Early-season efforts impact season-long weed management

Join us as we welcome Dr. Tom Peters, Extension weed scientist and sugarbeet agronomist, and Dr. Eric Yu, Extension crops educator, as they discuss preemergence herbicide tips and weed control economics use this spring. The session will be moderated by Dr. Angie Peltier, Extension crops educator.

Sessions run from 8:00 to 8:30 a.m. on Wednesdays from May 13 through August 12. For more information, visit https://z.umn.edu/Strategic-Farming.

We want to thank our sponsors the Minnesota Soybean Research and Promotion Council, along with the Minnesota Corn Research and Promotion Council.

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Angie Peltier: 00:00

Good morning, everyone, and welcome to today's Strategic Farming Field Notes session. This is the first session of the 2026 growing season. Before we get started today and introduce our featured guest, we'd like to go over some quick webinar tips. We're here to answer questions and have a bit of a discussion with you. Please use the Q and A box to type questions, and then you need to click enter in order for it to have, in order for your question to register on our end.

Angie Peltier: 00:36

Please use the chat button for technical issues. You can hover your mouse near the bottom to see your toolbar options, including that chat box and the q and a box. When you do log off today, there will be a very short three question survey. We really would appreciate it if you would answer those questions. It helps us in our planning and in reporting.

Angie Peltier: 01:02

This session is also being recorded and will be posted to your favorite streaming service later this morning. Again, we wanted to welcome you to today's Strategic Farming Field Notes program from University of Minnesota Extension. These sessions are brought to you by, by us and generous support from farm families of Minnesota through the Minnesota Soybean Research and Promotion Council and the Minnesota Corn Research and Promotion Council. We are happy to have you join us for today's session that's all about preemergence herbicides and weed control economics. My name is Angie Peltier, and I work as a regional crops educator in Crookston.

Angie Peltier: 01:48

And to tick kick off the twenty twenty six Field Notes series, we are delighted to welcome two of our own two weed scientists, Tom Peters. He is a sugar beet agronomist with both University of Minnesota and North Dakota State University Extension, and Eric Yu, my counterpart working as an IPM extension educator in Southeast Minnesota. With that, I will turn it over to Tom, and and he can start us off here today. Thank you, Tom.

Thomas Peters: 02:23

Good morning, It's a beautiful day in Fargo as I look out of the window. No wind today. So you may be listening to this podcast a few days later. Today is Wednesday and it's a calm day. So as Angie indicated, we're gonna spend a lot of time talking about weed control with a with great emphasis on our soil residual herbicides.

Thomas Peters: 02:53

I I think maybe the first question that you might have is are soil residual herbicides worthwhile? So we have a lot of wind and there's some consequences with application and effects of of herbicides and wind. So that's the first one. The second one is we need rainfall to activate these soil residual herbicides. Are we going to get that?

Thomas Peters: 03:24

And I guess I want to start by talking about the third one and that's what I call return on investment from using soil residual herbicides. So I want to focus on corn and soybeans and I want to start with corn. So corn grows tall and sometimes we don't get back into the corn and we realize that there is a lot of weed control challenges that are hidden behind that nice cornfield and I want to say that the critical stage for weed control in corn is the V5 stage. So that's when the corn plant is starting to think about kernel number and it's really important that we get good weed control early in the season. The information that I've seen that I've read has indicated that the return on investment from using herbicides, soil residual herbicides in corn is greater than a 100%.

Thomas Peters: 04:36

So you're gonna get value back from using those products. Now what we want you to use is a pre emergence program, preferably a combination of of two different products, and then to follow that up with a post emergence program. And Eric will talk into more detail about that in a couple of minutes. The return on investment in soybean is not as clear. It's eventually, it's essentially no return at all, so a breakeven proposition, or it can be as high as over 300.

Thomas Peters: 05:19

And it it really depends upon the weeds that you have and the stage when weeds come into your crop. So I I would encourage you to use soil residual herbicides in both in both situations. Eric, let's, let's transition to you. You're gonna get into some details about pre emergence herbicides.

Angie Peltier: 05:43

Before you do that, Tom, can I can I ask, what is the return on investment of using a pre emergence herb icide, for example, in in soybeans in a subsequent sugar beet or small grains crop? How do you think about it as a cropping system?

Thomas Peters: 06:04

So so say that again, Angie, in before sugar beets or after sugar beets?

Angie Peltier: 06:13

So I was thinking in in your soybean crop or in your, so if you use a pre emergent herbicide in your soybean crop, what is the return on investment throughout the entire crop rotation?

Thomas Peters: 06:26

Absolutely. Yeah. So it's it's even greater. And the reason for that is is unfortunately, we don't have very good sugar beet herbicides. So in many cases, Angie, we're relying on the crop rotation on what you're doing in corn and soybeans and that that clearly is is important.

Thomas Peters: 06:52

So I I think it's even greater. It's it's probably in that higher range that I mentioned earlier on as compared to maybe a more typical corn soybean rotation.

Thomas Peters: 07:08

Eric,

Thomas Peters: 07:09

let's let's transition to you and and let's get into more detail about preemergence herbicides.

Eric Yu: 07:16

Yeah. Thanks, Tom. So before jumping right into preemergence herbicides, I do wanna just make quick mention of some observations in the field. This past last week, late last week, we did already observe some waterhemp emerging, so a little earlier than we typically would observe them. And then some of our lamb's quarters has also reached a couple inches tall.

Eric Yu: 07:40

So those are some things that we should be concerned about because, by the time we get to some of our post emergence, applications, the lamb's quarters could get to a point where they're difficult to control. So just hammering in the idea that preemergence herbicides can be really important to get those unpredictabilities of, emergence of some weeds. Now Tom mentioned preemergence herbicides using two different products. This is pretty important, especially some research coming out of Wisconsin. There's a clear indication that reduced rates.

Eric Yu: 08:14

So if we cut some of our rates of our pre emergent herbicides, that's not where we wanna save our money. We wanna use closer to full rates. Otherwise, we won't get as good control. Now if we put in a second product or a second side of action, we do get a bump up in our control, even at the reduced rates, but we get nearly a 100% control when we use our full rates with two at least two sets of actions. So some of the questions might be what what can we do right now?

Eric Yu: 08:47

If your crops haven't emerged, then you have a little more flexibility. But if they have emerged, then we do still have some options for our preemergence options. But it is important to note that if you are gonna use preemergence herbicides, they are not gonna be that effective on weeds that have already emerged. So what can we do? We have something called overlapping residual, programs where we can actually include a soil residual in with your potentially early post, herbicide products.

Eric Yu: 09:23

So I actually got a little bit of a history lesson from Tom about kind of how some of this got popularized and started. And so I kinda wanted to share that because I think it's important to get a basis of where this started and then where that research has come to and what the recommendations are now. So, Larry Stekel, he did some early work at the University of Illinois where he used, more closer to half rate and two third rates. And so basically took a, a pre emergence herbicide or soil residual and kind of spread it throughout the growing season and, found some pretty good results with that. However, some of the research now is showing that, you want to go closer to that full rate label to get the most out of your products.

Eric Yu: 10:10

Now if you reach your label max, then the recommendation, coming out of Wisconsin is that we really wanna start switching to other chloracetamides. So this is your very long chain fatty acid inhibitors or your group fifteens. And I do wanna make mention why group fifteens are a really good option in something like this. This all most of these are gonna be also labeled for your early post, and I do have some of those cutoff dates kind of written down here so that we can at least get an idea of how much time you're looking at. So dual two magnum, this is your group fifteen.

Eric Yu: 10:46

You have, your cutoff at 40 inch tall corn or seventy five days pre harvest interval for your soybean. Outlook is another good option here where your corn cutoff is 12 inches, 36 inches with drop nozzle, and then your soybean at v five, and then Warrant at 24 inches, and then, before your r two stage for soybean, and then Zidual SC at v eight for corn and soybean, v six. So pretty forgiving times as far as, how much time you have before that cutoff date comes along. And so main thing is you don't wanna cut your rate here. You wanna use closer to full rates when we start to incorporate that into your post application timings.

Eric Yu: 11:33

And this is mainly to take care of all of your escapes that, or ones that weren't taken care of in your initial preemergence program, so already emerged weeds, as well as extend that, control time. So if you get it long enough, you can get your canopy to close, and a lot of that can, benefit you in terms of shading out those weeds. One last thing that I do wanna mention before throwing it back to Tom is that, there is research coming out of Wisconsin as well where if you have a really strong preprogram, you may not even need to do some of this, layering of group 15 herbicides. So they found that there was marginal improvement if you really had a really strong preprogram. So you can save money in a lot of ways just applying a really good pre.

Eric Yu: 12:22

So, Tom, do you wanna talk about maybe some of the, I guess, importance of activation and

Thomas Peters: 12:30

Yeah. You know what? We started out by saying that it's been dry, and we've also mentioned that we've had a lot of wind events. And the the question I get very frequently is what is the fate of a herbicide that's applied? How long can it be on the soil surface and still provide value?

Thomas Peters: 12:55

And in many cases, it's a grower trying to decide, I'm looking at the forecast and I'm not sure I wanna make this investment and I want you to do that and I wanna give you some reasons why. So so first of all, herbicides are are do not volatilize. The the products that you mentioned, Derek, do not vod volatilize and they generally don't photo decompose. So I did a survey recently as to what other state extension specialists are saying. And what their anecdotal data is is these products can remain on the soil surface for ten to fourteen days, get rainfall activated, and provide good weed control.

Thomas Peters: 13:55

Now interestingly, Eric, there was an experiment done at University of Arkansas where they used different soil residual herbicides and they use irrigation to activate those at different time intervals seven, fourteen and twenty one days and what they found is even at twenty one days, so laying on the surface for for three weeks, they were still seeing some partial activity from soil residual herbicides once they were activated by rainfall. So I wanna I wanna reinforce a point that you said because it's really important. Weeds may still emerge because they started the emergence process before activation. These products don't generally reach back especially the group fifteen's to provide weed control. Number two, I mentioned they don't volatilize or photo decompose but there is evidence that they do stick to soil and can move in the wind.

Thomas Peters: 15:10

So there is a possibility that the herbicide might be lost at some degree at some concentration just because of the effects of the wind. So I want you to think about making these soil residual applications with confidence knowing that if we get rain in a reasonable time frame that they'll be there and available for doing some work. Now maybe one side effect or unintended consequence of the herbicide being on the soil surface longer is it may lead to the layered program that Eric mentioned and it may impact some decisions that are made in the fall with the use of nurse crops and cover crops. And the best advice that I have is there's various materials that have been developed by states. So Wisconsin has developed materials, Iowa has and North Dakota has and it provides information about what the impact of soil residual herbicides that you're using this year in 2025 may have on the nurse crops and cover crops that you seed starting this fall or potentially using next spring.

Thomas Peters: 16:45

And I think we should make make an emphasis Angie in our our comments to include references so our our listeners can find some of that literature. Angie, where should we go from here? How do you wanna take, the next stage of the conversation?

Angie Peltier: 17:08

So we do have a question that just came in. But before we move on to to that question, what I what I'd like to ask you is so both of you have mentioned rates. Can you mention how both how how using that full label rate is important and and why it's important. And then, also, would you touch on some of the the research that was done in Illinois looking at comparing group 14 herbicides used as a a preemergence product versus postemergence given a group 14 resistant waterhemp.

Thomas Peters: 17:53

Yes. I think before we get into that, I think it's important to say that waterhemp, I think is on everybody's mind. I think statewide waterhemp is is an important weed management challenge for us. So Eric, I wanna just go through the list. Let's talk about resistance to waterhemp.

Thomas Peters: 18:21

So we have site of action two resistance, ALS inhibitors. We have glyphosate resistance and Angie's made mention to group 14 resistance PPO herbicides. Do we have other families of herbicide resistance that we need to be concerned about?

Eric Yu: 18:47

You know, there's a really good article coming out of, the University of Minnesota where one of, doctor Dublin Saranke, student of Jot Singh, kind of, goes through the extent of herbicide resistant waterhemp throughout the state. And we're seeing that, you know, with the most recent confirmation with Liberty resistance in, Dodge County, we're we're pretty much finding resistance to just about all of our post emergence herbicide products. So this is where, you know, incorporating those pre emergence herbicides is becoming important. So I I think we covered, you know, you covered most of what the resistance problem is here in Minnesota with confirmation.

Thomas Peters: 19:34

So let's get into Angie's question. So I I am a believer in mixing group fourteen and fifteen herbicides together. So in corn, there are some premixes of and I'll give an example, Sharpen is a combination of group 14 and group 15 herbicide. In soybeans we have other combinations of of fourteens and fifteens. I I think those strategies, those programs are especially effective for season long weed control.

Thomas Peters: 20:17

So I would highly recommend our listeners consider products that are a combination of both active ingredients. So so there's a few reasons for that. So first of all, multiple effective herbicides are, I think, the the approach to get to sustain weed control and and not to increase the likelihood of resistance. And I would recommend using the label rates of those. Now I'll just give you a sugar beet example.

Thomas Peters: 20:56

We typically use less than label rates of the chloroacetamides pre emergence because we have some safety challenges but then we also use that layered program that you mentioned and I started years ago working at two thirds the label rate following some of the guidelines from Illinois but in 2026 we're going to be increasing our rates to get closer to the full rates just to do a better job of getting waterhemp control. We don't want waterhemp making seed and a layered approach along with using full rates, I think, is the best approach to getting that combination.

Angie Peltier: 21:48

So we have a very timely question here. So how much, so some people do practice, minimal tillage or or no tillage. And I know that your your colleague, Joe Eichley, has done some work looking at at residue and cover crop residue and how that can affect preemergence herbicide act activity. How how much does residue affect PREs?

Thomas Peters: 22:21

Yeah. I'll I'll start, and Eric, maybe you can chime in as well. So I you know, every year we have some situations where we get less than expected control for various reasons. And I've come around to appreciate the impact that residue can have on impacting the level of waterhemp control or weed control in general that we expect. So our herbicides sometimes bind to residues.

Thomas Peters: 23:05

Corn stocks is an example of that or it could be straw residue from from last year's wheat crop. So and and I think the the conventional thinking was, well, eventually, it's gonna rain and and it's gonna wash those residues off. But, Angie, I I would argue that some of our performance challenges have been related to to that to those herbicides sticking to the residues. Now what about cover crops then or nurse crops that were spray that we're planting in the spring. And I think there's some evidence that we've had the same with those but at least in the case of sugar beets, my area, we've been pretty successful with using spring seeded nurse crops and also using chloroacetamide herbicides in combinations with those.

Thomas Peters: 24:07

So our nurse crops are not impacting our weed control, at least not appreciably. Eric, any thoughts from your side of the aisle?

Eric Yu: 24:18

Yeah. So I've I've done some research during my grad schooling with doctor Devlin Sarangi where we, had cover crops in the field, and we placed water, sensitive cards in the field below the cover crops and applied our, herbicides. So in this case, we were applying verdict, and we were seeing about a 50% reduction in the amount that reaches the soil, compared to our control bare plots. And, despite that 50% reduction, we were seeing still significant, weed control, waterhemp control. So for talking about in a situation where we're using a cover crop, I still wouldn't necessarily skip on the idea of adding a pre emergent herbicide.

Eric Yu: 25:06

So, in the cover crop aspect with cereal rye wouldn't, I would say it would still be it's not intercepting enough where it's not making a difference.

Thomas Peters: 25:20

There's a question that we received about the cheapest and best way to control water, hemp, and soybeans, and corn. And this is an important question to me. And I wanna go back and indicate some of what we've already mentioned. I think the the the cheapest way is to start with a pre emergence program and then to follow that up with a post emergence program. So start with pre, observe your results, and then decide what the best post emergence program is later on.

Thomas Peters: 26:01

You can also get there with a total post program. I think sometimes we run into challenges that might be weather related and we get into the situation that I mentioned earlier where we had interference from some of our weeds and then likely we're using a multiple post emergent strategy. So there aren't as many post emergent herbicides. We're putting more pressure on on the good ones that we have left. So a combined strategy of using pre programs and then following it up with post herbicides, I think are affordable approaches to weed control and there also are ways of maximizing weed control.

Thomas Peters: 26:52

So I would recommend those for our growers that are listening today.

Angie Peltier: 26:57

Another another question we have here that came in during the registration process was, it was about timing. So we've talked about timing for for the preemergence herbicides. We've we've, touched on the layered approach. But, how would you suggest that people consider, deploying postemergence herbicides and layering on those those herbicides? What what timing would you suggest?

Thomas Peters: 27:32

Great question. So, I think, you know, there's a mindset that, you know, if I wait just a little longer, I might be able to get by with one post spray. And then our weeds get big and again, we're all in on waterhemp maybe we'll use an example related to waterhemp again on weed size. As waterhemp grows, we see more and more growing points. So in order to control waterhemp, we've got to touch those different growing points and as it gets bigger and I'm going to say Eric bigger than three, four, five inches, I think you put yourself at risk at getting good coverage and potentially missing some of those growing points and getting unsatisfactory weed control.

Thomas Peters: 28:29

So Angie, my vote is going to be to go out there at two, three at the latest four inch sized weeds and and get them when they're small, get complete control, get good coverage and cover all of those growing points. Eric, anything you want to add?

Eric Yu: 28:48

No, I would I I would totally agree with you. I think once you get to the four to five inch range, you're starting to get into some trouble with control.

Angie Peltier: 28:57

And and, folks, if if, if you haven't tested your spray water quality, that's something that, Eric and others have written about in the Minnesota Crop News. We also had a Strategic Farming, Let's Talk Crops program dedicated completely to water quality, and herbicide efficacy. So, folks, if you haven't looked into that or gotten your spray water tested, that's something to consider as well.

Thomas Peters: 29:30

And, I hope we have time for one more question. Wanna talk about the crop rotation. So we've spent a lot of time here talking about chemical weed control approaches. We've added some cultural ideas with cover crops, nurse crops. What about the crop rotation itself?

Thomas Peters: 29:53

And I wanna say this, A corn soybean rotation is not an effective rotation and the reason for that is corn and soybeans, in many respects, we we treat them the same now. We plant them at the same time. We use some of the same products. The row spacing is similar. That's not an effective rotation.

Thomas Peters: 30:23

If you can, it would be nice to get a grass crop into the rotation. So whatever your favorite small grains are and and and the reason for that is is we grow small grains differently. We fertilize them differently, different row spacing and we use different pesticides. So we introduce different sites of action. Now if you can, it would be nice to introduce a fourth crop into that.

Thomas Peters: 30:56

For some it's sugar beets, for others it could possibly be alfalfa. Alfalfa is a wonderful crop at reducing the viability of some of our weed seeds that that are in the profile. And I'm probably missing out on some other crops that make sense in various areas But I'd like our listeners to look at trying to diversify as much as possible at max to maximize weed control.

Angie Peltier: 31:33

100%. I am in agreement, Tom. I think that up in Northwest Minnesota, having more crops in the rotation is our superpower. That is our our major advantage compared to the rest of of Minnesota. And so, include, incorporate a small grains crop into your rotation if at all possible.

Angie Peltier: 31:58

More crops in the in the rotation mean, fewer problems for each of those crops in the rotation in my mind. So thank you very much, gentlemen. This was a wonderful discussion. We really appreciate your time here with us today. Thank you all for attending this inaugural Field Notes program today.

Angie Peltier: 32:22

We would wanna thank our sponsors one last time. That's the Minnesota Soybean and the Minnesota Corn Research and Promotion Councils. As a reminder, when you log off today, please take the quick survey. We, also, as a reminder, we will be back next week again at 8AM sharp to discuss planting progress and growing conditions around the state, including, some areas that might have had some some frost injury earlier the this growing season. But have a great rest of your day, and we'll see you next week.

Angie Peltier: 32:59

Thank you.