Replacing Summer Fallow with a Forage Crop with Kendall Kahl


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Episode Transcription:

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 podcast app and leave us a review so others can find the show too.

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Drew Lyon: We regularly have guests from WSU and the USDA-ARS on the show. But today is one of a three episode series I have with colleagues from the University of Idaho for an extended discussion on the role of crop diversity in soil health. My guest today is Kendall Kahl. Kendall is a research specialist and part-time Ph.D. student in the Soil and Water Systems Department at the University of Idaho. She has a B.S. in community and international development from the University of Vermont and an MS in soil science from the University of Idaho. Over the past ten years, her research interests have focused on understanding links between farming practices and soil health and a range of different systems, including organic wheat production, small scale market garden systems, and typical Palouse no-till dryland production systems. In recent years, her research has expanded to include nitrogen and water use efficiency, work in the Landscapes Interest in Transitions Project, an interdisciplinary USDA-NIFA funded grant looking at a few ways to adjust cropping systems to be more resilient. Projected changes in climate. Hello, Kendall.

Kendall Kahl: Hello, Drew.

Drew Lyon: So tell us about the goals of the Landscape and Transition Project and the alternative rotations that you have studied.

Kendall Kahl: Sure. The motivation for the crop rotations in the LIT project came partly from a need to address common concerns in our dryland soils, like declining pH, declining organic matter, and reduced soil biological activity. And these concerns, in combination with increased variability in our climate, puts our farmland and our growers and really our whole food system in a vulnerable position. So the LIT team developed some alternative rotations that would help diversify and or intensify our dryland crop production as a strategy to address our climate, soil and other agroecosystems related vulnerabilities. Specifically, we looked at incorporating cover crops and winter peas and to coming crop rotations, and we did this in replicated strip trials and field scale plots in both the high precipitation zones, say over by Genesee, Idaho, and also in the intermediate rainfall zone near St. John, Washington. And the project was unique in that it was a really well integrated study in that we had scientists from different disciplines, all taking measurements from the same plots. So it helps us look at both the details of our agroecosystem and then also come together and to look and look at it as a whole. Today, I think it would be interesting to focus on our rotations with cover crops, partly because some of the others from our team will be sharing some interesting soil biological results from the winter pea rotations.

Drew Lyon: Okay. So let’s take a look at cover crops. If they’ve become a common pillar in discussions about improving soil health and sustainability, but often in places that have wetter climates than we have here. What what are the opportunities you use cover crops in our dryland rotations?

Kendall Kahl: Given the water limitations we have in our dryland systems, we looked at incorporating cover crops either in place of a cash crop or in place of summer fallow. And this is instead of sort of a traditional cover crop that might be grown over winter or in the early spring, which you can do and say arid irrigated agriculture or in wetter climates. So it’s really the way we grew them is really more of a forage crop and we chose a five way mix that would be suited for livestock consumption. And in the project, we managed it both as hay and then did some grazing and in the field scale trials. So in this way, we’re using again cover crops as a forward, more so, and this allows us to add some plant diversity but still provide some potential for income. And more specifically, we looked at replacing fallow with a spring forage crop in the intermediate rainfall zone that gets around 12 to 17 inches of annual precipitation a year. And there’s fallow every three years roughly. So not the driest part of our area. And then in the high precipitation zone, we replaced, we looked at replacing spring wheat with a winter forage crop to avoid the scenario of delayed spring planting when conditions are wet. Kind of like what we’re seeing this spring. And the thought here is that the forage crop could potentially provide more benefit to the rest of the crop rotation than the spring wheat would. And it would be competitive against spring wheat revenue, which is which can be low in a lot of years. So in this study, we’re using forage crops in the drier areas as a way to conserve water or in the wetter areas as a way to use some of that extra spring moisture with the hope that we’ll see some of the benefits of sort of nutrient retention and cycling further down in the rotation.

Drew Lyon: Okay. So using forage in both directions, both to use excess water and you have it or in the drier areas too to save it. I know in the Great Plains when I was doing similar work, the benefit of forages, at least from one perspective, is that they grow what maybe 4 to 6 weeks shorter time than if you take it to grain. So they therefore use less moisture than if you took the crop to grain so that we felt like forages would be one would be a better approach to try and conserve some of that water for subsequent crops. And it sounds like you’re seeing the same thing, or at least thinking along those same lines.

Kendall Kahl: Right, exactly. That was certainly some of the thought process.

Drew Lyon: So looking more specifically at fallow, what are the benefits and drawbacks of replacing fallow with a cover or forage crop?

Kendall Kahl: Yeah, so some are fallow is typically used as a way to refill the soil profile with water and store that water for the following crop. It’s also used to build up inorganic nitrogen in the soil, like nitrate and ammonium, through the conversion or mineralization of organic matter to plant available nutrients. But summer’s fallow is often not a very efficient way of starting water and nitrogen. We lose a significant amount of water to evaporation off the surface and also through leaching out of the system. And similarly, we lose nitrogen at the soil surface of the volatilization or off-gassing, or at least of that with the water. It can also leave the field at higher risk of soil erosion, depending on how much crop residue is left and how the field is managed during that fallow period. So replacing summer fallow with a cover crop or forage crop uses some water and soil nutrients, typically a little more than what’s lost in a summer fallow period. But it also provides that biomass that can be returned to the system providing additional carbon inputs or organic matter that again can be broken down into nutrients later on. So part of the question is what’s the tradeoff between the moisture and nitrogen loss during the summer fallow compared to what’s used during a forage crop?

Drew Lyon: Okay. So specific to your project, what did you learn over the last four years looking at replacing summer fallow with a forage crop?

Kendall Kahl: Well, it ended up being a good time to study climate uncertainty because annual precipitation and temperature patterns were highly variable over the past four years. As you may recall, from the really wet spring we had in 2020 to the drought we had last year in 2021. And so this allowed us to see how the system worked under a broad range of climate conditions. In summer fallow in the St. John area, water loss from the soil profile during the growing season was between 4 to 6 inches, whereas in the forage crop it was between around 5 to 10 inches again depending on the year. So for a few more inches of water, you can produce a forage crop that can provide some revenue through livestock gain or hay and get those organic matter inputs. Also, regardless of the years, soil moisture content in the spring of the year following fallow or forage where similar. So what we’re seeing is that precipitation over the winter refilled the soil profile water to similar levels going into the main part of the growing season. And this we saw in years with average annual precipitation between 13 and 17 inches. So it seemed to be relatively consistent. However, we did see lower average winter wheat yields following forage compared to fallow in the drier years, particularly 2019 and 2021. For example, in 2019 we got around 80 bushels per acre winter wheat following forage and around 100 bushels per acre following fallow, which was about a 20% decrease. But again, those averages include a lot of variability. Also, if we’re looking at the whole rotation, the question becomes whether the income we get from the forage crop as hay or weight gain and graze livestock, does that outweigh the slightly lower winter wheat yields that we see in some years? And replacing summer fallow with a forage crop is more efficient use of water use over the whole rotation as most of the available water is used to grow crops and produce biomass instead of that, that loss that we see during fallow. And in terms of improving sort of the overall efficiency, one thing to look into further is sort of when to terminate the cover crops to balance biomass production with leaving water for the next crop. Our economic analysis shows potential for income or at least breaking even on growing forages. But with prices and costs of inputs being all over the place the last few years, it’s hard to put an exact number on it at the moment. And my thought is that if you’re looking at income over the whole rotation, having a crop in the ground every year increases your chances at growing a profitable crop, especially with some of these crops that can still produce well under a wide range of moisture and temperature conditions. And so just the increase in that frequency of crops helps reduce your risk overall. As far as the impact of forage crops on nutrients, we were starting to see some interesting shifts. In early spring 2020, which again was that wet spring, we saw a slight lag in nitrogen uptake in the winter wheat following the forage crop which then recovered to similar levels by the end of June about a month later and yielded similarly to the winter wheat following the fallow by the end of the season. And we’re still scratching our heads a little bit over this lag in nutrient uptake, but it seems that it could be related to the decomposing forage crop residue sort of temporarily tying up nutrients and interestingly, we still saw the system recover and produce similar yields. So we are starting to see some of that impact of nutrient cycling of forage crop, we think. Then in 2021, in the drought year, looking further down the rotation in our spring wheat crops, we saw more biomass produced and greater nitrogen uptake in the spring wheat following the forage crop. So again, that would have been two years after the forage crop was actually in the ground compared to the spring wheat in the fallow rotation. And I think this suggests that we’re starting to see the decomposed forage crop impact, nitrogen availability again, as I was just suggesting. And we’re seeing that in the uptake in our crops. But again, there’s this timing dynamic that we’re still trying to understand. One other interesting sort of general observation from the study was that the mineralized nitrogen was high in all of our treatments during that 2020 year when we had a lot of spring moisture. And the idea again being that there was probably more decomposition and mineralization than normal because the conditions allowed for it. And without going into it too much, I think this suggests that when we have the water available to produce high yields, our cells can have the capacity to supply the needed nitrogen, which ultimately can mean we could get away with applying less fertilizer and still produce those high yields in the years when we have the water that allows for it. But this, again, is something we need to follow up on more, but it’s sort of a promising bit of evidence. So to summarize, replacing summer fallow with a forage crop seems to have a minimal impact in terms of soil water availability for the falling crop. And though sometimes we see a small decline in yields, at least in that intermediate rainfall zone. I think we’re also starting to see some increase in mineralized nitrogen in the forage crop rotation that has the potential to help us reduce fertilizers. We didn’t quite get there in this study, but it’s something to think about moving forward.

Drew Lyon: I know I’ve read some literature, I think it was North Dakota where they introduced legumes to this system, it actually took like six years before they saw that whole recalibration of the cycling thing. So it takes a little while to happen sometimes in soils. It’s not an immediate sort of thing. So maybe that’s just you’re just getting it started and you’ll see it as you if you carry on the studies a little longer.

Kendall Kahl: Right.

Drew Lyon: Maybe you’ll see that calibration. So that’s the the low rainfall areas or the intermediate with fellow what about the high precipitation areas? What was the impact of including a winter cover crop or forage crop in place of spring crops?

Kendall Kahl: Right. So in the high precipitation zone, we use winter forage two years before the winter wheat. So again replacing the spring wheat phase of the rotation and we did not see a decline in the winter wheat yields throughout the study period, which had been a concern of some of the growers who have expressed interest in using forage crops, again, likely tied to that change in nutrient cycling and maybe nutrients getting tied up. And similar to the intermediate rainfalls zone at the end of the study we started to see a shift towards greater nitrogen uptake in winter wheat and in the forage crop rotation. Again, two years we saw this in the winter wheat which was two years after the forage crop and we saw this during the drought year, even last year. So again, I think this merits more follow up research, specifically looking at the timing of the shifts in nitrogen availability and uptake in rotations with forages or cover crops with the hopes that this would help us reduce fertilizer needs and improve our nutrient use, efficiency and crop resiliency like we’re seeing in 2021 where under stress, the winter wheat following the forage seemed to to do better.

Drew Lyon: Okay, so let’s speculate a little bit here. In a really wet spring like we’re having this year in 2022. What do you think the role for a cover crop is in our crop rotations?

Kendall Kahl: Yeah, this is a great example of when it would work well as a replacement for fallow, where we’d have plenty of moisture likely at the end of this year if you’d would have a cover crop in our forage crop in this season. And similarly, if you had grown a forage crop last year, even during that drought year, it seems that this spring moisture can really help that following crop perform similarly as such as falling fallow, at least based on what we saw during our study. And then in the annual cropping zone, I think right now there’s a lot of fields left fallow just because you couldn’t get in to plant. And one of the thoughts is using a cover crop that you can plant a little bit later, still get some biomass production and use up some of that water. So we don’t have a huge erosion problem come winter when our soils are already full and we get all of that winter precipitation and can lead to quite severe erosion under the wrong conditions. So again, using cover crops, well using cover crops to either pull moisture or help conserve it, but I think they are they have a good place or they have a useful place, I should say.

Drew Lyon: Okay. And I know there’s a lot of questions out there on cover crops and forage crops and the mix between the two. And so this is really important research and I think that growers will be really interested. Is there a place our listeners can go to learn a little bit more about your project?

Kendall Kahl: We have a LIT website that will be a place for we have a lot of the research that is coming out of this project, most of which where we’re just working on summarizing now. So you’ll have to stay tuned and well, we’re hoping to get more out soon.

Drew Lyon: Okay. So LIT is Landscapes in Transitions. So if well, if you provide us with some links, we’ll get those in the show notes so our readers can go take a look at that as the information becomes available.

Kendall Kahl: Great.

Drew Lyon: Thank you very much, Kendall.

Kendall Kahl: Thank you, 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 podcast app. If you have questions or topics, you’d like to hear on future episodes please email me at drew.lyon — that’s — ( You can find us online at 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.

The views, thoughts, and opinions expressed by guests of this podcast are their own and does not imply Washington State University’s endorsement.

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