Bob Scott, University of Arkansas Extension Weed Specialist
There is a scientific process that weed scientists go through before we declare a weed resistant to a given herbicide. It involves looking at the suspect weed in comparison to a known “non-resistant” biotype of the same weed, looking at various rates and making sure that the resistance is heritable. The heritable part is why we grow plants out for two seasons and make sure the progeny are just as resistant as the parents.
My research counterparts at the University of Arkansas Campus in Fayetteville are working on these tests right now for PPO-resistant Palmer pigweed in Arkansas. They are not quite done yet, although it is becoming very apparent in the field that it’s here and possibly in other Mid-South states.
The PPO chemistry (group 14) includes: Blazer, Cobra, Flexstar, Prefix, Sharpen, Valor and Spartan/Authority. These products make up the more active ingredient on pigweed in many premixes such as: Envive, Prefix, Broadaxe/Authority Elite, and Fierce. These products, especially fomesafen or Flexstar, have basically been making it possible to grow conventional and Roundup Ready soybean for the past seven to eight years in the face of pigweed that is already resistant to the ALS chemistry, glyphosate and even Prowl and Treflan.
We have relied heavily on the PPOs for some time. A close second are the mitotic inhibitors such as: Dual, Outlook, Warrant and Zidua, to which little resistance has been documented thus far.
PPO-resistant pigweed species have already been documented in the Midwest and other areas around the U.S. and the world. But up until this year we seem to have been spared this problem. This year I am getting calls from all over.
I have been in several fields where it does appear that either Valor PRE or Flexstar POST, or both, have failed. The plants are burned and partially controlled but re-growing from the terminal and not being controlled. This can and does happen when applications are made too late, but I can verify at least two or three fields where the pigweed were legitimately 2 inches tall at application with good conditions for control. We retreated some in the field, and they also were “burned” but not controlled. Therefore, I am convinced it’s here.
Having a pigweed that is resistant to the PPO chemistry as well as glyphosate and the ALS chemistry is going to make conventional and Roundup Ready weed control almost impossible. There are very few options left. After thumbing through the MP44 I came up with a program approach in Liberty Link soybean: Boundry (Dual + Metribuzin) followed by Liberty. You could obviously substitute for the Dual with another mitotic inhibitor and add other products to it, but this is one program that would give me three modes of action to which no documented resistance occurs. It also might help prevent the seemingly inevitable development of Liberty or glufosinate resistance in Palmer pigweed.
In addition, growers are going to have to re-think some production practices. Wide-row soybean equals a heavier reliance on herbicides than drilled soybean. Twin row is better than a single row, 15-inch rows are better than 30-inch, etc. Narrow row spacing is better for pigweed control. Also, selecting a bushy versus more erect plant type can help.
Managing pigweed at or post-harvest can be critical to decreasing overall populations. Herbicide applications post-harvest to manage seed production, and the work that is going on in harvest weed seed destruction and post-harvest weed seed destruction will or has become more critical.
I wish I could tell you that the next best thing is just around the corner. I like the Enlist and the Xtend technologies, but until they gain market acceptance I would plan on trying to survive 2016 with current tools. My soybean research is funded by the Arkansas Soybean Promotion Board, and it is greatly appreciated.
Article use with permission, Delta Farm Press