Season-Long Weed Control Calls for Timely Application 

Getting rid of weeds can be frustrating—especially when the most problematic species make their presence known from planting through harvest. That’s why a successful weed eradication program begins with a plan that focuses on season-long control.

According to Purdue University Professor of Weed Science Dr. Bryan Young, the most problematic weeds today are those that germinate over a long period of time. And to make weed eradication even more challenging, growers can’t count on just one herbicide to keep troublesome species like pigweed at bay.

“The frustration is in determining how to combat a weed that doesn’t give up the entire season,” Young explains. “We can’t do it all in one or maybe not even in two applications. It’s going to take a dedicated approach to make sure you have the herbicides in the right place at the right time to manage the problem weed.”  

Thinking ahead with a proactive mindset is where weed eradication begins.

According to Liam Vincent, technical marketing manager for BASF, starting the season with a clean field is a good place to start. He says tillage or a burndown herbicide application are tools to help pave the way for having your field ready to plant.

Next, utilize a robust preemergence herbicide at planting to give your crop a competitive edge. Adding that preemergence herbicide buys you time and might extend your application window to put out subsequent herbicide applications.

Vincent poses the question, “What if we didn’t stop there? What if we applied residuals in a manner to not let weeds emerge so that we don’t subject them to selection pressure from our postemergence herbicides?”

The concept here is that of overlapping residuals. According to Vincent, it is a practice many Australian growers must abide by because they no longer have effective postemergence herbicide options.  Further, Vincent proposes that if a grower can have a plan in place and know how to implement the various residual herbicides at his disposal, a grower could accomplish the concept.

Being proactive in your weed control plan means having a plan and executing that plan. 

The concept of overlapping or layering residual herbicides refers to the practice where a grower has a line of defense in place to keep weeds from germinating all season long.  For example, a grower may apply his preemergence herbicide application ahead of his soybean crop in mid-April; by mid-May that herbicide is beginning to lose efficacy. To keep that line of defense in place, a grower would be sure to apply another residual herbicide before the first one loses its efficacy. If applied in this layered fashion, a grower greatly reduces or eliminates the number of weeds which may emerge.  The use of the terms “overlapping” or “layered” refers to the fact that there is always coverage in place to control emerging weeds and that there are no gaps in control.  By doing this, a grower does not allow weeds to emerge and instead controls all weeds using residual herbicides. 

A robust, long-lasting preemergence followed by subsequent complementary applications of residual herbicides is required to control weeds for the entire growing season.  Again, work with your BASF field representative to better understand what products you are applying, know your target, what weed species the herbicides control and how that specific product performs in your area.

Adding a second mode of action to your residual herbicide application creates an additive effect, according to Young. Complementary weed control is then achieved with the right amount of rainfall for at least one of the two active ingredients. Ensure there is no known herbicide resistance to the products you are applying. If there is, ensure you have multiple, effective sites of action.

“Make certain you don’t have resistance to both of those active ingredients or modes of action,” Young explains. 

In an ideal scenario, one application of a residual herbicide would control weeds all season. But that’s not the case.

That’s why a follow-up application is essential to successful weed control.

In soybeans, for example, another layer of residual herbicide prior to canopy closure provides weed control through the remainder of the growing season against late emerging pigweed species.

“Typically, that application might be three to four weeks after planting,” Young notes. “The one thing that’s difficult to understand is that application might occur when you don’t see any weeds in the field in that post-application phase. So, it’s very difficult to justify spending the money on a residual herbicide. However, that’s the discipline that’s necessary to make sure that we keep weeds down.”

Vincent adds that overlapping residuals prevent weeds from emerging so that a knock-down herbicide isn’t needed to control weeds.

“If you’re able to time your residual applications properly and follow with another residual herbicide before weeds emerge, then you can effectively control those weeds when they’re the most vulnerable, which is when they’re germinating,” Vincent explains. “Then, there’s only one growing point to control rather than a four-inch pigweed that has several more.”

Herbicide chemistries are not created equal, especially preemergence herbicide chemistry. Over the last five to 10 years, the crop protection industry has begun to release preemergence herbicides with multiple sites of action to give growers residual weed control which covers a wider spectrum of weed species and provides longer periods of control. That said, it is still the responsibility of the grower to understand his/her soil conditions and purchase the product that will provide them with the performance they expect.

“You need to know how long that residual is going to hold off weeds,” Vincent says. “That will dictate how quickly you need to put out an overlapping residual.”

Vincent also says understanding the rainfall activation requirement of the herbicide product you apply is critical when working with overlapping residuals. All residuals require precipitation to activate and control germinating weeds; however, some require more than others. For example, Outlook® herbicide requires only ¼" of precipitation to activate whereas other herbicides require ¾" or more. Further, if you do not receive activating precipitation within 14 days of application, you can expect reduced control or a complete failure of that herbicide application.  In this case, you need to be prepared to spray emerged weeds.

“The weed management plan needs to start with understanding your target weed and what resistance it has to herbicides because that will dictate what options you have available,” Young notes. “Consider your entire crop rotation and the herbicides you might implement so that you don’t focus on one herbicide or mode of action too frequently. In all cases, we ideally like to see multiple modes of action that are effective at each application.”

Timely planning of overlapping residuals used in tandem with other cultural and mechanical strategies can be useful tools in a grower’s weed management plan.

From effective chemical control and seed trait selection, cultural and mechanical practices, to practicing eradication diligence through spot spraying, mowing field edges and harvest management, pigweed eradication is best achieved with a varied approach.

Knowing weed species and paying attention to what’s been in the field—including what comes up early and what comes up late—are key to successful weed eradication, Vincent says.

After all, controlling weeds requires a season-long approach.

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