Herbicide Resistance ScreeningMichigan's Past, Present and Future

Herbicide resistance in weeds in not a new issue for Michigan, but these enemies are ever changing. Screening of weeds for resistance at Michigan State University (MSU) was initiated in 1980 with the confirmation of triazine resistance in common lambsquarters. For the past decade plus, a concerted and consistent effort to screen questionable populations has been ongoing thanks to collaborations between the Michigan Soybean Committee, Michigan Vegetable Council, MSU Plant & Pest Diagnostics and MSU weed scientists. Below are details on the latest trends in resistance in Michigan; all confirmed cases of resistance down to the county level can be found on an interactive map at www.canr.msu.edu/pestid/ resources/plant-and-weed-identification/map

Recent Concerns with Multiple Resistance

Multiple resistance is the loss of weed control efficacy from herbicides in two or more site of action groups. (Learn more about site of action group numbers at iwilltakeaction.com/weeds/siteof-action )

• Three-way multiple resistance to glyphosate(e.g. Roundup, Group 9), ALS inhibitors (e.g. Harmony, Group 2), and atrazine (Group 5) in Palmer amaranth (2012)

• Three-way multiple resistance to glyphosate, ALS, and PPO (e.g. Flexstar, Group 14) inhibiting herbicides in waterhemp (2020)

• Two-way multiple resistance to glyphosate + ALS inhibiting herbicides in marestail/ horseweed, Palmer amaranth, waterhemp, and common ragweed (Figure 1)

• Two-way multiple resistance to ALS and PPO inhibiting in common ragweed

• Glyphosate resistance in giant ragweed

Present Discoveries

In the past five years several new cases of resistance in Michigan have been noted. Not all of these were located in soybeans or field crop rotations.

Figure 1. Prevalence of two-way multiple resistance to glyphosate + ALS inhibiting herbicides in Michigan. Note, these findings are based solely on suspicious populations submitted to MSU, no statewide collection/screening has been conducted, therefore absence of letters or color does not guarantee no resistance exists.

• Clopyralid resistance (i.e. Stinger, Group 4) in common ragweed (2017)

• PPO inhibitor resistance in Palmer amaranth & waterhemp (2018 and 2020, respectively)

• ACCase resistance (e.g. SelectMax, Group 1) in large crabgrass (2019)

• New pigweed species, purple amaranth, identified (2019, Figure 2) and confirmed resistant to triazine and urea herbicides (e.g. Caparol and Lorox, Group 5)

• Glyphosate resistance (e.g. Roundup, Group 9) in giant ragweed (2016) annual ryegrass (2020)

• ALS inhibitor resistance in johnsongrass and fall panicum (2020 still being confirmed and 2021, respectively)

Future Directions

The total number of weed populations submitted for herbicide resistance has dwindled from a high of 70 populations submitted in 2016 to a new low of only one population submitted in 2022. While fewer in number, the requests for screens are getting more diverse. Different species are being submitted outside the usual five (i.e. marestail/horseweed, Palmer amaranth, waterhemp, common and giant ragweed and common lambsquarters), with a particular increase in grass species. The focus is also shifting slightly away from glyphosate as the main herbicide of concern, a trend that is expected to continue as new crop and herbicide technologies are introduced. While the turnaround time of these bioassay screens can be lengthy, the beauty lies in their flexibility to meet changing agricultural needs. We expect developing molecular technologies and alternative assays will increasingly aid in these efforts as well.

Should we be concerned about resistance to 2,4-D or/and dicamba?

YES, we should be on the lookout and screening populations for suspected resistance to 2,4-D and/ or dicamba with the increasing use of Enlist™ and Roundup Ready 2 Xtend® systems. Resistance to 2,4-D in wild carrot was confirmed in Michigan back in 1993 and we have since seen other species in the U.S. such as kochia, Palmer amaranth and waterhemp develop resistance to this herbicide. Dicamba resistance has also been noted in the U.S. as early as 1994 in kochia and since that time in Palmer amaranth, waterhemp and prickly lettuce. In several of these cases, particularly in the Amaranth/ Pigweed family, we are seeing 2,4-D and/or dicamba resistance appear in multiple-resistant populations.