Key weeds respond to residual herbicide options

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There are many good reasons to use a soil-applied residual herbicide for soybeans, according to weed scientists across the country.

Producers should not only be worried about weed resistance to postemerge herbicide applications, mainly glyphosate, but also some weeds being missed by postemerge application error or when some weeds are much bigger than label limits.

The big push recently is for eliminating weeds all together because of the volume of seed that each missed weed can produce at the end of a growing season.  

Dallas Peterson, weed management specialist, Kansas State University, has noted that using a residual herbicide can:

- Get early-season control of weeds and grasses to minimize early-season weed competition and provide more flexibility with postemergence treatment timing.

- Provide some residual weed control before and following the postemergence glyphosate.

- Provide some assistance to glyphosate in controlling certain hard-to-control or glyphosate-resistant weeds.

- Add a second herbicide mode of action to prevent or delay the development of glyphosate-resistant weeds.

Peterson stresses that it is key to know what weeds or grasses have to be targeted in each field in developing an alternative to the exclusive use of postemergence glyphosate treatments.

Common weeds and grasses that have to be targeted in Roundup Ready soybeans, from a Kansas perspective include:

  • Pigweeds (including waterhemp and Palmer amaranth). Glyphosate-resistant waterhemp has been confirmed and Palmer amaranth is creeping into Kansas. Palmer amaranth is a huge problem throughout the southeast and has been confirmed in a few Corn Belt states as of 2012. Fighting these weeds before emergence is key today.
  • Kochia. Kochia is a major weed problem in western areas and historically has been  difficult to control with glyphosate, especially as it gets bigger. A majority of kochia will probably have emerged prior to soybean planting, so controlling that kochia before planting is critical.  
  • Velvetleaf. Glyphosate has not always been entirely effective on velvetleaf; therefore, soybean growers probably should have been looking for help throughout their years of glyphosate weed control programs.  
  • Cocklebur. The most effective control of this week over the years has included a preplant and preemergence herbicide use.  
  • Marestail. This weed is probably the most widespread glyphosate-resistant weed in Kansas. Marestail control in Roundup Ready soybeans should begin in early spring by controlling fall-germinated seedlings and rosettes before they start to bolt.
  • Morningglory. Again, glyphosate sometimes has had trouble controlling morningglory, which means labeled preplant or preemergence a needed.
  • Crabgrass and other small seed grasses. Glyphosate has previously gives good control of most grasses, but producers should look at applying a foundation herbicide to control grasses early, then make just one postemergence glyphosate application later.

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Nathan Barnes (Crop Production Services)    
Concordia KS  |  February, 06, 2013 at 02:43 PM

In response to beginning in early spring to control glyphosate resistant marestail, one should think about applying the residual herbicide in the fall. Don't give marestail a start. Typically in the spring everyone is busy and when the issue is noticed it is so close to planting date that the grower may have to wait to safely plant soybeans after herbicide application.

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