Weeds AR Wild Series, Season 2 Episode 14. 
Title: Weeds AR Wild Series, S2 Ep14. Rainfall for Residuals: How much and how long?
Date:  May 19, 2022

[Music]:  Arkansas Row Crops Radio providing up to date information and timely recommendations on row crop production in Arkansas.

Jason: Welcome to the Weeds AR Wild podcast series as a part of the Arkansas Row Crops Radio. My name is Jason Norsworthy and I hold the title of University Professor in Weed Science for the University of Arkansas System Division of Agriculture. And today I'm joined by Dr. Tom Barber, one of our Extension weed scientists. Today, we're going to talk about activation of residual herbicide. With rainfall, there's a need for activation of a residual herbicide. At times we've been wet this year, Tom, early on and now we're starting to get dry. And as we get dry, there's a lot of seed that's going in the ground and there's residual herbicides that are being put out at planting to hopefully try to provide us a window, two weeks, three weeks into that crop, before we have to come back again and make another application.

Tom: Yeah, Jason, that's right. And it's crazy. This year, we were so wet for so long, then all of a sudden we've had an almost two week window now where we've been able to get work done. And I swear we can plant our 3.5, 3.6 or however many. Well, we’ve got more than that. Probably close to 6 million acres total, I guess, in the state. We can plant it all, I'm convinced, in about ten days. But everybody's been out working hard, getting the crop in where they can. And we've gone from way too wet to a drought almost in the matter of that time. So, it can change quick.

Jason: It is amazing. I’ve been in eastern Arkansas this week and over the last two weeks, two weeks ago, we had very little crop in the ground. Just overall I'm going to say, if you think of cotton, corn, soybeans – all the crops, I don't know if we were 10-15% planted two weeks ago. Just getting back in here late last night, it's just amazing to see where we are and how much crop we’ve planted in the last two weeks. And with that, again, there's a need to be putting residual herbicides out at planting because really that's the foundation. Your weed control program all starts with a residual herbicide at planting, and you've got to get that herbicide activated. And we haven't had a lot of rainfall over the last two weeks to activate those residual herbicides.

Tom: We really haven't, and it's been spotty where we have got some rainfall. It's been kind of spotty here and there. And you know, that's been one of the number one questions this week or over the last week is, “I'll put this out, whatever it was – you name your crop,” and we recommend residuals on all of them. “How long can it sit there? How much rain do I need to get it activated? If it if it stays out there seven days before activation, what happens?” So, I think this is a timely topic just based on the questions I'm getting. I say we back up and let's just talk about what is a contact versus residual herbicide and just talk about some of the differences there.

Jason: So when we go out and we're at planting and we make an application of a herbicide, if we have vegetation up in the field, we're trying to use a post-emergence herbicide to kill that vegetation. And, post-emergence herbicides, some of those have what we call residual activity. We may also spray some herbicides that have a residual activity and no post-emergence. In other words, they're only going to kill weeds as they're germinating and emerging. So let's start with activation, because I think that's really the cornerstone of what we're going to talk about today. What is activation? And so, activation just means that you're getting that herbicide, a residual herbicide, in solution in the soil so that germinating seedling can take up that herbicide as it's emerging. A misnomer for a lot of folks out there is they think that residual herbicides actually kill weed seed, but that's not true at all. If we actually kill weed seed, we would never have weeds emerging if we sprayed a residual herbicide, but those herbicides only kill those seedlings that are germinating in the soil seed bank and emerging. And that herbicide has got to be in solution in the soil so that the seedling can emerge through that treated barrier. So, the key component there is we need moisture. When we think about activating a herbicide, you mentioned here a few moments ago, well, seven days, eight days, nine days. When we apply residual herbicide, the longer that herbicide lies there on the soil surface without rainfall, the more and more of that herbicide that we're losing. And folks, that’s really a function. It differs from herbicide to herbicide. Some herbicides are broken down by light. Temperature can be important in the loss of some. We lose some due to volatilization. And so, the sooner that we can get a rainfall event close to that residual herbicide application, the more that we're going to have. So, when an individual says, “Well, can it lay there for seven days before we have a rainfall?” Well, yeah, it can lay there for seven days, but you might have 50% of that herbicide gone at the end of seven days. If you back up to three days, you're going to have more herbicide there at three days. You're going to have more herbicide there at one day. So, it's all about trying to get that herbicide activated as soon as possible. Also when we think about activation, rainfall is really not the only means that we can go about activating a herbicide. We can actually use irrigation. If you're planting on beds, no one really wants to roll out the poly pipe to try to get a crop up. But we've done a lot of work where we take a residual herbicide and we use furrow irrigation to activate it, and it's actually been very effective in activating most of our residual herbicides. I would contend that when you use a product post-emergence, like for instance the Metolachlor products, the Duals, the generic-type Dual products, the Warrants, things like that – our Group 15 herbicides. If you're coming over the top of cotton, you’re coming over the top of beans or even corn and you're using a residual herbicide over the top, and that crop is up, and you've got poly pipe out there, hey, turn that poly pipe on and start trying to run some water through that field just as soon as you make that herbicide application. Because we found out it really doesn't matter whether you're in the furrow or you're on top of the bed. If you can wick the moisture up to the top of that bed, you're going to be very effective in terms of activating those herbicides. What are your thoughts on some of that, Dr. Barber?

Tom: No, I agree. And that's some of the tricky part, is trying to slow down enough to roll some of that pipe out. But, I've had a lot of conversations with a lot of growers or farmers this week, and there's a lot of poly pipe being rolled out right now just for that reason. Especially, in row rice where we have to rely on so many more residuals it seems like in that type of environment. But we have ways to do it. It's not a great time to have enough people to get it all rolled out. So, we all want to time it by rain. But it is possible. We've done that as well in our research. We've been able to do it that way.

Jason: I know the word flush is a four-letter word in rice. But again, at times we need to flush these herbicides and we need to make sure that we have activity. Another misnomer, I think for a lot of folks, is that, “I activate a herbicide. So, once I have activated that herbicide, is it going to stay active?” And the answer to that is “not exactly.” Because as I said earlier, you've got to have the herbicide in soil solution. So as that soil dries and you have less and less moisture in there, you're going to have less and less herbicide that's actually in soil solution. More of it's going to be bound to soil. And as you lose soil moisture and you have more and more herbicide bound, there's less herbicide available for those seedlings that are germinating and emerging within that field. And that's the reason why also we need irrigation, why we're going to have to flush at times – because we've got to continually supply moisture to that field to keep that herbicide in soil solution. Now, I don't want to sit here and say, I mean, on some of our soils we can get too much water. I mean, some of these herbicides are extremely soluble. They don't bind tightly to soil. I’ll give you one example I know, me and you were talking about earlier, and that's herbicides like Dicamba and 2,4-D. Those are two herbicides that they're very, very soluble in water. And they're extremely active even on a dry soil, just due the solubility of those herbicides. We start getting water on a herbicide like dicamba or 2,4-D, we start driving those herbicides deep into the soil. And with that, we begin to lose the activity of the herbicide. So that's one thing. Those are two herbicides that I really like from a pigweed standpoint, in the sense that I can put those on dry soil and get some activity out of those herbicides. Whereas my other herbicides, things like our PPO inhibitors, things like our Group 15, our chloroacetamide type herbicides, our PSII herbicides – I'm going to have to have a minimum of a half an inch, if not an inch of water. Several of these herbicides, we need at least an inch of water to really get them fully activated. And I can basically have with 2,4-D/Dicamba, I can have some activity up front while I'm waiting on moisture with these herbicides that I just mentioned here previously.

Tom: That's right. And, you know, that's some of the frustrating calls I've gotten over the last week, is they’ve spent a lot of money. As a cotton example, going out with Brake and Cotoran, which has been a Cadillac treatment really for us over the last several years in all of our cotton weed control comparisons. But you've got to get rain to get those products activated. You spend a lot of money on those residuals. We've got a seven, eight, maybe nine-day period here where we haven't gotten any rain. We're going to have pigweeds and other weeds break through that, just because there's been no activation. And, that's another reason why I think pairing these residuals together is good, because in that same scenario, you've got Cotoran, which is a little more water soluble than Brake is, I guess. And it can be activated with less rainfall than Brake. Brake generally requires a little more rainfall than Cotoran. And so they work together pretty good in that type of scenario.

Jason: I really like what you said, pairing herbicides together. And, that's why I like at planting a Dicamba. I like Dicamba at planting because I can mix it with a herbicide, whether it be cotton, whether it be soybean, I can mix it with a herbicide that’s going to need a good bit more rainfall to get going. And then the other thing is, we talk about resistance management a lot for these podcasts. Again, just having two herbicides that’s targeting these weeds. Weeds like pigweed, definitely it’s going to be a strength for us.

Tom: Absolutely. And I think just as a system, it works better when we – without even talking resistance. Which we know with resistance, we need them. But we seem to get better activity broad spectrum, not just on Pigweed when we have some paired residuals up front regardless of the crop. And so, the next question I get sometimes, Jason, is people think, “Well, I got to plant my crop first before I can spray.” Or, a lot of farmers might think that. “I’ve got to plant my crop and then I can bring the sprayer.” But we're rolling into a weekend now where we've got a pretty good chance of rain coming up Friday night through Saturday, maybe, it looks like across the whole state. You never know. But we can always go out in front of that planter, right? And spray our residuals out and get those activated and then drop the planter in there and plant.

Jason: We can. Back a few years ago, I was a little bit skeptical. We did a lot of work at several locations, several site years where looked at pre-plant herbicides going out 7, 14 days prior to planting and came back in and compared that to a true pre-emergence where we plant and we spray the day of planting or maybe we spray within a 24 hours of planting. And you know, my thought going into that was, “Well if I spray 14 days prior to planting, I've lost 14 days of residual from the time that I plant on. I've wasted 14 days of that herbicide.” So, I was a little bit skeptical about recommending some of these pre-plants and I put a lot of emphasis on pre-emergence herbicides instead. And, I was really shocked when I saw the results over a lot of site years. Consistently, the pre-plant herbicides outperformed the pre-emergence herbicides. And the reason, looking back at it now, I know exactly why that happened. We put a pre-plant out and we have 7 to 14 days. In a 7 to 14-day period, we're more than likely going to get that pre-plant activated. We're going to have a rainfall event that's going to activate that pre-plant. So now, we're planting into an activated herbicide. Now I can come back in, and I can run Gramoxone at planting. I can make sure that I'm clean, and I have an activated herbicide that I planted into. Whereas if I put a pre-emergent herbicide out at planting, I may go 7 days, I may go 10 days without rainfall and weeds are emerging in that 7-day period. Folks need to understand. You're going to have weed emergence until that herbicide gets activated. And for that reason, if you have 7 days of emergence, you're generally not going to get a reach back with a lot of these residual herbicides. In other words, you get them activated and they're active from that point on, until you get extremely dry conditions again. But those weeds that emerged at planting, you've got to kill those now with post-emergence herbicides. And so, I'm a big fan. Looking back at the data. All the trials that I've conducted over the last probably four or five years, consistently, these pre-plants outperform these pre-emergents. Now with that, I also want to tell folks, you can't go out and spray a pre-plant and then knock down your beds. Spray them on top of a bed, knock down your beds at planting, thinking you still have activity with that pre-plant herbicide – because you don't. So there is some give and take. Some folks don't like to put a pre-plant out because they want to conserve moisture. And when they knock that bed down, you're going to start losing moisture pretty quickly. So there is some give and take there when you think about pre-plants versus pre-emergence herbicides. But I am a big fan. If you can knock those beds down and you still can have moisture at the time of planting. These pre-plant residual herbicides have done a very good job on sustaining residual control over into the crop.

Tom: Well, I think it just gives us some situations where it may give us a little more flexibility if we kind of change our train of thought to that. Because the field may be a little too tacky to plant, but we could get a ground rig across it to spray – or we've got a situation coming up. A lot of these guys may be out of moisture right now. I don't know everybody's situation. But again, we got a pretty good chance of rain coming. We can still get a herbicide active and then come back in. If those beds, if the field is ready, everything's ready to go and you get a herbicide active before we bring that planter. I think it's just a good opportunity, if we haven't got any out before this pretty good chance rain coming in this weekend. Go ahead and get some out.

Jason: I would love to see some sprayers running. Of course, having said that, I do know that I was looking at the forecast. I think they're calling for 20-30 mile per hour winds. [laughter] That does scare me here in the coming days. But yes, if conditions are suitable and you can get a herbicide out prior to a rainfall event, make use of that and then look at planting into that once conditions are dry enough to plant.

Tom: Well, that's another great point because we always talk about coverage, and I know you were talking about drift. And we don't ever want to recommend somebody to spray something that will drift, at all. But we think coverage is just important maybe for post-emergence applications, but really, it's just as important for pre-emerge or residual applications as well – getting that soil covered by that residual herbicide.

Jason: It is. We definitely want to make sure that we hit our target. Now, we've actually done some work. I think Dr. Butts probably has done some as well, as I know there were some in Mississippi State here a couple of years ago. I have looked at nozzles and droplet sizes and I haven't seen a major effect. We can go with a TTI nozzle. The one thing I tell folks in terms of applying a residual herbicide, I do like to use high spray volumes. If I'm on a ground rig 15 to 20 gallons of water, but I don't necessarily need just a standard flat, thin nozzle. I can run something that's going to produce a course, even an ultra- coarse droplet and still get very good residual control. We haven't seen a lot of differences among our droplet sizes. But we've got to make sure that we have the spray volume necessary, that we get that material to the soil surface. And in speaking there of soil surface, and I get questions about residual herbicides. There’s a lot more talk about cover crops these days and planting into cover crops. And folks ask me, “Well, do I need to use a residual herbicide if I've got a cover crop out there? Because I'm afraid that residual herbicide isn’t going to get to the soil surface.” And we've done a lot of work looking at residual herbicides where we have cover crops in place, in cotton as well as in beans. And yes, you still need a residual herbicide. When I say residual herbicide, I'm talking a pre-plant or a pre-emergence herbicide. What I have found is, especially as that planter goes through the field, you're going to have double disc openers. You're going to have a coulter there cutting, and you're going to have some areas that the soil is bare. It may not be much. It may be 1%, it may be 5% depending on what your cover crop looks like. But you're going to have some bare areas of soil. And those bare areas of soil is where the weeds are going to emerge. What we have found is that we cannot go in and effectively use a cover crop without a residual herbicide down. When we put a residual herbicide down, at planting or pre-plant, we see tremendous benefit of that in a cover crop system. And those will outperform not using a pre-plant or pre-emergent herbicide. Nine out of ten times we get better weed control results where we utilize residual chemistry. So, if you are using cover crops, don't take your residual herbicides out of that. It doesn't give you an option of just pulling out a residual chemistry.

Tom: No, I agree with that 100%. I mean that's what we've seen. We've done a lot of cover crop termination studies in two or three different crops, and adding that residual in there makes all the difference. I think they actually stay active a little longer because that moisture is not depleted as fast. That soil stays moist longer in those scenarios, it seemed like to me. I think they become a little more active in a lot of those scenarios.

Jason: I think they do. I've seen some data before also where some of that herbicide, it's intercepted by residue and it may be tied up there initially, but over time, that herbicide with rainfall, with irrigation, that herbicide breaks loose from some of that residue that's binding that material. And as you mentioned, as it does break loose, what we have found – we've got a lot higher moisture content where we have residue. So I think once it does get to the soil surface, we get a lot more activity out of those high moisture content residue surfaces than where we have these just slick fields that we have no infiltration of water. I was talking to a guy yesterday, and we just talked about the amount of water leaving a field. You have a half-inch, one inch rainfall event that's going to come this weekend. If you don't have any residue there, we get a tremendous amount of runoff. With that, you just don't maintain that moisture for long periods of time in these fields.

Tom: Well, and you think about the mechanisms of breakdown you talked about earlier – moisture, temperature, heat. Well, you're going to be a little cooler in that residue scenario with the cover crop. You're not going to get as much direct sunshine, you know, sunlight to break it down as quickly, and you're going to have a more moist environment. So, to me, it makes more sense to put residuals in a cover crop scenario, I guess. It may provide a lot more protection. You may have less post-mergence applications because of it.

Jason: Yeah. No, I think you will have less post-emergence. What we've seen is when we use residual herbicides in combination with cover crops, we have fewer weeds that we're having to kill with post- emergence herbicides. I hate to come back and harp on resistance, but everything ties into resistance. If you're trying to prevent herbicide resistance, the fewer weeds that we're trying to kill with post- emergence herbicides the less risk that we have a resistance evolving to those herbicides. And I think cover crops work well with residual chemistries.

Tom: Absolutely.

Jason: Anything else that you're any other questions that you're getting as it relates to activation and rainfall, residual herbicides, that you’d like to see us talk about today, Dr. Barber?

Tom: No, I think we've kind of covered it from kind of the questions I'm getting from the field. I just still get asked, “Should I go ahead and put a residual in there?” And my answer is always, “Yes.” [laughter] Regardless of which crop you're going to ask me that. My answer is always going to be yes, put a residual in there.

Jason: I don’t care if it’s corn, soybean, rice, grain, sorghum – any of the crops that we have here in the state of Arkansas. If you're going across that field, there are very few instances that I could think of that you need to go across a field and not have a residual herbicide in there. It just doesn't make sense. And just as we end here, at some point we're going to get in the season, and the time that we don't need a residual herbicide – I tell folks, is we don't need a residual herbicide when we get 100% crop canopy formation, okay? Or we eventually, for instance, flood a rice crop. But if you're out there two weeks prior to canopy formation, three weeks prior to canopy formation, you need a residual herbicide. If you're going to flood rice and it's going to take you more than 24 hours to flood that field, you need a residual herbicide. And I don't think there's too many folks that’s listening to us today that’s going to flood a rice field in 24 hours. So you need that residual herbicide to try to carry you, until you can get a flood established on that rice field. So, in all instances, for the most part, when you come across a field, if you're spraying a post-emergence herbicide, put in a residual herbicide. I think you're going to be pleasantly surprised at what you see from a weed control standpoint.

Tom: Absolutely.

Jason: So with that, again, I want to thank you, Dr. Barber, for joining us today to talk about activation and talk about residual herbicides, and I want to thank everyone for joining us for this episode of the Weeds AR Wild podcast series on the Arkansas Row Crops Radio. And we look forward to you joining us again next week.

[Music]: 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.uada.edu.