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Posted 26 June 2014. PMN Crop News.

Post Weed Control in Soybean

Source: Penn State University Press Release.

University Park, Pennsylvania (June 18, 2014)--Postemergence soybean herbicide applications are starting to happen 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. We 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/clethodim, Select Max, 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.

For horseweed/marestail: Use the highest rate possible of Classic, Synchrony (use highest rate on STS bean only), and FirstRate and full adjuvant systems (see label) may help control glyphosate-resistant marestail. However, there are populations in PA that are both ALS- and glyphosate-resistant. In these cases, the above products will not provide control. High rates of glyphosate Liberty can be effective postemergence on marestail if Liberty Link soybean varieties were planted. According to Mark VanGessel, Univ. Delaware, horseweed plants are generally not very tolerant of shade and most soybeans will begin to canopy over the horseweed and outcompete them. Additional glyphosate applications will provide some suppression of horseweed and sometimes the soybeans have a chance to outcompete them. It is always best to treat the horseweed plants soon after they start regrowing from the burndown application.

Classic and Snchrony added to glyphosate will help provide residual control of burcucumber. Again, rates will vary by product.

Contact type herbicides such as Cadet, Cobra, Reflex (or Flexstar GT), Storm, 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/will 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. Anthem, Dual, Outlook, and Zidua 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.

William Curran