“If you're in a region where corn grain harvest typically occurs around, say, the historical first frost date and fall growing degree days are going to be limited post harvests, you’re really likely limited sowing to cereal rye or another winter cereal. The conservation benefits associated with overwintering cover crops, so erosion and fall nitrogen scavenging are going to be limited. If that describes your region, then interseeding and cover crop might be a viable alternative. Interseeding of the cover crop and establishing that cover crop earlier also opens up opportunities for increasing biodiversity, because most of the legume and brassica cover crops that growers are interested in need a longer fall growing season window to establish than of post-harvest permits in corn grain systems.
— John Wallace, University of Pennsylvania
This week’s episode of the Cover Crop Strategies podcast features John Wallace, an assistant professor of weed science at Pennsylvania State University.
Wallace talks about why the V4-V5 stage of corn growth is the best time to put cover crops on 30-inch corn. That period allows for cover crop development before the canopy closes. In colder climates, this serves the purpose of lengthening the growing window for cover crops, allowing growers to maximize the benefits covers can bring.
Wallace also talks about how equipment and fertility management can affect the decision to seed or interseed and more.
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Full Transcript
Brian O'Connor:
Hello and welcome to the Cover Crop Strategies podcast. I'm your host, Brian O'Connor. Today's guest, John Wallace, an assistant professor of Weed Science at Penn State University. He spoke with Sarah Hill about using a high clearance grain drill to interseed covers in V4 form.
Sarah Hill:
To get us started, tell us a little bit about yourself.
John Wallace:
Sure. I'm a extension weed scientist working in field and forge crop systems here at Penn State. So growers in Pennsylvania have adopted no-till and cover cropping practices are pretty high rate. So a good bit of my research program focuses on addressing weed management opportunities and challenges associated with conservation practices like interseeding.
Sarah Hill:
Fantastic. Well, let's go ahead and get right to our topic for today. Talk a little bit about the best timing for interseeding into corn.
John Wallace:
Well, I think interseeding timing's really going to depend on the type of equipment you have available to interseed. So most of my research is focused on interseeding earlier in the corn growing season using a high clearance grain drill. So if you have access to a high clearance grain drill, and the plan is to try to establish that cover crop between 30 inch rows, research at Penn State has shown that the best timing's probably going to be around the V4 growth stage. So interseeding at that timing allows the cover crop to establish and achieve some root growth before canopy closure. And so we'll probably talk more about this, but the other considerations about that timing is herbicide programs and fertility management as well.
Sarah Hill:
Great. So why might some growers prefer to interseed cover crops rather than trying to establish them in the fall?
John Wallace:
Yeah, that's a great question and an important point. So if you're in a region where corn grain harvest typically occurs around, say, the historical first frost [inaudible 00:02:14] and fall growing degree days are going to be limited post-harvest, you're really likely limited to sewing cereal rye or another winter cereal. And the conservation benefits associated with over wintering cover crops like soil erosion and fall nitrogen scavenging are going to be limited. So if that describes your region, then interseeding a cover crop might be a viable alternative and the interseeding of the cover crop and establishing that cover crop earlier also opens up opportunities for an increasing biodiversity because most of the legume Nebraska cover crops that growers are interested in need a longer fall growing season window to establish than kind of post harvest establishment permits in corn grain systems.
Sarah Hill:
So have you seen that cover crop establishment rates are better when cover crops are interseeded?
John Wallace:
Yeah, that's a good question. I think it depends on what your standard practice is after corn. In no-till systems drill interseeding is going to improve [inaudible 00:03:23] soil contact and establishment rates relative to say broadcasting cover crops early or even late in the season. And my experience has been that the persistence of the cover crop through the kind of period of environmental stress or warmer period during the summer that's going to occur after corn canopy closure is a much bigger driver than establishment rate. So we typically see pretty good initial establishment when we're drill interseeding cover crops into corn. So if you're comparing drill interseeding cover crops into corn at earlier growth stages compared to say post-harvest establishment using a drill, it'll just depend on what your fall growing season looks like post-harvest. If you have a nice long window post-harvest, you'll likely have greater consistency establishing cover crops post harvest. If you have a short season post-harvest, you might find greater success by drill interseeding.
Sarah Hill:
What are some best management practices that growers should keep in mind when they're interseeding?
John Wallace:
Yeah, that's a great question. It's always good to think about the management goal for interseeding because that's going to help determine the species selection and the approach and some other considerations. So our research suggest that timing is kind of everything. So best management practices need to consider things like labor logistics. So given all the other things that might be going on with farm operations early in the corn growing season, do you have the capacity to be out there at the right stage of corn to really optimize interseeding? So you need to be able to prioritize that practice at the right time. And so from a agronomic perspective, herbicide selection, fertility approach, and crop management decisions like hybrid choice and population drop are also going to be important to consider.
Sarah Hill:
You mentioned earlier that legumes and brassicas seem to work well for interseeding simply because they need that longer growing period. Are there other species of cover crops that work best for interseeding?
John Wallace:
In the Northeast. And I think it's probably also similar in some parts of the Upper Midwest. When we interseed at those stages, we find the greatest consistency with annual Ryegrass, medium red clover and crimson clover seem to do well. Forge radish also seems to do well. These species seem to be shade tolerant enough to persist under corn canopies on a fairly consistent basis. But I'm always interested to see what others are trying because some folks are having success with other species and some folks are trying really high diversity cover crop mixtures. But for example, it'd be great if we could consistently establish cereal rye as an interseeded cover crop and have it persist given its fit ahead of a soybean crop. So a nice rotational fit. But here in the northeast we've had pretty inconsistent results with cereal rye, where in some other locations I think they may do better with cereal rye.
Sarah Hill:
What have you seen is the return on investment for interseeding cover crops?
John Wallace:
Yeah, that's a tough question... answer for a number of reasons. So first, most of our work has really focused on drill interseeding rather than broadcast seeding. And so that's going to require a specialized piece of equipment. And so there's some upfront kind of capital expenditures to consider when you're thinking about a return on investment. But from a crop production standpoint, there doesn't seem to be either a yield drag or much of a yield benefit to the corn crop. And I don't believe there's a body of work that has really measured interseeding impacts to a subsequent crop in the rotation. But from a soil health perspective, we can at least frame or think about this question based on the standard practice, whatever the standard practice is after corn in your rotation, whether that be leaving fields [inaudible 00:07:54] or post-harvest seeding [inaudible 00:07:56] cover crop. And so if interseeding results in continuous living cover, greater biomass potential or more diversity in the rotation relative to the current practice, then we should expect to see some long term returns on the investment that come from building soil health.
Sarah Hill:
You mentioned earlier that timing is a really important factor in interseeding. What other factors can impact how successful interseeding will be as a practice?
John Wallace:
Yeah, that's a great question. So agricultural scientists talk about G by E by M interactions. So genetics by environment by management interactions. And so for interseeding corn and cover crop genetic, genetics probably matter, management certainly matters, but I think environmental conditions are probably the biggest driver of success. So in warmer climates with high corn yield potentials, those conditions are just more challenging to establish cover crops by interseeding. Whereas northern climates, like here in the northeast or parts of the Upper Midwest that are cooler and may have lower corn yield potentials seem to be a better fit for this practice.
These kind of northern climates are also where we struggle to integrate winter cover crops after corn grain harvest. So interseeding is a practice that really should be investigated in those regions.
Sarah Hill:
How does interseeding affect weed management?
John Wallace:
I think it's useful to think about weed management decisions from two different starting points when it comes to interseeding. You can either design herbicide programs around your cover cropping strategy, including which species you interseed or you can design your cover cropping strategy to fit with the herbicide program that you need to meet those crop protection goals. And so as a weed scientist, I'm inclined to suggest that the weed management decision comes first. So we know that residual herbicide programs that we use in quorum have to be reduced in order to interseed due to this potential for cover crop injury as a result of kind of persistent soil [inaudible 00:10:26] activity of certain residual products. And so the presence of multiple resistant weed species like common waterhemp or palmer amaranth, they're going to make it difficult to interseed and meet crop protection goals because soil applied residuals are really essential for controlling these species.
But I also see many farms that have diverse crop rotations and cover cropping is really prioritized in their rotation. And so diverse crop rotations are really the foundation of sustainable weed management. And these farms then tend to just do pretty well with simple herbicide programs, and that really opens up even more opportunities for how they can integrate cover crops like interseeding.
Brian O'Connor:
We'll get back to the conversation between John Wallace from Penn State and Sarah Hill in a moment. First, I want to take a moment to thank our sponsor. SOURCE from Sound Agriculture unlocks [inaudible 00:11:27] the nitrogen and phosphorous already in your fields so you can rely less and expensive and hard to find fertilizer. This foliar application is a low use rate. You can mix it right into your tank, check out SOURCE. It's like caffeine for microbes. Learn more at www.sound.a. Now back to John Wallace in Sara Hill.
Sarah Hill:
What herbicides are best to apply to the crop that aren't going to impact the cover crop that has been interseeded?
John Wallace:
Yeah, so the goal is to start clean at interseeding. And so we typically recommend a post emergence pass of glyphosate in the Roundup Ready system or glufosinate could be used if you have a liberty link trait because these products don't have any residual activity and provide pretty good post emergence weed control and allow you to start clean. But this kind of simple one pass program is often not enough. And so we've been doing field trials for probably the past six to eight years here at Penn State to help identify residual herbicide programs that can be used kind of as a setup program and a two-pass herbicide program approach.
And so there's several short lived residual herbicides that can be used, but many [inaudible 00:12:57] the pre-programs that you see in corn that includes some combination of atrazine, a group 15 herbicide like [inaudible 00:13:07] and a group 27 herbicide like Callisto or [inaudible 00:13:11], those types of programs are going to increase the risk of injury to an interseeded cover crop.
We do know a fair amount about corn herbicides and how they differ in soil persistence or the length of activity in the soil, but we still don't really have a complete picture of where there's differences between cover crop species in their sensitivity to any one particular herbicide that you might be using in a corn program. So right now, my program's completing greenhouse studies to determine the relative sensitivity of different cover crop species that common corn herbicides in hopes that we might find groups of cover crop species that have some tolerance to key herbicides, and that might help facilitate the ability to interseed cover crop mixtures while still using good pre-soil applied residual herbicides in your corn program.
Sarah Hill:
When a cover crop is interseeded and it has more sunlight, more hours of heat units applied, how much more biomass is produced when cover crops have that extra jumpstart from being interseeded?
John Wallace:
I think that's going to depend on when you're looking at those cover crops. So when we're interseeding around V4, V5 there will be this establishment phase [inaudible 00:14:38] we'll see some cover crop growth, but then we'll see growth kind of stop or slow significantly after corn canopy closure through kind of perhaps late July, early August. And then resume growth after we start to see corn dry down and some leaf drop. So total biomass production is also going to depend on species. So a good stand of say, interseeded annual Ryegrass. If you're looking at biomass production at harvest, you may only find about 300 pounds of dry matter per acre out there, but that can translate to a lot more biomass productivity in the spring if you've really have a nice established cover crop there in the fall.
I've also seen forge radish, for instance, exceed a thousand pounds of dry matter biomass at the time of corn harvest. Typically this would be on a high fertility dairy farm, but that amount of in-season productivity also means that it likely out-competes winter hardy cover crop species that you might have in that interseeded mixture, which can reduce spring biomass productivity.
Sarah Hill:
How does the date when the corn variety is planted and then harvest, how does those dates influence how successful interseeding might be?
John Wallace:
Well, I'm not sure that we have a complete picture of which corn management practices have the greatest impact on interseeding success. Certainly in a 30-inch [inaudible 00:16:18] system, there's planting day considerations, there's hybrid day length, there's plant architecture genetics to consider, like [inaudible 00:16:28] or hybrids, plant populations, all those things can be adjusted. But in general, we should be thinking about how these cultural practices for corn management can make room for cover crops kind of both in space and time without incurring yield penalties. And so probably more specifically, how can we manage corn so cover crops can withstand the environmental stress between or beneath the corn canopy that is closed during August, which is our hottest and driest month.
So getting back to planting date and harvest date. If earlier corn planting dates mean a longer window of cover crop establishment and growth before a sustained period of environmental stress in summer, then that might be a viable cultural practice. And if a shorter day hybrid means earlier fall dry down and leaf [inaudible 00:17:20], which allows that cover crop to get going again after that period of environmental stress [inaudible 00:17:25] late summer, then that also might be a good cultural practice.
Sarah Hill:
So how does the corn being used for maybe grain versus maybe being chopped for silage, does that impact interseeding as well?
John Wallace:
Sure, it does. We typically see better performance of interseeded cover crops in silage systems for a few reasons. First, that earlier harvest in silage systems lengthens that fall growing season window. But perhaps even more importantly, when you're harvesting for silage, you [inaudible 00:18:00] removing all that crop residue. Whereas in grain systems, we see issues related to burial of those interseeded cover crops when there's a lot of fodder left on the field post harvest and so corn residues in those grain systems can be a pretty significant issue, particularly in those high yielding environments. But one other note about silage systems, post-harvest establishment of cover crops after silage is typically pretty consistent in my region where many growers are using double crop [inaudible 00:18:37] to corn silage rotations.
So the interest in interseeding from dairy producers or in dairy systems is driven in part by this interest in increasing fall forage production or fall grazing potential. Rather than thinking about needing to get those conservation benefits of an overwintering cover crop.
Sarah Hill:
What should growers do to manage soil fertility in such a way that it better facilitates interseeding?
John Wallace:
I don't believe there's been a whole lot of work done on soil fertility management in interseeding. I think the biggest consideration might be interseeding timing relative to side dressing if you're using those split applications. And so modifying equipment, so interseeding and side dressing can be accomplished in a single pass around that V4 growth stage has the potential to reduce the labor and fuel costs and can make interseeding that much more economical. We typically side dress before we interseed because we don't want to run down newly emerging cover crops or risk foliar burn from nitrogen applications.
Sarah Hill:
What have you found is the best plant population for corn to facilitate interseeding? And then on the other side of that coin, what is the best cover crop plant population when you're interseeding as well?
John Wallace:
So some of my colleagues in the northeast completed a study several years ago that did evaluate corn population rates when you're using [inaudible 00:20:22] hybrids. And that work found that you could reduce seed drop from perhaps 32,000 per acre by about 15% without having an impact on yield and reducing the population did produce a measurable difference in light penetration into the canopy. But that study was done at only a few locations with just a couple hybrid comparisons. So to really understand how much of a benefit we might get by adjusting plant populations, I think that type of work really needs to be expanded. On the cover crop seating rate side, we're typically in the neighborhood of 20 to 25 pounds per acre when we're interseeding smaller seated cover crops. If we're interseeding annual Ryegrass alone, we might be in that 15 to 20 pound range.
If we're interseeding a mixture that would include something like red clover and forge radish, we might have 15 pounds of annual Ryegrass or maybe even 20 pounds of annual Ryegrass with four or five pounds of a clover species and just a pound or so of forge radish. We have found that you can go at too high of a rate forge radish, which can cause some competition, and you don't get the expression of those winter hardy cover crops. So we like to keep our forge radish rates around a pound if it's in a mixture.
Sarah Hill:
Have you seen that corn yields take a hit at all when interseeding is done?
John Wallace:
Sure, that's often one of the first initial questions we get for folks that are interested in the practice, you can interseed too early and see a yield penalty. We've seen yield penalties when interseeding really early around the V2 stage. Part of that yield penalty when you're interseeding that early can be attributed to increased weed pressure because weeds are breaking or emerging with the cover crop at that time. And so waiting a few more weeks to interseed to the V4 stage means that you're going to get that early season weed flush to occur prior to your last herbicide spray, and you'll be more likely to be able to start clean with a good herbicide post pass.
And in all our work that's focused on interseeding between, say, the V3 to V5 growth stage, we're much more likely to see a loss of the interseeded cover crop due to too much corn competition than we are to see corn yield penalties due to cover crop competition.
Sarah Hill:
Have you seen if it's more advantageous to interseed maybe a mono-crop cover crop species or a cover crop mix?
John Wallace:
That's a great question. So I think it depends on your management objective. Certainly interseeding is an opportunity to increase diversity in your rotation. There's also some support, I think, to the idea that interseeding a mixture increases the consistency of interseeding performance over time and space because some species will do better [inaudible 00:23:33] one set of environmental or soil conditions, and another species might thrive under another set of conditions. But as a weed scientist, one of my concerns is that these high diversity cover crop mixtures add some constraints to coming up with a good residual herbicide program that we might be able to use and have the necessary safety for all those species in the mixture. But again, it gets back to management goals. So if your management goal is soil erosion protection, nitrogen retention, or scavenging or forged productivity, a monoculture of something like Ryegrass might be your best approach.
I would just suggest that interseeding is a practice that has to be tailored to your region and your production system. And so I know many of the land grant institutions are researching this practice, and so you're likely to find some good information through those extension services. There's several farmer groups that are really focused on advancing cover crop practices, like Practical Farmers of Iowa or the Pennsylvania No-Till Alliance in my state. And so those groups can be a really valuable resource. And so finally, I think, soil water conservation districts are starting to acquire these high clearance grain drills that might be available to a grower for a small fee for service that might allow them to try that on their farm and to get a better understanding of whether it's a good fit for their system.
Sarah Hill:
All right. Great. Well, thanks so much for joining us, John. Appreciate your insights.
John Wallace:
You're welcome. Thanks for having me, Sarah.
Brian O'Connor:
That's it for this week's episode of the Cover Crop Strategies Podcast. As always, be sure to like or follow us on Facebook or Twitter to get live updates about where we're at on the road. You can also visit our website, www.covercropstrategies.com to subscribe to our weekly email updates. And thanks again to SOURCE for helping us keep the lights on. Thanks for listening [inaudible 00:25:41] cover up.
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