Title: Weeds AR Wild, Ep. 02: Resistant Palmer amaranth (pigweed) discussion and 5 key points for management plans in Arkansas row crops (3-10-21)
Arkansas Row Crops Radio, providing up-to-date information and timely recommendations on row crop production in Arkansas.

Tom Barber: Welcome to the Weeds are Wild podcast series as a part of Arkansas Row Crops Radio. My name is Tom Barber & I am an Extension Weed Scientist for the University of Arkansas System Division of Agriculture.

If you paid any attention to social media or read popular press articles, you may have heard or read about the recent finding of glufosinate-resistant populations of Palmer Amaranth, commonly called pigweed, in Arkansas. 

Today I will be discussing key management points for growers or consultants to be thinking about prior to making their weed control program decisions for this upcoming season. Many of these you probably already heard before, but they bear repeating.

First, let's get one thing straight. Glufosinate or Liberty resistance is not widespread at this time. Currently we have confirmed three separate populations. Two in Mississippi County & one in Crittenden County. Likely there are other fields out there that contain populations or populations of pigweed that have some level of resistance to glufosinate, but this issue is not widespread by any means at this time. 

In addition we know that pigweed populations that are resistant to group 15 herbicides, specifically metolachlor, have also been identified in isolated areas. But both the metolachlor resistance & glufosonate resistance, let me repeat, are not wide spread at this time. They are both found though in northeast Arkansas. In addition, pigweed resistance to PPO herbicide, such as Flexstar, Valor or Sharpon, has become more & more prevalent over the last couple of years. Especially in Northeast Arkansas. But statewide resistance to PPO herbicides is still not as widespread as what we see with glyphosate or Roundup resistance at this time. 

In addition, I spoke with my counterpart, Larry Steckle recently & he tells me that dicamba resistance has been confirmed across the river in Tennessee. So after only 3 seasons following the release of the Dicamba technology, a resistance has been identified & confirmed in a neighboring state. Again I feel like a broken record, but this continues to drive home the point that relying on a single herbicide mode of action postemergence, is not the answer for pigweed. A systematic approach is needed. It is though, very important to know what herbicides will or won't work on the pigweed populations on your farm. 
However, regardless of which resistance you think you might have, the following five key points of recommendations still apply. And if you don't think you have PPO, metolachlor or glufosinate resistance at this time, it is still critical that you follow these key points, rotate your herbicide modes of action to reduce the selection pressure from any single herbicide group.

Alright, so again, five key points here. This is going to be regardless of crop planted. Now some herbicides that I mentioned will be specific to crop, but regardless of crop that we plant, #1 thing that we need to make sure & do is start clean. We can do that a couple of different ways through tillage. Most recently in our stale seedbed systems, a lot of tillage is not an option, so we need to start clean by using nonselective herbicides or a herbicide that is active on the pigweed populations at planting. Most common herbicide use at planting is paraquat or Gramoxone. Second most common is probably Dicamba where we're planting Xtend crops or Xtend technology. So either paraquat or dicamba at planting to make sure that we are clean & getting started on a good note there. 

Point #2. Residual herbicides at planting with that, either paraquat or dicamba application. These are going to be your pre-emerge herbicides. What we've found over the years with these multiple resistant populations is two preemerge herbicides are much better than one, so multiple is better. 

If we're in soybean, we've said over the last several years that metribuzin needs to be in that system upfront with soybean. And again, that requires us to do our homework & make sure that our soybean cultivar that we're planting is tolerant to metribuzin. In the past we've recommended a group 15 with that. But really, if you're in a non PPO resistant area, this could be a Valor or flumioxazin  with metribuzin. The point is metribuzin belongs in the system for soybeans. 

If we're in a cotton system, this is where  Cottoran or Caparol or Diuron or Brake belong upfront. Again, two of these are better than one. And continuously in our research, what we're seeing is 1pt/A of Cottoran followed with, not followed with but plus a 1pt/A of Brake, has been a cadillac treatment on our lighter soils the last several years. So mixing two residuals instead of one upfront has given us better residual control of pigweed in both cotton & soybean. And again, this is regardless of the resistance levels on your farm. You know also, we've talked about crop rotation before, so if we're rotating these fields to a corn system, it may be a little easier to manage these pigweed populations in corn, but we still need that upfront residual herbicide followed by a timely post even in a corn system. So I just wanted to throw that bit out there with this particular point.

Point #3. Timely post applications. It doesn't matter if we're talking about an Enlist system, a Liberty system or an Xtend system using dicamba, pigweed size is very important in terms of how successful we are in controlling that pigweed. Pigweed that are 5 inches or less are much easier to control regardless of the technology we are using than pigweed that are larger than 5 inches tall. And so, we've also said in the past that in the Enlist system being able to tank mix glufosinate with Enlist One has also been very effective in one application controlling our pigweed population. But many times what we find if we miss it early, meaning we spray when the pigweed are too large, we're going to miss those. And then sometimes when we come back with our second application, we miss them as well. So that first application is critical in season long pigweed control, but we need that timely application post emergence prior to pigweed getting much more than 5 inches.

Point #4. Use residuals with that post herbicide application. And again it doesn't matter which crop we're talking about here. Most of our pigweed fighting is done in soybean, cotton, corn & peanut. So using these residuals posts or using these residuals post, are a key in our overall management of pigweed. And we're talking about our group 15 herbicides here. So that's Anthem & Zidua, which contain pyroxasulfone. It's very effective on pigweed populations. This would include Dual, Outlook, or Warrant Now we mentioned earlier that we have some group 15 resistance. Specifically to metolachlor in scattered areas of the state in Northeast Arkansas. We have found on these populations that products like Anthem or Zidua or Outlook, in that group 15 class, seem to work better on these metolachlor resistance populations than Dual, which is metolachor, so that makes sense. But one thing's for sure, we need to rotate these where possible & use multiple of these with each of our posts. So we put one out early post, mid post, etc. And again, like I said we have used them upfront pre with metribuzin in soybeans.

Point #5 would just be remove the escapes that we have towards the end of the season. We know that a pigweed plant without competition may produce more than a million seed. In competition with a crop more like between a hundred & three hundred thousand seed. That's still a lot of seed. If we're ever going to get over our pigweed issues, we're going to have to start focusing on the pigweed seed bank. Pigweed has one weakness that I've found. Actually two. One is water, so if we're growing rice or rotating rice, we can usually flood them out. The second one is seed longevity. Pigweed is not going to survive much more than three or four years in the soil. And so if we can manage that seed soil bank. We'll be ahead that following year on our pigweed population. So whatever it takes removing those plants, rouging them from the field, if we can mechanically move them, fine, but we need to reduce the seed numbers going back to the seed bank with these pigweed populations. 

So again, just recap. We're going to start clean, we're going to use residual herbicides & we're going to use more than one. Again depending on the system, we're going to pair two residual herbicides together with either Gramoxone or paraquat or dicamba in Xtend crops. We're going to use timely post applications, we're going to time our post before those pigweed get bigger than 5 inches tall. We're going to use a residual with that post application & then we're going to be back with our second post within a 14 day window. And a lot of times, 10 days may be better, just depends on thecontrol or the activity you get on your first application. 

Again, we're going to use those group 15 residuals with those post applications & we're going to remove our escapes prior to harvest before they make viable seed that they raindown on the soil.

So again, I thank you for listening today. We're always available for questions. Phone or e-mail. Join us next week as Dr. Jason Norsworthy will be discussing barn yard grass herbicide screening results, & best control options for getting a jump on grass & sedge in our rice crop. 

But I want to thank you for joining us for this episode of the Weeds are Wild podcast series on Arkansas Row Crops Radio.


End notes: Arkansas Row Crops Radio is a production of the University of Arkansas System Division of Agriculture. For more information please contact your local county extension agent or visit uaex.edu.