I have spent a lot of time in the heart of pigweed country over the past few weeks. My most recent visit was to west Tennessee and the observations fit well into my comments on soil residual herbicides.
The person I met with farms a considerable acreage and also works at a retail outlet. He had taken the pigweed resistance issue to heart last winter and decided to change his program for this year. He used Ignite with a residual mix of Prefix and Sencor (another forgotten herbicide with some pretty good pigweed activity) on his Roundup Ready, LibertyLink and conventional soybeans at planting.
He also sold his retail clients on using a soil residual herbicide in their programs. Some were reluctant at first to spend the upfront money, but most did.
Boy, are they glad they did! Almost without exception all of the fields where a residual was used were clean, while I photographed hoe crews in some that did not. In fact the Ignite, Prefix and Sencor treatment worked so well for the guy I am picking on this week that the only postemergence herbicides required in any of his soybeans were for touchup work. He never had to spray any of his Roundup Ready soybeans with glyphosate.
Because residual herbicides require moisture, you can expect them to be erratic from year to year. This year we have seen them at their best. However, this year has also shown why we have to have them for pigweed control.
If you are going to attempt to plant conventional soybeans where you have pigweed problems, you must make a residual program work, because postemergence options are limited. To make a residual program work in conventional soybeans, I believe, will take multiple residual applications.
In conservation tillage programs, I like starting with a residual applied in a burn-down application a couple of weeks prior to planting. If pigweeds have emerged, consider switching the burn-down treatment from glyphosate to Ignite.
The Valor-containing residual treatments do not have much grass activity, so you may want to factor that into your pre-emergence treatment.
At planting, Dual (or a generic), Sencor, Prefix, Authority MTZ, or a mixture of some of these are among the options you have.
In a conventional program, Flexstar and Ultra Blazer are the only viable postemergence options you have, and Flexstar has the best activity. My advice would be to plan that you will need two applications. If so, you will need to use something other than Prefix pre-emergence to keep from exceeding the yearly amount of fomesafen you can use.
Some who plant using a conventional seedbed are considering trying some incorporated treatments. In some situations there can be a place for Dual incorporated, for example, or perhaps just going back and trying something like Treflan. Based on previous experiences, my guess is the results will be mixed.
Another place to get a residual into a program is to tank-mix Dual with the first postemergence application. It is labeled up to the third trifoliate stage of soybean growth. This treatment can sometimes provide a little boost the residual activity a little later into the season.
There are a lot of ways you use residual herbicide combinations in a program. I have tried to illustrate some examples. Perhaps I have beat the topic to death, but I have attempted to show them at their best both in university research programs this year and also in the field. However, you likely have seen them at their very best this year.
I also want to prepare you for the fact that sometimes they are not going to work. They were not the total answer to pigweed control before Roundup Ready and they will not be now. While it may sound contradictory, I believe you have to use residual herbicides in a pigweed control program, but you have to also assume they are not going to work. I will start here next time.