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Post Weed Control In Soybean

Postemergence soybean herbicide applications are happening across the state.

It seems like growers frequently are trying to wait until they think all the weeds have emerged before they make these applications which can be late or at least less than optimum to maximize yield. I believe it is better to spray a little on the early side than risk later applications and potential yield loss and poor weed control.& Assuming you start clean either with a burndown application or tillage, it is important to remove weeds by 5 to 7 weeks after soybean planting which is generally when annual weeds are 6 inches tall or less. If more severe infestations of annuals exist or if perennials are the primary concern, applications may need to be adjusted one way or another by a week or so.

Glyphosate can be applied over the top to Roundup Ready soybeans up to 1.5 lb ae/acre. This is equivalent to 44 fl oz of Roundup PowerMax/WeatherMax, 48 fl oz of Touchdown Total or Durango DMA/Duramax or 64 fl oz of most generic types of glyphosate. You can use up to 66 fl oz of Powermax equivalent in your soybean crop (44 fl oz for corn). Use the higher rates for perennials and large or hard to control annuals. Glyphosate can be applied to RR soybean from emergence through flowering (R2 stage – ends when a pod is 5 mm or 3/16 inch long at one of the four uppermost nodes on the main stem). Include an appropriate surfactant if the product is not fully loaded and AMS for hard water or when tank mixing. You can tank mix a number of products with glyphosate to broaden the spectrum. Here are some suggestions:

  • For volunteer corn, common mixtures would include: Assure II, Targa, Fusilade, Fusion, Select, and others. Rates will vary by products.
  • For increased broadleaf activity and/or for residual control, Classic, Harmony, FirstRate, Pursuit (or Extreme), Raptor, Scepter, and Synchrony can be added. Classic, Synchony, and FirstRate will help control glyphosate resistant marestail. Classic and Snchrony added to glyphosate will help provide residual control of burcucumber. Again, rates will vary by product.
  • Contact type herbicides such as Aim, Cadet, Reflex (or Flexstar GT), and others have been promoted in tank-mixture for improved control of certain glyphosate tolerant or resistant weeds (morningglorry, nightshade, pigweeds, ragweeds, etc.). Be aware that these herbicides can cause some leaf burn and also antagonize glyphosate performance on normally susceptible weed species
  • Increased rates of glyphosate can help improve perennial weed control. For weeds like pokeweed, be sure the spray boom is above the weeds and use spray tips that maximize coverage.
  • Finally, several residual grass herbicides can be tank mixed with glyphosate products. Warrant is one of the newer products containing encapsulated acetochlor. Both Dual and Outlook can be applied post, but only up to the third-trifoliate leaf stage.

Most potential glyphosate tank-mix products should be applied before soybean bloom or have days to harvest restrictions, so check labels for specific recommendations.

Double-Crop Soybean Reminders: With the barley coming off now and wheat in the near future, double-crop soybean planting will follow. Weeds are generally less of a problem in double-crop soybean because of the delayed planting date resulting in fewer weeds emerging, but when they are present, the combination of weed competition and potential dry weather can greatly impact yield potential. A weed-free start is one of the most critical aspects of weed management in double-crop soybean. One of the biggest weed concerns in is managing glyphosate-resistant marestail or horseweed. Some of these populations may also be ALS-resistant. Marestail that are cut off during small grain harvest will recover, branch out, and probably be even more difficult to control. In full season soybean, we have been strongly advocating the use of 2,4-DLVE in the burndown program. However, most growers are not willing to wait the necessary seven (7) days after 2,4-D application in double-crop. Alternatives include:

  • Add 1 oz of Sharpen (or equivalent rates of Optill or Verdict) to glyphosate.
  • Add FirstRate (or Authority First or Sonic) or a chlorimuron containing product (Authority XL, Canopy, Envive, Synchrony, Valor XLT, etc.) to improve control if the population is not ALS-resistant.
  • Use Liberty at 29 to 36 fl oz in the burndown application. This is fairly good on marestail, but not that effective on other weeds such as large annual grasses and Canada thistle. You can tank-mix with other herbicides to improve performance and provide residual control (Sharpen, metribuzin, etc.)
  • Plant a Liberty Link soybean and apply Liberty POST as needed.
  • Plant a Roundup Ready soybean and apply glyphosate POST if marestail is not a problem.
  • For growers interested in non GMO soybeans, a number of POST herbicides are still available, but many of the broadleaf products have some potential for soybean injury and they are not effective for control of marestail.

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