3 - 2021 South Central Ag Lab Field Day - Cover Cropping Impacts on Weed Dynamics

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08/30/2021 Added

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2021 South Central Ag Lab Field Day - Cover Cropping Impacts on Weed Dynamics, Elizabeth Oys, UNL Graduate Student

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[00:00:07.590]Hi
[00:00:07.910]My name is Elizabeth Oys.
[00:00:09.320]I'm a master's student here at the University of Nebraska Lincoln in the
[00:00:13.490]department of agronomy and horticulture. I'm advised by Dr.
[00:00:17.510]Basche in the resilient cropping systems lab.
[00:00:20.240]And today I'm going to be discussing cover cropping impacts on weed dynamics.
[00:00:25.310]This is the third presentation of three of the cropping systems theme originally
[00:00:30.260]presented at the south central ag laboratory field day in late July of
[00:00:34.730]2021. You can refer to Dr.
[00:00:37.250]Kohler Kohl's video and Chris annual's video for the remaining
[00:00:42.140]topics in cover cropping and cropping systems.
[00:00:45.770]So let's get started.
[00:00:48.710]So after.
[00:00:49.760]You've watched Dr. Kohler Kohl's video and Chris Annuo's video,
[00:00:54.590]we talk a lot about cover crop benefits.
[00:00:57.080]Katia talks a lot about that nutrient cycling component.
[00:00:59.990]And Chris talks a lot about building organic matter with cover crops,
[00:01:04.190]and you can kind of think of cover crops as a multifaceted tool,
[00:01:07.700]kind of like a Swiss army knife as we see here.
[00:01:10.610]And one of the benefits that I'm talking about today is we control and weed
[00:01:14.690]suppression with cover crops on weed dynamics.
[00:01:19.130]So when we're talking about cover crop mechanisms of weed suppression,
[00:01:22.880]we're going to think of four main things.
[00:01:25.130]The first being that early season competition with the actual growing cover
[00:01:29.090]crop,
[00:01:29.870]the growing cover crop acts as a competitor for resources in light for those
[00:01:34.160]weeds that are already up.
[00:01:35.060]So maybe winter annuals that have already emerged or also those early emerging
[00:01:39.530]summer annuals. And then that residue itself,
[00:01:43.220]once the cover crop is terminated, actually shades the soil.
[00:01:46.490]Like you might see there on the right. And so by shading the soil,
[00:01:49.940]you're reducing the sunlight that gets into the top soil layer. And so,
[00:01:54.800]siblings need light, to be successful,
[00:01:57.920]to continue growing and also seeds need sunlight to germinate.
[00:02:02.450]And so we hope by shading the soil, you're going to delay weed emergence,
[00:02:06.920]but if those weeds do emerge,
[00:02:08.630]they have to expend a lot of energy to get up through that residue mat that you
[00:02:12.350]might see there on the right. And by expanding all of that energy,
[00:02:15.770]we hope that their biomass in turn would be reduced. Now,
[00:02:19.970]the third way is reducing soil temperature that also has to do with the shading
[00:02:24.080]effect from the cover crop residue.
[00:02:27.230]And so when you lower the soil temperature,
[00:02:30.170]you reducing the amount of weeds that can germinate or delaying that germination
[00:02:34.010]onset because just like sunlight is needed for weeds to germinate and be
[00:02:38.180]successful. So is temperature.
[00:02:40.460]So you're delaying weed germination that way. And then the fourth way,
[00:02:44.300]which we're not really going to get into today,
[00:02:46.340]but does occur is with allelopathy and an allelopathic cover crop would
[00:02:51.170]be cereal rye. And when a plant is allelopathic,
[00:02:54.410]what it does is actually excrete a chemical out into the soil environment around
[00:02:59.020]it.
[00:02:59.650]And that chemical can actually act as a growth inhibitor for any of the
[00:03:04.480]plants around it, but also a germination inhibitor.
[00:03:08.860]So how can we.
[00:03:09.940]Actually quantify the impacts of cover crops and weeds?
[00:03:13.270]And with this experiment, we do this in two different ways.
[00:03:16.240]The first being in season weed observations,
[00:03:18.430]that's the most apparent just like you might go out into a field and crop scout
[00:03:22.450]for weeds, identify weeds that way,
[00:03:24.250]or maybe count weeds to get an economic threshold.
[00:03:27.280]The same concept is held true with this experiment here.
[00:03:30.970]and what we do is we count the weed densities,
[00:03:33.370]the amount of weeds that have germinated, and also we're clipping biomass.
[00:03:37.510]as I referred to,
[00:03:38.890]if those weeds do emerge through that cover crop residue,
[00:03:42.430]then we hope to see differences between,
[00:03:45.010]that cover crop treatment and that control.
[00:03:47.650]And then the second way that we're assessing weeds is with the soil,
[00:03:50.410]weed seedbank,
[00:03:51.160]and this can kind of get complicated the way our experiment addresses the seed
[00:03:55.180]bank is through the germination method.
[00:03:58.060]So we're directly germinating out those weed seeds present in the seed bank,
[00:04:01.840]but you can also use elutriation which is the physical removal of,
[00:04:06.100]seeds from the soil.
[00:04:07.990]but that can kind of get complicated when we're interested in identifying the
[00:04:10.960]species in that seed bank.
[00:04:13.270]but we can kind of think of the seed bank as that we database.
[00:04:16.420]so the maximum number of species and the total number of weeds that are present
[00:04:20.860]in that seed bank, that's,
[00:04:21.760]what's probably going to emerge out in the field for those in-season weed
[00:04:26.410]observations, but overall,
[00:04:28.180]we want to assess how those total populations are going to shift due to a cover
[00:04:31.420]crop presence or lack thereof.
[00:04:34.750]And so we can't take a holistic picture,
[00:04:37.510]a big picture of weeds what's going on out in the field without having both,
[00:04:42.460]we don't get a clear snapshot of what's going on out in the field.
[00:04:47.020]So that being said, we did sample the weed seed bank,
[00:04:50.500]and we did this at the south central ag laboratory,
[00:04:54.460]which I'll refer to as SCAL near clay center, Nebraska,
[00:04:57.310]and also the Eastern Nebraska research and extension center,
[00:05:00.910]which I will refer to as ENREC in Yutan, Nebraska.
[00:05:04.780]And now we are also conducting this experiment at four on farm locations,
[00:05:08.590]which I won't talk about today.
[00:05:10.420]But if you're interested in the on-farm component of this experiment,
[00:05:13.360]feel free to reach out to me.
[00:05:15.400]So what we're doing is we're assessing three different cover treatments,
[00:05:17.890]the cereal rye, the vetch, and the no cover as our control. Cereal rye,
[00:05:22.450]If you watch Dr. Kohler Cole's video,
[00:05:24.900]you'll understand that cereal rye accumulates a lot of biomass.
[00:05:28.540]It's also a really common cover crop in the state of Nebraska.
[00:05:31.510]And then we're assessing vetch to assess a legume cover crop.
[00:05:34.270]It's also common in Nebraska. And then of course our control,
[00:05:37.300]no cover. Three reps for each of those treatments.
[00:05:39.910]So a total of nine plots that we were taking samples from,
[00:05:43.090]and all the treatments are in a no-till corn soybean rotation.
[00:05:47.710]The previous year's crop was soybean, and this year's was corn.
[00:05:52.480]And so what we're doing is we're going out really early spring,
[00:05:55.510]basically as soon as we can, as soon as soil thaw occurs.
[00:05:59.450]and we're taking 20 random soil samples using a JMC probe, that's a soil probe.
[00:06:04.160]That's just a little wider than what you might take a standard soil sample with.
[00:06:08.540]We're going to a depth of 10 centimeters and as I mentioned,
[00:06:12.620]this is an a no-till system.
[00:06:14.090]So that seed rain that comes off of those seeds are off those weeds,
[00:06:20.510]will land on the soil surface or the first few centimeters of that soil.
[00:06:24.260]So 10 centimeters is really generous for this experiment.
[00:06:27.440]And then we're going to take that soil back to the lab, to the greenhouse,
[00:06:31.070]and we're going to sip it through a half inch mesh screen so we can get the soil
[00:06:35.660]really fine, break up some of those soil clods,
[00:06:38.750]and then we're going to lay them out into trays. Like,
[00:06:40.640]you'll see here on the right, and we're going to let all of the weeds,
[00:06:44.420]germinate and water those trays every day. and so again,
[00:06:47.750]we're doing this to assess the species and those population shifts that might
[00:06:50.780]occur due to different cover crops being present.
[00:06:53.660]And it's going to be our reference for those in-season measurements. And so,
[00:06:57.880]what we're going
[00:06:57.990]to do is wait even a week and the trays
[00:07:02.910]start to look something like this.
[00:07:04.090]So just a crazy number of weeds that emerge in these trays.
[00:07:07.510]You wouldn't expect it.
[00:07:09.220]but what's really interesting is that we can already see some differences
[00:07:12.010]between the trays between the treatments just visually.
[00:07:14.950]And so as we'll come to find out, when I share the data,
[00:07:17.470]we see a huge abundance of pigweed species. So we're talking waterhemp, palmer
[00:07:22.000]amaranth and Redroot pigweed in there. but then of course,
[00:07:26.590]grasses and array of other broad leaves so that what we do then is we identify
[00:07:31.240]them by species. We pull them out and dispose of them,
[00:07:34.420]and that's how we get that running count of the seed bank.
[00:07:36.910]And we'll do this until the seed bank is completely exhausted.
[00:07:42.070]And as I mentioned before as well,
[00:07:43.900]we did take those in season weed observations and samples.
[00:07:47.980]And so what we do is we go in before and after post emergence herbicide,
[00:07:53.290]and we're just counting the weeds in a small
[00:07:55.600]quadrat three different places in each of these plots.
[00:07:58.840]And then we're clipping biomass as well in nearby location.
[00:08:01.990]We also take soil moisture and temperature readings along with each of those
[00:08:06.250]samples. And so to better visualize why we're going out, when we're going out.
[00:08:11.260]Yeah. I have these great graphics from Dr. Salvador Ramirez. He's a post-doc,
[00:08:15.400]with me in the resilient cropping system lab.
[00:08:19.060]And so we go out initially right around V2.
[00:08:23.320]So after that pre-emergent herbicide is put out.
[00:08:26.350]And so we're getting that first flush of weeds out in the growing season,
[00:08:30.550]go out at V2 with the corn,
[00:08:32.500]and we take the weed biomass and density samples.
[00:08:35.890]We will let that post emergence herbicide be sprayed roughly around V5.
[00:08:40.450]And then we'll wait a couple of weeks for any of those residuals,
[00:08:43.000]with the herbicide to break down.
[00:08:45.070]And we take the second emergence counts and biomass samples right around
[00:08:49.420]V10.
[00:08:50.410]And so we're going out at V 10 because canopy closure occurs,
[00:08:54.920]roughly around V11 in corn.
[00:08:56.580]So we're hopefully capturing that second flush of weeds right around V10,
[00:09:01.290]before sunlight is limited down in that lower canopy.
[00:09:06.630]So.
[00:09:07.470]with this experiment,
[00:09:09.030]we're not necessarily saying that cover crops are doing all of the heavy lifting
[00:09:12.750]here.
[00:09:13.350]So we're taking a look at weeds in the context of the normal management system
[00:09:17.130]what's typical in the Nebraska.
[00:09:19.860]and so all of these prescribed herbicides were still applied in all of the plots
[00:09:23.790]that we took samples from.
[00:09:26.330]and if you're interested in more of the details of the agronomic management at
[00:09:29.850]each of these sites, you can reach out to me or Dr. Kohler Cole,
[00:09:33.030]who also has managed this site before.
[00:09:36.450]And so now we're going to get into some of the preliminary results and I'm
[00:09:40.410]sharing the data today in the form of box plots so that we can more easily
[00:09:44.040]observe the trends with the data. And so getting into it,
[00:09:47.280]we have the legend here on the right. So red is the check.
[00:09:51.570]Blue is rye green is vetch. And so,
[00:09:55.980]when we're taking a look at scowls seed bag, initially,
[00:09:59.400]we don't really see a difference across the total weeds that
[00:10:02.850]germinated out of the seed bank across the three treatments.
[00:10:05.760]It's when we start to look at the proportion of pig weeds in that seed bank is
[00:10:09.600]when we start to see a difference.
[00:10:11.190]It seems that the proportion of pigweeds is higher in the cover crops,
[00:10:15.600]seed bank itself. And then, when we analyze these results,
[00:10:19.980]we did not yield a P value under 0.05.
[00:10:24.030]So it's not statistically significant,
[00:10:26.040]but the trend in the data is still interesting.
[00:10:28.890]When we start to look at those early season weed observations,
[00:10:32.730]that first time we went out in the growing season, again,
[00:10:35.190]we don't really see a difference in the total number of weeds that are merged
[00:10:38.940]out in those plots.
[00:10:40.680]but when we look at weed biomass and particularly pigweed biomass,
[00:10:44.700]that's when we see a significant reduction of pigweed biomass in both the rye
[00:10:49.650]and the vetch treatments and that asterisk there just notes that that was
[00:10:53.880]statistically significant. So very interesting results there,
[00:10:57.810]opposite trend of what we saw with the seed bank.
[00:11:00.540]And then when we take a look at those later season weeds, again,
[00:11:03.870]we don't see a huge difference in the total number of weeds emerge, but again,
[00:11:07.440]we see that downward trend with the pigweed as being somewhat reduced,
[00:11:11.490]albeit not statistically significant,
[00:11:14.790]with both of those cover treatments. So from the south central ag laboratory,
[00:11:19.170]the things I want you to take away is that the Piggly biomass seems to be
[00:11:22.410]reduced where the cover crops are present,
[00:11:24.390]but the total weed density doesn't seem to be impacted. Now,
[00:11:28.710]when we're looking at ENREC in Eastern Nebraska, near Yutan Nebraska,
[00:11:33.120]when we look at the seed bank,
[00:11:34.170]we see that the total number of weeds in the seed bank and the proportion of
[00:11:37.950]pigweeds in the seed bank is greater in the cover crop treatments.
[00:11:42.300]And particularly the proportion of pigweeds in the vets treatment is quite high.
[00:11:46.710]I'll be at not statistically significant.
[00:11:48.660]This trend is really interesting because when we look at those early season
[00:11:52.410]weeds,
[00:11:52.710]we see this trend reversed with the total weeds seeds and pigweeds emerged.
[00:11:57.600]So we're not really seeing a difference in the total weed biomass across those
[00:12:01.500]treatments.
[00:12:02.880]but we are seeing that downward trend that reduction in the rye and the vetch
[00:12:07.530]cover crops. Now, when we look at the later season weeds,
[00:12:11.610]we see this trend continue with the total weeds emerge and pigweeds emerge.
[00:12:15.810]However,
[00:12:16.290]it's when we take a look at the total weed biomass is when we see something
[00:12:19.680]interesting. And so, remember that this,
[00:12:22.590]the later season weeds that's after the post emergence herbicide is applied.
[00:12:26.340]So basically what we're seeing here is that that that residue from that cover
[00:12:29.820]crop is really effective at holding back weeds and their,
[00:12:34.110]their biomass is reduced,
[00:12:36.990]with the later season weeds with the total weed biomass there.
[00:12:39.900]So the takeaway from Eastern Nebraska research and extension center is that
[00:12:44.820]the total weed densities seem to be reduced,
[00:12:48.090]where the cover crops are present,
[00:12:49.560]but the biomass differences that occur are with those later season weeds.
[00:12:54.390]So I'm going to leave you guys with a few thoughts,
[00:12:58.410]and I just want to reiterate that yes, these results are preliminary.
[00:13:03.000]the experiment really only wrapped up, a couple of weeks ago.
[00:13:07.500]And so we're continuing to build statistical models to further analyze this
[00:13:11.070]data,
[00:13:11.820]but the results are promising because we're showing that cover crops can impact
[00:13:16.050]both weed densities and weed biomass. Of course, this was dependent on the site,
[00:13:20.340]but,
[00:13:21.630]this can also be promising because it could be a possible tool at combating,
[00:13:26.520]aggressive weed species, especially pigweed species,
[00:13:29.520]where we did see some differences, both in weed, biomass and densities.
[00:13:33.540]And so ultimately when we have,
[00:13:35.940]an additional tool to help diversify our weed management, cover crops,
[00:13:40.560]being that cultural tool integrated into our cropping systems,
[00:13:44.160]we help mitigate the risk for further herbicide resistance and pigweed species.
[00:13:48.930]As many Nebraskans are well familiar with,
[00:13:51.810]they can have multiple resistances to multiple modes of action of herbicides.
[00:13:56.250]So mitigating the risk of further resistance is really important for,
[00:14:01.200]any farmers or growers in the state of Nebraska. And then again,
[00:14:05.040]we're not saying that cover crops are necessarily doing all of the heavy lifting
[00:14:08.430]in this case.
[00:14:10.470]but rather what we're finding is that integrating cover crops into your cropping
[00:14:13.620]system, along with a successful herbicide program,
[00:14:16.860]meaning that you have diversified herbicides,
[00:14:20.100]many modes of action at different times of the year and varying that year to
[00:14:23.940]year is really what's going to lead to the reduction in in-season weeds
[00:14:28.110]and that total weed biomass. Okay. So thank you for joining me here today.
[00:14:32.880]If you have any questions, about this experiment,
[00:14:35.910]the results or how the experiment was designed in general or the on-farm
[00:14:39.330]component,
[00:14:40.020]feel free to email me or reach out to the resilient cropping systems lab.
[00:14:43.890]Thank you.

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3 - 2021 South Central Ag Lab Field Day - Cover Cropping Impacts on Weed Dynamics

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