16 - 2020 Soybean Management Field Days - Soy Planting Date – Transpire Water
Soy Planting Date – Transpire Water - Jim Specht, UNL Emeritus Professor of Agronomy and Horticulture covers the following: In all crop plants, photosynthesis (uptake of CO2) and transpiration (loss of H20) are inextricably linked, which means crop yield and transpiration are highly correlated. A soybean crop yielding 75 bushels per acre in NE will generally require about 20 acre-inches (27,154 gallons per inch) of seasonal water. Let’s examine how and why.
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[00:00:01.192](bright upbeat music)
[00:00:07.730]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
[00:00:18.150]in addition to Genetics Genomics,
[00:00:20.080]but today we're gonna focus on physiology
[00:00:22.260]and production of soybean.
[00:00:24.130]So, in this segment, we're gonna talk about
[00:00:27.320]one of the biophysical reasons why you could expect
[00:00:30.600]an early soybean planting date to increase potential yield.
[00:00:34.480]And one of those issues is the amount of transpiration
[00:00:37.180]you can get from an early planted soybean field
[00:00:40.410]that will transpire more water during the summer season.
[00:00:44.810]And we know from biophysical research done
[00:00:47.300]by many micro-climatologists is that
[00:00:49.970]there is a direct relationship between soybean yield
[00:00:54.320]Indeed, there is such a direct relationship for all crops,
[00:00:57.670]including cover crops.
[00:00:59.900]So, let's talk about that.
[00:01:03.100]Here, you can see a slide here,
[00:01:06.130]on the left we have estimated soybean ET
[00:01:11.075]in the vertical axis and day of the year on the bottom axis.
[00:01:14.200]And across the top, you can see the vegetative
[00:01:16.440]and reproductive stages.
[00:01:18.810]You see that water use by soybeans reaches a,
[00:01:23.810]could reach as much as 0.3 inches of ET,
[00:01:27.660]just before R3, before it declines later
[00:01:30.200]in the season.
[00:01:32.290]So, what we're talking about here,
[00:01:34.710]if you take 0.3 inches of ET and divide that
[00:01:39.630]by the inches, by the 27,154 gallons per inch of water,
[00:01:47.430]you're talking about this field transpiring about,
[00:01:50.790]or having ET of about 9,000 gallons of water a day, right?
[00:01:55.480]Just before R3.
[00:01:59.890]That's a fair amount of water and we'll discuss that
[00:02:01.950]in a moment.
[00:02:05.800]So, that's mentioned on the right.
[00:02:07.959]So, we're really interested in T
[00:02:12.080]because the linear relationship is between transpiration
[00:02:15.230]and soybean yield.
[00:02:16.220]So, if we can increase the fractional amount of T
[00:02:20.246]that is in ET, we're gonna be much better off
[00:02:22.780]in terms of getting a higher yield.
[00:02:25.230]So, if we take the data from this side and cumulate it,
[00:02:28.400]which is on, this one's in inches per day,
[00:02:30.430]we're gonna talk in the next slide,
[00:02:31.610]cumulative inches per day.
[00:02:34.210]You can see you transform that peak curve
[00:02:37.030]into a sigmoid base curve showing...
[00:02:40.410]By sigmoid we mean it's slow at the first,
[00:02:42.570]but accelerating and reaches an inflection point
[00:02:46.120]where it's nearly linear before it turns around
[00:02:48.900]and decelerates to reach a final amount of water use.
[00:02:53.510]For this Lincoln Nebraska example in 2006,
[00:02:57.560]you can see that rate should be,
[00:02:59.550]based on that predicted ET from the prior curve,
[00:03:03.270]or could probably use almost 19 inches of ET
[00:03:06.250]to grow that crop.
[00:03:07.083]That's a 140 day crop.
[00:03:09.820]And you can see that if we also plot in the dash line,
[00:03:12.930]the cumulative amount of precipitation
[00:03:15.020]that would be expected historically normal at this site.
[00:03:18.970]You can see that's just a straight linear line.
[00:03:22.260]So, around R3, you can see that
[00:03:25.200]we have more than enough water as presenters in this,
[00:03:29.830]in this field day have pointed out
[00:03:32.480]to satisfy growth early on in the season,
[00:03:35.680]but later in the season we're gonna be deficit
[00:03:38.190]in terms of supplying rain, using rainfall
[00:03:41.560]to supply soybean water use ET on a cumulative basis.
[00:03:46.520]In fact, I think we're talking about,
[00:03:48.570]about a four inch difference here.
[00:03:50.980]And just a quick stop here,
[00:03:55.870]just to mention how much water a crop use,
[00:03:59.620]a soybean crop with 125,000 plants per acre
[00:04:04.130]could produce 70 bushels of soybean per acre
[00:04:07.350]in 140 day growing season.
[00:04:10.150]And that would require about,
[00:04:11.560]as we saw in the prior slide about 19,
[00:04:13.630]we'll just round it up to 20, acre inches.
[00:04:16.100]And so, if they're 27,154 gallons of water in an acre inch,
[00:04:21.227]you're talking about that crop is gonna need
[00:04:23.937]543,000 gallons of water just
[00:04:27.500]to supply its seasonal ET needs.
[00:04:30.740]If you look at a small town in Nebraska or elsewhere,
[00:04:34.420]the water tower there only has enough room
[00:04:37.080]to hold 300,000 or so.
[00:04:39.310]So, that crops is gonna use more water
[00:04:41.270]than it's in this tower, water tower for a small town.
[00:04:45.130]And also keep in mind that all that water for transpiration
[00:04:48.050]and ET has to come from root hairs
[00:04:49.980]and root hairs are the only way that water gets in
[00:04:52.570]or nutrients get into the plant.
[00:04:54.350]Everything else in the root is basically suberized tissue
[00:04:58.800]so water doesn't escape or is permitted to enter.
[00:05:02.740]So, remember that about point, at about R3,
[00:05:08.120]the Peak ET reaches about 33 inches, acre inches,
[00:05:13.660]a third of an acre ranch,
[00:05:14.840]and that's about 9,000 gallons of water.
[00:05:16.870]And I think I transport that into
[00:05:18.940]how many cups would be needed.
[00:05:21.630]So, on the left is a textbook diagram of a crop root,
[00:05:26.460]a young one, showing the growing tip, the root hair region,
[00:05:31.070]and an above region which is stationary and suberized.
[00:05:34.240]On the right I have the final photo
[00:05:35.870]that shows an actual soybean seedling,
[00:05:38.610]that's after germination.
[00:05:40.680]Actually shows the root hairs
[00:05:42.090]are so fine that you can hardly see
[00:05:43.760]without some magnification.
[00:05:45.780]In fact, when you pull out a root,
[00:05:47.750]all the road hairs are shut, the root hairs are shut off.
[00:05:51.810]So, soybeans grows down pretty quickly.
[00:05:53.960]In the root system you can see the taproot system here
[00:05:56.610]and some work done by my graduate student, how deep it is.
[00:05:59.370]It's 10 inches by the time
[00:06:00.790]the soybeans pop up their cotyledons,
[00:06:03.460]they are shown on the left
[00:06:05.130]and they go downward pretty rapidly.
[00:06:07.630]And as the taproot heads down and the taproot heads down,
[00:06:11.000]you'll see lateral roots coming out of the tap root.
[00:06:13.908]And you'll also see tertiary roots coming out
[00:06:15.950]of the lateral roots and each one of those tips
[00:06:18.910]has root hairs that will forage for water.
[00:06:22.710]Jessica, my postdoc found this relationship between the top
[00:06:27.560]above ground growth and the below ground root system.
[00:06:32.670]So, at the top you see that we're monitoring Vn, R stage
[00:06:36.650]in this soybean crop that was planted in early may.
[00:06:40.040]And you can see down below we're using these glass fields
[00:06:43.130]glass open tubes to see where the root tip system is.
[00:06:46.370]So, if you start on the day of planning,
[00:06:48.940]you can see there that roots,
[00:06:51.500]the radicals already are emerging
[00:06:53.710]from the soybean seedling, before it even emerges
[00:06:57.840]and it's headed downward in a straight line at a rate of
[00:07:01.600]1.2 centimeters per day.
[00:07:05.000]And that's equivalent to about 0.47 inches per day.
[00:07:08.730]So, it's heading down pretty rapidly.
[00:07:10.970]The lateral roots are a little bit behind
[00:07:12.850]because you have to wait about 10 inches behind
[00:07:15.680]the root tip before they come out,
[00:07:18.140]and the tertiary roots also don't come out
[00:07:20.020]until the lateral roots are established.
[00:07:23.030]So, if we look here, you can again see that
[00:07:26.770]what we have here is soybean roots.
[00:07:29.560]We can use that prior scale
[00:07:30.980]to determine where our root system is.
[00:07:33.440]And if you go to SoyWater,
[00:07:34.790]the online device for scheduling soybean irrigation,
[00:07:38.210]you can see what the root depth is
[00:07:39.850]for each stage of the crop you're growing
[00:07:42.380]assuming this model holds.
[00:07:45.170]And keep in mind also that in this calculation,
[00:07:50.681]ET data from four sites in the U.S. that comes from Purcell
[00:07:55.290]and his colleagues, Thompson and Claire,
[00:07:57.550]you can see the transpirational water use efficiency
[00:08:00.340]is much, much greater earlier in the spring
[00:08:03.010]than it is later in the summer as because temperatures
[00:08:06.180]are cooler in the spring and so you're able
[00:08:08.610]to transpire without, at lower rates
[00:08:12.670]or at better water use efficiency.
[00:08:15.310]So, I've drawn the blue lines in there
[00:08:17.120]for the may 1 planting, and then also the blue lines
[00:08:20.790]for the may 31 planting.
[00:08:23.970]And this would also apply to your cover crop
[00:08:26.870]'cause remember it's growing on,
[00:08:28.230]it started coming out of the ground and transpiring
[00:08:30.470]on April 15th.
[00:08:31.480]So, it's water use early on
[00:08:32.870]is also gonna be day efficient as well.
[00:08:36.050]So, what's the take home points here.
[00:08:38.460]Early planning allow soybeans to, soybean crop to capture
[00:08:42.490]and use available early season rain for transpiration
[00:08:46.400]at a high water use efficiency due to lower temperatures.
[00:08:50.760]And because of that transpiration, rate of transpiration,
[00:08:54.210]the known relationship between soybean yield
[00:08:57.260]and transpiration can come into play.
[00:08:59.880]And therefore you're gonna end up increasing yields.
[00:09:02.030]So, this is basically the second reason
[00:09:03.940]behind solar radiation as the first reason
[00:09:06.200]why you'll get higher yields when you plant soybeans early.
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