Larry Steckel, University of Tennessee Weed Specialist

 

When the non-farm press call for an interview it is always interesting.

The first thing they typically want to discuss is “superweeds.” There is a definition of superweeds in the Oxford online Dictionary which states that a “superweed is a weed which is extremely resistant to herbicides, especially one created by the transfer of genes from genetically modified crops into wild plants.”

In other words, Roundup created superweeds. This totally misleading and factually wrong definition is typically what many non-farm press think a superweed is. The truth is that Roundup did not create superweeds, but rather superweeds defeated Roundup.

Palmer amaranth is a great example of this as it was a superweed long before Roundup or any herbicide for that matter was used. The pigweeds in general, and Palmer amaranth in particular, have long been fellow travelers with man. They evolved in flood plains where spring and summer floods remove all the existing vegetation. They emerge and grow very quickly, produce an abundance of seed that can float and are very competitive if the water is limited as it often is in sandy areas of flood plains. 

Now, replace a flood removing all vegetation with tillage, or even Roundup, and you have essentially the same environment. It is no wonder the pigweeds and Palmer amaranth have gone on to cause agriculture so many headaches.

The developing resistance to herbicide, particularly Roundup, came long after Palmer amaranth evolved into a superweed.

The real question should be how do we stop, or more realistically delay, superweeds from developing resistance to any more herbicides? There is recent research that addresses this question, which studied the evolution of glyphosate-resistant waterhemp (a pigweed species) across the land scape level published in the Journal of Pest Management Science(Evans et al. 2015). 

The research examined factors related to landscape, weed and management from 105 Illinois grain fields, including over 500 site-years of herbicide application records. Results of this study indicated that management practices were the best predictors of glyphosate-resistant waterhemp among all the variables examined. The occurrence of glyphosate-resistant waterhemp was greatest in fields where glyphosate had been used in over 75 percent of the seasons, where fewer MOA (modes of action) were used each year and where herbicide rotation occurred annually. 

I was not surprised by the first two findings. However, the last one --where herbicide rotation between years increased likelihood of glyphosate resistance -- was surprising. In other words, simply rotating herbicide MOA from one year to the next actually increased the frequency of resistance. This may be due to the common rotation in central Illinois which is ether corn and soybeans or continuous corn. You would likely not have glyphosate resistance develop in continuous corn but you sure could have atrazine resistance and I understand this is very common.

In my mind, the most important result reported from this study was finding that exposing populations to multiple MOA through tank-mixtures greatly reduced the selection for glyphosate-resistant waterhemp. A field in which 2.5 MOA per application were used was 83 times less likely to select glyphosate-resistant waterhemp within four to six years than a field in which only 1.5 MOA per application were used. I have long thought that this management tactic is a major key to resistance management and now there is data that proves it.

In a year with low commodity prices it is a hard sell to tell growers that they should use two effective herbicides on Palmer amaranth when one would likely get them by this year. However, both from a resistance management perspective as well as consistency of control, utilizing two herbicides effective on Palmer amaranth is the way to go.

There are many good premixes in soybeans that can be applied PRE that contain at least two herbicides effective on Palmer.

From a POST emergence perspective in Roundup Ready soybeans, there is really only one effective MOA: the PPO herbicides (fomesafen, Ultra Blazer, etc.). However, adding cultivation or a chopping crew could effectively be another. In LibertyLink soybeans, adding Prefix in with Liberty POST provides several effective MOA on Palmer.

Article used with permission from Delta Farm Press