Dakota Farmer

Using preemergence herbicides can help prevent the spread of resistant weeds.

January 31, 2020

2 Min Read
A pre-emergent herbicide is applied to a field
PUTTING ON PRE: A preemergent herbicide is applied to a field to be planted to corn. Lon Tonneson

It is always good to start a weed control program with a preemergence herbicide, says Paul Johnson, South Dakota State University Extension weed science coordinator.

Using a pre helps prevent weeds from becoming resistant to widely and frequently used postemergent herbicides.

The chemistry of the pre product should be different than the post product, he says. Also, applying a pre will buy you some time before the post treatment is needed to control weeds.

Johnson lists four other things to know about preemergent herbicides:

1. Rain. Most preemergent herbicides need about one-half to three-quarter inch of moisture to be activated once they are applied. If weeds germinate before the preemergent herbicide is activated, there may be some weeds that will continue to grow. A postemergence herbicide will be needed to control them before they get too large.

2. Kick-back. Some pre products will kill some small emerged weeds. Atrazine is a preemergent herbicide with the largest kick-back window. Check product labels for specific information. If a pre doesn’t have kick back, consider mixing it with a burn-down herbicide it to take out emerged weeds, or consider doing one more tillage pass before planting, Johnson suggests.

3. No loss. Once a pre 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 a rain to activate it. Even if the field has some temporary flooding, the product is usually still there.

4. No more. After the first labeled application of a pre, dom’t add more of the same product to the field unless it is recommended. It may injure the crop. Always read the label for information proper use.

Source: SDSU, which is solely responsible for the information provided and is wholly owned by the source. Informa Business Media and all its subsidiaries are not responsible for any of the content contained in this information asset.

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