Source: University of Delaware
By Mark VanGessel, Extension Weeds Specialist
May 9, 2012 — I have had a number of calls about burndowns not being effective for no-till soybeans. Poor control can be attributed to a number of reasons, weeds were too large, gallonage was too low, and wrong products or adjuvants were used. But the question is what to do now.
First, determine what was not controlled. In most cases it is marestail or horseweed, and it needs to be controlled before you plant because there are not effective postemergence herbicides for it (unless you are using Liberty Link soybeans and use Liberty 280). It’s too late to rely on 2,4-D to control marestail because you need the 1-quart rate to provide effective control.
The 1-quart rate of most 2,4-D products require 4 weeks before planting and in too many places sensitive plants have emerged. If you are on an appropriate soil type, Sharpen, with all the required adjuvants, is an option (see the label).
Sharpen is not an option for coarse-textured soils because it also needs 4 weeks between application and soybean planting. Liberty or Ignite can be used, but it works best on days with full sun shine and requires excellent coverage (at least 20 g/A and medium droplet size).
A chlorimuron-based herbicide is another option, but you need to use rates that will provide good suppression/control. Chlorimuron rates equivalent to 1.5 oz of Classic is needed (see table below). In most situations, the chlorimuron-based products should be used with a burndown herbicide (glyphosate or Gramoxone) and refer to their label for adjuvants.