April 12, 2013
The auxin herbicide (dicamba, 2,4-D) tolerant traits in soybeans will likely become available to at least a limited extent next year.
Specifically in soybean the herbicide tolerance package will be either dicamba + glyphosate which will be marketed as Roundup Ready 2 Xtend or 2,4-D + glyphosate + glufosinate marketed as Enlist.
The tolerance in cotton we are told will be about a year or so behind the soybean.
There is a lot of hope that these new technologies will help us rein in Palmer amaranth and marestail. There is also a host of questions about how they can best be utilized. We have a number of years of research in soybean and cotton designed to address these very questions. Based on this research I have no doubt weed management utilizing these technologies will help us more consistently control Palmer amaranth and marestail. However, until the summer of 2012, I was not sure how the whole system would fit together.
It is always best to evaluate weed management programs over a series of years and locations because you can learn something in every environment.
This past year was very valuable from a research perspective, as it clearly showed the value that an auxin herbicide can bring in soybeans and cotton from a soil applied residual stand point.
I am going to speak of dicamba from here on because we have conducted more studies on what it can bring to the table from a soil applied residual stand point than we have with 2,4-D.
During the very dry spring of 2012 we evaluated dicamba at a quart per acre (1 pound per acre) pre applied in soybeans compared to the standard many use today of Valor at 2 ounces per acre. We received essentially no rain for the first month after planting and the Valor treatments never got activated and quickly became grown up messes of Palmer amaranth. However, the dicamba at the 1 pound rate provided great (95 percent) residual control of Palmer for almost 30 days. Now, we had follow-up treatments of a pint per acre of dicamba or quart of Prefix among others applied early post emergence.
Regardless of the early post treatment we never could control the Palmer that blew through the non-activated Valor treatments. However, we were able with all the post treatments (both dicamba and conventional herbicides) to control the smaller and much fewer Palmer amaranth in the dicamba pre treatments.
The results of this research, along with walking many soybean fields last spring that were injured from dicamba applied 30 to 10 days before the soybeans were planted, convinced me that one of the biggest values this new technology can offer is in providing good residual control of broadleaf weeds in dry springs.
We also know from previous years that dicamba does not last long in a wet spring. However, most other pre applied herbicides we utilize today do work better in wet springs. So in the new dicamba-tolerant soybeans utilizing a tank-mix of dicamba plus say a Valor or Dual or Warrant or even the new Zidua applied pre-plant or pre emergence should provide much more consistent residual control of Palmer amaranth than what we can do today on our dry land acres.
Of course, dicamba can be sprayed over the top of dicamba tolerant soybeans. However, there will be fields or environments where dicamba should not be sprayed post emergence due to off-target concerns and a traditional PPO herbicide like Flexstar or Cobra could be used instead. These conventional herbicides should work better just because the pre will have worked. Moreover, dicamba utilized this early will minimize off-target movement concerns because sensitive crops are less likely to be present.
I really feel the dicamba-tolerant soybean system could look very similar to what we do today in Tennessee where most acres receive dicamba 30 to 14 days before planting. It will be a little different in that the rates will be higher and can be applied closer to planting than what we can safely do today. Moreover, we will have the flexibility to utilize dicamba in-crop but traditional herbicides we are currently using in-crop like the fomesafen-based products (Flexstar, Prefix) or Cobra should work well in this system too.
In my mind, one of the most intriguing aspects of this new technology is what herbicides like dicamba can bring to the table from a residual point of view. I feel it will be a key in best utilizing this new technology.
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