Mark VanGessel, Extension Weed Specialist; mjv@udel.edu
This is the point in the season when I get calls asking what can be done to rescue soybean fields from weeds that “got away”. Unfortunately, there are not a lot of good options for control of most large weeds. Of course, there are always exceptions such as Velvetleaf (Aim, Cadet, Resource), volunteer corn (Poast, Select etc.), and most grasses (glyphosate, Poast, Select). However, when it comes to glyphosate-resistant species the options are very limited.
There are situations where someone “took out large resistant weeds” with a given herbicide. However, there are a lot more situations where these large resistant weeds were not controlled with the same herbicides. So, first, be realistic about your expectations. Do you want (or need) to kill the weed that is in the field? Do you want to limit (or stop) seed production? Alternatively, do you just want to set it back to limit its competitiveness?
If looking at killing large Palmer amaranth or common ragweed, do you have the right genetics in your soybeans. It’s July, so unless you are in a unique situation dicamba and 2,4-D should not be discussed. ALS-inhibiting herbicides like Classic, Synchrony, FirstRate or Pursuit are generally not an option due to resistance.
If you have soybeans with the Liberty Link or Enlist trait, you can spray Liberty. Otherwise, the options are limited to Reflex, Ultra Blazer, or Cobra and they are not good choices since they are not very effective on large weeds. All of these products are contact herbicides and require good spray coverage (high gallonage and medium sized droplets). If you need a high level of control, then it may require two applications; and the applications should be made within 7 to 10 days of one another. In on-going trials with Liberty, we did not see any benefit with including Cobra or Reflex as a tankmix partner. Refer to the labels for allowable number of applications (Reflex can only be applied once every two years), total amount of product allowed, and allowed crop stage for late applications.