With most of Mississippi receiving large amounts of rain during early June, opportunities for herbicide applications in soybean have been rare up to this week. Several calls have come in regarding herbicide options in situations where the weather has prevented timely postemergence (POST) applications. Each situation is likely different depending on the growth stage of the soybean crop and whether residual herbicides have already been applied at an early POST timing.
No POST application to date: With respect to Palmer amaranth, herbicides such as Cobra, Flexstar (or other fomesafen products), Prefix, and Ultra Blazer seem to provide the most consistent control when Palmer amaranth is small (< 2 to 3 inches). In situations where additional residual activity is needed, Prefix may be preferred. Additional options for added residual in POST applications are metolachlor products (Dual Magnum, etc.) or Warrant. In a Roundup Ready soybean system, tankmixing one of these products with a surfactant-loaded glyphosate is recommended for additional control of other species. Liberty 280 is an additional option for effective Palmer amaranth control, but is only for use in a LibertyLink soybean system.
2nd POST application: If a POST application containing fomesafen was applied at an earlier timing, Cobra and Ultra Blazer are two additional options to try to clean up any escapes.
For more detailed information on application rates and timing, target weed size, adjuvant requirements, etc., please see the 2014 Weed Control Guidelines for Mississippi.