Fall manure application: Optimal strategies for Minnesota farmers
Advancing Nitrogen Smart, from the University of Minnesota Nutrient Management Podcast:
“Fall manure application: Optimal strategies for Minnesota farmers”
October 2, 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 Advancing Nitrogen Smart, a special series that connects the science of nitrogen in the environment to practical, efficient nitrogen fertilizer management for Minnesota farmers. I'm Jack Wilcox, and I'm a communications generalist here at U of M Extension. As always, here to explain all this, we have Brad Carlson, an Extension educator. Today we have a special guest. Melissa Wilson, manure management specialist, also here at Extension.
Today we're going to talk about fall manure application - important dos and don'ts, nitrification inhibitors, soil temperatures, and more.
Brad Carlson:
Thanks, Jack. We've been following through some of our curriculum material for Nitrogen Smart with these podcasts. Those that have attended in-person at these meetings know that it's not just always Dan and I. Particularly, one of our first advanced sessions that we did was specific to manure. Glad to have Melissa here this morning. Our first one, but not going to be the last, because that was a fairly large curriculum. We've been sort of timely with these podcasts. Yet on the other hand, we want them to endure for future use, also. Today we're going to deal with a subject, at least at the moment, is current. For future reference, it's going to be out there for a while.
Melissa Wilson:
Yeah. Thanks for having me. It's always fun to do podcasts with this crew. Thanks.
Brad Carlson:
Melissa, the topic we're talking about today is going to be dealing with fall application. In the presentations, when we do these live, you had some survey data that shows that roughly about 65% of manure is applied in the fall. Then we were looking up some data prior to recording this, and that's changed a little bit. We know that the work year for most farmers is dependent on what the weather is like and the amount of time that's available, and so forth. Fall, typically speaking, once harvest gets rolling around, there tends to be a window in there. Obviously, the chance of frost becomes high by the middle of September, so our growing season's over. The ground's really not frozen most years, until we get to, oh, I think the averages tell us early December. It gives us a long window. That just tends to be what works for a lot of farmers.
Melissa Wilson:
Yeah. I would say that's probably one of the main reasons why we see manure application in the fall. It also gives time to muck out the barns, clean everything up heading into winter for the animals for health and safety reasons, too.
Brad Carlson:
Yeah. We've looked at this over the years, as far as management practices, and we talk about the four Rs. Obviously, when you're using manure, your source is stuck. You're going to have manure. Then it's a matter of modifying rate, and the timing, and application, and how you apply based on what your realities are.
But because of some of that stuff, the fall application does end up being what works for a lot of farmers. I guess one of the things that we've been looking at is, particularly with all the issues relative to water quality, is wanting to, if we can, mimic our fall application practices to the same things that we do relative to commercial nitrogen applications. However, because it takes time to get the manure applied, and in a lot of cases, you're using commercial applicators who can't just show up the day you want them. They've got their other customers too, that they've got to fit in.
There's a wider window there in the fall. Then it does require a little extra care to try to prevent that manure from getting in the water, and to make sure it's there for next year's crop.
Melissa Wilson:
Yeah, absolutely. I think one of the hardest parts about fall application is just that shifting temperatures. The one complicating factor with manure is that you don't just have inorganic nitrogen, like you do with fertilizer. It does have an organic portion. The ratio of organic to inorganic does change, depending on what species you have. With our manures that inherently have more ammonium, these best management practices are going to be really important, particularly timing in the fall.
As we're seeing our falls in Minnesota have been getting progressively warmer, in some cases progressively wetter. Of course, this changes from year to year. But overall, we've been seeing this warming and wetting trend. That also complicates this manure application timing thing, because you want to get in in good conditions. But you also want to make sure you're keeping that nitrogen in the form where it's not going to be lost to the water.
Brad Carlson:
Well, I know relative to the soil being wet, a lot of farmers feel, and it's kind of true, that if you go out in the fall and it's wet, and you compact things, and you chunk it up, and so forth, you got freeze thaw in the winter, and then you got a chance to do secondary tillage in the spring and clean that up. Well, that turns problematic in the spring because, in the average winter, we start the spring wet and saturated. Now you're playing a time game, as far as when does the soil finally get fit to be able to go out and do that, and when do you need to get planted.
Melissa Wilson:
Yeah. We're going to be starting some research where we're looking at timing aspects, including fall versus spring. But we already do know some things about fall for some of the best management practices. One of those is waiting until temperatures are cool, if possible. There's also the aspects where you can use nitrification inhibitors, too.
The timing aspect is really interesting. We've actually done some experiments with jars of soil, different soil types, and we've added manure or not added manure to them. One set we kept on the counter in the lab, so it was roughly 70 degrees. Another set, we put in a refrigerator. What's that, 40 degrees Fahrenheit, or so? We just let it sit for a month, and then came back and measured nitrate ammonium in the soil. It was actually astounding how much more nitrate there was in the jars. It didn't matter, soil type or manure type. There was still way more nitrate in those soils in the jars that had been left on the counter than the ones that had been left on the refrigerator. Even just doing this little experiment ourselves, we actually saw that it really does make an impact, the temperature.
Brad Carlson:
Yeah. The other aspect of that, we talk about this really with everything with fall nitrogen applications, is the temperature is a major factor. But the other thing you have to realize, if the soil's warm, typically we're talking about earlier. The other factor is time. When you're dealing with the warm soils, you're also usually dealing with earlier, and that means you've got more time for bad things to happen. That's also a double whammy there.
The one aspect, and we're not going to spend a lot of time talking about this today because it's going to be a topic for future discussion, but when we are looking at these earlier applications, we're also hoping to incorporate a cover crop in there, trying to arrest some of nitrogen, if it turns it into nitrate in the fall. But we should at least mention that that's part of the management for applications when there's still enough warmth and days left to get a cover crop germinated, that that could be part of the situation, also.
We have dealt with, in a lot of the discussions we've had, one of our Advanced Nitrogen Smart sessions is dealing with climate. When we look at the rate of mineralization based on the soil temperature, while we recommend the soil temperature being below 50 degrees for fall nitrogen applications, getting back to this whole time thing is, the other part you realize is the days are getting shorter as we move farther, and it's getting cooler. The temperature is going to continue to fall. When you apply at 50, actually the curve of mineralization and the rate of mineralization at 50 is not great. It's certainly a lot better than it is at much warmer temperatures. But the thing about it is, a week later, it's probably more like 45. And a week later, it's probably at 40, and another week later potentially down to freezing. The rate of nitrification doesn't completely goes to zero at all, but it gets almost zero by the time we freeze the soils.
It's a combination of what the soil temperature is, and then how long it is before we freeze soils. On average, we're looking, at least in southern Minnesota, most of the time these days, this last decade or so, we're freezing about the first week of December where it's frozen and stays frozen.
Melissa Wilson:
Yeah. One of the things that Brad alluded to as well is it's like a continuum. It's not a hard stop where, at 50 degrees Fahrenheit, these microbes in the soil just stop doing their jobs or their work. It's a continuum. If you're applying early in the fall, 60 is better than 70, 65 is better than 75, and so on and so forth. The closer you can get to that 50, the better. If you can wait until you get to 50, that's our best management practice. But realize it's a continuum.
Brad Carlson:
I think particularly the farmers that do their own application really got to be keying in on that. If you're using a commercial applicator, you're at their mercy for when they can get to you. But when you got your own equipment, you really should be keying in on that 50 degree temperature. You're probably watching the weather forecasts, also. If it looks like you've got some major, a wet pattern coming in, that might be a different story. But all things being equal, you probably want to key in on 50 and later.
Melissa Wilson:
Yeah, absolutely. I think one of the other things that is important to consider with manure too, is that, again, because of those different nitrogen forms, we have two processes that are happening that are temperature dependent. The mineralization, which is the organic nitrogen into ammonium. Then the nitrification, which is the ammonium into nitrate.
In manures where we have a lot of organic matter, think your bedded packs, anything with a lot of bedding in it, those have that two steps that they have to get through before they get to nitrate. Nitrate is the one that we worry about. Versus something like our swine manure, our liquid dairy manures, which tend to have a lot more ammonium to begin with, they're much more likely to get to nitrate faster in these warm conditions.
These are the ones we're talking about being particularly important for this temperature aspect, to make sure. Those, you definitely want to wait as long in the fall as you can, until the soil temperatures are cool.
Brad Carlson:
What do you think about poultry compost?
Melissa Wilson:
Poultry manure is a little bit of an enigma. It has all the bedding in it, but it typically has so much nitrogen in it that even if it's a small proportion that's mineralizing and nitrifying, it's still a lot more than some of our other manures. That one, we definitely recommend waiting until later as well. But note that it does have more of a reserve of nitrogen in it than some of the other manures too, just because there's literally so much more nitrogen in a turkey litter, or something.
Brad Carlson:
I know that a lot of ... I guess it depends on where you're at and what your source is, but there's a lot of farmers who, that gets hauled, and then it just waits in a pile until they do the application. I do tend to see that going on a lot later in the fall.
As far as other concerns with fall application, Melissa, I guess we've talked about nitrification inhibitors as being a little bit of a part of this puzzle. What role do you think that they play these days? We've been back and forth with those, as far as whether they are actually recommended or how often do they work. Some of our neighboring states have had some mixed results. Let's talk a little bit about that.
Melissa Wilson:
Yeah. Nitrification inhibitors are basically something that's, again, stopping that nitrification process. But they only work for so long. They can probably buy you a couple extra weeks before you get to that 50 degrees. They're not going to buy you months at a time, though. Keep that in mind. Some of our research has shown that if you use nitrification inhibitors in September, but soils temperatures don't get cold until November, the nitrification inhibitors aren't going to make it. However, more of an early October thing, and then soil temperatures get cool in November, those usually last a little bit longer. It has to do with, again, those microbes in the soil. They're breaking down the nitrification inhibitor, too. Again, the warmer and longer you have it out there, the more likely it's not going to last.
But you alluded to whether it works with certain manures or not. First off, we only really see it being applied with liquids, because it can be mixed well with the liquid manures. We've seen mixed results with dairy, but we've seen decent results with swine manure. Research in Minnesota and Iowa has shown pretty decent results with the swine manure. It's out in Wisconsin, where they started working on it with dairy manure, that we've seen mixed results.
Brad Carlson:
Yeah. It should be pointed out, I think the primary product that's out there is Instinct, or one of the forms of Instinct, which is nitrapyrin. Because that's classified as a pesticide, there's been some issues about using that product and having to be a licensed applicator.
If we go back, when that product first came out on the market, there was some suggestions that perhaps you could just simply apply the Instinct, and then put the manure on. One of the things that we've gone through with some of our other Nitrogen Smart sessions, and Dan did some calculations looking at the concentration of nitrapyrin in the soil for it to be effective. We really don't recommend that practice.
That applying Instinct, first of all, it's volatile and it degrades in the sun real fast. But in addition to that, when you spread it out like that, it's questionable whether it's even in the concentration to make it work. Putting it in, loading it in with the manure ensures that it's at a higher concentration and in the same location that the manure is at. I think that's an important point to emphasize.
Melissa Wilson:
Yeah. I think most of the research that's been done has injected the Instinct with the manure so it's in high enough concentrations to actually work.
Brad Carlson:
Yeah. There was, early on, I think in some cases, the product was being actually put in with the manure, and agitated with it. There's been some questions about the corrosive nature of nitrapyrin, and not necessarily wanting to run that through the system. I think most manure applicators now are set up to just trickle that into the stream as it's getting pumped out. Is that correct?
Melissa Wilson:
Yeah. At least, the systems or operators that I've seen using it have a separate tank on top of their tank. Then it injects a stream of nitrapyrin into the manure stream as it's exiting, rather than having it mixed in with the entire stuff.
Brad Carlson:
Let's talk a little bit about what research has been done. I know there's been projects in Minnesota, and Iowa, and Wisconsin are probably the ones that are most applicable to the folks in Minnesota. Give us a little bit of an overview of what's been looked at.
Melissa Wilson:
Yeah. Jeff Vetsch, down at the Southern Research and Outreach Center, did this research over four years. They compared manures, liquid swine finishing manure they applied in October or November. They did without Instinct, that's our standard practice. With a full rate of Instinct, and then with a double rate. Because, you know, us scientists are like, "Let's just double it and see what happens."
What they found is they looked then in the spring at soil inorganic nitrogen down to a foot. Then they looked at yield. The soil inorganic nitrogen in the spring, whenever Instinct was used, or nitrapyrin was used, there was a lot more nitrogen left in the ammonium form than in the nitrate form, compared to where there was no nitrapyrin used. In that case, it was almost 50/50, the ammonium versus nitrate. It seemed like it was doing a pretty good job of keeping the nitrate in the ammonium form, which is what it's supposed to do.
But the question always is, does this actually translate then to yield? Because you're paying more for this product, so do you get more yield then?
Brad Carlson:
Well, one of the issues always with nitrapyrin, it's never a question of whether it works. It always works. It's a question of whether it worked long enough and at the right time for it to have made a difference. Did it wear off before there was still a critical point in the potential for losing nitrate?
Melissa Wilson:
I guess the question always is too, when you're putting it on the in the fall, that's such a long time period.
Brad Carlson:
Right.
Melissa Wilson:
People are always worried that it's not there. In this case, averaged over four years. They had all kinds of different seasons. It was still there and still working in the spring.
Brad Carlson:
The one aspect, and we just got done talking a little bit about soil temperature, is that I've always found intriguing is, they did two timings of application. They did October and they did a November application. While they found that the Instinct made a big difference in October, it didn't make as big of a difference as if they just simply waited to apply in November.
Melissa Wilson:
Yeah.
Brad Carlson:
The November also made a difference. It's not just a matter of, "Well, we could put on Instinct, and then we can apply early." Yeah, if you applied early, Instinct helped a lot, but it didn't help as much as just simply delaying the application until the soil temperatures got cool.
Melissa Wilson:
Yeah. That's exactly what we found, or what they found in the yield results is that, if you did not use Instinct in October, yield was 15 to 20 bushels lower than when you did use instinct. The rates didn't really matter as much, the full rate was sufficient compared to the double rate. But if you went into November, the yields were almost the same, regardless of whether you used Instinct or not. In that case, maybe it doesn't make sense. If you can wait, maybe it doesn't make sense to use the Instinct.
Brad Carlson:
That could also be a portion of what you talked about earlier, that in a lot of cases, we're also trying to mineralize that nitrogen first, and then it's got to be converted into nitrate. That extra step might be some of that difference. How does that contrast with what they found in Iowa and in Wisconsin?
Melissa Wilson:
In Iowa, it was almost the exact same study, a very similar set up. They found very similar trends. The difference was in Wisconsin, when they moved to dairy manure. They did it first where they broadcast the dairy manure. That was just their first test, with and without Instinct. They think that it was, like we talked before, the concentrations just weren't high enough so they didn't really see good results.
But then, they did move into injecting the manure. They did it at three different sites, two different years each year at three sites. They did early fall, late fall, spring application with and without Instinct. They actually only found a difference in two treatments out of all six of those site years. In this case, it's just so variable that they don't think it makes sense to recommend it in Wisconsin. I tend to agree. I think that's probably because the dairy manure has a lot more in the organic form. The Instinct isn't acting on that. The Instinct is working on the inorganic portion, and that's a lower part of the manure when it comes to dairy.
Brad Carlson:
Sure. All right, we're wrapping this up. I'll guess, I'll summarize the soil temperature stuff and say we'd like you to wait until it's 50 degrees, but if you can't, 55 is better than 60, and so forth. The cooler, the better.
Melissa, maybe you'd like to just summarize what our recommendations are for using nitrapyrin and manure applications?
Melissa Wilson:
Yeah. For nitrapyrin and nitrification inhibitors, if you want to put on a few weeks early, you think the soil temperatures are getting close, but they're not quite there, but you need to put on the manure because we know that happens. The nitrification inhibitors can work for liquid manures, particularly liquid swine manure. For dairy, it's still out whether it's useful or not. Unfortunately, we don't have a product like that for our solid manures at this time.
Jack Wilcox:
Have a question about something you see on your farm, send an email to either Brad or Melissa at nutmgmt@umn.edu. Thank you for listening. 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 checkoff investment through Minnesota Corn.
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