Don't Wait to Wage the Weed War

When you're attempting to control broadleaf weeds in fields of herbicide-tolerant soybeans, Southern States Cooperative's Ken Sechler warns against the temptation of delaying in-season herbicide applications.

“Look, if those weeds break through the canopy, you're just not going to be able to control them,” says the regional agronomist. “You can't wait to spray, plain and simple.”

Whether a grower has planted Liberty soybeans with tolerance to glufosinate, RoundUp Ready 2 Xtend® soybeans with tolerance to glyphosate and dicamba, or even Enlist E3™ soybeans with tolerance to glyphosate, glufosinate and 2,4-D, the situation is the same. The effectiveness of all of these systems is sensitive to weed height. Simply put, the taller the weed, the less likely the labeled herbicide rate will provide control.

“Some farmers want to kind of drag their feet a little bit and make sure any weeds have germinated, but with all the newer chemistries, smaller is better,” Sechler says. “Marestail is our worst problem here, along with Palmer amaranth and some common ragweed. You let any of those get tall, and you'll be really challenged to kill them. Sometimes you end up just making them sick for the rest of the year rather than taking them out. And that has the potential to lead to future herbicide resistance.”

Two-pass plans work best

Regardless of the herbicide-tolerant traits that growers elect to plant, Sechler recommends they adopt a “two-pass” system for their herbicide programs. While the specific products applied can vary, the goal is to gain weed control through the combination of both pre-emergent and post-emergent soil-residual herbicides with multiple effective modes of action.

“I tell a lot of growers they should go in with a really strong pre-emergence program,” explains Sechler. “To me, the most important herbicide pass you're going to make is that pre-emergent pass. With it, some may even avoid a second pass.”

A pre-emergent herbicide can be applied before, during or just after soybean planting. Depending on any herbicide resistance that may already be present in a field, these products could include Authority®, Boundary®, Fierce®, Prefix®, Sonic®, Trivence®, Valor® or Zidua Pro®, to name a few. The timing of this application should be based on field conditions.

Closing the canopy

“Old-time agronomists always say that 50 percent of weed control is shade, because none of these weeds like shade,” Sechler says. “Now, you're not going to get a full soybean canopy to create that shade until almost eight weeks after planting. So it comes down to scouting your fields and timing that post-emergent herbicide pass.”

Sechler advises farmers to come back with the herbicide application four to five weeks after planting, especially when dealing with potentially herbicide-resistant weeds. The timing of the application is relative to the size of the weeds. The best opportunity to kill weeds is when the plants are four inches tall or less, roughly the same height as a standard soda can.

“It comes down to timing,” Sechler says. “With post-emergence herbicides, being timely with application is key to get the best control possible.”

Using the appropriate adjuvants and nozzles, as well as following guidelines for field buffers and application timing restrictions, is crucial. “It's important that we comply with the label,” Sechler adds.

When a two-pass strategy is successful, the soybean canopy closes and weed control is achieved. Should weeds survive post-emergence herbicide applications, options are limited.

“We're legal to come back in with glyphosate, but a lot of these weeds are tolerant to it,” Sechler says. “In all seriousness, there aren't many options. If it gets away from your initial post sprays, you're kind of stuck.”

While rogueing out surviving weeds is always an option, the agronomist says the technique isn't the most practical for today's agricultural operations.

“Sure, if you can count the few hundred plants and get them by hand, that's great,” he adds. “But most of the time, if your herbicide application wasn't effective, it's beyond a few hundred plants.”

Timely application is key

Weed pressure can impact soybean yield, so it's important to apply herbicides correctly and at the right time to ensure their efficacy. For assistance improving your soybean herbicide program, contact your nearest Southern States agronomy team member for help.

Always read and follow all label directions when using and applying herbicides.
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