Why use pre-emergent herbicide?
It is always good to start with a pre-emergence chemical to help prevent weeds from becoming resistant. Usually this is a different chemistry than what you are using post-emergence. It also will buy you time on doing a post if the pre-emergence is activated. With the wet cool spring, some weeds may now have germinated before the pre-emergence product is applied after planting.
Most pre-emergent products need about ½ to ¾ inch of moisture to be activated once they are applied. Once this happens the product is now ready to kill the weeds. If weeds germinated before the pre-emergent was activated there may be some that will continue to grow and will need a post-emergent treatment to control before they get to large.
Some pre-emergence products do have the ability to kill some small emerged weeds. Atrazine is the one with the largest window to control emerged weeds. To insure the product being used does have kick back control check you label. If not consider putting a burndown with the pre-emergent to take out emerged weeds, or consider doing one more tillage pass before planting. But remember once the product has been activated it will start to control germinating weeds and should work as normal from this time forward. In most cases no chemical is lost waiting for activation.
In all cases read the label for more information on how you product works. Do not add more of the same product to the field unless it is recommended as this may cause injury to the crop. Even if the field had some temporary flooding the product is usually still there.
Source: Paul O. Johnson, South Dakota State University, iGrow