I ended the last article on pigweed control in conventional soybeans re-stating the need for residual herbicides but pointing out the need for a postemergence follow-up program.
The amount of pressure put on the postemergence program can range from light (if the residual program worked well and the pigweed infestation was not severe) to extreme or overpowering (if the pigweed infestation is severe and/or the residual program fails).
A postemergence program in conventional soybeans is essentially limited to Flexstar or Ultra Blazer. There are several ALS inhibiting herbicides that can be used postemergence in conventional beans. However we had developed so much resistance to the ALS inhibitors before Roundup Ready, I always assume there will be enough resistance there to prevent the ALS inhibitors from being successful when used alone.
Both Flexstar and Ultra Blazer can have good activity on small pigweeds. I prefer to use the word “tiny” rather than “small.”
I hear speakers in meetings and at field days stating that you can kill pigweeds up to 2 to 3 inches tall with Flexstar. In my experience, I could kill some of them that big sometimes.
My preferred timing on the first flush of pigweeds is 0.5 inch tall or less when the soil surface turns red when they first emerge.
The main problem we had back when we had to depend upon Flexstar, Reflex and Blazer for the postemergence pigweed program was there just was not much margin for error. In a year like this one, we tend to forget about the potential effects of drought stress (we dumped another 6 inches out of our rain gauge this morning). However in a more normal year, as a weed gets older or larger it, often gets more drought stressed.
Perhaps the best way I can summarize the Flexstar-type treatments is they can be good pigweed herbicides if the population is not overpowering and if you get them applied very early under good conditions. However, if you miss the timing a few days and if conditions become dry, the activity drops dramatically.
In the days before Roundup Ready, if we did not get the first postemergence application out within the first 14 days after soybean and pigweed emergence, it would fail and there was no fallback position.
In situations with severe pigweed populations, the treatment may provide 80 to 90 percent control, but the plant population numbers would simply overpower the treatment.
I have taken several articles to cover potential control in conventional soybeans because I get a lot of questions about the possibility. In addition, the comments I have made on control in conventional soybeans also provides the background for upcoming articles on pigweed control in Roundup Ready soybeans.
If you are thinking about planting conventional soybeans for pigweed control, my advice would be to pick fields that still have low pigweed populations or those where you can activate residual herbicides with overhead irrigation.
To be successful in a conventional program you will need multiple applications of residual herbicides and they must be activated within a few days after application.
If you do not have a good preplant residual treatment activated before planting, the at-planting treatment will need activating three to five days after planting. If this does not happen, things get interesting in a hurry. You will have a flush of pigweeds emerge with the soybeans with a fallback position of Flexstar or perhaps Ultra Blazer. These can be a good treatment but without much margin for error.
If you miss the timing or the environmental factors go against you and the treatment fails, it is essentially over. While the program can sound simple, it is difficult to get it to work consistently.
We do not yet have the pigweed plant populations built back up to where they were before Roundup Ready. Because of this I am cautiously optimistic that we can manage a lot of the resistant pigweed fields with conventional herbicides.
However, we also have a good pigweed crop going to seed in a lot of areas this year. Therefore, I will state again that while I am glad we have these conventional options available, one reason Roundup Ready took much of Arkansas by storm was because we could not control Palmer pigweeds in conventional weed control programs.
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