By Paul O. Johnson
It is always good to start with a pre-emergence program to help prevent weeds from becoming resistant. Usually this is a different chemistry than what would be used post-emergence. It also will buy time before doing a post-treatment if the pre-emergence is activated. With the spring starting out below normal in subsoil, there are questions if a preemergent herbicide will work.
Most preemergent products need about ½ to ¾ inch of moisture to be activated once they are applied, and when this happens, the product is now ready to kill the weeds. This requirement is there every year, so subsoil moisture that’s good for growing crops has little effect on a pre-emergence product’s ability to perform. So, if weeds germinated before the pre-emergent were activated, there may be some weeds that will continue to grow and will need a post-emergent treatment to control before the weeds get too 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 ensure the product being used does have kick back control, check the label. If not, consider putting a burn-down 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 then on.
In most cases, no chemical is lost waiting for activation. In all cases, read the label for more information on how your 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 : sdstate.edu