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PFR Report: Making Your Soybean POST Trip Count

Published on Wednesday, June 5, 2019

As you start to plan your post-herbicide application trip for your soybean crops, you may be looking for ways to maximize weed control. With the ever-growing list of resistant weeds, proper weed management techniques are more important than ever. Heavy spring rains may cause the pre-emergent herbicides to break down earlier in some fields this year. Therefore, another layer of in-season residual herbicide will be critical this year.

Why Group 15 Herbicides

Group 15 herbicides such as Dual II Magnum®, Outlook®, Warrant®, and Zidua® are very effective on grasses and small-seeded broadleaf weeds such as waterhemp. And while Group 15 herbicides have excellent crop safety, sometimes we see a crop response that looks like heart-shaped leaves (Figure 1). To learn more about soybean crop response to chloroacetamide herbicides, click here. 

Figure 1. Heart shaped or draw stringing soybean leaves

Typically, we see these products break roughly 28 days after treatment (DAT). Group 15 herbicides only have residual or pre-emergence activity, making them a great option for an in-season residual. The main function of an in-season residual is to provide an additional layer of protection after the pre-emergent herbicide is exhausted, resulting in increased weed control until crop canopy closure. 

Figure 2. Waterhemp control at Beck’s SIL PFR site from 2016-2017 at harvest across multiple herbicide trait platforms. Group 15 herbicides resulted in 5% fewer plants going to seed and adding seed to the soil seed bank.

Which Group 15?

Since Group 15 herbicides have no post-emergence activity, they must be paired with products that do. Therefore, your post-emergence application tank mix partners will have a heavy influence on which Group 15 herbicides you can use. It’s important to remember that not every Group 15 herbicide is labeled as a tank mix partner with XtendiMax® or Engenia®.

What about the growth stage? Each Group 15 herbicide can be sprayed at different timings (Table 1), so weather and crop size can influence your decision on which product to select. What about pre-emerge applications? Many premixes on the market will have Group 15 herbicides in them as well. Therefore, it is important to understand which actives are in your pre-emerge so the maximum amount is not exceeded in the growing season.

Table 1

Other factors that can influence your product selection include encapsulated acetochlor products, such as Warrant, that have an advantage in high biomass scenarios. Products such as Zidua have a very low water solubility when compared to other Group 15 products such as Outlook, so more water is needed to activate Zidua. However, some research has shown Zidua can provide greater control later into the season compared to Outlook. The Zidua use rate is also much lower compared to all other Group 15 herbicides.

What is the difference in s-metolachlor vs. metolachlor? Metolachlor consists of two isomers; an s-isomer and r-isomer at a 1:1 ratio. However, the s-isomer is much more effective and results in the majority of the herbicide activity. S-metolachlor formulations primarily consist of the s-isomer. Therefore, pound per pound, it takes 35% more metolachlor to achieve the same level of weed control as s-metolachlor. Click here to learn more. 

Resistant Weeds

In the past year, there have been two cases of pigweed resistance found in the U.S. Dr. Aaron Hager from the University of Illinois reported two waterhemp populations in Illinois that showed some level of resistance to all four Group 15 herbicides. You can access the report here. The resistance was ranked from greatest to least: S-metolachlor > dimethenamid > pyroxasulfone > acetochlor. The second report (click here) of resistance to Group 15 herbicides was reported in Arkansas. A Palmer amaranth biotype was reported to be resistant to s-metolachlor. 

With the recent documentation of resistant pigweeds to Group 15 herbicides, it’s important not to over-utilize these products. Because Group 15 herbicides play a key role in weed management, utilizing multiple sites of action (SOA) is critical when developing your corn and soybean herbicide programs.

Where Do We Go From Here?

In recent years, more and more pigweed biotypes have been reported to be resistant to one or more herbicides. Last year, a six-way resistant waterhemp biotype was reported in Missouri, demonstrating the importance of proper weed management. Thankfully, farmers today have the flexibility to select from more post-emergence herbicide options than ever before, making weed management seem easy once again. One way to ensure that post-emergence platforms remain effective is to use appropriate pre-emergence products to take selection pressure off of the post-application herbicides. The other key is preventing late-season emergence by using an in-season residual. It’s also important to remember other cultural weed management tactics from cover crops, row spacing, etc. There is no simple answer to effective weed management, but by developing a solid plan you can make the most out of your applications.




Zidua®, Engenia®, and Outlook® are registered trademarks of BASF. Warrant® is a registered trademark of Monsanto Technology, LLC. Dual II Magnum® is a registered trademark of a Syngenta Group Company. XtendiMax® is a registered trademark of Bayer Group.


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