Case Study: Herbicide Resistance to Waterhemp Threat Escalates

Herbicide resistance, especially with waterhemp and palmer amaranth, has been well documented and continues to grow.  What makes these two weeds unique is that they have all of the key characteristics that can accelerate resistance to herbicides.

1. Both are dioecuis, which means male and female flowers are on separate plants and cannot self-pollinate. Instead, they are cross-pollinated combining the genetic traits from two different plants resulting in more genetic diversity. Waterhemp is known to outcross with other pigweed species as well. More genetic diversity means there is an increased chance of developing resistance.  
2. Both are prolific seed producers, so the more seeds and plants produced, the greater the chance for genetic diversity.
3. Wind-blown pollen from the male plants can remain viable for several days and can travel up to one-half
4. mile giving a higher chance of transferring resistant traits.
5. Both are easily spread as it has also been documented that seeds are readily transported by waterfowl giving the ability for movement across the entire midwest, as well as by contaminated farm equipment and seed.

📸: BASF Contributing Writer/Featuring a closeup of Waterhemp.

To get another look at the current landscape of herbicide resistance to waterhemp, a university of Minnesota study evaluated seven post applied herbicides comprising six different sites of action for the frequency and distribution of waterhemp resistant biotypes across Minnesota. 

They included 2,4-D, atrazine, dicamba, fomesafen, glyphosate, imazamox, and mesotrione. Of the herbicides tested, resistance to imazamox was found 81 times, glyphosate 79 times, atrazine 40 times, fomesafen 28 times, mesotrione 7 times, 2,4-D 7 times, dicamba 2 times, and none with glufosinate.  

📸: BASF Contributing Writer/Featuring herbicide multiple resistance results from 90 Waterhemp accessions across 47 counties in Minnesota.

Herbicides Evaluated

All the herbicides tested are generally used as post applied products. Although waterhemp and other weeds are able to develop resistance more often to post products, resistance to soil applied herbicides can certainly happen as well. The list of herbicides tested are commonly used across the entire midwest, so although this research was done in Minnesota, the frequency and distribution of similar waterhemp resistant biotypes has been reported by other midwestern university researchers.

• Cross Resistance: When weeds are resistant to more than one herbicide having the same herbicide site of action, they are considered cross resistant.  So, if waterhemp is found to be resistant to one herbicide, other herbicides with the same site of action will also not control the waterhemp population. Cross resistance with herbicides sharing the same site of action is very common, but not automatic. There are numerous examples of both scenarios.

• Multiple Resistance: When waterhemp are resistant to more than one site of action, they are considered to be multiple resistant. The presence of multiple resistance exponentially eliminates many herbicide options compared to resistance to only one site of action. It was no surprise that almost all of the 90 waterhemp accessions from 47 counties evaluated had waterhemp that were “two way” resistant to both glyphosate and imazamox, as growers have adapted to that issue relying on herbicides other than glyphosate or any ALS site of action for control.

• Three, four, five, and six way multiple resistance: The disturbing find was that of the 90 accessions, resistance to more than two sites of action was found 54 times.

1. Three way resistance was documented 30 times; 18 times with additional resistance to atrazine, seven times with the additional resistance of fomesafen, and five times with the additional resistance of mesotrione. 
2. Four way resistance was documented 13 times; eight times with the glyphosate, imazamox, atrazine and fomesafen resistance, two times with the additional resistance of fomesafen and mesotrione, two times with glyphosate, imazamox ,atrazine and mesotrione, and once with imazamox, atrazine, fomesafen, and mesotrione.  
3. Five way resistance was found seven times; four times with glyphosate, imazamox, atrazine, fomesafen, and mesotrione, two times with glyphosate, imazamox, atrazine, fomesafen, and 2,4-D. and once with glyphosate, imazamox ,atrazine, mesotrione, and 2,4-D.
4. Six way resistance was found four times; twice with all products tested except for dicamba and glufosinate, and twice with all products tested except for glufosinate. There was no single nor multiple resistance detected with glufosinate.

📸: @fotokostic via Canva/Featuring a sprayer spraying soybeans crops.

• Management Options:  You can see from the examples of multiple resistance in the table that understanding what herbicides will work is imperative prior to assembling a successful herbicide management program for waterhemp. Without the type of intensive testing done in Minnesota, it is difficult to identify all the multiple resistance scenarios in your field until you notice a failure.

• Soil Applied Herbicides: Certainly, one solid recommendation is to keep a strong soil applied herbicide in your program for waterhemp, as effective soil applied herbicides used in conjuction with others offer a good multiple site of action program. 

• For waterhemp that can emerge over a long time, layering residuals at planting and again tank mixed with post herbicides offers the best continuous control. Group 15 herbicides such as Outlook® herbicide and Zidua® herbicide, group 14 herbicides such as Sharpen® herbicide, and combination group 15 and 14 soil applied herbicides such as Verdict® herbicide and Zidua® Pro are all strong on waterhemp and have shown to be less likely to develop resistance.  

• Post Applied Herbicdes: A second recommendation is to carefully choose postemergence herbicides based on your field scouting and experience. As you can see from the chart, in many cases several post herbicides without an X still offer good control in many locations, so identify those and mix with effective options. It is also notable that the only post applied herbicide showing no evidence of waterhemp resistance is glufosinate. Liberty® herbicide which contains glufosinate, can be used with most of todays soybean varieties, can also be used in selected corn hybrids, and can be tank mixed with Outlook or Zidua herbicides.

• Cultural Practices: Weed management also means diversifying with more than just herbicides, as crop rotation, use of cover crops, and deep tillage can all help reduce the number of weed seeds that make it into your seedbank.

The use of multiple sites of action in each herbicide application is imperative as it has been documented that growers who averaged 2.5 herbicide sites of action per application were 83 times less likely to have glyphosate resistance than growers who averaged 1.5 sites of action.

📸: @DS70 via Canva/Featuring an Iowa soybean field free of Waterhemp and all weeds..

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