Wallaces Farmer

Part two of a series on how to successfully plan your weed management strategy for 2019.

December 21, 2018

5 Min Read
waterhemp infested with giant ragweed and other weeds
WATERHEMP: For most Iowa farmers, the target weed is waterhemp; others may have problems with giant ragweed, horseweed or other weeds.

By Bob Hartzler and Meghan Anderson

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 farmers in Iowa, the target weed is waterhemp. Other farmers may have problems with giant ragweed, horseweed or other weeds. Waterhemp is the target weed in the following example, but you should consider what your most problematic weeds are, so you can run through this exercise for yourself.

Things to consider in determining if a herbicide is effective against your target weed: 

herbicide is labeled to control the weed

target weed is resistant to the herbicide group

In this example, look at herbicides as if waterhemp is the weed that is causing the most issues in your field. The table on herbicide groups used in Iowa crops contains a lot of options. But if your target weed is waterhemp, you only have nine herbicide group (HG) choices for control in corn and soybeans:

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 soybeans but provides unacceptable waterhemp control.

Related:Use multiple sites of action in herbicide program

HG 19 contains one chemical, diflufenzopyr, and is only used in combination with dicamba in herbicide premixes. This chemical does not provide waterhemp control alone.

HG 22 includes paraquat, a nonselective 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.

waterhemp herbicide table

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

Find best herbicide option
Most waterhemp weeds in Iowa are resistant to HG 2, so you know those herbicides won't kill your waterhemp. You know that the presence of other resistances in your fields depends highly on the history of herbicide use.

For example, if you’ve heavily used products from HG 14 postemergence in soybeans in your 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 your field is resistant to these herbicides.

Be careful about continuing to rely on those products. Look for other options — possibly glufosinate (HG 10) — as alternatives 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. Then carefully evaluate if they are still controlling weeds as effectively as in the past. What herbicides are you using that your waterhemp are surviving?

Narrowing down choices
After analyzing all herbicide options, you 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.

Weed resistance is a possibility with any herbicide. As you plan your herbicide program for 2019, 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 you rely on these tools, but you can delay resistance problems by using multiple, effective herbicide groups.

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

The most recent weed control guide from Nebraska is available for purchase online, as well as the most recent weed control guide from Ohio, Indiana and Illinois. These documents contain significantly more detail than Iowa's guide, including numerical efficacy ratings and more detailed information about specific herbicides.

Iowa State University’s Weed Science Program has published their 2017 weed control results (2018 not yet available). You can also search for past years' data in the ISU Extension Store.

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

University of Missouri has a Weed ID Guide. It’s important that you correctly identify the weeds infesting your fields. Are the herbicide groups you are now using effective against your target weeds?

Hartzler and Anderson are weed management specialists with ISU Extension.

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