By W.S. Curran, Penn State Cooperative Extension Weed Science
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.
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. In 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, 48 fl oz of Touchdown Total or Durango DMA or 64 fl oz of most generic types of glyphosate. You can use up to 66 fl oz of Powermax equivalent in crop. Use the higher rates for perennials and large or hard to control annuals. Glyphosate can be applied to RR soybean from emergence through flowering. 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. A post grass herbicide may be tank-mixed if glyphosate resistant volunteer corn is present. Several products are available including 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. 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 weeds (morningglorry, nightshade, pigweeds, etc.). Be aware that these herbicides can cause some leaf burn and also antagonize glyphosate performance on normally susceptible weed species.
Finally, several residual grass herbicides can be tank mixed with glyphosate products. These products will not control emerged weeds, but provide residual control of annual grasses and small seeded broadleaves. Warrant is a new product from Monsanto containing encapsulated acetochlor. It can be applied after soybean emergence and before soybeans reach R2.
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.
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