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What Farmers Need to Know About Burndown Herbicides

Time is Tight to Control Marestail Before It Bolts.

Farmers who weren’t able to apply herbicide last fall should consider a burndown treatment as soon as possible this spring.

“The earlier we get them out on small weeds, the better,” says Joe Bolte, a Practical Farm Research (PFR) operator and herbicide specialist for Beck’s Hybrids based in Effingham, Illinois.

Fall treatments are particularly preferable for winter annuals like marestail, as they nix small weeds that otherwise would come back in the spring. Marestail rosettes “bolt” in the spring into upright stems that reach up to 6-foot heights and produce up to 200,000 seeds, according to Pennsylvania State University weed scientists. 

However, it was all many farmers could do last fall to harvest their crops, let alone apply herbicides to control winter annuals like marestail. Although time is tight, a burndown application now can kill marestail before it bolts, he says. 

Burndowns are critical for no-tillers, he adds. “If you don’t apply a burndown pass, you get a weedy mess out there (at planting),” he says. “By doing a burndown pass, you make sure those weeds are dead before no-tilling.”

For those who mix in tillage, adding a burndown followed by vertical tillage can control weeds better than just a tillage pass alone, he says. 

Burndown treatments have other benefits besides weed control, Bolte adds.  Winter annuals like henbit provide overwinter cover for soybean cyst nematode, for example. 

“If you don’t feel like you have time for a burndown and plan to till and then plant, put down a residual (herbicide) and then add something that has post activity – such as Gramoxone or Liberty,” he says.

What Burndown Treatments Should You Use? 

If it looks like there will be a window of time between your burndown and planting, consider a burndown herbicide with residual activity like Sharpen, says Bolte. A downside is residual burndown herbicides can have plantback restrictions. 

If the time window is tight between burndown and planting, Bolte advises applying a herbicide with no plantback restriction, such as Gramoxone, says Bolte.

An old standby burndown herbicide combo is 2,4-D and dicamba. “Those are cheap and effective options, but they also have plantback restrictions,” he says. Several 2,4-D products can have plantback restrictions ranging between 7 to 30 days, depending on application rate.

These burndown options contain the same chemical as is contained in Xtendimax/Engenia/Fexapan (dicamba) and Enlist Duo/Enlist One (2,4-D choline) applied postemergence in herbicide-tolerant soybean weed-management systems. Using the same herbicide site of action in burndown and preemergence applications as in postemergence applications increases selection pressure on weeds, says Bolte.

“Every time you apply a herbicide, you select for resistance,” Bolte says. “It is critical to look at other burndown options so we can save growth regulators (Group 4) for in-season use,” says Bolte. 

Gramoxone, for example, is a Group 22 herbicide that has a different site of action then the Group 4 herbicides like 2,4-D and dicamba. 

The good news is that several options now exist for postemergence applications in soybeans,” says Bolte. “Now we have Enlist, Liberty, and Xtend (herbicide-tolerant weed-management systems). Ten years ago, we didn’t have as many options.” 

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