What Happens to my Soil-Applied Residual Herbicides if it Stays Dry?

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• Precipitation is an important piece of the equation for successful residual weed control.
• Herbicides retained on the soil surface can be exposed to volatilization and photodegradation limiting the amount available when an activating rainfall occurs.
• Weed control will be compromised without activation and may require an earlier postemergence application.

Soil-applied residual herbicides need rainfall to be effective in controlling weeds. The dry weather in areas may prompt questions about the value that these applications may have in dry soils. Two things must happen for residual herbicides to be effective: 1) sufficient rainfall within 5-7 days of application to activate the residual activity and, 2) sufficient soil moisture to keep the herbicide dissolved and in solution to be taken up by the emerging weeds. Without rainfall, both herbicide activation and dissolution are severely impacted.

Rainfall plays a key role in not only activation, but also incorporation of the herbicide into the soil. Without incorporation some herbicides are prone to surface loss via volatilization and photodegradation. Volatilization is the process of the herbicide turning into a gaseous form and therefore less is available for weed control. Photodegradation is a process where an herbicide is broken down from UV rays. Precipitation helps to incorporate the herbicide for weed control and mitigate the risk of loss by either volatilization or photodegradation.

What happens if a rainfall activates the herbicide, but then conditions are persistently dry afterwards? What about fields that are planted and have not received any precipitation since the soil applied residual was sprayed? The herbicide active ingredient(s) must be in soil solution for weed control. If it becomes dry after the activating rainfall, residual weed control will be reduced due to a lack of active ingredient(s) in solution. Conversely, negligible control would be expected in cases where no rain occurred at all. One thing to note is-in dry soils weed seeds need moisture to germinate and therefore weed germination rates will be slow or non-existent.

When soil residuals do not receive the rainfall needed for activation, scouting is advised. Scouting is necessary to determine if an early and timely post-emergence herbicide application is needed. When scouting, it is imperative to scout the whole field to make sure timely applications can be made and weeds do not get to uncontrollable sizes. Both scenarios have been observed in the past and often require an earlier postemergence application. One note would be to consider adding a postemergence residual herbicide to the tank mix like a group 15 herbicide to help with weed control through to crop canopy.

Residual herbicides provide a great deal of value in controlling our tough-to-control weeds like waterhemp. In areas that are experiencing dry weather, scout fields to ensure adequate weed control and realize that an early postemergence application may be necessary. For questions or concerns about weed control contact your local FS Crop Specialist.
 

https://crops.extension.iastate.edu/cropnews/2021/04/preemergence-herbicides-dry-soils-and-rain
https://crops.extension.iastate.edu/blog/bob-hartzler/what%E2%80%99s-happening-my-pre-herbicide-soil-surface