Weeds take the top spot as the most problematic pest problem farmers manage. Evolution and spreading of herbicide-resistant weeds, strategies for weed control in conventional corn, and preserve the new products or premixes for future use contribute to the challenges. 

Failures to control waterhemp – a troublesome weed – are frequently reported in Minnesota. Are failures due to increased herbicide resistance or other application factors? To help answer this, Dr. Debalin Sarangi, University of Minnesota Extension weed specialist, collected 120 waterhemp samples from 56 counties for resistance screening.  

Each waterhemp population was exposed to full (1x) and high (3x) rates of eight commonly used postemergence herbicides. When 40% of a population survived the high or 3x rate, that population was considered ‘resistant’ in this research work. This research was conducted by Mr. Navjot Singh, a Weed Science graduate student at UMN, and funded by the Minnesota Soybean Research & Promotion Council.

What did they find in their first two years of screening? Out of 85 waterhemp populations tested so far, all of them were resistant to at least one herbicide. Two-thirds were resistant to glyphosate.  “Though we didn’t find many populations showing resistance to Xtendimax or Liberty, there were some individuals that survived those herbicides at full and high rates,” Sarangi explains, “we have to be careful as the auxin resistance is on its way.”

The number of effective options for waterhemp control dramatically decreases as resistance to multiple herbicides develop.  In the screening, over half of the populations were resistant to at least two herbicide sites of action. One population even survived six herbicide sites of action in the screening. In situations like this, very few herbicide options for postemergence waterhemp control remain in corn and soybean. 

There are two major types of herbicide resistance, target-site and metabolic. Of the two,  metabolic resistance is more challenging to manage. Resistant plants are able to increase their metabolism of the applied herbicide or detoxify it. The worrisome thing, according to Sarangi, is that weeds are rapidly evolving and resistance development is complex, unpredictable and on the rise. As a result, integrated weed management becomes even more important. 

What can producers do? Mixing and rotating herbicides is still recommended, but this strategy alone will not prevent metabolic resistance. Other practices include using full labelled rates; spraying when weeds are less than 4 inches tall; using non-chemical strategies, like tillage, crop rotation and cover crops; controlling weeds in fencelines; and managing the seed bank.

Another area of research for Sarangi is controlling grassy weeds in conventional corn. While grasses can be controlled with glyphosate, applying it is not an option in conventional corn, making weed control challenging.  

The first comparison included only preemergence herbicides to get at baseline performance. They provide a strong foundation for weed control, but adding other herbicides with multiple types of action will broaden control and help manage resistance.

The second trial expanded into a program approach with herbicides applied at different times:  1) preemergence application only, 2) early postemergence application (V3), and 3) a combination of preemergence herbicide followed by a mid-postemergence application. 

While a couple of the early postemergence applications showed excellent early-season grass control, weed competition later in the season reduced corn yield. In contrast, the highest yielding treatments included preemergence herbicides in the program.  

Sarangi has also fielded many questions about Enlist E3 soybeans that offer several herbicide treatment options. In addition to tolerance to 2,4-D choline, E3 soybeans are also tolerant to Liberty and Roundup. Conventional herbicides are also options.  The sheer number of choices raises questions about what to apply, when to apply it and what the sequence should be.

To help answer these questions, he initiated a waterhemp control study with and without a preemergence herbicide in combination with a variety of early-, mid- and late-postemergence applications. 

In postemergence only herbicide programs, waterhemp control was highly variable – it could be 50% or it could be 90%. High levels of variability even existed in the two-pass postemergence-only programs. 

When the preemergence herbicide was added to the programs, waterhemp control was both better and more consistent. This also translated into about a 10 bushel soybean yield advantage, compared to treatments that didn’t include the preemergence herbicide. Similar to the other studies, preemergence herbicides provide a good foundation for starting clean, so that fields have a better chance at staying clean and maintaining yield.

Finally, Sarangi stated that detailed reports on the these studies will be available for the growers and other stakeholders through MN Crop News in the coming months.

Thanks to the Soybean Research and Promotion Council and the Corn Research and Promotion Council for their generous support of this program.

Disclaimer: Reference to commercial products and trade names in made with the understanding that no discrimination is intended and no endorsement by the University of Minnesota is implied.

For more information from the University of Minnesota Extension, visit extension.umn.edu/crop-production.