Title: Weeds AR Wild, Ep. 03: Current Status of Herbicide-Resistant Weeds in Arkansas Rice (3-17-2021)

Arkansas Row Crops Radio, providing up-to-date information and timely recommendations on row crop production in Arkansas.

Jason Norsworthy: Welcome to the Weeds AR Wild podcast series as a part of the Arkansas Row Crops Radio. My name is Jason Norsworthy & I'm a weed scientist with the University of Arkansas System Division of Agriculture. 

Today, I'd like to talk to you about the current status of herbicide-resistant weeds in Arkansas rice. And next week Dr. Tommy Butts is going to follow with a podcast on the control options for each of the weeds that I highlight today.

I think many on this podcast would agree & also based on the survey work that we recently conducted that barnyardgrass is one of the most problematic weeds that we have in Arkansas rice. When we take a look at barnyardgrass today, it has resistance to several herbicides that we typically use in rice. 

First, it's resistant to propanil & that resistance was documented back in the late 1980's, early 1990's. And today better than 50% of the samples that are sent into me each year test positive for propanil resistance. 

Shortly after finding propanil resistance, a lot of growers in Arkansas switched over to quinclorac or Facet. And today we also have wide spread resistance to this herbicide & similar to what we see for propanil, better than 50% of the samples that I screen each year test positive for quinclorac resistance.

One thing I want everyone to realize when we talk about quinclorac or Facet is that the herbicide, while it's not effective on a resistant population from a postemergence standpoint, it still does have utility from a residual standpoint & it's not uncommon for us to see a 60-70% control over the resistant biotype several weeks after we spray quinclorac preemergence, or from a residual use of that herbicide. So we still have some utility of that herbicide even on resistant populations.

The next herbicide to which we have resistance within the state is clomazone. Today we have about ten fields in the state of Arkansas for which we have clomazone resistance. A lot of folks ask me why is it that we don't have wider spread of clomazone resistance like what we see with propanil, quinclorac & some other herbicides. And I really think the reason is that we start out with all of acres with clomazone & then we begin to follow that with other herbicides & in instances where we may have a clomazone-resistant barnyardgrass, we do a very good job of coming in & trying to control those plants & prevent seed production of those clomazone-resistant biotypes. 

The next case of resistance that we found in the state was ALS-resistant barnyardgrass. When I say ALS resistance, that could be resistance to herbicides like Newpath, Preface, Beyond, Postscript, Regiment, & Grasp. And today in the samples that we have come in or in the last couple of years, we're seeing that about 40% of the samples will test positive for some type of ALS resistance. I want you to understand that 40% of the samples testing positive does not necessarily mean that 40% of the barnyardgrass populations in the state are going to be resistant to the ALS chemistry. Instead I think that number is probably going to be somewhere closer to between 10-15% of the barnyardgrass that we have across the state is probably going to have some type of ALS resistance within the field. 

I'm going to come back here in a moment. We're going to talk a little bit more about ALS resistance & what that means. After we found ALS resistance, we then found some ACCase resistance within our barnyardgrass populations. 

We have resistance to herbicides like Clincher, Ricestar, & Provisia. And what we have seen the last couple of years is that the amount of resistance that we have in the samples that we have sent into us is generally less than 10%. So overall, I would contend today that ACCase inhibitors have a tremendous amount of utility on a lot of barnyardgrass populations across the state, even though there are some resistance that does exist within this weed. 

The last herbicide for which we have barnyardgrass resistance is going to be florpyrauxifen-benzyl or Loyant. Loyant was a herbicide commercialized in 2018 & what we quickly saw was that we had some populations scattered across the state that did lead to failure of this herbicide. 

Now I want to come back & talk a little bit about these ALS inhibitors because a question that I commonly get from a lot of folks is, well when we find we have some ALS resistance in the field, does that mean that barnyardgrass is going to be non-responsive to all of the ALS herbicides that we have at our disposal in rice? The answer to that is not always. When we look at the herbicides that we have, ALS herbicides for use on barnyardgrass, we have herbicides like Newpath & Preface, Beyond, Postscript. These are imidazolinone herbicides. We also have Regiment, which is a pyrimidinylthiobenzoate herbicide. And then we have Grasp, a triazolopyrimidine herbicide, & lastly, we have League, which is a sulfonylurea herbicide. So, these are 4 different families of ALS herbicides that are available in rice for the control of barnyardgrass. What we have seen in our research is that sometimes when we have resistance to Newpath or Beyond. Sometimes that is also going to result in resistance to herbicides like Regiment & Grasp, but not always. 

Now in my barnyardgrass screening program, we currently only screen against the ALS herbicide Newpath. And with that said, we don't really understand whether we have resistance to Regiment or resistance to Grasp when we have an ALS-resistant population that has been confirmed resistant to Newpath. So what I would tell growers across the state is there is some likelihood that Newpath is not working, Beyond is not working, because of ALS resistance. There is a good likelihood that Regiment & Grasp will not work. However, we cannot say with certainty that that's the case. And really it depends upon the mutation within the ALS gene & what the substitution is within that gene as to whether we're going to have resistance to all ALS herbicides or just certain ALS herbicides. The best way of going about & kind of putting this in layman's terms or giving an example of this is, if we look at STS soybean, many folks will be familiar with STS soybean & you'll understand that we could spray Permit or halosulfuron, over the top of STS soybean. So soybean has some ALS resistance within it. Soybean is naturally tolerant to herbicides such as Newpath & Beyond. But if we drift Regiment or Grasp over the top of those beans, or even spray 1X rate of Regiment or Grasp over an STS soybean, it's going to result in death of that soybean. In other words, we don't have tolerance to all ALS herbicides even though there's a mutation within that crop that allows sulfonylurea herbicides to be used over the top of an STS soybean. Be mindful that just because you get back a result from the sampling, the screening that we conduct each year where it says you have ALS resistance to Newpath, that does not necessarily mean that Regiment & Grasp are non-effective on that barnyardgrass population. 

Also I want to touch on glyphosate. We have a lot of interest in using glyphosate in our burndown. We screen against glyphosate in the barnyardgrass samples that are submitted each year & to date we have found no resistance to glyphosate, but what we've definitely seen over the last several years is an increase in the reduced sensitivity of these populations to glyphosate. Again, we don't have what I would consider field level resistance to date, but I can say with a high degree of certainty that some of our barnyardgrass populations out there are going to require repeat applications of glyphosate, if we are going to have effective control of them in the field.

I want to now move on to the second most problematic weed of flooded rice in Arkansas. And I'm going to put these together as a group of weeds, & we are going to call these the ALS-resistant sedges. Within this group we are talking about weeds like yellow nutsedge, rice flatsedge & smallflower umbrellasedge, which appears to be an increasingly common weed here in Arkansas rice. We take a look at the use of clomazone on most of our acres, really to set up the initial grass control. What that's leading to is an increase in the occurrence of sedges. Sedges are extremely tolerant to clomazone on the front end & for that reason, in most rice fields across the state, we're going to have some sedge that exists & we're going to have to control that sedge if we are going to be successful in managing it in rice. Permit or halosulfuron has been a mainstay in rice for many years now & as a result of our overusing that herbicide or using that herbicide repeatedly for the control of rice flatsedge & smallflower umbrellasedge, today we have populations that are resistant to Permit or halosulfuron as well as League, which is imazosulfuron. About 50% of the rice sedge populations that I find across the state of Arkansas are going to be resistant to the ALS chemistry. Same for smallflower umbrellasedge, which actually I'd say it's better than 50% of the populations are going to have some resistance to the ALS herbicides. Just want to note that we have now found in the state a couple of populations of yellow nutsedge that have ALS resistance. These are extremely sparse overall. I do think that herbicides like Permit, Gambit, Permit Plus, League, these are herbicides overall are going to still be very effective on yellow nutsedge, but there are some populations, a couple of populations in the state for which we have confirmed resistance. 

I want to move on now to weedy rice. Weedy rice is third most problematic weed in flooded rice in Arkansas. Really with weedy rice it's all about the ALS resistance that we find. In the samples that I screen on a yearly basis, we see that about 95% of those weedy rice samples that are sent in will test positive for ALS resistance. And this would be resistance to herbicides like Newpath, Beyond, Preface, Postscript. I want everyone to understand that if you were not effective in controlling weedy rice in a Clearfield system where Newpath & Beyond were used, switching to a Fullpage system & the use of Preface & Postscript is not an effective option for the control of ALS-resistant weedy rice. If you have resistance you're going to have to think about switching over to a system centered around the Provisia trait or Provisia rice. And then another option that we have in 2021 is the use of Gowan's new herbicide called Rogue. We have a Section 18 that was granted for Arkansas for the 2021 growing season & we are going to be able to use Rogue in a water-seeded system only on about 25 thousand acres across the state, so if you have some interest in trying to control ALS-resistant weedy rice or weedy rice that's not resistant to herbicides in a conventional system or in a Clearfield system, give Gowan a call if you have some interest in the use of Rogue. 

The fourth weed that I want to mention today is the second most problematic weed of furrow-irrigated rice & that is Palmer amaranth. Today 99% of our Palmer amaranth populations in Arkansas are ALS resistant. This means herbicides like Newpath & Beyond are not going to have any activity. A herbicide like League is not going to be an effective option. We also have PPO resistance throughout most of the rice-producing counties in Arkansas. If the PPO herbicides are still working, you ought to consider yourself extremely fortunate & if they are working, I would suggest that you try to rotate in some other herbicides. Consider some alternative herbicides like Loyant, which is very effective on Palmer amaranth. If you are in an area where 2,4-D is not restricted, consider using it in rice. If you're going to catch Palmer amaranth early, what we've seen is Grandstand plus propanil can be a very effective option & in most instances if you follow that again with Loyant, it can be very effective. But you need to mindful, if you have PPO-resistant Palmer amaranth you're going to see no postemergence activity out of herbicides like Sharpen & Ultra Blazer, & Sharpen is going to provide very limited residual control of PPO-resistant Palmer amaranth.

Lastly today I just want to mention Pennsylvania smartweed. A few years ago I confirmed a Pennsylvania smartweed population a few miles north of the Arkansas state line in a southern Missouri rice field & it was confirmed resistant to Regiment. We now have another population this year that was confirmed resistant to Regiment from a Jackson county rice field. So we have a population in Arkansas for which we're confident we have resistance. It's not only resistance to Regiment but we have also seen that we have resistance to Gambit on this resistant population. There are some other populations of smartweed across the state that we have some concern about, however it's been very difficult to get Pennsylvania smartweed seed to germinate in the greenhouse & get a good evaluation of some of these populations for resistance. What I would tell you is If you have ALS resistance or again if the ALS chemistry is not working, consider switching over to propanil plus Basagran & apply that early an realize you are going to need a sequential application of propanil plus Basagran if you're going to have success on large Pennsylvania smartweed. 

Again, I appreciate you joining me today for this podcast & I ask that you please join us next week to listen to Dr. Butts talk about some effective control of the weeds that I've highlighted today. And again thanks for joining us for this episode of the Weeds AR Wild podcast series on the 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