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Drew Lyon: Hello. Welcome to the WSU Wheat Beat podcast. I’m your host, Drew Lyon, and I want to thank you for joining me as we explore the world of small grains production and research at Washington State University. In each episode, I speak with researchers from WSU and the USDA-ARS to provide you with insights into the latest research on wheat and barley production. If you enjoy the WSU Wheat Beat podcast do us a favor and subscribe on iTunes or your favorite podcasting app and leave us a review while you’re there so others can find the show too.
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Drew Lyon: My guest today is Dr. Dave Huggins. Dave is a USDA-ARS soil scientist and research leader of the Northwest Sustainable Agroecosystems Research Unit in Pullman, Washington. He obtained his PhD at WSU and has been working with conservation farming systems and precision agriculture for 39 years. His research specialties include soil carbon sequestration, nitrogen use efficiency, and soil health. Hello, Dave.
Dr. Dave Huggins: Yeah. Hi, Drew.
Drew Lyon: So we just kind of finished up harvest here not too long ago, and as I drove around, I noticed people with balers out there baling straw, people out there with disks disking straw, and it seems like this time of year, we always come up with questions about straw production. Should you be taking it off? Should you not be taking it off? To start off, how much residue does wheat actually produce?
Dr. Dave Huggins: Yeah, Drew. Good question. And you’re right. There’s just a considerable amount of thought about and activity concerning straw removal from fields and also either mechanically via harvesters, you know, and baling operations or burning as well. and so when we start talking about, well, how much residue does wheat produce, the kind of rule of thumb is, for every bushel that you have, there’s 100 pounds of residue. So let’s just use an example of 100-bushel winter wheat crop that would produce about 10,000 pounds of residue or about five tons of residue. Of course, that varies considerably across the field and can be much more than that in high-producing areas and then much lower than that in low-producing areas.
Drew Lyon: It seems to me that when the rains come can affect that, you know. So sometimes you can have really low-yielding but lots of straw. There’s a lot of variables. So 100 pounds is a good rule of thumb, but that’s all it is. It’s a rule of thumb.
Dr. Dave Huggins: Yeah. It’s just a rule of thumb. And again, you’re right. You know, it comes back to the harvest index, you know, how much grain is produced, you know, divided by the amount of residue in grain. That’s called the harvest index, and that can vary quite a bit depending on where you are in the field even and then the kind of year that we’ve had.
Drew Lyon: Okay. So how much residue is needed to maintain soil organic matter? Do we need all of that five tons from that 100 bushel or can some of it come off and some of it remain, or do we know that?
Dr. Dave Huggins: Yeah, good question. If you look at the amounts of residue that needs to be returned to the soil every year in order to maintain soil organic matter, it comes up to about 4,000 to 5,000 pounds or 2 to 2.5 tons of residue. That’s every year, okay? So you have to consider not just the winter wheat crop but the other crops that are also in sequence or in rotation that you may have and whether or not they’re meeting that bar in terms of residue production. And, of course, you know, if you’re fallowing, of course, there’s no residue produced that year, so you have to have other years that kind of compensate for the lack of residue that’s produced. Also, grain legumes tend to produce a lot less residue, as everyone knows. So, you know, a ton of garbs, for instance, can produce maybe 2,000 to 3,000 pounds of residue. So you don’t even have, like, a ton or so of residue during that portion of the crop sequence that you may have. So you need 2 to 2.5 tons, and I just mentioned, for 100 bushels, we got five tons. So for the winter wheat residue production, we’re probably twice as much as we need for that year. And then you have to start thinking about the other years. And often, the winter wheat has to compensate, then, for other crops in the rotation that don’t produce enough residue in order to maintain that soil organic matter levels that you currently have. And, of course, you can also say, okay, do we have enough soil organic matter now? And over time, of course, we’ve, you know, declined in soil organic matter, and we still are across the Palouse. So there’s definitely locations where we don’t have enough soil organic matter, particularly, you know, those upland positions that have been historically eroded, et cetera, and don’t produce enough or a large amount of residue even now just because they’ve been impacted in terms of their yield. So yeah, to answer your question, there’s portions of the field where we’d really like to have, at least in the annual cropping region, about 3% organic matter, and we’re not there in terms of the amount of organic matter now. And if we’re harvesting from those low-yielding areas, that really still, you know, projects us downward from the standpoint of soil organic matter, whereas some of our bottoms, of course, where we produce lots of organic matter, you know, lots of residue, then it’s not as much or a concern in terms of maintaining those soil organic matter levels there because a lot of residue is often produced if we’re not fallowing.
Drew Lyon: Okay. So in a lot of these systems, we aren’t producing a lot of excess residue, if you think across the whole system, three-year rotation here in the high rainfall or intermediate or the two-year rotation out west, so —
Dr. Dave Huggins: Yeah, definitely. If you have fallow in the rotation at all, you’re not producing enough residue if you’re harvesting residue, and I think I mentioned already that the harvesting is about 50% efficient. So, you know, that can change, too. There’s some baling operations that are more efficient than that, upwards of 75% or more. And in those cases, even more residue is being removed. So that’s kind of the assumption based on — Or that’s the data based on that assumption of harvest efficiency. And you’re right. The winter wheat kind of compensates a lot in terms of the residue that we produce to make up the difference in terms of what we need per year when we have low-residue crops like grain legumes or fallow. If we were just talking about canola or spring wheat, we produce just about the right amount to maintain — If we’re looking at about a ton or more, then we’re probably maintaining levels of residue for those particular crops in rotation.
Drew Lyon: Okay. So in addition to carbon for soil heath, soil organic matter, residue also contains nutrients. They don’t all get transferred into the grain. Some of it’s still in the straw. How much nutrients is in the straw, and how much are we hauling off then if we take the straw off?
Dr. Dave Huggins: Yeah. So you can look at the concentrations of various nutrients that are in the straw itself. And you’re right. Some of the nutrients, particularly nitrogen, et cetera, a lot of that is translocated up to the grain. And so the concentrations in the residue itself tend to be pretty small in those cases, but then we’re hauling up a lot of residue. And so we can kind of go through in that 100 bushel example, again. So say we have the, you know, 10,000 pounds of residue. And if you look at carbon that you mentioned first, it’s about 43%, the residue’s about 43% carbon. So from that standpoint, that’s, you know, what, 4,300 pounds of carbon that’s produced, then you’d be hauling off about half of that in a typical baling operation. And that is about — you know, it’s about a little over 2,000 pounds of carbon that you’d be hauling off, and that is about — and leaving as well [ laughter ]. And so that is about the amount of carbon that you would need to maintain organic matter levels. But again, remember that this carbon, it’s reduced carbon. This is the source food for [ laughter ] all of microorganisms that are decomposing, residues that are releasing nutrients, and the healthy soil really needs these carbon inputs in order to remain healthy. It’s a real driver of soil health and soil organic matter. And so, you know, that carbon source is a really important one from the standpoint of being food for a lot of microbes. Nitrogen, again, the concentrations are going to go way down, about .4%, okay? The residue is typically about .4%. So going back to our example of 100-bushel wheat, 10,000 pounds of residue. That’s about 40 pounds of nitrogen that’s standing in that standing residue that’s out there. And if we take half of that, that’s about 20 pounds, and it’s being removed in a typical baling operation. So again, .4% small. You can do the math yourself in terms of how many, you know, bushels per acre you’re producing and how much residue that represents. And you know, for that 100-bushel residue, the kind of rule of thumb there would be about 40 pounds of nitrogen, and about half of that with a 50% baling operation. Moving on to phosphate, that’s even less concentration, the residue. It’s about .15%. So doesn’t sound like very much, but when you start multiplying it by 10,000 pounds, then that is about 15 pounds of phosphate that’s in that residue for that 100-bushel crop. And again, half of that, you know, seven /eight pounds would be hauled off in terms of a baling operation. Sulfur, it’s about .1%. And so, again, we got about 10 pounds then of sulfur that’s actually in that residue and about half, five pounds or so that would be hauled off in a baling operation. And then potassium’s another one, and it’s a little bit interesting in that potassium occurs as a free ion, and much of it doesn’t go up into the grain. It remains in the residue itself, and the residue is about 1%, give or take, of pot ash. And so there’s about 100 pounds, then, of pot ash in the residue. But that’s before it rains [ laughter ]. And since it’s a free ion in the residue, if we have, you know, upwards of a half-inch of rain or more, then much of it gets basically leeched right out of that residue and goes back into your soil. And so consequently, if you’re harvesting after it rains [ laughter ], which no one likes to do, and taking the residue off, then it’s going to have a lot less potassium in it, but if it’s before, if it’s dry conditions and we’re hauling it off, then yeah, that 1%, and that’s about 100 pounds or about 50 pounds of pot ash that’s being taken off. Many of our soils have a lot of pot ash in them, and so it may not be a concern, particularly as we go west, but there are upland areas and eroded areas in our fields now that are potassium deficient, and we’re seeing more applications of pot ash for a fertilizer in those locations.
Drew Lyon: Okay. Yeah. So sounds like that’s something growers should really factor in as they’re thinking about removing straw. Just, what is it going to cost me to replace those nutrients, and can all of them actually be replaced before they decide they’re going to haul off their straw for some other reason?
Dr. Dave Huggins: Yeah, it’s definitely — You know, I’m a soil scientist, Drew, so it’s definitely a soil scientist perspective. There’s tradeoffs here for sure, you know, in terms of removing residue definitely can facilitate subsequent tillage or planting operations. And so, you know, there’s definitely reasons why farmers would want to remove residue just from a logistic, production perspective, but from a soils perspective and from the standpoint of maintaining soil organic matter, soil erosion also starts to figure into here. If you don’t leave enough residue that’s there to cover the ground in subsequent tillage operations, then you are going to have to look at, you know, maintaining at least 30% to cover to prevent erosion from residue cover of the soil. And there’s other factors, too, that are, you know, less tangible. Leaving standing residue in the field tends to capture snow. And so you have much less variable snow distribution across your field if you have standing residue, and that could be beneficial for some of those upland areas in terms of recharging those soils. Interestingly enough, even the annual cropping area, some of those upland positions don’t recharge as much as they could simply because — particularly if much of the precipitation in the winter is coming as snow, it blows off, et cetera. But the standing residue helps to hold it in place and can contribute quite a bit of soil water storage during those years. So yeah, and the replacement value, you know, looking at those nutrients, of course, depends on the cost of those various nutrients, but you could start to add it up, and it can add up. And will that occur that year, you know, directly? No. A lot of those nutrients in the residue cycle over time, but if we remove residues, you know, year after year, then we’ll definitely be impacting soil organic matter levels as well as the nutrients that were in those residues over time.
Drew Lyon: Okay. We’ve kind of focused on baling or straw, but you mentioned earlier burning. How does burning compare to baling? I know there’s probably a lot of variability, right? You can have a really good burn, you know. You can have something that burns a lot more residue than another burn might have, but in general, do you have a sense for how those two things compare, burning versus baling?
Dr. Dave Huggins: Yeah, it’s a good question, and you’re right. A fall burn can be a hot burn and burn more residue than, say, a spring burn, which we kind of refer as a cold burn or, if the residue’s wet, can be a pretty cold burn. If you look at — But the burning doesn’t — As most of you probably know, the burning isn’t 100% efficient. It’s not like it’s going to burn all your crop residue in that field. In fact, we’ve done studies on this, and often, we’re looking at, you know, on average, 60 to 70% of the residues being burned. And, you know, of course it leaves behind the ash, so it’s a whole different process rather than bailing as a direct or a physical removal, right? The burning or losses are due to the oxidation basically of nutrients that are in that residue and the carbon, so that happens during the burning process. And then we also experience losses with the soot that comes off of there. So they’re kind of the particulates that are in the smoke that we also lose, and this was something that actually took us by surprise. And that’s a major loss of potassium that goes out through the burning process just in the soot, the smoke that’s leaving the field. And we can lose about 40 pounds of pot ash that way per acre with a burning process in the fall. Now, in the spring, if we do that same burn, the pot ash loss goes from 40 pounds down to about 3 pounds because what happens during the course of the winter is that the water comes, leeches the residue, and it takes the pot ash right out of that straw. So that’s returned, then, to the soil itself. So the timing of that burn has an impact on what kinds of losses we have in terms of carbon in particular, but also some nutrients. Now, nitrogen losses, we’ve done field scale averages. You’re looking at between 10 and 15 pounds of nitrogen loss. Phosphate, much less, about one pound or so, as well as sulfur, about the same. It’s in that one-pound range. So pretty low levels for the most part in terms of losses that accumulate, but, you know and then — Yeah. So that kind of gives you an indication that baling is probably a little bit higher losses overall from the standpoint of nutrient export than the burning itself.
Drew Lyon: Okay. So just something growers need to be aware of, that there may be reasons, financial or otherwise, for baling, but that they should be aware that there are some costs, particularly in the long term with soil health and nutrients. Is that fair to say?
Dr. Dave Huggins: That’s correct. And I’d add in, you know, just the water conservation and the soil conservation aspects of it in terms of erosion processes that need to be considered as well as just water conservation.
Drew Lyon: All right. Well, an important topic. Thank you for spending some time with us to talk to us about crop residue and the effects of removing it from our fields.
Dr. Dave Huggins: Yup. Thanks, Drew.
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Drew Lyon: Thanks for joining us and listening to the WSU Wheat Beat podcast. If you like what you hear don’t forget to subscribe and leave a review on iTunes or your favorite podcasting app. If you have questions or topics, you’d like to hear on future episodes please email me at drew.lyon — that’s firstname.lastname@example.org –(email@example.com). You can find us online at smallgrains.wsu.edu and on Facebook and Twitter @WSUSmallGrains. The WSU Wheat Beat podcast is a production of CAHNRS Communications and the College of Agricultural, Human and Natural Resource Sciences at Washington State University. I’m Drew Lyon, we’ll see you next time.