Weeds AR Wild Episode 11 with Dr. Tom Barber 5/12/21
Welcome to the Weeds AR Wild podcast series as part of Arkansas Row Crops Radio. My name is Tom Barber and I’m an Extension Weed Scientist with the University of Arkansas System Division of Agriculture.
Today I’m going to focus on a topic that seems to be pretty popular every year, which is residual herbicide discussion and to try to answer some of the key questions about the amount of rainfall needed for activation and how long these residual herbicides actually sit and wait for a rainfall and how active will they be after sitting for a while and waiting on that rain. 
But first I want to announce that as of today, May 11th, Arkansas has an approved section 18 emergency exemption label for the use of Brake herbicide applications in peanut. There’s a blog post up on our Arkansas Row Crops blog discussing some of the specifics for this section 18 and how we need to use this on some of our peanut acres in Arkansas for growers that are interested. So just quickly some of the key points from the section 18 Brake label. Again this section 18 is targeted acres where we have the multiple resistance Palmer Amaranth or Palmer Pigweed, specifically PPO resistance populations of Palmer Amaranth is where we need to focus applications of this section 18 use of Brake herbicide. So the rate that we are going to be looking at with Brake in this situation is a range of 12 to 16 ounces. It’s not that big of a rate range. Personally we’ve only looked at this two years, but personally I would look at that lower rate on some of our really sandy soils in northeast Arkansas where we’re growing some of these peanuts, maybe the 12 ounce rate there. If it’s more of a sandy loam then I might extend it to 16 ounces. And again that’s not a big difference in rate, but we have seen some injury and I go in a little depth of that injury on the blog post. So this is going to go out pre-plant or at planting or within thirty eight hours following planting according to the label as we recommend Brake in cotton, Brake in peanuts is going to work better if we mix it with another residual herbicide that gets activated a little easier. So Valor at 2 ounces plus Brake at 12 to 16 ounces looks really good as a system up front in peanut acres. Now we have gone up as high as 16 ounces of Brake with 3 ounces of Valor and again that has looked well for us on those sandy loam soils. We do see some injury following that 16 ounce rate. And generally it’s less than fifteen percent, but it is possible to get more injury than that if we overlap in the field anywhere to double that rate or if we overlap it on the ends and more or less double the rate on the ends we’re going to get some severe injury with Brake. So we need to be careful when making these applications. I would not overlap anywhere in the field. It’s going, again, extend the amount of stunying and bleaching with Brake on peanuts if we do that. So be careful with the applications, 16 ounce max rate, don’t overlap. Also according to the label, if we’ve made applications of Brake in cotton in the past or let’s just say last year, you made an application of Brake in cotton, rotated the peanuts this year and want to use it again, that’s fine, but then the next year we need to use another herbicide in our cotton crop other than Brake. So you get two years in a row and then you need to rotate to something else if we’re using Brake on the acre. Also if you’re not familiar with Brake as a herbicide and you use it the first time in your peanut crop, if you fail a stand after applying Brake herbicide then you have to replant to either peanuts or cotton because it’s going to be way too active on any corn or soybean or other crop that you’ve planted on that acre within that year. So again if you failed a stand for whatever reason, come back and you’re either going to have to plant either peanuts or cotton on that acre. Again, to me, best fit areas where we have pigweed issues, 12 to 16 ounces depending on your soil type there. The lighter sands I’d go probably on the 12 ounce rate plus 2 to 3 ounces of Valor. I think that’s probably the best program at this time. We’ve looked at that multiple years and time in and time out it’s pretty good. Again more info regarding the Brake label can be found on our blog, ArkansasRowCrops.com, if you just search for that or the actual website I think is ArkansasCrops.uada.edu, so if you want to look there.
Another topic that most of you have heard about I’m sure by now that has also got a write up on the blog that Dr. Tommy Butts put together about the new dicamba rules for Arkansas. So if you’re not familiar, within the last week the plant board met and extended the window for dicamba applications in Arkansas specifically in Xtend or Xtend Flex crops out to June 30th. So from May 25th, was the old cutoff date, now June 30th is the new cutoff date. There are buffers that continue to apply and so to know the buffers, if you haven’t followed it or haven’t heard, again there’s an article on our blog discussing buffers and our recommendations with this new rule in place of what we recommend from a weed control standpoint. So check that out while you’re looking at the blog as well. 
Alright so I’m going to move into what I planned to talk about today and it seems a little funny or silly or ironic or something to cover this topic as rain is beating down on our shop right now, but every year we do get a lot questions about residual herbicides and if it doesn’t rain immediately or with the first couple or three days after planting, what now. Is it going to be active, how active will it be if I get a rain, how long will these residuals sit there and wait for a rain, etc. And then again, if I get the rain and have some weeds emerge, do these residuals have any reach back to control those weeds that actually came up in the meantime. And so the answer to all of this is real simple, and that is, it depends. And so in general this is real easy paint brush to use for all of our residual herbicides, but in general a half inch will cover most of them but if we go up to an inch, between a half inch and an inch of rain will activate every herbicide that we have. But we can break it down a little further than that and the other thing that plays a role in this is also temperature. Getting back to how much rain, some chemistry doesn’t require much at all so if we look at these Xtend crops, we’ve been talking the last couple of years or longer that dicamba’s a very effective at-planting herbicide because it’s active without rainfall. If we pair it with Cotoran in cotton or something like, for example Boundry or something in beans, it will be active until we get the rain to activate the other Pre that we pair with it, so it pairs together good with our other residual herbicides, but it kind of works backwards than our others in that it’s active until we get the rain & when we get the rain, the activity from dicamba starts to decrease. And this is very similar with Enlist One herbicide or other 2-4D containing herbicides as well. So there’s some chemistry out there that doesn’t require rain for activation but most residuals do. So let’s talk about some of these. We move into groups like the PPO’s. I’d say the next group that doesn’t require as much rainfall, this would be most of our PPO’s. So herbicides like Valor, Sharpen, Reflex. Dr. Norsworthy did some work probably almost ten years ago now, looking at the various cotton herbicides and the amount of rainfall required and these were actually required less than some of our others, so a quarter of an inch of rain as it turns out usually is enough to activate Valor or PPO’s. Valor, Sharpen, Reflex, etc. I’d also list Cotoran that we use in cotton in this group and also based on the label and some of the work we’ve done, you might loop Metrabuzin in there that we use in beans as well. At least a quarter or three tenths inches of rain should activate those products that we use. These are common products that we use quite often. Most of our residuals by a whole is going to fall into that half inch window of rainfall needed for activation. That half inch amount. So these are products like Dual, Dual Magnum, Metolachlor in generally. Ziduas, Acidachlor or Warrant, Direx, Caparol, many others that we commonly use are going to fall into this half inch of rainfall needed for activation. Again we’ve seen this in some of our research data and also on the product label. So if there’s any one herbicide in this Group 15 chemistry that probably requires a little less than that in my opinion is Outlook and again just based on recent data conducted across Arkansas it seems to be a little more active with less rainfall than some of the other Group 15’s and so again I think a lot of growers have looked at Outlook a little bit in recent years because it does have more activity on some of our Metolachlor tolerant resistance populations of pigweed than metolachlor but it also can be a little bit more active with a little less rainfall. But again we’re going to need at least a quarter to three tenths to get it activated. Now some require a lot more rainfall. Some of our residuals, we’ve already talked about one this morning and that’s Brake. Brake is going to require anywhere from a half to three quarters of an inch of rain for the best activity. Others that fall in this group based on what’s listed on their label, League is an example that we use sometimes in rice and then also our yellow herbicides, so Prowl and Treflan if we’re going to use rainfall or overhead irrigation to activate those. Our yellow herbicides like Prowl and Treflan are going to require much more rainfall to do that. We also know that those yellows are the ones that if we really want good activation from, we probably need to incorporate. The next question is, and I can’t touch on every herbicide here, but those are some key ones that I get questions about.
So the next question is, well if I don’t get rainfall right away, how long will they sit on the soil and still be active while we wait on a rain? And the answer I thought after I was digging through some research and looking to answer this question from label standpoint is really most of them will sit there fourteen to twenty one days. Now the problem is although they will sit there and still be active once that rainfall occurs we will have a major weed issue in that fourteen to twenty one day window while we wait for activation. While the weeds are up it doesn’t look like there’s going to be any activation from that point forward, but they actually are still somewhat active. In general, personally, I’d like to get activation within seven to ten days, no longer than fourteen days. Ideally two to five days where we get activation, but most of these will sit on the soil and remain there waiting for activation for longer than you might think. But a lot of that is depending on temperature so in the spring months, when it’s cooler, less microbial activity in the soil, they will probably sit there a little longer than if we’re talking about June and July. The other side of this is photo degradation, so herbicides like Treflan and Prowl, especially any yellow herbicide, very sensitive to breakdown through sunlight and so those need to be incorporated, rainfall or through tillage within forty-eight hours of application for the best activity. And again this is due to the amount that’s degraded by sunlight or photo degradation. The big problem with all of this, is we never know when it’s going to rain or how much it’s going to rain. The more rainfall we get to actually flush some of these out of the area that we need them for activity, but if we don’t get enough then some of them may not be active. So we play a game, it’s not really a game but roll the dice I guess would be a better way to put it, that we’re going to get the amount of rainfall we need to activate these residuals and that’s one reason why it’s hard to just rely on residuals, especially for barnyardgrass control in rice or pigweed control in cotton or soybeans because we don’t know how much rain we’re going to get, we don’t know how much germination we’re going to get in between activation and so that spurs the next question. How much reach back am I going to have for some of these herbicides? Unfortunately, just through personal observation I guess I can answer this, but again it’s going to depend on which herbicide we’re talking about. A lot of our Group 15’s, such as Warrant, Dual, Zidua, Outlook, are going to have very little if any reach back on pigweed. I have seen Zidua have some activity or reach back on ryegrass as we use it in wheat, but in general I don’t plan on reach back on any of those on broadleaf weeds especially pigweed. One of the main ones we get questions about all the time is Command on grasses in rice because it will actually bleach the grass and so you’ll see the bleaching of the little grass coming up and you’ll think, man, I’m going to control that. I just say sometimes we get lucky and I’d rather be lucky than good, because I think sometimes we get it. Nine times or eight times out of ten, I’d say that that grass may be stunted, it may be bleached, but eventually it’s going to come back and we’re going to be required to make a post application to control that grass in rice. So PPO herbicides, we’ve seen this on our crop, can have splashing injury to both crop and weeds and so if the weed’s small enough or we get a heavy rain, could we splash enough herbicide to get control of some of some weeds, sure that’s possible. I don’t like to bank or depend on that, but it is possible to get reach back I guess looking at it from that standpoint. 
And the other one we’ve already talked about today again was Brake. I’ve seen Brake be applied and not get enough rain for activation and then morningglories or something come up. It’s very active on morningglories, at least it’ll bleach them out real easy, and we get that rainfall and then it begins to bleach the morningglories out. Some of our photo synthetics such as Diuron or Cotoran can also do that. Not as much as bleaching but give you that photo synthetic activity on weeds after they’re up and so we don’t like to rely and I would never recommend somebody just relying on that reach back activity. I think sometimes we get lucky and it works in our favor. But we need to plan on our post-emergence application controlling any of those weeds that come up regardless of the crop, in between when we spray that residual and when we get activation. Again, all of these situations can vary based on how active the herbicide is in the soil and the infinity of the herbicide to bind to the soil and so sandier soil, these herbicides, some of them may be more active. Soil pH can cause some of them to be more active and the moisture present throughout, for example the Cotoran label for cotton says that it’s active basically if there’s enough moisture in the soil to get the cotton seed to germinate and that’s one reason we like to pair it with Brake or Caporal, is that Caporal requires a little more rainfall, Brake requires more rainfall than Cotoran to get activated. Pairing two products together like that can really help spread our risk from a residual standpoint at the beginning of a growing season. But at the end of the day we like to plan the best we can in our residuals. Regardless, they are going to work best if we get them out and then we’re lucky enough to get a rainfall in the next two to three to five days, or if we can run the water to flush or run overhead irrigation, that’s the best method or plan going forward to make sure that those herbicides get activated and we get off to a good start. 
So, thanks for listening today, I’d like to invite everybody to join us next week as Dr. Jason Norsworthy will be discussing expectations for impregnated Loyant  herbicide in rice, as well as any other key management issues that may pop up between now and then. So again thank you for joining us for this episode for the Weeds AR Wild podcast series on Arkansas Row Crops Radio.