Judging by rating corn trials this week the answer depends upon the herbicide. Atrazine or dicamba used pre plant started breaking last week about 14 days after application. Many of the other common pre applied herbicides like Lexar, Corvus or Bicep were still holding.
The treatments in our studies have seen 12” of rain since they were applied. Some of the pre applied herbicides in corn across the state have experienced even more than that. With that kind of rainfall, do not expect any pre applied herbicide to last like it did last year. As we saw last spring, under dry conditions a pre applied herbicide can provide weed control for 5 weeks or more. This year with the amount of rainfall Tennessee has experienced expect pre applied tankmixtures to start to play out about 3 weeks after application.
With the warm and wet environment in the forecast for this weekend, I would expect to see weeds start to break through. If they do, I would manage these corn fields as if no herbicide has been applied. What I mean by that is plan to re-apply a herbicide that can provide some residual control. There are a number of good herbicide options that can be applied post in corn. These include atrazine, Status, and the popular premixes Halex GT, Capreno and Realm Q, just to name a few. A more thorough discussion of these corn post options was posted on an April 11 blog and can be found here: (Corn Pre Applied Herbicide Considerations).
Where this gets complex is the season use limits on these products. An example would be that, by the label, a field can have 2.5 qts/A of atrazine applied if split between two applications. So a field will in reality have 1.5 qts/A of atrazine on it when 2 qts/A of Bicep was applied pre emerge. So another 1 qt of atrazine can be applied post in the next week to 10 days if a new flush of weeds appears. Please be mindful that atrazine can be applied post only up to 12” tall corn. All the other herbicides mentioned above have season use limits as well.