbial activity and chemical reactions in the soil.

Rich Zollinger

Extension Weed Specialist

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