There have been a lot of people questioning “how much of my residual herbicide is left after all this rain?” The answer to that is anyone’s guess but a safe assumption would be that if you received some of the 8 to 13” of rain that some areas got then the residual herbicide you applied is likely gone. How much residual value is left may depend on what kind of herbicide was used. Some residual herbicides like Treflan, Prowl and Valor are very insoluble in water meaning they would rather adhere to soil particles than be dissolved in water. These insoluble herbicides may still have some residual activity on pigweed and other weeds but the length of residual will likely be shortened after all the rain. On the other hand products like Dual, Warrant and Sencor are very water soluble and are more than likely floating down the river by now. Similarly, the solubility of some of the ALS-inhibiting herbicides like Classic, Scepter, FirstRate and others is much greater in higher pH soils. If the soil pH is 6.5 or higher then it is likely these herbicides are gone as well.
So what are our options now? If the soybeans are emerged we must scout intensively for escapes. The only way to know if your residual is working is to get out there and look. We must be scouting these fields on a 2 to 3 day interval looking for signs of the residual herbicides breaking down. If you see pigweed or other weeds emerging make sure that you are timely in making postemergence applications (2-3” in height). Remember, there is nothing in our herbicide arsenal that will take down a glyphosate-resistant pigweed >6” tall. Consider trying to overlay an additional residual herbicide with these postemergence applications. This will help take up some of the slack of deteriorating or lost preplant residuals.
If the soybeans have not been planted or need replanting I would highly encourage the use of another residual herbicide. Be mindful of season restrictions on herbicide applications. We may not be able to use the same herbicides that we used earlier on. I would also encourage using a different herbicide mode-of-action (MOA) than what was applied previously. For instance, if you used Valor or Authority (Class 14 herbicides) earlier I would recommend going with something like Sencor (Class 5 herbicide), Dual/Warrant (Class 15 herbicides) or a combination of these herbicides.