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Research Highlights
Weed Management Tips Relevant for All Producers

Photo: United Soybean Board

By Carol Brown

Any farmer who has dealt with weeds in their crop fields knows it’s a never-ending battle. Illinois farmer Jared Greuel offers tips for weed control that are applicable for any farmer.  As a guest on the IL Soy Advisor podcast, he addresses weed management considerations, some of which are highlighted here. To hear the podcast in its entirety, go to: https://www.ilsoyadvisor.com/on-farm/ilsoyadvisor/podcast-herbicide-treatments-and-weed-management

Q: Should growers use a residual herbicide to manage weed pressure?

Greuel: Residual herbicides are not a question of whether to use one, but which ones to use. They are a requirement regardless of what soybean system is used except in organic production.

Q: What are some risks associated with controlling weeds using only a post-emergence application?

Greuel: When using only a post-emergence application, you are multiplying the selection pressure on these individual products. For a long time, producers have used products such as Pursuit®, Roundup®, and the ALS products as stand-alone solutions for controlling weeds. We would spray one product and be done. But as we know, it doesn’t take long for resistance to form. When using a post-emergence application only, we are jeopardizing these products.

Once that weed has emerged, it competes with our crop. With weeds such as waterhemp, marestail or lambsquarters — you may see a weed, but I see a sink for profits. It is taking yield and profitability away from your crop. Our weed goals should be to not let them come up at all, and with just a post-emergent application, we’re losing that goal.

Q: What should growers consider when choosing a residual herbicide?

Greuel: There are a lot of things to consider, but I preface everything with one main consideration: read the herbicide label. It’s a lot easier to fix things before the herbicide is applied than afterward.

The second thing is: know your products. Know which chemical families offer residuals and know the individual characteristics of each. If I ask what the difference is between Zidua® and Outlook® herbicides, most growers don’t really know. They know they’re both good, but don’t know the unique characteristics of either of them. This just sets up a person for failure. Know the active ingredients, know what they do and won’t do, and know what the residuals are.

Q: Should growers be concerned with herbicide carryover into the following crop or cover crop?

Greuel: Absolutely. There are so many things we don’t think of that are carryover from the previous year. Carryover can come in different forms. For instance, Flexstar® has a 10-month plant back. If you spray it, technically you shouldn’t plant corn in that field until 10 months later.

Q: Why is it important to understand the active ingredients of a selected herbicide program?

Greuel: When we understand the characteristics of the active ingredients, it is beneficial because we’re using them as tools rather than rolling the dice to see what will work.

For example, look at the differences between Zidua and Outlook herbicides. Zidua needs a 2-inch rain to get it working. Outlook only needs a quarter-inch of rain to activate, but its residual length depends on the amount of rainfall received. The more rain we get, the less effective it is. In 2012, Outlook was a good option because it didn’t take much rain to get activated and it kept working. If you applied Zidua in 2012, you very likely had weeds break through your program, and that’s no fault of the product. Zidua was sitting there needing more rain to activate.

Zidua also has the longest residual window of up to 56 days of the Group 15 herbicides. Last year, people said Zidua was effective, because we had 18 inches of rain in July. There was a lot of rain to keep it working. Whereas those who used Outlook started to see weed escapes.

If the overlapping windows expire and your product hasn’t activated, there’s now a window for the weeds to get started. You need to be cognizant of those active ingredients.

Q: What does it mean to layer soybean residuals?

Greuel: I’ve been using the term ‘reinforced’ soybean residuals rather than layers. Layering residuals means two things. One is having more than one active residual at a time. We don’t want just one herbicide working on a specific weed species, in particular anything from the amaranth species. You need to know how long each product is effective and make sure you’re getting back out in the field before the windows of control expire.

The other component is we can’t rely on only one residual. Chemical families that have residuals to them — Groups 3, 4, 14, 15 — we should be using all of these as part of a residual program. Understanding the characteristics of the active ingredients will help to know where they fit into this puzzle.

Q: What are some general recommendations to help growers avoid giving up soybean bushels to weeds?

Greuel: Never let the weeds get started. The definition of a weed is any species that’s competing with the desired crop, which is why corn is a weed in some years. Volunteer corn in soybeans is competing for nutrients and moisture and providing a haven for overwintering disease. Is it practical to want 100 percent control? No, we’re going to have escapes. But we need to learn from those escapes. The best form of weed control, short of getting canopy in the crop, is never letting them get started and letting the seeds expire from the soil seedbank.

Greuel is an Illinois Soybean Association 2021 Soy Envoy and grows non-GMO soybeans and corn in south-central Illinois. He is the owner of Greuel Farm Service and a Certified Crop Advisor (CCA).

For more information about the Illinois Soybean Association Soy Envoy program, visit: https://www.ilsoyadvisor.com/on-farm/ilsoyadvisor/soy-envoys