This week I have some general comments on soil residual herbicides. I have written in the past several articles that you should have realistic expectations for residual treatments.
Where you already have glyphosate resistance problems, it is doubtful that residual herbicides will do much to help you solve that problem. Rotating crops and rotating to an alternate mode of action like LibertyLink are going to be required there.
However, if you are in a Roundup Ready program and do not yet have a glyphosate resistance problem, or if you are planting your first LibertyLink soybeans, residual herbicides have a definite place.
There are several ways to make effective use of residuals that require rainfall for activation.
You can add a residual herbicide to a burn down herbicide applied before planting. Most weed scientists I talk to feel the ideal timing for this application is a couple of weeks prior to planting.
This timing represents a pretty good compromise between not applying it so early that it plays out too soon, and applying it so late that you may as well be using a pre-emergence treatment after planting.
An example treatment would be glyphosate plus Valor 14 days prior to planting. There are numerous alternative herbicides or herbicide mixtures containing Valor that can be substituted here. The early treatment has the advantage of getting activated prior to planting and, in the case of Valor, less chance of crop injury compared to Valor applied at planting.
Most of the Valor and Valor combination treatments will be more effective on broadleaf weeds than grasses.
Another use of residual herbicides is the pre-emergence treatment applied after planting but before weed emergence. Rainfall or overhead irrigation is required for activation — before the first flush of weeds emerge — for them to be effective.
Under normal weather patterns (whatever that is anymore) in Arkansas, pre-emergence herbicides are generally more effective earlier in the soybean planting season than later. Rainfall chances are generally better earlier in the spring and weed emergence is also slower.
The later in the planting season we get, the rainfall chances usually decrease and the weeds also emerge much sooner.
Example treatments are Dual (or comparable generic) or a combination treatment such as Prefix. Dual alone will be primarily a grass herbicide while Prefix, for example, can add greatly to the pigweed activity.
I hear a lot of talk that pre-emergence treatments “will buy time for the first postemergence treatment.” In order for this to happen, they first must get activated and they must be 100 percent effective. I do not count on this happening.
However, what they can do is reduce the number of weeds you have to fight postemergence and also reduce the re-infestation between postemergence treatments.
Another place you can often effectively use a residual treatment like Dual or a generic is tank-mixed with the first postemergence treatment. Examples would be glyphosate plus Dual in a Roundup Ready program or Dual plus Ignite in a LibertyLink program. While these may not be standalone treatments, if rainfall or irrigation occurs shortly after application, the residual will reduce the amount of re-infestation of grasses and small seeded broadleaf weeds.
In some situations you may choose to use a combination of residual herbicides in a Roundup Ready or LibertyLink program. For example, you may choose to add Valor or combination herbicide containing Valor to a burn down treatment prior to planting, and then either use a pre-emergence herbicide or add a residual to your first postemergence treatment. That may seem extreme to some and certainly will not be used in every field. However, I believe we are further into a herbicide resistance problem than others do; if we are, soybean weed control will not be as simple as it has been the past 10 or so years.