Pre-emergent Herbicides: Will they still work as normal?

By Paul O. Johnson, SDSU Extension Weed Science Coordinator, SDSU Plant Science Department.

With the nice spring, some farmers worked the ground ahead of time but were waiting to plant corn. Then considerable portions of South Dakota received some much needed moisture this past weekend. In many cases some weeds germinated before the pre-emergent product was 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 able to kill the weeds.

Effectiveness on early emerging weeds

If weeds germinated before the pre-emergent was activated there may be some weeds that will continue to grow and will need a post-emergent treatment to control them before they get too large. Some pre-emergent products do have the ability to kill some small emerged weeds (kick back). Atrazine is the active ingredient with the largest window to control emerged weeds.

To see if your product does have kick back control check the product label. If not, consider applying a burndown with the pre-emergent to take out emerged weeds, or consider doing one more tillage pass before planting.

Post-activation considerations

Remember 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, and even if the field had temporary flooding the product is usually still present. 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 crop injury.

Pest Management Guides

For more information on herbicides labeled for use in South Dakota, consult the following resources:

2016 Soybean Pest Management Guide
2016 Corn Pest Management Guide
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