UMN Extension Nutrient Management Podcast

Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:
“Sidedressing nitrogen: What growers should keep in mind when applying in-season”

June 24, 2024

Written transcripts are generated using a combination of speech recognition software and human transcribers, and may contain errors. Please check the corresponding audio before referencing content in print.

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Jack Wilcox:
Welcome back to the Advancing Nitrogen Smart podcast series at University of Minnesota Extension. Today we're going to discuss what to consider when planning to sidedress. I'm Jack Wilcox communications generalist here at Extension. Our experts today are Extension educator Brad Carlson, and Extension nutrient management specialist Dan Kaiser.

Let's start broadly with growth stages. Dan, what are the growth stages that farmers need to know about when planning to sidedress?

Dan Kaiser:
Well, one of the things about sidedress is that we see more and more that with the high clearance equipment that's out there, that there is the ability to go in later in the season for it. Now, the question is, when we start talking about growth stages is, when is the nitrogen taken up by the plant? Typically, our recommendations would say that we should, if we're planning on sidedress applications, finish those applications by V10. We have what we call the V, or the vegetative growth stages.

There's a number of vegetative growth stages in all, and it equates back to the number of leaves, the number of fully developed leaf collars on the plant. VT is what we call tasseling. And then beyond VT, then it's reproductive stages, so that's silk development and then ear development progresses on that.

But generally with sidedress the vegetative stages are the more critical. And a lot of that has to do with the fact that the majority of the nitrogen we have that's taken up in the plant really comes into between what we call V6, or when ... I would say about when we're looking at optimal or kind of our optimal sidedress timing. Through VTR1, we see roughly about 80% of the nitrogen that the plant's going to take up is done at that point in time.

So, we see a really high daily demand where it's pretty easy to short the crop if we don't have the nitrogen there. That's one of the key points why we recommend sidedressing early, is because we have to get the nitrogen applied, but we also have to get it into a form that the plant can take it up, which is typically nitrate, which moves into the plant with water. It takes time to do this, so we can't just expect that we apply it and it goes directly into the plant.

That's one of the reasons why with the planned application V6, we really just look at ... especially if you're dealing with a sidedress application that's not in a sandy soil, is that starting when you can start growing the crop, because it's about two, three days, Brad, in your estimation essentially with some of those that they're putting on a leaf?

Brad Carlson:
Yeah, pretty much. Pretty much. Obviously, it's going to be heat-dependent, but it happens so fast. I think really the key, and most farmers know this, is while maybe when the corn first emerges the first few weeks, it grows kind of slowly. But once it starts getting up to in the 70s, into 80 degrees in the day, it grows so fast, you're only talking about a few days difference in growth stages.

And so, really from a fertilizer application standpoint, there's not a lot of difference in just a couple of days, but you can see the corn change pretty rapidly over that time.

Dan Kaiser:
V6 is really, I think, the critical one because then you really start seeing things ramp up, because we start getting to situations where the temperatures are higher, particularly the overnight temps, we start really putting on some GDUs. But a few key corn growth periods, if we look at essentially when the plant should need nitrogen at about the V3 stage or about three leaf is the end of ... when the seed is being used as the main food source of it.

So, generally it should have enough to get it to that point with the seed that's in the ground. Then beyond that, it depends on what's actually in the soil itself. We typically recommend with ... particularly if you are sidedressing with continuous corn, that you want to have at least 40 to maybe 60 pounds of nitrogen down at planting, just to stave off some of the issues with immobilization with that, because there just isn't enough at that point.

Brad Carlson:
Right. And I think a lot of farmers are familiar with that graph that Ross Bender from the University of Illinois put together, that looks at total nitrogen uptake over the growing season. It breaks it out into the parts of the plant, but I guess for our standpoint, we're not that worried about what part of the plant it is, we're just simply worried about whether the crop needs it. And while we talk about wanting the nitrogen on by V10, you look at that chart really only about 30% of the nitrogen is taken up.

But the key is that the corn is growing so fast at that point, you're at a lot of risk if it's not there. Things can happen just so fast. That's really the key is to have that nitrogen on before the crop starts really growing fast. And as I just said, we're only talking about a couple of days, you might as well just get it on.

Dan Kaiser:
No, and you look at it, V4 to V6 is about the point in time where we're seeing some initiation of the ear in the tassel, and then the kernel row in numbers are being determined at that point in time. So, if you're short then, it's essentially lost yield that you're not going to get back. When you look at corn, it's really about the number of kernels per acre and the size of those kernels.

Yes, we can make some of that back maybe when we see it towards V17 or one week before silking, when a number of kernels per row are set. Maybe we can ... later on you can see that the kernel size can be changed slightly, but it's a numbers game with it. So, if you're looking at it in terms of risk mitigation, really it's better to err earlier than err later. And we see more emphasis that ... we know from some of that bender work that about 20% of the nitrogen is taken up post-tasseling from the crop.

In many cases when we look at Minnesota, I think a lot of that comes from mineralized organic matter. We're really feeding at that point because the daily demand, it tends to level off where you're not seeing that peak demand where you need a lot of available nitrogen. If you look at a lot of the data that Fabian Fernandez has right now, if you look at nitrate concentration in the soil over time, you tend to see it really flat-line once we get into those reproductive stages in corn, in that it's just drawing out as much as it can.

It doesn't necessarily need more at that point in time, but just with the daily mineralization we're getting, I think it can do most of what we need in terms of meeting what that crop is requiring at that point in time.

Brad Carlson:
Right. And if you're looking at the data on ... if we look at residual soil nitrate following harvest, sometimes we can find very large amounts of nitrogen in that top six inches foot of soil. Well, that's not leftover unused fertilizer. That's nitrogen that came from the mineralization. Now, after the crop black layers and reaches maturity, it doesn't take it up anymore. It's left there.

But the point being, it's supplying all that nitrogen during that late part of that growing season to that plant, and it's a lot. And so, we really need to get the fertilizer in place prior to when we finally really start kicking it in gear for mineralization. Now, if you have perfect weather conditions, and it stays moist and it's nice and warm, that might happen a lot earlier. But then there's a lot of other years where we can have some weird weather, and so forth.

So, you want to play the averages or at least play a little risk management, and just make sure that that nitrogen is on early enough to not run out.

Dan Kaiser:
Well, looking at it, a lot of growers, as I said before, you see some interest in late-season applications. And I guess the way I view a lot of these post-tasseling applications more as rescue treatments, and it's just a question of whether or not they're going to be effective. Because again, going back to those key growth periods, if we starve the crop at that point in time, there might be situations where we can't really bring it back.

And if you look at, I'd say, the last 15 years, there's been some work. I think Peter Scarf, who was at the University of Missouri, was doing a lot of work in the Corn Belt looking at late applications of nitrogen. And a lot of the work they were doing here in Minnesota, I think maybe one year they were able to pick up a little yield with some of those late applications. But in general, I think there's a point of no return for us with that.

And that's one of the things that I think is challenging for a grower, because you want to still be able to make up some yield, but you've also got to look at the investment, what you're putting in time and money at that late. And the fact that if it's not utilized, it's just going to be there in September and October in the soil as nitrate, and there's nothing that's going to be able to take it up.

Brad Carlson:
Right. Dan, I think the way I look at it is in-season applications and nitrogen fall into three buckets. There's the split application where we know what kind of a rate we're going to apply, and we just simply plan to apply some of it pre-plant and some of it after the corn emerges from the ground.

There's the variable rate treatments where we apply a set amount, and then we try to use some kind of a technology, whether that be sensors, soil testing or what have you, to determine how much nitrogen we're going to apply on an overall basis or site-to-site basis. And we make those decisions on the fly in season. And then there's rescue treatments, which you've just talked about.

And from a management standpoint, there's similarities based on the fact that we're doing applications during the growing season, but really from a philosophical standpoint and how we approach these, these are three very different things. And so, we started by talking about just the plain sidedress. Whereas, say, we applied 80 pounds prior to planting and we're going to apply 70 pounds afterwards. And that's just simply a matter of making sure you get it on.

Now, the rescue treatments, like you were just talking about, Dan, as far as those late-season ones, and like you mentioned, we don't see a lot of benefit for just scheduling an application that late. Really it's just a situation where we know that there's a crop issue out there. We lost nitrogen probably because of something related to the weather. In all likelihood it got extraordinarily wet.

And there's going to be some interactions there between when you applied the fertilizer, how much you applied, what type of fertilizer you used. Did you use a nitrification inhibitor and some of that kind of stuff there? But the rescue treatment, you're never going to achieve full yield. At that point you're just simply hoping to get something back that otherwise is lost if you leave it alone.

Dan Kaiser:
And I think a lot of times ... a lot of that where it comes into is probably seed weight, and there's something to that since it's interesting with corn, since it's sold by the pound yet we market it by essentially volume. But when it comes down to it, yeah, we can make something back, but you're not going to get it all back. And that's the thing. So, it's how much do you want to throw at it. And really looking at it, what are options for assessing that?

It is a little limited looking at ... when we know to sidedress there, meaning we do have in-season soil tests that people do use. The pre-sidedress nitrate tests is one of them. We don't have a very good set of guidelines for that, other than in general, if you're above a certain concentration of nitrate in a one-foot sample, you have to look at the pre-sidedress nitrate test, different from what we call the pre-plant nitrate test, which is done ahead of planting, which is a two-foot test, that you can't use the same set of recommendations.

And the issue with ... especially the in-season tests, like the pre-sidedress nitrate test, is we don't always know what to do once you get a number back. We generally know if you're above a certain number, you're probably fine. So, I think a lot of times it's 25, 26 part-per-million in a one-foot sample that you should be okay. But that's one of the things we're looking at now, because looking at what options do we have instead of just flying blindly and just doing something because we think we need it, it's nice to have something to back up our decision.

And the in-season tests I think aren't a bad option. The other option that's been out there has been sensors, like canopy sensing. There's a few, like when you start looking at the remote sensing from satellites or drones. There's also what we call active sensors, which emit their own light sources. They're different from satellites and drones, which rely on ambient light from the sun to take their measures.

The issue I've seen with a lot of these sensors has really been timing. For us here in Minnesota, if you look at when you start to see your crop start starving for nitrogen, it tends to be later in the growing season, at key points beyond where we tend to recommend sidedress applications. The question then is, do we see the problem well enough before where we can make a decision to actually get some yield back where we're not actually hurting things? I think it's really the big question with these.

I've looked at this. We had a number of trials. Interestingly enough, there was a study we had by New Richland back in 2014, and that was an interesting study because we had a foot slope, side slope and a top slope area in this particular field, and especially the bottom or the lower area of the field was extremely saturated. And if you looked at the sensors, we had eight nitrogen rates. You couldn't see any difference among them in that low area.

But if you got to the side slope and the top slope, you could pick out some of the lower rates, where you could see somewhat of a response curve like you would expect. So, I would have really looked at making a recommendation for probably a full rate of nitrogen at that toe slope area. But when we got to the end of the year, the response then was ... if you looked at the corn response it was exactly the same, but I just got less yield.

I couldn't have put any more nitrogen to that to get any additional yield out of it, just the damage was already done at that point. That's really the assessment on a lot of this when it comes to weather. You just have to know, is it ideal where we can actually get any of this yield back? And that's one of the things that's really difficult right now. I don't think we've got a good answer to that, Brad, just for being able to figure that out.

Brad Carlson:
The difficulty with a lot of this really is ... and there's been a lot of interest in drones. What are you actually finding when it's out there? Part of the problem with the drone is, not only is the interpretation of that data, has it been challenging as far as making a recommendation once you see differences?

That's not that hard, but then making a recommendation or calibrating it to what to do about it, that's been hard. But beyond that, you've got the issues with if you fly the drone, somebody's got to go out and do that. That takes time. Then it comes back with a computer file, and they usually got to run back to their office and get it on a computer. And then it's got to run through some kind of a program, a GIS program, and straighten the field boundaries out and then interpret the differences.

And then, if you've got some kind of a recommendation algorithm that predicts based on the color how much nitrogen you need, you still got to order that then from your dealer and have it applied. So, we're dealing with several days. And so, in a lot of cases ... we just talked about the corn's growing really fast that time of year. If we're short of nitrogen, it can be a risky proposition trying to use a drone to do that.

We're not really trying to trash on them, it's just the reality of the situation. It's been very challenging. I know, Dan, you've done a little bit of looking at some of that technology that looks at leaf pattern instead of overall color. That seemed like it held a little bit of promise at one time. Has that gone anywhere?

Dan Kaiser:
Well, a lot of the issue when you look at that work, that work was geared towards sandy soils where we started to see a lot of deficiencies early on. And that's been one of the things I've run into with several of my studies. It's amazing how far we can get into the season with what's already in the soil and when it starts to run out.

And I've been caught a few times where I've been into around silking or later where we start to run out of nitrogen. Where it's gone that long, where it was able to survive, but then was a point at which I couldn't really do anything because of the timing, and it just was too late to come back in to make up some of the yield. And the thing with a lot of this imagery is you have to look at imaging later on, because this is similar to soil testing, in the fact that you're using one snapshot in time to forward-predict what's going to happen.

The thing with soil testing, and I'm going to say I'm biased towards it since I do a lot of it, it just seems like it's better in terms of a risk tool, management tool, versus some of this drone imagery, which can be affected by hybrid leaf architecture. Brad, you were talking about other nutrients too. Yellow, there's a lot of things that can cause the plant to be yellow. It's just knowing what you're looking at. There's a lot of interpretation.

But I will say with a lot of the satellites that are out there, they've gotten a lot better for return times where they can come every day or so. They will fly over where you can get images, and you can get some very high-quality images with very, very high resolution on it. It is advancing. The issue with it is the technology.

It's like anything. The technology is just advancing quicker than our understanding of how to use it and make recommendations out of it. So, you can use it. I don't know where a lot of the recommendations are coming from right now.

Brad Carlson:
Right. I think of a couple of experiences during my career. I know one, Jeff Vetch had that study looking at cover crops, and they were measuring chlorophyll in the corn plant. And in a case where they had a lot of residue from a cover crop compared to one that didn't have any. And at tasseling time, they found no difference in the chlorophyll readings on the plant. But then when it got to the third week of August, there was a big difference and there was a big yield hit.

And so, that's when you see it. Basically the crop ran out of nitrogen that late in the growing season or something causing nitrogen stress that late. Very difficult to predict that by using some kind of an image. That was more of a case of knowing what happened out there management-wise, and seeing ahead to the fact that that could happen. And I also know early in my career, I remember a year when ... Dan, this goes back to before you were even here.

There was a year where all of the upper leaves of the corn plant turned yellow. And a lot of us who were in soils were on the radio talking about, well, nitrogen is mobile in the plant. The crop probably ran out of nitrogen this year, but we didn't think it was going to hurt yield, blah, blah, blah. Well, then lo and behold, here come the plant pathologists and tell us all that this was a leaf disease that caused it.

We saw it singularly through our lens. We weren't seeing it through somebody else's. And so, that's the other thing you got to be very careful about if you're using sensing technology, is what exactly is it that you're even looking at?

Dan Kaiser:
And I'm going to go back to this. When we start looking at just higher nitrate concentrations in the soil, really it's more critical earlier in the growing season. Predominantly the data that I've seen, if you look at where we have at the end of the season the optimal nitrogen rate, that we don't see a lot of nitrate in the top two feet at that point, if we start talking about tasseling or later.

Again, we know that that crop is extracting as much as it can, but it doesn't need to have these high elevated levels that late in the season. Because if we apply it too late, we can see essentially a greater risk for elevating the nitrate, and the plant not utilizing it. And that's just not what we want to do.

And in a lot of cases, while a lot of growers like to see it green to the ground, we don't really ... it's one of the things like some of these other crops like sugar beets, where it isn't the worst thing in the world. Actually, with beets it's better actually if it starts to run out towards the end of the season with that.

Brad Carlson:
Right. And we've tried to send that message to a lot of farmers, that it's normal, it's natural to see those lower leaves yellow and dry out, so, don't feel like you shorted the crop. Now, if that yellowing extends all the way up to the ear, then we might have had some issues. Really, that's the key. Well, Dan, we talked ... I guess to put a bow on this talk about rescue treatments.

Going again way back early in my career, we developed this University of Minnesota Supplemental Nitrogen Worksheet for corn. And basically this is a tool that's available to assess risk, it looks at what your application timing and rates were, what the weather's been like since your application was made. And you score a certain number of points on that.

And based on that sheet, it gives you an indication of whether it's likely that you've lost nitrogen and you need to do a rescue treatment. But it's a little ambiguous too. It doesn't always ... it doesn't really tell us exactly what to put on or so forth. It just gives us a clue that we've got some problems. It's a good place to start. That web sheet's available on our website, our Nutrient Management website.

If you just go to extension.umn.edu and click on the nutrient management and look under corn, you can find that. That's probably the easiest way to describe it instead of giving a big long URL. But what's been your experience in using that worksheet, or how should farmers think about using that?

Dan Kaiser:
Well, it's what I call a qualitative assessment. Essentially what it'll do is hopefully give you some idea of whether or not it's worth going in and looking at sidedressing. But again, Brad, you hit the nail on the head there. That said, it doesn't give you a good idea on how much to apply. I think it recommends 30 pounds if you're corn following soybean. If it deems that you're in a situation where the score you have weren't sidedress, or it might be 40 to maybe 70 for corn following corn, I can't remember what exactly it says.

For somebody wanting to know the exact rate to apply and make adjustments based on different scenarios, it doesn't do a good job with that. But that's one of the things with nitrogen. When you look at a lot of the issues with water quality, if we could get down to the point where you can get within five, 10 pounds of the actual nitrate within a field, whether it's a sidedress supplemental or it's a pre-plant application, that'd be great. We just can't right now with the technology.

Brad Carlson:
Right.

Dan Kaiser:
That's the problem with that. So, we've got to look at some of these other things and when it comes to some of our four Rs to try to at least mitigate some of the potential risks for loss. One of the things about timing, if you look at the data, and I want to go back to this, because I think it warrants just talking about quick. When we start talking about optimal timing, it really depends on our summer conditions.

So, situations where we get high spring and lost potential, these would be sandier soils that delaying our timing to say, V6, V7, V8. Really with sands, I would probably be doing three splits anyway, or even ... I know we've got some growers that are going with more splits with fertigation, which there's nothing wrong with that because you've got the moisture and the water there, hopefully to get the nitrogen down to the plants.

And with UAN, you've got to remember that 25% of the nitrogen that you're applying, as UAN is already in the nitrate form, which will freely move if you have water with that. But in other situations, if you look at a dry summer, obviously there's really nothing there to warrant sidedress applications. And if you are sidedressing, earlier is better. We get beyond V6, we know we start seeing some issues with yield penalties.

I think Jeff Vetch and Giles Randall did a study just right before I started in Minnesota, where they did a number of locations, and they saw more decreases in yield from sidedress application. And a lot of those situations where the growers were going too late, and it's a situation where they weren't getting much precipitation within two weeks of that application. So, the nitrogen would have been there, but it would be positionally unavailable to the crop.

Brad Carlson:
That's an interesting point, Dan, because we do look at sidedressing nitrogen as being risk mitigation against the potential for losing that nitrogen. So, in a sandy soil or a heavy soil we might see denitrification. By split-applying it, if it's not out there, you can't lose it. And so, the idea is, well, if you just put it on later in the year, then it wasn't subject to whatever environmental pressure happened to make it be lost.

But that's not without risk, as you just pointed out. There's years where it gets really wet in early June and it's difficult to get back out in the field. And then there's also years where we've had some borderline drought conditions, and sometimes that nitrogen, if you lay it up on the surface, say you top dressed urea, A, you could be at risk for volatilizing it if it doesn't get watered in soon enough.

But, B, it could just sit there because once the nitrogen turns into the nitrate form, it moves into the plant with water movement. If there's no water up there, it might just sit there. And so, there is risk for doing a sidedress also.

Dan Kaiser:
And if I was in central Minnesota, looking at ... if I have a planned sidedress application, really, I'd be looking at sidedressing ... after you get done with beans, you get maybe some of your spraying done, then start on sidedressing. The data shows, if you look at clearly what Fabian Fernandez has done is, if you look at it, a similar yield in situations where there was a situation to warrant sidedress application, that yield was maximized with the sidedress at V2, V4, V6, V8, it all got about similar yields.

It's when they delayed to V12, they started to see yield drop, so that was too late. That's one of the things where I think where some growers get caught is, we know there's an optimal window in terms of sidedressing, but they just wait too long. And then you have a situation like you're talking about, Brad, where all of a sudden then it gets too wet and they can't get back in the field.

So, really if you're in, let's say, a medium to heavier soil, really there's no reason to wait that long. Start at V2 or earlier. Even after spiking, start getting that fertilizer on, because I think it's not worth the risk of not being able to get back out there. Because we know that as quick as that crop is growing, demand's going to peak. I don't think there's really any more risk for going earlier than waiting till later.

Brad Carlson:
No, and we get it. We've been harping on this point for a long time. There's no such thing as spoon-feeding the crop. The crop does not care when the nitrogen was applied. It's either there or it's not there. And so, if you're split applying nitrogen, say you plant it on May 1st, you get to about the third week of May and it's at V2, why are you waiting until the first week of June for it to be at V6?

It just does not matter. For one thing, you're not likely to lose that nitrogen between the third week of May and the first week of June. The weather conditions would have to be extreme. And not only that, but if they were that extreme, you probably could pick that up on a weather forecast at that point in time. If the conditions are fit, just get it out there. That's what's important.

Dan Kaiser:
And you hear of some growers in some of those years having to resort to aerial application with airplanes. That's just not the most ideal in terms of a spreading aspect.

Brad Carlson:
Yeah, it swirls around.

Dan Kaiser:
You're better off just starting early if you can. And that's one of the challenges I know we ran into some of our on-farm trials, is getting the co-op out there. Because a lot of times they're using their spray rigs, and then they switch them over to spreader boxes for the in-season applications. So, they're focused on spraying early on before they can get to you for sidedress.

That's one of the nice things if you look at with the products we have right now, particularly with a lot of the urease inhibitors, is that we have a pretty wide time range with urea, with UAN also for applications. If you're looking at a time crunch, say, in the fall or the spring, delaying application in season with the planned split, it isn't a bad option to give you a bigger window.

Looking at the numbers in terms of efficiency, that's one of the things we'd like to see, essentially is a greater efficiency where we can reduce our rates with a split application. But in many of the medium defined textured fields, if we look at central, southeast, southwest Minnesota, in the West central is we just don't see a difference in the amount of nitrogen required with a split versus a single pre-plant application.

So, the efficiency isn't always there. I would say it would be on sandy soils, but not as much on some of these other ones. Really then it comes down to, I think, a lot of logistical issues. If you're trying to get planted early, if you don't have the time to get the fertilizer on, then that split, I think, works in your favor in lengthening out your application window to get nitrogen on, particularly if you're focusing more on a spring application.

Brad Carlson:
Yeah. I guess to wrap up this conversation about sidedress, we didn't spend a lot of time talking about the variable rate nitrogen stuff, because we'll cover that in a different podcast. But really the split application part, if you just simply are applying a fixed rate of nitrogen, you're split applying it, just simply get it on.

And beyond that, if you're using the split application as a way of adjusting your rate with a variable rate treatment, you do need to focus on what that technology is that you're using to give you that advice, and make sure that you're not violating one of the premises of split application. That being make sure you're not shorting the crop or getting it put on too late. And so, that's really just the keys.

Jack Wilcox:
Brad Carlson Extension educator, and Dan Kaiser, Extension nutrient management specialist thank you for this information.

Brad Carlson:
Thank you.

Dan Kaiser:
Thank you.

Jack Wilcox:
If you have questions for either Brad or Dan, please email us at nutmgmt@umn.edu. Thank you for listening, and we look forward to seeing you next time.

Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota, and their corn check-off investment through Minnesota Corn.

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