Resistant pigweeds — residual herbicides
Sep 3, 2009 11:02 AM, By Ford L. Baldwin, Practical Weed Consultants, LLC.
I hope I did not end last week’s article on a negative note when I wrote that I did not recommend Valor-containing products at planting — that was not my intent.
The pigweed situation in soybeans is going to cause a lot of farmers to learn or re-learn a lot of things as we move away from glyphosate-only programs for control. If you have a resistant pigweed problem and plant conventional soybeans or Roundup Ready soybeans, your program is going to require the use of residual herbicides.
If you plant LibertyLink soybeans, even that program can benefit from the use of a residual treatment.
With residual herbicides, not everything that happens with them is always good. I sometimes use the old football cliché about a pass play — three things can happen and two of them are bad.
The good thing that can happen with a residual herbicide is you apply the right one, get a nice rain on it at the right time and you get excellent weed control with no crop response.
This was an excellent year for soil residual herbicides in soybeans. When you attend University Field Days and look at the pigweed plots, most of the residual treatments look very good.
However, from our experiences with residual herbicides since the 1960s, things do not always go by the book. If you apply them and do not get activating rainfall within five or so days after application, during the planting season, a flush of pigweeds will emerge. The flush that emerges with the crop is the most competitive and it is also the flush that puts the most pressure on your postemergence program.
If, therefore, you plant, spray a pre-emergence treatment, and it does not rain in five days or less, you are in a firefight from the start.
The other thing that can happen with residual herbicides is you sometimes get a crop response. In the days when we had to use a lot of residuals and a lot of postemergence herbicides like Flexstar and Blazer, farmers were used to seeing some crop response.
Roundup Ready caused most growers to forget what a real crop response looked like.
As I frequently state, we have a generation of farmers, county agents, and consultants that does not know any soybean weed control program except you spray the weeds with glyphosate and they die.
This gets me back to where I ended last week’s article about the Valor-containing products. Apply them around 14 days prior to planting is an excellent way to begin a pigweed control program. The preplant application gives you two weeks to get a rain for activation before you plant.
In addition, the chances of a crop response are much less compared to an at-planting treatment if a rain occurs just prior to or at emergence.
Some growers this year applied Valor at what they thought was 14 days prior to planting and it turned out to be six weeks or more due to continuous wet conditions. The result was the treatment “broke” right about the time they actually got to plant. That is just another fact of life with residual herbicides.
If you do not get the preplant treatment applied, and choose to use a pre-emergence treatment at planting, Prefix can be an excellent choice. It is a mixture of Dual and Reflex. If you get it properly activated it is a very good pigweed treatment.
Authority MTZ also looked good in the University of Arkansas research plots I have seen this year.
There is another aspect of residual herbicides we have not had to worry about for years, and that is carryover. If you choose Prefix, for example, you may limit the amount of Flexstar you can use later to avoid a carryover into a rotated crop.
I will pull the thoughts on residual herbicides back together into a control program for conventional soybeans next week.
e-mail: ford@weedconsultants.com
