Is it time to apply my pre-emergence herbicide?

The short answer is, “Yes.” It’s after April 16, so as the weather straightens out, we are at calendar time to begin applying pre-emergence herbicides. Your crop protection retailer is the best source of information about your pre-emergence chemistry, but here are a few things you may want to consider.

What rate should I run? 
Most pre-emergence herbicides have a rate range on the label, so how do you choose what rate you run? Remember, the reason we run a pre-emergence herbicide pass is to keep your field clean of weeds until the post herbicide pass can be applied. My personal goal is to never see a weed in the field. This means I don’t want any actively growing weeds in the field even when the post application is being made.

So far, the weather pattern for 2018 seems to be fickle. This means we apply the pre-emergence herbicide pass and because of weather delays we may not get to plant for several weeks. If you think this may be the case in 2018, I would consider raising the rate of your pre-emergence herbicide pass to a rate level that can provide weed control until the crop is planted and emerged (even though we don’t know how long that may be). For Bayer’s pre-emergence products, Balance® Flexx and Corvus®, I like to start at a minimum of 4 oz. per acre. Balance Flexx can be applied up to 6 oz. per acre, and Corvus can be applied up to 5.6 oz. per acre.

How water-soluble is my pre-emergence herbicide? 
If you want your pre-emergence herbicide to provide weed control for the length of time it takes to plant and have the crop emerge, (not knowing how long that will be), then knowing the water solubility is important. We all know that we have a larger chance of heavy rain events in April and early May than later in the growing season. In a dry spring, a higher water solubility would be preferable since it takes less water to activate.

If you are using Balance Flexx or Corvus, you should rest assured that that water solubility is not a concern. When these products are applied they are in the form of isoxaflutole, which is fairly water insoluble. This means you can get a heavy rain and they will still be there. However, isoxaflutole will break down into herbicidal metabolite called dikenonitrile (DKN), which is water-soluble. Because you get a water insoluble and a water-soluble herbicide in one product, you do not have to worry as much about the weather with Balance Flexx or Corvus herbicides.

Where should I place the pre-emergence herbicide in the soil profile?
This depends on your driver weed. For most of us our driver weed is waterhemp. As we know, waterhemp germinates in the top 1/8 inch of soil, so the best place to place chemistry to control this weed is on the soil surface. However, if your driver weed is giant ragweed, which can germinate from several inches deep, then you would want to incorporate that chemistry to get it closer to where this weed is germinating.

Having a combination of water-soluble and insoluble active ingredients means that some will remain near the surface and some will move down a couple inches.

Will pre-emergence herbicide provide control of emerged weeds? 
If you are not clean tilling before planting, you may be expecting your pre-emergence chemistry to “burn down” any weeds that have emerged. Be sure to know exactly what adjuvants or tank mix partner herbicides your pre-emergence chemistry will require to provide burndown weed control. For example, if you are using Balance Flexx or Corvus, these products require crop oil concentrate to provide burndown of weeds up to 2” in height.

If weeds are larger than 2” in height, then these products would require a tank mix partner such as 6 – 10 oz. of DiFlexx for broadleaf weeds, or glyphosate if you have both broadleaves and grass weed species emerged. However, crop oil concentrate, DiFlexx® or glyphosate may only be tank mixed with Balance Flexx or Corvus when applying before the corn emerges. Once your corn is emerged, Balance Flexx or Corvus can be applied (up to V2), but atrazine is the only tank mix partner labelled.

Weed control is achievable if you know your driver weeds, have a thorough application plan to control them from emergence on and be ready to improvise if weather throws a curve

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About the Author: Eric Ifft

Eric Ifft has been a technical consultant with Bayer CropScience since 2008. Contact him at or 309-825-3730.