Marestail (or horseweed) is growing rapidly this spring across much of the state. Be sure you are scouting your no-till soybean fields this spring and early summer and selecting appropriate control measures.

Before you plant soybeans or corn, consider overall marestail management. Extension weed specialist Dwight Lingenfelter tells us this weed is mostly a problem in no-till and more of a problem in soybean than corn.

It is also common along the margins of some fields where herbicide programs are usually more hit or miss.

Much of the marestail in the region is glyphosate resistant, and pockets of Group 2 or ALS-inhibitor (Classic, FirstRate, etc.) resistant biotypes also exist.

With the widespread herbicide-resistant marestail across the state, it is increasingly important to tank-mix herbicides to improve the spectrum of activity for successful control, and to use multiple modes of action.

In most cases, glyphosate plus 2,4-D ester applied close to planting will not provide effective control of marestail, as it did previously.

Postemergence herbicide options in soybeans are limited and require upfront preparation using either LibertyLink, XtendFlex, or Enlist E3 soybean varieties.

The use of residual herbicides to control marestail that emerges in crops is becoming more common, and six to eight weeks of residual control may be necessary for complete control.

Residual herbicides that contain flumioxazin (Valor), sulfentrazone (Authority) or metribuzin are effective on germinating marestail and are key for controlling ALS-resistant biotypes.

Ideally, burndown applications in April and early May should include a residual herbicide to control later-emerging marestail.

In some cases, the residual herbicide can affect the utility and timing of the burndown herbicides.

If you plan to tank-mix Sharpen and a residual product that includes another Group 14 (PPO inhibitor) herbicide such as flumioxazin or sulfentrazone, you must wait 14 days to plant soybeans.

Unfortunately, if all the burndown and residual herbicides are applied in one pass a couple weeks or so ahead of planting, that means two weeks of “wasted” herbicide residual activity.

Once the crop is planted, it needs to have as much residual herbicide available, for as long as possible, to provide a weed-free environment to get established and increase its growth and development for optimal yield.

In Xtend or Enlist E3 platforms, registered dicamba or 2,4-D products, respectively, can be tank-mixed with glyphosate and PPO (and/or other) residual herbicides and applied at planting without a wait period.

Keep in mind that applying dicamba-containing products like Clarity, Banvel and generic dicamba-DMA/DGA and then immediately planting Xtend(Flex) soybeans is off label.

For dicamba products like this, the label states that at least 28 days plus 1 inch of rainfall or irrigation is necessary before any kind of soybean is planted if 1 pint of product is applied. (The labels of the specific products may vary slightly but essentially state similar information.)

Since Xtend soybeans are not specified on their labels, this longer wait period must be applied.

Currently, the only dicamba products that can be legally applied near planting or over the top in Xtend brand soybeans are Engenia, XtendiMax with VaporGrip Technology, and Tavium plus VaporGrip Technology.

Similar regulations hold true for the use of 2,4-D products in Enlist E3 soybean. Enlist E3 soybean varieties can be planted immediately after an application of Enlist One or Enlist Duo (choline salt) in a burndown program.

Any other 2,4-D product (LV4 ester, amine, etc.) must adhere to the seven- to 15-day wait period before planting, depending on use rate.

Once the E3 soybeans emerge, only Enlist One or Duo can be applied as an in-crop application. No other 2,4-D products can be sprayed over the top, and doing so would be in violation of the technology use agreement.

As previously mentioned, marestail is generally more easily managed in corn than in soybean. However, here are a few considerations for successful control in corn.

Effective burndown programs usually include, Sharpen plus glyphosate; Gramoxone plus 2,4-D plus atrazine or simazine; and/or Elevore.

Make sure to include atrazine for residual control of germinating marestail seedlings.

Aside from atrazine and Balance Flexx or Corvus, no other active ingredients provide effective residual control of this weed in corn.

Leon Ressler is a Penn State Extension educator based in Lancaster County, Pennsylvania.


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