17 - 2020 Soybean Management Field Days - Soy Planting Date – Transpire Water- Cover Crop
Soy Planting Date – Transpire Water- Cover Crop - Jim Specht, UNL Emeritus Professor of Agronomy and Horticulture covers the following: Much of the seasonal water supply needed by a soybean crop comes from water stored in the soil prior to soybean planting, in addition to rainfall occurring during the growing season. Inserting a cover crop prior to a soybean crop (including delaying soybean planting) results in water use by the CC, which in some dry springs could be problematic for planting the soybean crop. Let’s examine this possibility.
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[00:00:07.730]Okay, my name is Jim Specht.
[00:00:09.660]I'm Professor in Agronomy now retired
[00:00:12.140]at the University of Nebraska,
[00:00:13.600]and my specialty was
[00:00:15.115]Soybean Physiology and Production
[00:00:18.160]in addition to Genetics Genomics
[00:00:20.080]but today we're gonna focus on
[00:00:21.590]Physiology and Production of Soybeans.
[00:00:24.475]So we just discussed a while earlier
[00:00:26.440]soybean planting date
[00:00:28.090]allows the soybean to capture and use
[00:00:30.213]more Spring rainfall
[00:00:34.320]So let's talk about
[00:00:35.610]the impact of adding a cover crop sequence
[00:00:39.040]before you plant your soybeans
[00:00:41.706]and we'll talk specifically
[00:00:43.560]about our Winter Annual Cereal Rye before then.
[00:00:48.040]So again, here's the situation here
[00:00:51.990]you're talking about
[00:00:53.216]inserting a cover crop,
[00:00:54.836]right at this position
[00:00:56.560]so you have a cover crop
[00:00:59.310]which can be harvesting sunlight
[00:01:01.780]pretty much from the time that
[00:01:03.177]emerges in the Spring
[00:01:05.310]until its termination date
[00:01:06.800]where soybeans can only capture
[00:01:08.550]the radiation after its seedlings emerge.
[00:01:12.420]So the question of when to terminate,
[00:01:14.210]when to plant
[00:01:15.060]aside from planting in grain,
[00:01:17.270]or doing other manipulations here
[00:01:19.620]as an obvious one,
[00:01:21.820]and my colleague Aaron Nygren provided this group
[00:01:26.480]with his slides showing
[00:01:28.280]the amount of water use
[00:01:29.960]that relates to the termination of a cover crop
[00:01:33.100]just before planting.
[00:01:34.660]I would urge you to go to his presentation
[00:01:38.810]to see what the data he presents
[00:01:40.844]but the key thing here I wanna mention is again,
[00:01:44.030]is that if you delay the start of soybean growth,
[00:01:48.750]you're also delaying the start of soybean ET
[00:01:51.410]as showing here.
[00:01:52.250]The red, blue, green and brown lines
[00:01:54.310]again, represent what I projected with soy water
[00:01:57.280]would be the starting dates
[00:01:59.690]and the lag times
[00:02:00.712]and the total amount of transport ET
[00:02:03.510]that would be possible
[00:02:04.620]if you planted on May 1,
[00:02:06.617]May 15, May 31 or June 15
[00:02:10.560]and you can see there's definitely less water
[00:02:13.730]and because of the known relationship
[00:02:16.180]of crop yield with transpiration,
[00:02:18.480]at least the T part of ET
[00:02:20.397]you can see you're bound to get lower yields
[00:02:23.260]because you're not utilizing the whole season
[00:02:27.490]but when we talked about,
[00:02:31.610]using that early season rain
[00:02:34.050]for soybean transpiration,
[00:02:36.380]we can also say that the cover crop
[00:02:38.670]does the benefit also
[00:02:39.930]because it's doing transpiration
[00:02:41.490]in mid April already.
[00:02:44.100]So it's doing it's transportation
[00:02:46.620]at a very higher,
[00:02:48.180]even than soybean
[00:02:49.292]water use efficiency rate,
[00:02:51.160]because that's doing earlier in the season.
[00:02:53.560]Now this slide up is applied just to soybeans
[00:02:56.610]but if you did one for crop
[00:02:58.221]Winter Annual Cereal Rye crop
[00:03:01.987]you can probably find the same thing.
[00:03:05.173]what we're talking about here
[00:03:06.072]is you're gonna
[00:03:08.640]delay a soybean planting
[00:03:10.480]from May 1 to April 15
[00:03:12.850]and seeing a loss
[00:03:15.100]of solar radiation capture
[00:03:16.820]a loss of ability to use
[00:03:20.600]early May to mid May rain for transpiration
[00:03:23.860]but, you know, again,
[00:03:25.000]you get the advantage of the cover crop
[00:03:30.170]which may be above normal
[00:03:31.570]to convert that to transpiration,
[00:03:33.690]which can keep your soils
[00:03:34.940]from being too water logged
[00:03:37.520]in the Spring before planting.
[00:03:42.370]So again, if you look here,
[00:03:43.590]you can see again,
[00:03:44.423]we're talking May 15
[00:03:46.460]and again, you can see
[00:03:49.480]also, once we get canopy closure in soybean,
[00:03:51.770]we can do better
[00:03:53.400]fractionating of the ET
[00:03:55.903]into more T and less E.
[00:03:58.500]So what are the take home points?
[00:04:01.310]CC when it greens up in mid April
[00:04:03.090]will transpire water
[00:04:04.290]it collects from the soil by its root hairs
[00:04:06.160]until it's CC termination date,
[00:04:09.130]which if we assume 5-15
[00:04:10.940]would be the traditional time
[00:04:13.660]for the CC termination
[00:04:15.423]and the soybean to start planting date.
[00:04:19.770]So the CC will rely
[00:04:21.030]on stored slow water
[00:04:22.750]and the issues where you have
[00:04:23.780]historical rainfall amounts,
[00:04:26.424]as Aaron has shown in his presentation
[00:04:28.750]that's not likely to be a problem
[00:04:31.310]with historical normal rain
[00:04:32.750]of that happening
[00:04:33.583]nor in the irrigated soils
[00:04:35.100]where the moisture in the Spring
[00:04:36.650]is gonna be higher probably
[00:04:38.110]and at the end of the prior crop season
[00:04:40.660]it's gonna be higher anyway
[00:04:41.660]but it look could be a risk
[00:04:43.950]if it's not necessarily a rule
[00:04:46.500]but an exception,
[00:04:47.460]that the CC crop
[00:04:48.930]can dry out the upper soil zone,
[00:04:51.570]forcing a delay in soybean planting
[00:04:53.428]in rain-fed fields
[00:04:55.208]and forcing an irrigation
[00:04:57.048]in irrigated fields.
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