Herbicide program development: Using effective herbicide groups

February 11, 2016 9:44 AM
Blog Post

This is part two of a four-part series on using multiple, effective herbicide groups at effective rates as part of a long-term weed management system.

Part one: Using Multiple Sites of Action

Part three: Herbicide Rate Calculations

Part four: Long-Term Planning

After you've started working on a program that contains multiple herbicide groups (sites of action), you need to make sure you're using multiple herbicide groups that will be effective against your target weeds. For most people, the target weed will be waterhemp. Others may have problems with giant ragweed, horseweed, or other weeds. Waterhemp is the target weed in my example, but consider what your most problematic weeds are to run through this exercise for yourself.

Things to consider when determining whether a herbicide is effective against your target weed include (1) whether the herbicide is labeled to control the weed and (2) whether your target weed is resistant to the herbicide group.

Let's look at herbicides as if waterhemp is the weed that causes us the most issues. Here's a table of herbicide groups used in Iowa crops.

Herbicide Group Number Site of Action Herbicide Group Number Site of Action
1 ACC-ase 10 Glutamine synthetase
2 ALS 13 DPX synthase
3 Tubulin 14 PPO
4 Auxinic binding sites 15 Unknown (long chain fatty acid synthesis)
5 D1 protein 19 Unknown (Auxin transport)
6 and 7 D1 protein 22 Unknown (Photosystem I inhibition)
9 EPSP synthase 27 HPPD

The table looks to contain a lot of options, but when I consider that my target weed is waterhemp, I can start removing choices and find I only have nine HG choices for waterhemp control in corn and soybean.

  • HG 1 consists of only grass-killing herbicides.
  • HG 6 and 7 are not commonly used in these crops, and they have limited activity on waterhemp.
  • HG 13 (clomazone) is labeled for use in soybean, but provides unacceptable waterhemp control.
  • HG 19 contains one chemical, diflufenzopyr, is only used in combination with dicamba in herbicide premixes. This chemical does not provide waterhemp control alone.
  • HG 22 includes paraquat, a non-selective contact herbicide, which could only be used prior to crop planting or as a crop desiccant. It would control emerged waterhemp, but due to its non-selectivity, it could not be used to kill weeds in an established corn or soybean crop.
Herbicide Group Number Site of Action Herbicide Group Number Site of Action
1 ACC-ase 10 Glutamine synthetase
2 ALS 13 DPX synthase
3 Tubulin 14 PPO
4 Auxinic binding sites 15 Unknown (long chain fatty acid synthesis)
5 D1 protein 19 Unknown (Auxin transport)
6 and 7 D1 protein 22 Unknown (Photosystem I inhibition)
9 EPSP synthase 27 HPPD

After I removed some herbicide choices due to lack of activity or usefulness against waterhemp, I need to consider what my waterhemp population looks like. Do my fields have any herbicide resistances? Weeds that are currently known to be resistant in Iowa can be found at http://weedscience.org/. Iowa has biotypes of waterhemp resistant to HG 2, HG 5, HG 9, HG 14, and HG 27.

Most waterhemp in Iowa are resistant to HG 2, so I know those herbicides won't kill my waterhemp. I know that the presence of other resistances in my fields depends highly on the history of herbicide use. For example, if I've heavily used products from HG 14 postemergence in soybean in my battles with waterhemp, and it seems these products are providing less consistent waterhemp control than in the past, there is a good possibility that the waterhemp in my field is resistant to these herbicides. I should be careful about continuing my reliance on those products and need to look for other options - possibly glufosinate (HG 10) - as options against waterhemp. 

The presence of resistant waterhemp will vary from field to field, often based on individual management tactics used by farmers.  Waterhemp is very efficient at accumulating multiple resistances. In Iowa, at least one population has been identified that is resistant to group 2, 5, 9, 14 and 27 herbicides. It is important to determine what herbicide groups you have relied on for managing waterhemp in the past on your farm(s), and then carefully evaluate if they are still controlling weeds as effectively as in the past. What herbicides are your waterhemp surviving?

After analyzing all herbicide options, I see we only have FOUR herbicide groups that are effective against waterhemp and that waterhemp (in Iowa) is not known to have developed resistant to: Groups 3, 4, 10 and 15.  However, HG 4 resistant populations of waterhemp have been identified in Nebraska and Illinois.  In addition, a close relative of waterhemp, Palmer amaranth, is known to have populations resistant to HG 3. Resistance is a possibility with any herbicide. 
As you plan for your herbicide program next year, make sure you're including multiple sites of action that are effective against your target weeds. Evolution of herbicide resistance is inevitable in plant populations when we rely on these tools, but we can delay resistance problems by using multiple, effective herbicide groups. 

Pages 8 and 9 of the most recent Herbicide Guide for Iowa Corn and Soybean Production has herbicide efficacy ratings.

The most recent version of the Weed Control Guide for Ohio, Indiana, and Illinois may be of use to you as well and can be purchased online. This document has lots of detail, including efficacy ratings and specific herbicide information.

The Iowa State University Weed Science Program has also published their 2017 weed control results. You can also search for past years' data in the ISU Extension Store.

In order to determine effective herbicide groups, you have to properly identify your enemy. The Weed Science Society of America keeps a page of weed identification resources from many universities and other good resources.

Are your herbicide groups effective against your target weeds?


Meaghan Anderson Field Agronomist in Central Iowa

Meaghan Anderson is a field agronomist in central Iowa and an extension field specialist at Iowa State University Extension and Outreach. Educational programming is available for farmers, agribusinesses, pesticide applicators, certified crop advisors, and other individuals interested in...

Bob Hartzler Professor of Agronomy

Dr. Bob Hartzler is a professor of agronomy and an extension weed specialist. He conducts research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also teaches undergraduate classes in weed science and weed identificatio...