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Frequent information updates for agricultural audiences

Marestail and Waterhemp Control Stategies - from Mike Roegge

Posted by John Fulton -

It was pretty evident during this past summer that marestail and waterhemp populations across the area are resistant to glyphosate. Thus many will need to utilize a different weed control strategy to control these weeds in soybean fields. The process can begin this fall.

Marestail can germinate in the fall and the spring. So if using a fall application in these fields, be sure to include 2,4-D in the mix. It will provide excellent control if used at the 1# per acre rate before the plant becomes completely dormant in late November. It would be best to include glyphosate in the tank to control emerged winter annuals such as henbit and chickweed as 2,4-D does very little for these two specific weed species.

If you include a residual herbicide in the fall mix, do not expect control to last into the late spring and summer to provide any control of waterhemp. There is simply too much time between application and emergence. Waterhemp can emerge from very shallow depths and it's tough to keep this weed under control with spring applications much less from fall applications.

In fact, because waterhemp can emerge over such a long period of time, a residual application at or before planting time coupled with an application of residual at post emerge timing may be necessary. Many will ponder the necessity of such extreme measures, but because of the widespread resistance to glyphosate, we'll be relying upon the postemergent applied PPO herbicides (Cobra, Phoenix, etc.) to provide control of any escaped weeds. So it makes the job much easier if there are fewer waterhemp plants to control, thus the need for residual herbicides to help keep the waterhemp populations low.

And as you recall from pre glyphosate days, these PPO herbicides are contact products and work best on very small waterhemp that can be completely covered with the spray mix. Liberty Link soybeans are another option for waterhemp control, but again, they rely upon a contact herbicide as well, and work best when small waterhemp is treated.

We're in a new era of weed control. You can't keep doing the same things this year that you did last year. We need to treat these resistant weeds differently.

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