To prevent weed resistance, it is always good to start with a pre-emergence program, said Paul O. Johnson, SDSU Extension Weed Science Coordinator.
"Usually the pre-emergence is a different chemistry than what would be used post-emergence - and it will buy time before doing a post treatment if the pre-emergence is activated," Johnson explained.
Due to current wet, cool conditions, spring 2018 many growers may see weeds which have already germinated prior to the application of a pre-emergence product after planting.
"In order to activate most pre-emergent products, they need about one-half to three-fourth of an inch of moisture. So, if weeds germinate before the pre-emergent was activated, there may be some weeds that will continue to grow. These weeds will need a post-emergent treatment for control," Johnson said.
However, there are some pre-emergence products on the market which can kill some, small emerged weeds. Atrazine is a pre-emergence product with the largest window to control emerged weeds.
"To insure the product being used has kick back control, check you label," Johnson advised. "If it does not, consider applying a burndown with the pre-emergent to take out emerged weeds, or consider doing one more tillage pass before planting."
Johnson reminds growers that 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," he said. "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."