15 - 2020 Soybean Management Field Days - Soy Planting Date – Sun Capture - Cover Crop
Sun Capture - Cover Crop - Jim Specht, UNL Emeritus Professor of Agronomy and Horticulture covers the following: When a winter annual cover crop (CC) precedes the soybean cash crop, the CC can capture sunlight from about Apr 15 (when it greens up), but the CC must continue to capture sunlight from May 1 to at least May 15. Let’s examine how the soil health benefit of an April 15 - May 15 CC is offset by the cost of a lower yield when delaying soybean planting from May 1 to May 15.
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[00:00:07.171]Okay, my name is Jim Specht.
[00:00:09.650]I'm professor in agronomy now retired
[00:00:12.120]at the university of Nebraska
[00:00:13.620]and my specialty was soybean physiology and production
[00:00:18.130]and in addition to genetics genomics,
[00:00:20.090]but today we're gonna focus on physiology
[00:00:22.260]and production of soybean.
[00:00:24.240]So what we're gonna talk about next
[00:00:25.590]is the impact of a cover crop
[00:00:27.000]on that solar radiation capture.
[00:00:30.160]So this is a slide I borrowed from Justin,
[00:00:32.560]one of the presenters here
[00:00:34.170]and what we're talking about here is
[00:00:35.840]the timing of the termination of the cover crop
[00:00:38.920]that's just before soybean and in this case,
[00:00:40.820]we're talking about our winter annual cereal rye
[00:00:43.160]and the planting date of the soybean.
[00:00:46.030]So the question is, when do you do that termination?
[00:00:49.000]And when do you do the soybean planting?
[00:00:51.540]You have some other options here.
[00:00:52.979]You can plant your soybeans green to get them started
[00:00:57.370]as, while the cereal crop is still growing,
[00:00:59.571]or you can terminate the cover crop a little earlier,
[00:01:02.640]if you wish.
[00:01:03.910]So we're gonna talk about those issues.
[00:01:06.480]Here at the Eastern Nebraska Research and Extension Center,
[00:01:12.820]they're located near me.
[00:01:14.660]You can see we had four plots last year,
[00:01:16.970]demo plots that were,
[00:01:18.015]that were set up to be planted on,
[00:01:21.680]on April 15th,
[00:01:24.470]May one, May 15 and June one
[00:01:29.250]or late May,
[00:01:30.083]I guess May 31st.
[00:01:31.560]And so this is a picture showing those four plots
[00:01:36.010]that had been planted with a fall planted cereal rye
[00:01:39.320]back in November, mid November I believe.
[00:01:42.600]So I've yellowed some of the points there.
[00:01:44.748]The key thing there is that this is a 4-15
[00:01:48.970]April 15 photo and as we look North,
[00:01:52.580]we'll see the soybean plant planting dates were scheduled
[00:01:55.680]to be the top most one on the same day as today
[00:01:58.190]and then the second one, two weeks later,
[00:02:00.020]and the third one, four weeks later,
[00:02:01.720]and the fourth one, six weeks later.
[00:02:05.610]So there's not much of a swiping or much of a cover crop.
[00:02:10.230]Biomass here is just started to green up basically.
[00:02:14.857]So here's the photo then on May one,
[00:02:18.780]which we plant later that day,
[00:02:20.890]showing that there was a little more cover crop
[00:02:23.350]on early May.
[00:02:24.967]But there's still not really sufficient amount of cover crop
[00:02:28.760]to create the biomass that most people would want
[00:02:31.360]at a cover crop to put carbon into the soil.
[00:02:34.286]So that one was planted on 5 May,
[00:02:37.750]which is the second one from the top, okay?
[00:02:41.670]And here is the photo, a date of mid may,
[00:02:44.337]and you can see there's much more cover crop.
[00:02:48.040]You can see on that date,
[00:02:49.200]the cover crop has a greater biomass than
[00:02:53.380]and maybe it's sufficient in terms of having enough
[00:02:56.800]to add carbon successfully to the soil,
[00:02:59.640]so that we plant soybeans now.
[00:03:02.380]My colleague was there, Keith Glen,
[00:03:04.280]and he's planting the soybeans for me on that day,
[00:03:06.570]in that particular plot.
[00:03:07.880]So you can see the plots on the West side
[00:03:11.195]and each plot on the West side has the cover crop
[00:03:14.070]and on the East side, just a plain, no till.
[00:03:17.305]Now, if you wait until May 31,
[00:03:20.930]you have a lot more cover crop to deal with
[00:03:23.380]because that cover crop has also been harvesting the sun
[00:03:26.244]in addition to the soybeans.
[00:03:28.444]And you can see there, it has a huge amount of biomass
[00:03:32.810]that could be used to really
[00:03:33.900]add a lot of carbon to the soil.
[00:03:39.050]So again, I go back to this slide
[00:03:40.810]because what you're seeing here is one benefit of
[00:03:43.317]the cover crop, is that it can capture solar radiation too.
[00:03:48.100]That soybean doesn't capture when you plant soybeans
[00:03:50.420]on May one.
[00:03:51.910]So one advantage of the cover crop,
[00:03:53.344]it's early, is gonna capture some of that solar lighting
[00:03:56.916]and solar sunshine and make a dry matter.
[00:04:00.060]On the other hand to get,
[00:04:01.470]because it's cold then to get you a higher biomass
[00:04:03.893]that the cover crop people would want,
[00:04:06.360]you have to delay soybean planting until the 15th of May.
[00:04:09.647]And so you can see that spread there with a black arrow
[00:04:13.120]just above the Equinox line.
[00:04:17.796]So again, we're back to planting soybeans on,
[00:04:20.927]on about May 15 to accommodate our cover crop,
[00:04:25.310]needing to get to a sufficient biomass
[00:04:27.660]that you can kill it and incorporate as carbon
[00:04:30.880]into the soil.
[00:04:32.820]So you can see there the difference here,
[00:04:35.010]but you can see the blue, mid-May,
[00:04:38.080]a planting date there and how much soybean bio-mass it has
[00:04:42.206]versus the biomass it had
[00:04:44.990]if you planted on May one or late April.
[00:04:49.500]So what are the take home points here?
[00:04:51.810]A cover crop requires about a month
[00:04:53.680]from it screen up on mid April to about five mid may
[00:04:58.565]to obtain sufficient biomass
[00:05:00.880]to really be an effective means
[00:05:03.940]of adding carbon to the soil.
[00:05:06.240]This is a soil health benefit that is of value.
[00:05:10.570]So the profit that goes the,
[00:05:14.600]the amount of benefit that you get in terms over-time
[00:05:20.280]is probably large.
[00:05:21.907]On the other hand,
[00:05:22.920]that also shows that a soybean planting date
[00:05:26.580]must be delayed and so there is a loss
[00:05:29.170]in capture of solar radiation by the sun soybeans.
[00:05:33.280]So that's an opportunity cost trade of about
[00:05:35.880]a half bushel per acre, per day,
[00:05:37.950]and certainly in the irrigated fields
[00:05:40.110]and maybe a little less or none, maybe in ghrelin fields,
[00:05:43.760]depending on the amount of August stress.
[00:05:46.960]So my point here is that
[00:05:48.780]the cover crop provides two benefits.
[00:05:50.850]It's captures the sunlight, one light you can't use anyway,
[00:05:54.930]by planting soybeans that early,
[00:05:56.650]and it puts some of that two weeks of that sunlight
[00:06:00.360]into the dry matter that will go into ground storage.
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