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Identify and Manage Waterhemp

February 12, 2019

Waterhemp growing in Iowa corn field

Simply put, waterhemp is one of the most common yield-robbing weeds found in corn and soybean fields today. And now that resistant waterhemp is becoming more widespread, growers are having a more difficult time controlling it. Moreover, many growers may think they’re dealing with waterhemp, when in fact they’re dealing with a completely different weed.

Waterhemp is often confused with Palmer amaranth. The Iowa State University Extension and Outreach says waterhemp is usually shorter and thinner than Palmer amaranth, but that there is a lot of overlap in their general appearance. Due to control failures because of herbicide resistance, waterhemp often reaches a flowering stage that makes it look even more like Palmer amaranth. The best way to tell them apart is to look at the modified leaves at the base of the flowers. In Palmer amaranth, they’re narrow, ending in a sharp point, and usually 3 to 7 millimeters long. In waterhemp they’re almost always less than 3 millimeters long.

Other ways to identify waterhemp include:

This agronomic image shows identifying characteristics of waterhemp.

This agronomic image shows identifying characteristics of waterhemp.

This agronomic image shows identifying characteristics of waterhemp.

The good news is that many pre- and post-emergence herbicides control both waterhemp and Palmer amaranth. The best approach is to use a herbicide program with multiple effective sites of action (SOAs). This will help manage the current weed population and prevent further resistance from developing.

In corn, we recommend a 2-pass application of Acuron® or Acuron Flexi corn herbicides or an application of Acuron or Acuron Flexi as a pre-emergence herbicide, followed by Halex® GT herbicide plus atrazine or dicamba as your post-emergence herbicide.

  • Acuron has 3 effective SOAs (Groups 5, 15 and 27) and Acuron Flexi has 2 (Groups 15 and 27).
  • Both Acuron and Acuron Flexi contain bicyclopyrone, which complements the other active ingredients in the premix to deliver broader, more consistent management of tough weeds that other products are missing.
  • Applying Halex GT post-emergence following a foundation rate of Acuron or Acuron Flexi adds another SOA (Group 9).

In soybeans, we recommend a pre-emergence application of Boundary® 6.5 EC or BroadAxe® XC soybean herbicides, followed by a post-emergence application of Flexstar® GT 3.5 herbicide.

  • Boundary 6.5 EC contains 2 effective SOAs (Groups 15 and 5).
  • BroadAxe XC contains 2 effective SOAs (Groups 15 and 14).
  • Applying Flexstar GT 3.5 post-emergence following a foundation rate of Boundary 6.5 EC adds another 2 SOAs (Groups 14 and 9).
  • Applying Flexstar GT 3.5 post-emergence following a foundation rate of BroadAxe XC adds an additional SOA (Group 15).

These programs overlap multiple effective SOAs to offer residual control, meaning there is less of a chance of building up the weed seed bank.

For more information about how to identify and manage waterhemp, check out this video from our “How It’s Done” series, and learn more about how to fight weed resistance by visiting Resistance Fighter®.

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