Title: Weeds AR Wild, Ep. 05: Weed Control and Carryover Questions in Corn and Grain Sorghum with Tom Barber (3/30/21)


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

Today for this podcast episode IÕm going to focus on corn weed control programs & the importance to start clean, use residuals & include multiple herbicide modes of action where possible. IÕll also touch on grain sorghum weed control & answer a few questions IÕve been getting the last couple of weeks on potential carryover issues of various herbicides to both corn & grain sorghum. 

First, IÕd like to talk about carryover from a herbicide called Select or any herbicide that contains the active ingredient clethodim. So due to the increase spread of glyphosate resistant ryegrass from south Arkansas to north Arkansas over the last several years, we get calls questioning the replant interval from Select or clethodim herbicide products. So any product containing clethodim has become a key component to these burndown applications for ryegrass over these several years again because glyphosate resistant ryegrass has become so prevalent. 

The recommended rate for ryegrass control in our burndown program is Select Max at 16 oz. per acre. The replant interval to both corn or grain sorghum to that rate is 30 days. We have to remember formulations & if you didnÕt use the name brand product Select Max, which is a one pound per gallon formulation, you likely used a generic. So some generic clethodims are two pounds per gallon active & there may be some that are three pounds per gallon of active ingredient clethodim. So itÕs important to know which formulation of clethodim was applied, but as long as it is equivalent to the rate of 16 oz. of Select Max, which would be 8 oz. of a two pound generic, the 30 day replant interval applies. If higher rates are used, then likely more time will be required prior to planting corn or grain sorghum, or you may see some carryover injury, or there will be some injury potential. 

We are currently updating our replant guide which is the MP 519 publication. So look for it soon on our website, which is www.uada.edu, for more information on that in the future. And you can go to the search bar on that website & just type MP 519 & that publication will pop up & be available to you. Also there will be several of these in your local county agentÕs office, so you can go to your local county Extension office & pick those up.

So when we look at corn weed control & compare corn to other crops, really in my opinion, corn is absolutely the easiest one to develop a herbicide program that controls the weeds that we have to deal with in Arkansas, which includes pigweed. Where we really get in trouble with corn though is when we donÕt put a pre-emerge out or an at-planting herbicide and we get behind because thereÕs so many things to do during that time of year, all kinds of crops that we want to plant & get in the ground in a timely fashion, we forget about that corn that we planted & when we get back to the field with the sprayer, our weeds are 6-8 inches tall.  I can tell you time & time again where I see scenarios like this playing out driving across the state every year. One thing to keep in mind, the first 6 weeks following corn emergence is absolutely critical from a weed control standpoint to make sure we maintain our optimal yield production & reduce competition from weeds which again protects our corn yields from weed competition or interference.

So most growers I talk to want to accomplish their corn weed control programs in one shot or one herbicide application that includes multiple products. And again Atrazine, as weÕve always stated, is still a backbone of any of our corn herbicide programs. Like I said earlier, there are many products that work & there are many ways we can develop a herbicide program & itÕs more or less what IÕd call easy. ItÕs just the timing that really kills us. So again, I like & what weÕve been recommended for the last several years, is to put a residual herbicide out at planting. It doesnÕt have to be anything major, it can be a generic metolachlor herbicide. Due to the recent finding of metolachlor resistant pigweed populations, I would say that if youÕre in one of those farming regions, which would be in northeast Arkansas where we have identified some of those, you want to include another product. And Atrazine is an easy choice, itÕs cheap, itÕs easy to include, so Dual plus Atrazine in those areas. In other areas, you might could get by with just Dual. But again, two products in any crop are better than one, especially where we have a severe pigweed infestation, a pint of Dual, a quart of Atrazine is cheap and provides good residuals to get us to our 12 inch stage or around V4, V5, which is our Atrazine cutoff for our POST. So this two shot program would look like again Verdict or Dual or Dual plus Atrazine. Several things we can do there. 

Then comeback by V4, V5 or before 12 inch corn with our total POST, itÕs not a total POST, but our complete POST program is probably a better way to phrase that. And this two shot program has always been the best. Overtime when we look at comparison to any of our applied research trials, that two shot program will 9 times out of 10 provide our best weed control throughout the season & also secure our higher corn yields by harvest. And again, like I said, Atrazine is the backbone of any corn herbicide program. We get two & a half quarts of Atrazine we can use each year & I think we should use all of it, in my opinion. If we use some up front, like a quart, we get a quart & a half in our POST application.
There are all kinds of different herbicide programs that we can develop that work in corn & a lot of times we cater them to the weeds that we have. But just again, our general weeds in Arkansas, historically, products like Halex GT which is a premix product of Roundup, metalochlor & an HPPD mesotrione plus Atrazine is a complete program. Another popular one may be Roundup plus Capreno, which is the HPPD portion of plus Atrazine. Armazon, there are several HPPD products. Shieldex is a new one that we can include in this POST window. And all of them can work in my opinion.  

Morningglories are starting to become more of an issue than they have in the past. And mesotrione from a residual standpoint in my opinion has more morningglory activity than some of the other HPPD herbicides. So anything with mesotrione in it, which is in Callisto & other products, again generally based on our data has a little better residual on morningglories. But we know morningglories again have been an increasing problem & a lot of it is just basically due to the fact that weÕre putting all of our herbicide out from a POST emergent standpoint out early. Our big residual application, mixing several modes of action is going out at V4 or V5 & sometimes earlier, & we expect that to last all season. Well itÕs just not going to happen. We canÕt put anything out that early thatÕs going to last from a residual standpoint all season. The reason the corn is predominantly clean is the corn will outgrow & shadow the ground preventing light penetration thus preventing weed emergence. However morning glories are very good at finding the holes in the corn canopy & will start emerging about mid-season. Again if the total POST program goes out around V4, V5. 

So in our research if youÕve got a morningglory hole or field that is real bad with morningglories, applying mesotrione or Callisto, again Halex GT is a popular mix over the years out over 30 inch corn, if we get enough coverage, we can many times prevent those morning glories from coming up mid-season. Not all the time. IÕd say youÕve got about a 50/50 shot. Most of the time we need to break out those drop nozzles which nobody has in the south. I think drop nozzles is a four letter word in the south, but we really got to get the drop nozzles out & get good coverage over the ground later in the corn growing window to get that residual to prevent morning glories from coming up. The later we plan on spraying that mesotrione mix the more residual itÕs going to provide on morningglories & then what comes up we just have to deal with at harvest with harvest aids, etc. 

So again, two shots are better than one with corn. It buys us time, timely applications are key, using multiple products, which corn gives us the flexibility to use a lot of products that we donÕt get to use in other crops, so thatÕs a good from a rotation standpoint. Where we hurt ourselves in a corn rotation in terms of pigweed control is after we we put a good mix out that controls pigweed most of the year, corn starts drying down, light breaks through the canopy, pigweed start coming up late season. Once we harvest the corn we see those pigweed plants everywhere. And what we absolutely canÕt do if weÕre trying to reduce our pigweed population on the farm, is let those pigweed go to seed. So whether itÕs coming through with a shredder or tillage equipment following harvest, two weeks or putting a residual herbicide out to prevent new flushes from going to seed, thatÕs what we need to do. We need to protect our soil seed bank at the end of the corn growing season from various pigweed populations that can pop up late in the year. So thatÕs really where we lose pigweed control in corn is actually after harvest, & thatÕs a hard pill to swallow to spend more money that time of year because weÕre at the end of the season but itÕs really providing insurance that weÕre not adding more pigweed seed to that soil seed bank. WeÕll talk more about that toward the end of the season on a podcast. Things that we can do to prevent pigweed emergence?

Alright, letÕs move over to grain sorghum. Based on what IÕm hearing & calls IÕve been getting, IÕd say that the grain sorghum acres are going to significantly increase in the state this year. The biggest question I get or that IÕve gotten lately on grain sorghum is about metribuzin carryover. Metribuzin is a herbicide we use on probably 80-90% of our soybean ground now just because of our tremendous pigweed population that weÕre dealing with & resistance in those pigweed populations. So metribuzin is a key component in soybeans over a lot of acres. Trade names may be Tricor and Boundary, there are various other products. Most of these metribuzin product labels indicate a 12-18 month replant interval following applications to grain sorghum. So thereÕs been a lot of concerns about whether or not weÕre able to plant grain sorghum on an acre that weÕve used metribuzin In a significant amount in the last few years.  Through funding from the corn & grain sorghum promotion board, we have published some research recently & it was multi-year, multi-location research looking at various ps2 herbicides PRE for a potential Atrazine replacement. Now Atrazine we hope is not going anywhere, but you never know. You see various reports in the media about Atrazine & groundwater & whether the EPA will re-register it or not from time to time. We donÕt think itÕs going anywhere but you do see those reports, so it was a concern we had & we wanted to evaluate various ps2 herbicides to see if we could find a replacement. So metribuzin was included in this research & we sprayed it PRE & we put it in at 6 oz. of a Tricor equivalent per acre which is 0.25 lbs. ai/A, PRE to grain sorghum along with various other ps2Õs. 

Now the results over this period of time that we conducted this research showed that we did get a little injury at the end of the year. Over all most of the ps2Õs other than Atrazine reduced yield maybe between 5 & 8%. It was not significant in some cases. Some cases it was. But again thatÕs putting metribuzin out PRE or at planting, not the year before. So based on that research & conversations with others & their experiences, I feel like if we put metribuzin out the previous season, I donÕt think that weÕre going to have any issues from a replant standpoint coming back with grain sorghum. A lot of times, these extended periods of time where you see 18 months on a replant interval on a herbicide label, a lot of times is just because of the residue work may not have been to reduce that interval. And again we know that we need metribuzin in our soybean system but with this data that we have, we feel confident that if weÕre following up from last yearsÕ metribuzin application that we can go ahead & plant grain sorghum on that acre with minimal injury. Honestly IÕm more concerned with Flexstar carryover, or fomesafen. We see that more common than probably anything else in both corn & grain sorghum. Where weÕve gone out late with a Flexstar application or maybe doubled up on some ends or overlapped, we can see some severe carryover from Flexstar on corn & grain sorghum in those areas. IÕm more worried about Fomesfen or Flexstar than I am metribuzin at this point. 

And just to touch briefly on grain sorghum weed control. I know we havenÕt had a lot of grain sorghum in the state the last several years. Number one to me is first we donÕt want to plant it in a field where we have Johnsongrass or Texas Panicum as a main weed problem because we have absolutely nothing to control those two weeds in a grain sorghum crop, with most of the seed we are planting now. There are new technologies being developed where herbicides are available to control those species but thereÕs not much seed on the market for that right now, so at this time weÕll just recommend not planting in those acres because thereÕs not much available for control there.

Number two, we want to plant concept treated seed. I think this is a given & I donÕt know that you can buy grain sorghum without the concept treatment, but we need that treatment to provide protection against Dual or metolachlor at planting. So make sure the seed has concept treated or itÕs labeled concept treated on the bag. We need to use Dual Magnum or another metolachlor product plus Roundup at planting. And the main reason for that is to control any grasses that may be emerged or may be coming up or could come up after planting the grain sorghum. Grass is the key weed on our grain sorghum acre & we donÕt have many products that have good activity on grasses. At least in or conventional grain sorghum system. So donÕt plant fields infested with Johnsongrass or Texas Panicum. Make sure you have concept treated seed & we need to use a metolaclor or Dual Magnum product at planting plus Roundup for any grasses that are up, & then weÕll also provide residual control with that Dual. 

We can use Atrazine as well with the Dual. HereÕs one thing IÕd caution you. Number one if we get much over a quart of Atrazine, we could see more injury to the grain sorghum, especially in cool, wet conditions, so be careful with that. And sandy soils, be careful with that too. Grain sorghum is very susceptible to Roundup & over the years weÕve seen a lot Roundup drift on to grain sorghum. If you put that Atrazine out PRE, then youÕre locked in the grain sorghum or corn crop that year, if you have to fail the stand for whatever reason, so I like Dual by itself upfront. If you want to put Atrazine with it, fine. Be careful on sands & watch your rate depending on the environmental conditions.

 Timely POST on grasses is important in grain sorghum, so if we have any grass weeds come up, once the grasses get about 3 to 4 leaf, they become tolerant to Atrazine for the most part, so weÕre not going to get them. Facet is not very good on larger grasses, we know that from a rice standpoint. So, if weÕre going to control grasses, itÕs either Facet or Atrazine & they need to be applied to small grasses, & thatÕs why we have the Dual out there Pre, because the metolachlor or the Dual is probably one of the better residuals we have for grass control.

There are many more options for broadleaves, including Huskie, and Atrazine. I had several questions this year on dicamba, & I put this in a blog post earlier, that you can check on ArkansasRowCrops.com.   We thought with the new dicamba formulations Xtendimax & Engenia, the new labels for those this year, donÕt allow other crops other than the Xtend or Xtendflex crops, but the plant board regulations are written that if a herbicide is labeled for use in grain sorghums, so if there is a dicamba herbicide label for use to grain sorghum then we can use that herbicide as long as we stick to the buffers set forth by the plant board for use of dicamba & Xxtenda crops. So check the plant board website for those rules & regulations. But again, several options for broadleaves, not many for grasses. Timely applications and spray weed small. And again grass is going to be our biggest concern in grain sorghum. 

So if you have any questions on this feel free to text us, write us, e-mail us. Our information is on-line, again at uada.edu & I want to thank you for listening this week & tune in next week as Dr. Norsworthy will be discussing cover crop termination timing and strategies for control of the cover crop to provide the best protection against weed emergence.  Thanks for joining us for this episode of the Weeds AR 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.