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Welcome to University of Minnesota Extension's Nutrient Management Podcast. Each month we bring you the latest research in nutrient management for crops and how you can incorporate the latest tips and best management practices to your farm.
Hello and welcome to Advancing Nitrogen Smart, the special podcast series from University of Minnesota Extension. I'm Jack Wilcox in communications here with Extension, and I'm joined today by Brad Carlson, Extension educator out of Mankato, and Anna Cates, Extension soil health specialist here in Saint Paul.
Jack Wilcox:Brad and Anna, let's talk about how covers affect residual nitrogen and water quality.
Brad Carlson:Well, we've talked a lot in the past about a lot of our water quality issues stem from residual soil nitrate when the soil freezes up, and that particularly our soils continue mineralize nitrogen as long as the environment is good, as long as it's moist and it's warm, it's going to still mineralize and release nitrate into the soil from the organic matter. Our crop typically, most of our corn crop is done taking up nitrogen, you know, for the most part around Labor Day, but it's not gonna... On average, it's not cooling down till toward the end of October. And so we got, you know, nearly, depending on the year, nearly two months where we're mineralizing nitrate out of the organic matter and then just leaving it behind. You know, in addition, there can be some inefficiencies with the crop, so there could be some carried over fertilizer that was applied and so forth.
Brad Carlson:And so one of the big theories out there is that, well, let's grow a cover crop and take that nitrate back up out of the soil so it's not just loose to be lost. You know, so it is the ultimate, it depends how well it works because it really kinda comes down to did it grow and how much did it grow, and then how much of that... How efficient was it in getting that nitrate picked up out of the soil.
Anna Cates:That's right. And that's gonna depend on the crop, the timing of growth, the amount of nitrogen that's in the soil to scavenge, probably three or four other things I'm not thinking of right off the top of my head. But everything affects this outcome. And, you know, cover crops in Minnesota, if you plant them late and you don't get a lot of spring growth, you might just get 500 pounds of biomass, which then if you apply that 1% nitrogen in the biomass, that's like five pounds of nitrogen you took up. So it's not that much necessarily.
Brad Carlson:Yeah. And you know, and I think ultimately we're hoping eventually that we have some guidelines that just says do this, this, this, and this, you know, the whole cookbook thing. You know, obviously, there are guidelines out there. I mean, we'll talk a little bit about that as far as what we recommend and where we recommend it and some of those things. But nothing is really set in stone that, you know, you're growing a corn crop and do this, and now you're going to solve a big portion of your problems.
Brad Carlson:We just don't really have that answer yet. And think ultimately, you know, like I said, stand establishment and early growth are the key. We've been doing a lot of looking at early season crops. That's kind of the simple one because if what we're worried about is getting germination and growth, obviously, if you got small grains out there, you're able to plant a cover crop in August. If you got canning crops, you know, peas particularly are getting harvested anywhere from July and into August.
Brad Carlson:Those are easy crops to get something planted provided it rains, which, of course, that's not always guaranteed either when we're talking about into August. But even corn silage, you know, if that's being harvested the second week of September beyond the fact that you're... Now the crop is gone and you can plant something out there, you're not dealing with all the residue that's left on the surface from harvest that you normally would. You know? So there are some scenarios where I think it's just I don't know if we wanna call it a no brainer, but we're almost to the point where we just...
Brad Carlson:That's just an automatic recommendation.
Anna Cates:Yeah. I think that for those those early harvested crops, then that's easy to get the cover crop out there. You probably are gonna save yourself some time cleaning out your ditches because you're gonna reduce the wind erosion over the winter, where otherwise you just have a big empty field after harvesting silage. So I think that it starts to make more and more sense for a number of reasons, not just for perspective.
Brad Carlson:Yeah. We're not gonna go deep into the the issue of livestock, but, you know, a lot of the producers who are harvesting silage are planting cover crops, and they're actually getting a a harvestable forage out of that crop yet in the spring as another crop. You know, we don't we don't look at having a lot of opportunity to double crop in Minnesota, but but this is this is one of the few instances I know, you know, particularly, there's there's some that I'm aware of in Southeastern Minnesota. They're getting some pretty significant growth of of rye, and then they're able to harvest it and then silo it and and actually have a a forage crop that that they're putting up and then coming back with with soybeans afterwards.
Anna Cates:Yeah. We've been tracking a lot of cover crop biomass the last few years, and certainly sometimes you get in those spindly few 100 pounds an acre that... Where you still see a lot of bare ground, but, you know, in a good growing area, you'll get a whole ton of biomass out there. And certainly, the guys who are growing silage and then the cover crop, they're prioritizing getting that cover crop in, planting it effectively, and making sure they get their money's worth in forage quality out of it.
Brad Carlson:Well, and you're probably going to invest a little more effort. I mean, the seeding technology on cover crop, you know, some folks have been messing around with just blowing it out over the over the the crop with an airplane. If you're if you're harvesting silage and you want a forage crop to harvest next spring, you are probably going to go in there with a drill and try and ensure that you're getting something to grow too. Yes. I think it's also important to talk about, you know, the fact...
Brad Carlson:You know, obviously, we're talking Minnesota here. A lot of the experience with cover crops nationally has come from farther south where we've got... There's different growing conditions. And I know particularly, you know, the the the the radishes and the brassicas were really popular in some of the central states, really haven't been able to do much with those in Minnesota because they die. You know, they freezes and dies.
Brad Carlson:And so whether you're growing an annual... Planting an annual like that, they've got their place, particularly, like, if, we have prevented planting or or for some reason we're gonna end up fallowing an area, they may have a good spot. But for the most part, we're we're... When we're talking performance for water quality purposes in Minnesota, we're we're we're talking about winter annuals.
Anna Cates:Yes. Absolutely. And I think that where I see people using winter annuals, one, people are excited to see some diversity. Maybe they're gonna graze animals on it in the fall, and they wanna have something besides a grass out there. And then the other is where people are working with programs where they want a diverse mix because they qualify for higher cost share, and they don't wanna plant that many pounds of cereal rye that sometimes is required by a cost share program.
Anna Cates:So throwing in a brassica at a low rate can give you a little bit of diversity and a little bit less biomass that you're gonna have to plant through and terminate and deal with in the spring because you have to have a plan for that biomass. That biomass is great for water quality but it is another thing to manage for the agronomy of the farm. And I think when you say they're the ultimate it depends for cover crops, I think in some ways they're just as much of an it depends as any other seed you're putting in the ground, right? Like it could go great, could not go great depending on what happens, all the things out of your control after you put that seed in the ground. So we always tell people, like you say, if you're serious about planting a cover crop, get the seed in the ground, get it adequately covered, and, you know, if you can give it moisture in an irrigated system, great.
Anna Cates:If not, you know, cross your fingers or whatever your strategy is.
Brad Carlson:Well, let's talk about some specific plant species here because you collected some data. I know you were... You told me when I got the data from you, be careful. This is just... These were just grab samples.
Brad Carlson:This is just an idea. It's not replicated. But there's a big difference in terms of the percentage of nitrogen in some of these, but then there's also a big difference in whether these things are going to grow and how they're gonna perform. So, you know, some of these species that have a high nitrogen percentage probably aren't greening up in the springtime, which is kind of what we've discovered is is really key for reducing nitrate loss to water is if we're not getting something growing in the spring. That's when a lot of the data shows from our drainage plots at Waseca and other places, The majority of our nitrogen is lost in April and May and into early June, and so we're pretty concerned about whether these things are functioning in the spring.
Anna Cates:Right. A four or five inch tall plant April 1 is going to be a lot more effective at preventing that nitrate loss than a two inch tall plant, and having 10 plants per square foot is gonna be more effective than three plants per square foot. Right? Just more biomass April 1 is gonna be more effective during that window. When you think about which plants have a higher nitrogen content, it's usually not because they've taken up more nitrogen from the soil, a lot of the ones on your list are things like field pea and clovers that are legumes, right?
Anna Cates:So they're taking up some soil nitrogen but they're also fixing their own nitrogen. So that's not really a great way to address a nitrate loss issue. The way you get your best bang for your buck on nitrate loss is growing more biomass of something that grows quickly like oats or cereal rye that's gonna have more biomass at a lower concentration, I think.
Brad Carlson:Yeah. So so, I mean, I think we've pretty well evolved to the point now where in Minnesota, just everything is pretty much rye. I mean, there's been a lot of messing around with other stuff, but it seems like we just keep coming right back to to rye. And and, I mean, is... Am I correct in that, or is there any reason to even be looking at anything else because of kinda where we're at with the water quality stuff and the desire to have a greening up in the spring?
Anna Cates:From a water quality perspective, rye is is great. I think people like triticale for forage quality, excellent overwintering but a little better forage quality, and actually they like it because it grows a little slower in the spring. No one studied whether that also was, you know, having a big issue, big effect on how much nitrate it takes up. Presumably it takes up less because it grows more slowly. But people get worried that your rye is gonna take off on May 7 when you didn't have a chance to terminate it right at planting and you're suddenly having a huge amount of competition for your cash crop.
Anna Cates:So, yes, rye is the best one for water quality that we've got in our playbook in Minnesota.
Brad Carlson:There's, I I've, in a previous life I did some work with organic growers and there was always a lot of discussion about the allelopathic effects of rye and weed suppression. Is there a reason to have any concern about allelopathy with rye and it potentially negatively impacting the crop you're growing next year?
Jack Wilcox:We have to play a quick game of "Jack Doesn't Know What That Word Means". What is allelopathy?
Anna Cates:So allelopathy is happening when the rye is growing, right? So if you are having your rye plant growing next to your cash crop plant, then you might have an issue. If you've terminated it two weeks beforehand, you're not gonna have an issue. Allelopathy refers to when a plant secretes chemicals to kill other plants around it like the reason you can't have a garden under a black walnut tree. In the case of rye though, rye can cause yield issues especially in corn for a number of reasons.
Anna Cates:One is that it's tying up nitrogen as it decomposes so starter nitrogen is a solution for that. Another is water if there's a lot of, if it gets dry after planting and your rye has taken up a lot of the profile water while it was growing. And another is disease and there's great work out of Iowa showing that when you have rye right next to your corn seedling you are more likely to have pythium in that corn seedling. So something like strip till where your corn gets a little space from rye, something like having termination two weeks before you plant your corn, those solve all those. But I think a lot of what we have historically called allelopathy with rye could be any number of other issues in competition.
Brad Carlson:Yeah. That's a pretty good point. Let's talk a little bit about some of the research that's been done as far as how well cover crops perform. I know I was involved personally, and I think this was before you came here, Anna, that there was... We had a project that was looking at the whole concept of that we need to or we want to apply liquid manure preferably as late as possible because that prevents it from getting mineralizing and getting into the water.
Brad Carlson:However, cover crops need to get established as early as possible. And so we did a project in Minnesota where we were coming in following corn silage harvest, and we were planting the cover crop and then waiting to do manure application until it was what we would consider good timing. We're waiting until the soil temperature got below 50 degrees and basically tearing apart the cover crop. And this was an interesting study because we did this in Central Minnesota and Southern Minnesota and did it for two years, and I can't even remember how many sites we had. I think it looks like there was maybe as many as 20 of those.
Brad Carlson:But what we discovered though was that the... Where there was a cover crop growing, okay, number one, when we tore that apart to do the manure application, it tended to come back. We didn't usually lose that stand.
Anna Cates:It's very resilient.
Brad Carlson:Yep. And so that was not a big concern. It was like, okay, go ahead, plant cover crop. If it it looks like you tore it apart when you did the manure application, actually, you just kinda moved it around. It rerouted and grew back again in those spots.
Brad Carlson:But the other thing is is we looked at the soil test data showed that we reduced the soil nitrate content in the soil by 50% in the spring, and that's huge. Not huge. Because, I mean, we're looking at wanting to reduce the amount of nitrate running down the Mississippi River by 45%. So if you're reducing the amount of the soil by 50%, now that's not loss, that's potential. You know?
Brad Carlson:Good start. But but at 50%, I mean, that's that's beyond what our target is for nitrate reduction, and so that could really be a big deal.
Anna Cates:Yeah. And it gets back to the question of timing because where did that soil nitrate go? It went into your cover crop. Right? So now it's in an organic nitrogen form.
Anna Cates:It's gonna take some time to break down. It'll be released later in the season. Now, we're terrible at saying exactly when and how much will be released for your cash crops use, but it is not vulnerable at that time of year when we have a lot of leaching losses, it's inorganic forms.
Brad Carlson:Right, and when we looked at the yield in this study the following year, we discovered that there was virtually no difference. And so, you know, it kinda gets back to though what we said before, that whole low hanging fruit, that we're doing this following silage. This wasn't following the grain crop where we were able to reduce by 50% and then had no yield. This was following silage, but it does add credence to it. If you're doing silage, you really need to be looking at putting in a cover crop afterwards.
Anna Cates:Yeah. And the regulations are moving in that direction too. Right? You have more of a window now in terms of when you can apply manure if you have that living cover crop there. Right?
Anna Cates:Everyone's catching up to the fact that your cover crop will survive, and it'll take up a lot of your manure nitrogen.
Brad Carlson:Right. Well, then let's move on to one of my favorite spots in the world, the the drainage plots at Waseca because I think one of the first things I ever did with a university paycheck was climb down the ladder into one of those pits and hold a beaker underneath the tile with a stopwatch and and time how much water was coming out.
Anna Cates:I've seen that data point, Brad. That's a terrible data point.
Brad Carlson:Yes. It really was. So it was so accurate that they since automated that whole process. It really was quite the deal, though, when every day you had to go in for each one of them tiles and you had to Wow. Beaker and you timed it with a stopwatch to see how much the flow was coming out.
Brad Carlson:Anyway, you know, a lot of studies have been done over the years on those drainage plots. And because of the potential for cover crops and because we kinda answered a lot of other questions, we moved into doing research on cover crops on those drainage plots starting in 2016, and the data there kind of reconfirmed a lot of what we already knew. That that was just quite simply that it needed to grow in order to get performance on that. And and I I remember that very first year, Jeff Vetsch was talking about how excited he was because they... The the the reductions that that they had in in in nitrate.
Brad Carlson:That particular study, they had they had obviously a treatment with no cover crop on it, but they were looking at a blend of annual brassicas and I think other things in there. And then they had the cereal rye in there. And when they looked at the residual soil nitrate numbers, it was almost an identical situation to the dairy manure project that we just talked about that, you know, come April 17 the following year, it was almost in half, and that that really was getting towards what we wanted to see for a goal, you know. Then in addition to that, when we looked at the nitrate loss from that particular study, we saw quite a similar trend to that. However, that also produced an interesting result that first year, and that was that we actually discovered we needed a higher nitrogen rate out there to maximize the yield in the plots where we had the cover crop going.
Anna Cates:Right. And that gets back to what we've talked about already in terms of tie up as that cereal rye cover crop is decomposing. If you have all that biomass, it's got that organic nitrogen. It's not available at corn planting or right away.
Brad Carlson:And so that's really a point, guess, you know, a lot of farmers do have that concern. If I grow a cover crop, am I hurting next year's yield? Back to the whole it depends, I mean, it's really a matter of what is that following growing season like. If it's cold, you're gonna have a problem. If it's dry, you're gonna have a problem.
Brad Carlson:If you have perfectly ideal growing conditions, you maybe aren't going to have a problem. You know, when we're talking about, though, an extra 30 pounds of nitrogen, realistically, if that nitrogen is applied early in the growing season, the odds are pretty good that that's not going to end up in the water. The timing of that application and such, I mean, unless unless it's really a great year and the crop just flat out only needed, like, 90 pounds or something, then it's gonna leave it behind residual. That's not very predictable. So that was that was a factor.
Brad Carlson:However, you know, while that looked like it was really producing some fantastic results, what we discovered then coming up after that in 2017 and 2018 and 2019 was we got no growth out of the cover crop, but it's pretty hard to study cover crops when you get no growth out of the cover crop.
Anna Cates:Yeah. Those are never my favorite datasets.
Brad Carlson:Right. Right. I mean, it's almost like, well, what are we what are we even doing here? You know? Because there's nothing even to look at.
Brad Carlson:You know. And so ultimately, you know, ultimately speaking, if you look at the if you look at the graph of the the total nitrate loss, In the end, after three years, the reduction for having the the cover crop out there was about four pounds of nitrogen per acre. It's something. It's something. However, almost all of that occurred in the spring of twenty seventeen, and after that, there was really no difference between them because we had no cover crop growth.
Anna Cates:Yeah. And I wanna point out, so your biomass level in that spring of twenty seventeen was around, you know, 200 pounds an acre so you couldn't look at that and think wow, that's a huge sand of cover crop. That's very moderate. So you're able to achieve about four pounds of biomass lost with a real moderate cover crop. I would say 500 to seven fifty pounds of biomass are attainable in most systems, and then you would probably get 10 pound savings out of your nitrate loss.
Anna Cates:And then the other thing I think that is cool about this study to point out is that it has both soil nitrate and that tile nitrate loss because sometimes when we think about how we are assessing whether cover crops are taking up nitrogen we just look at that soil nitrate. But this also has the edge of field loss so they correlate really nicely in this study but the effect actually looks a little bigger in the soil nitrate than it does on the edge of field loss. So when we think about the data we have access to, this edge of field stuff is so valuable.
Brad Carlson:And I think it's important to mention that this was just in regular field corn and soybeans, so it's the scenario that we've been talking about how difficult it is to get to function. I know when this project was ready for a reset, and I had a conversation with Jeff, and I said, in fact, what I was really encouraging was using sweet corn. But I said, you know, we really... What we really wanna do is we would like to find out what we could achieve as a best case scenario. If we can if we can figure that out, now we can look at when we're using just a regular corn soybean rotation.
Brad Carlson:We can compare compare it to a best case scenario. And so the project since then has been looking at silage harvest and some other things. You've been involved in that, so I don't have the data incorporated into this presentation that I did a couple of years ago because it... We were just still right early into that new project. What have we been seeing since 2020 in that project?
Anna Cates:Well, we're getting a little bit better at growing rye biomass. It was about a foot tall in the spring of twenty twenty five, so I'm sure that that was over a thousand pounds an acre.
Brad Carlson:Well, and I know that I was there helping soil sample this spring, so I know that there was actually really good cover crop growth here in the spring of twenty twenty six also.
Anna Cates:So I think we could say when we're looking at these cover crops over the last several years in the drainage plots, we're not taking up huge amounts of nitrogen in the biomass. Right? You know, maybe maxing out at 10, 15, 20 pounds of nitrogen in that rye biomass in particular, and the annual blend is usually behind that. So that's a little bit, it's not like you're gonna, you know, take a nitrogen credit based on that, but it is a little bit of nitrogen that we can physically see coming up. And then sometimes that has an effect on the tile drainage.
Anna Cates:We have had some dry years so we're not always seeing a ton of drainage in these plots, but sometimes that cover crop does drop the nitrate in the drainage, the nitrate load.
Brad Carlson:And and I I guess that I don't have the the most recent numbers, and and I know part of the problem has been... This has always been a problem with those drainage plots. If you don't get drainage, then you're not getting any numbers. You know? That...
Brad Carlson:That's one of the reasons why we do soil nitrate testing also. You know? So, like, for instance, last fall, in the fall of twenty twenty five, it was pretty dry, and there was no numbers to be collected. And similarly, there was a general moisture deficit coming into 2026. So there's there's really not a lot of data to show on that.
Brad Carlson:However, if we look at some of the older data going up through through, August 2022, there was a decent reduction. The the the average amount of nitrogen lost on a... You know, what we've talked about frequently is pounds of nitrate nitrogen per acre because it's easy to think of that way with respect to applying nitrogen on pounds per acre. Roughly about 10 pounds per acre where there was no cover crop and a reduction down to about six and a half for where we had the cereal rye, and that's pretty good. That's a 35% reduction.
Brad Carlson:So again, we're trying to hit a 45% reduction for our water quality goals. You know, if you can couple some other practices together with this, you know, this does have the potential to have a pretty serious impact in the long run.
Anna Cates:Definitely. Definitely. And in that silage system that we've discussed as being particularly leaky, it's a really good thing to have.
Brad Carlson:Yeah. So ultimately, I guess we'll just have to kinda stay tuned as we try and fit this into our regular corn cash crop and and soybean system. We need to work it out best for the the short season crops first, and then we can start moving on to those other crops.
Jack Wilcox:That was Brad Carlson, Extension educator from Mankato, and then also Anna Cates, Extension soil health specialist in Saint Paul. Brad and Anna, thank you very much.
Anna Cates:Thank you.
Brad Carlson:Thanks.
Jack Wilcox:Do you have a question about something on your farm? Just send us an email here at nutmgmt@umn.edu. Thanks a lot for listening, and we look forward to seeing you next time.
Jack Wilcox:Advancing Nitrogen Smart is proud to be supported by the farm families of Minnesota and their corn checkoff investment through Minnesota Corn.