As LibertyLink acres increase, protect the technology

Bob Scott, University of Arkansas 

This year we with the help of an assay provided by Pat Tranel and Aaron Hagar, weed scientists at the University of Illinois have confirmed that at least five counties in Arkansas have populations of pigweed that are resistant to the PPO herbicides (Blazer, Cobra, Flexstar, etc.) or group 14 family of chemistry.

This is not good news, because we have come to rely so heavily on them (specifically fomesafen, the active ingredient in Flexstar) as our post option for pigweed in both conventional and Roundup Ready Soybean. The development of PPO resistance will severely limit options for those growers in 2016. As you drive through Arkansas you will see that the problem may be worse than we know from testing. I have been accused of being an alarmist, but the fields look a lot like they did back in 2010. They are just as variable from one field to the next as they were back then in terms of how grown up they are in pigweeds, when Roundup resistance really broke out.

I fear we may be at the start of widespread resistance to post applications of fomesafen (Flexstar), our most widely used last-shot tool for pigweed control in soybeans today.

Post and pre

There is still some debate as to whether or not resistance to post applications of group 14 PPO herbicides like Flexstar (fomesafen) equals resistance to pre applications of group 14 PPO herbicides like Valor. Other than circumstantial evidence I do not have any data yet.

This is something we will be looking at in the greenhouse over the winter. This is important because we have come to rely on Valor and the Authority brands (products with the group 14 PPO herbicide sulfentrazone in them) heavily for pre-emerge pigweed control. But whether they are resistant to either pre and post PPO applications or just post ones does not matter to me at the moment the time to act is now.

If you have not you must include products other than group 14, PPO herbicides in your pigweed control programs for soybeans. I have been shocked at the number of growers who told me that Valor followed by Flexstar or Flexstar and Roundup was their herbicide program after all the training we have been doing on weed resistance management.

Again, both Valor and Flexstar are group 14 PPOs and offer no alternative modes of action in that system because pigweed is already resistant to Roundup.

I will be recommending that all growers try to incorporate both a mitotic inhibitor or group 15 herbicide (Dual, Outlook, Warrant or Zidua) and some metribuzin (group 5) in their programs in 2016 for pigweed resistance management. Outside of Liberty (group 10), these are the only two modes of action left that are effective on our resistant-pigweed and to which we do not have documented resistance in other areas, yet.

More coming

There are a lot of premixes available that include these two groups and more products are on the way. I know that new herbicide-tolerant varieties are also on the way that will let us use 2,4-D, dicamba and the HPPD chemistries on soybeans, and these products will be very effective for pigweed.

But until these varieties get global approval and completely go through the regulatory process, we will not be using them. I will go forward with recommendations based on what I know farmers can get and use in 2016.

I am hearing from several of seed company representatives that interest in LibertyLink soybeans is at an all-time high. Based on conversations that I have had with some growers, it would not surprise me if these acres doubled in Arkansas and the Bootheel of Missouri next year.

This is a dramatic increase. We need to be careful as we move into a glufosinate (Liberty) program so that we don't just start selecting for resistance to that product as well.

When used properly, Liberty will control pigweed and in our university trials the varieties have been as good or nearly as good as the Roundup Ready varieties we have looked at. Sometimes even better.

Not Roundup

I do not support those who claim there is a yield drag with this technology. However, Liberty is not Roundup and must be applied early to small pigweeds (1 to 2 inches) to be effective.

In addition, I do not recommend that Liberty ever be the sole herbicide used in LibertyLink soybeans. In fact, I prefer to start with a good residual program with two modes of action as mentioned above and then come back on small pigweed with my first Liberty application.

I like to tank-mix Liberty post with a group 15 mitotic inhibitor for extra residual on pigweed and grass. I also think that tank-mixing the final application with Flexstar is the one last step towards preventing resistance that should be considered, even though we are seeing some resistance to Flexstar. Why? Because it is our last post option and the best we can do for resistance management in LibertyLink soybeans.


Many states, including Arkansas, are looking into pigweed populations that may have already started to express some "tolerance to glufosinate. We can kill them, but it seems to take a higher rate on some populations than others. This tells me that we cannot fool around with single and sequential applications of Liberty alone, or we may be having a discussion next year about glufosinate resistance instead of PPO resistance.

We must apply Liberty early and use residuals, period. This is especially true if, in fact, Liberty acres increase to the degree that I am hearing.

The main funding for my soybean research program comes from Arkansas soybean producers through the Arkansas Soybean Promotion Board. Without this support my work would not be possible. I also accept industry support and collaboration and regardless of what you may have read in the press lately, these collaborations are good for agriculture and the environment. The day I feel I cannot provide an unbiased opinion to help Arkansas farmers, I will do something else.

Article used with permission. Bob Scott, University of Arkansas | Delta Farm Press