Controlling waterhemp is a challenge for growers across Illinois and the Corn Belt and the No. 1 subject of questions I received during the last week of June. So I decided to do a quick review of the 10 sites of action that make up 99.9% of what we have available for controlling weeds in corn and soybeans.

  1. Group 1 – ACCase inhibitors – These are products like Select Maxx®, Assure® II and Fulisade®. They are selective herbicides that only control grass weed species postemergence. Our main usage is controlling volunteer corn in soybeans. Because these do not control broadleaf weeds, they will NOT help us with waterhemp in soybeans.
  2. Group 2 – ALS inhibitors – This is wide range of products for pre and post for corn and soybeans that includes products like Classic®, Synchrony®, Accent® and Raptor®. These products are used in combination with other modes of action for controlling weeds in both corn and soybeans, however, waterhemp has now developed resistance to this herbicide site of action, so it no longer helps us with waterhemp in either corn or soybeans.
  3. Group 3 – DNAs – These are products like Prowl®, Treflan® and Sonalan®. While these products have good activity on waterhemp pre-emergence, they cannot be used postemergence, so they do not help us with waterhemp at this time of the growing season.
  4. Group 4 – Synthetic auxins – Until Roundup Xtend™ soybeans get approved, this site of action that includes dicamba products can only be used on corn, so it doesn’t help us with a waterhemp problem in soybeans in 2016.
  5. Group 5,6,7 – Photosynthetic inhibitors – These three sites of action are so similar that I lump them together. For soybeans this group would contain Sencor® and Basagran®. Sencor is becoming more popular in pre-emergence mixes (Boundary® and Matador®, for example) but doesn’t help us with waterhemp postemergence. Basagran is a postemergence applied product, but only controls weeds when they are very small and no pigweed species are labelled as controlled by this product.
  6. Group 9 – Glyphosate – This is our major site of action, but waterhemp has figured out four different ways to be resistant to glyphosate. Resistance to this site of action has caused this active ingredient to no longer be a tool that can totally be relied upon for waterhemp management in RR2 Yield soybeans.
  7. Group 10 – Liberty – This is the site of action that is being utilized in 2016 on LibertyLink™ soybeans for controlling glyphosate-resistant waterhemp, but because it is a non-selective herbicide. Liberty cannot be used on RR2 soybean varieties.
  8. Group 14 – PPO inhibitors – I separate this site of action into soil applied PPOs like Valor® and Authority® (which cannot be applied now) and then postapplied PPOs like Flexstar®, Cobra®, Ultra Blazer® and Phoenix®. This is the main site of action group of products that can be added to glyphosate to enhance control of glyphosate-resistant waterhemp. This is why products like Flexstar GT are popular, as it is a premix of glyphosate and Flexstar.
  9. Group 15 – Mitosis Inhibitors – This site of action includes products like Harness®, Dual®, Degree Xtra®, Outlook®, Warrant and Zidua®. Warrant and Zidua could be added to glyphosate and/or Flexstar, but they only provide residual control. They will not help control a waterhemp that has emerged.
  10. Group 27 – HPPD®s – This group contains Corvus®, Balance® Flexx, Laudis®, Callisto®, Impact® and Armezon®. Until Balance GT soybeans are released, these products are corn herbicides only, so they do not help us with waterhemp in RR2 soybeans.

So, as you can see from the above list, group 14 PPO inhibitors are the only herbicide site of action that has postemergence activity on waterhemp and can be tank mixed with glyphosate on RR2 soybeans. Be aware that resistance to the PPO site of action is becoming more and more prevalent in waterhemp. If you have sprayed glyphosate and a PPO herbicide like Flexstar, Cobra, Ultra Blazer or Phoenix and not even fazed your waterhemp, you most likely have glyphosate AND PPO resistant waterhemp.

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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.