28 - 2020 Soybean Management Field Days - 26 - Scheduling Late Season Irrigation in Soybean
Scheduling Late Season Irrigation in Soybean - Steve Melvin, Nebraska Extension Educator, Cropping Systems discusses late season irrigation scheduling is very important in soybean because our goal should be to apply enough water to produce top yields while leaving the soil profile as dry as possible. The reason we want to leave the soil dry is to prevent soil compaction from harvest equipment and allow as much water storage room as we can to hold off-season precipitation.
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[00:00:07.140]Hi, I'm Steve Melvin, extension educator based
[00:00:09.250]in Merrick and Hamilton counties,
[00:00:10.960]and I work primarily with irrigated cropping systems.
[00:00:13.610]And today I want to talk about end of season
[00:00:15.530]irrigation scheduling with soybeans.
[00:00:17.760]I want to talk about this for a couple of reasons.
[00:00:19.530]One, it's that time of year when we need to be starting
[00:00:21.560]to think about how we wrap up the irrigation season
[00:00:24.320]and try to use as little water
[00:00:25.819]as we can and get full yields.
[00:00:28.120]And also the fact that if you've watched the other videos
[00:00:30.850]on the water use you can see that we typically
[00:00:33.970]deep percolate some water and irrigated fields
[00:00:36.160]and we try to like to use up as much as we can
[00:00:38.320]so we minimize deep percolation and save some water.
[00:00:41.700]So let's jump into the slide.
[00:00:43.080]There's a couple of different types
[00:00:44.520]of irrigation scheduling strategies that we can use.
[00:00:47.110]The first one's kind of just a growth stage type
[00:00:49.600]of a strategy.
[00:00:50.433]We've used that in Nebraska quite a bit.
[00:00:52.157]Basically we just wait until later in the season
[00:00:55.012]and start to irrigate.
[00:00:56.550]And there's some advantages of those.
[00:00:58.320]Certainly if the soil, is silt loam
[00:01:01.070]or a deeper, higher water holding capacity soil
[00:01:04.578]we can use this strategy.
[00:01:06.780]The soil can support a four foot root zone
[00:01:10.130]you know, where we've got a good deep soil.
[00:01:11.801]We certainly don't have those everywhere in the state
[00:01:13.940]but where we do, we can use this strategy pretty well.
[00:01:16.540]If the soil is at field capacity at planting,
[00:01:19.340]or sometime shortly after, it works well.
[00:01:21.699]And an irrigated field, that's typically always the case.
[00:01:25.030]Rain is somewhere close to normal during May
[00:01:27.132]and early, through early July.
[00:01:31.480]The capacity of the irrigation system is more
[00:01:33.800]than one and a half inches per week.
[00:01:36.190]Then we can use this strategy where we can pretty much
[00:01:38.520]delay irrigation until about R2 or R3,
[00:01:41.923]and end up with a full yield.
[00:01:44.250]And then just irrigate fully during the rest of the season.
[00:01:47.830]Another strategy I want to spend the rest
[00:01:49.490]of the time thinking about today, is based on soil water
[00:01:52.470]levels and the soil.
[00:01:54.500]And really the goal of this is to keep the soil level
[00:01:57.790]between field capacity and 50% of plant available water.
[00:02:02.010]And this is based off a system where we're talking
[00:02:04.150]about field capacity being 100% of plant available water.
[00:02:07.860]And if we had all of the water used up, it'd be zero.
[00:02:10.010]So 50% would be about when half of it is gone.
[00:02:14.440]Best for lower water holding capacity soils,
[00:02:16.570]or works better there.
[00:02:18.480]Certainly works on a really great soil as well.
[00:02:21.802]Best on dry years.
[00:02:24.080]Now we're farther West in Nebraska.
[00:02:25.760]Capacity for irrigation system is less
[00:02:28.200]than a one and a half inches per week.
[00:02:30.130]In other words, a low capacity well that we have to
[00:02:33.100]be kind of careful about that we don't get too far behind.
[00:02:36.130]And then use caution if irrigation is needed
[00:02:38.240]during the flowering stage 'cause certainly we can end
[00:02:40.270]up with some white mold potential irrigating
[00:02:42.570]during that time.
[00:02:44.170]Soybeans watered frequently early in the season
[00:02:46.950]often require us to continue to irrigate the rest
[00:02:49.450]of the season.
[00:02:50.283]So if we started irrigating we kind of need to
[00:02:51.750]keep going a lot of times.
[00:02:54.910]So, let's set up a little scenario where we can talk
[00:02:57.810]about on figuring out late season irrigation.
[00:03:00.530]So let's say that today's date is August 11th.
[00:03:03.776]Soybeans are our R5, beginning seed stage.
[00:03:07.150]We can allow the soil now to go
[00:03:08.760]onto about 40% of plant available water.
[00:03:11.080]Remember 100% is field capacity.
[00:03:13.550]So it's using up 60% of the plant available water.
[00:03:16.320]We can get them dried down fairly well.
[00:03:19.160]And then a location is here at Shelby
[00:03:21.100]in east central Polk County.
[00:03:23.410]So if we take a look at our readings that we went out
[00:03:25.820]and got and we just happened to be using watermark sensors
[00:03:28.060]here for the example, and you can see that the top one
[00:03:30.430]was at 35 and these are set up on a one foot layer.
[00:03:34.430]So it's six inches.
[00:03:35.263]The next one's at 18 was 120, 90 and 60.
[00:03:39.200]So we're gonna use this chart that's in the middle
[00:03:41.300]that's been created here the last few years
[00:03:43.130]by University of Nebraska and take a look
[00:03:45.740]at how we can use this to do some irrigation scheduling
[00:03:48.350]with these numbers.
[00:03:49.620]So this is set up for a silt loam soil.
[00:03:52.320]And with the silt loam soil is what we've got here,
[00:03:56.250]but there's also 10 other charts
[00:03:57.870]for different soil types across the state.
[00:04:00.610]Our first column on the left is a watermark reading
[00:04:03.530]or Sena bar readings.
[00:04:04.645]We can also have these with volumetric
[00:04:06.780]water content as well.
[00:04:08.540]The next column then is a percent of plant available water.
[00:04:12.580]And that's based off of field capacity.
[00:04:14.550]Remember we said 100% was field capacity.
[00:04:16.920]So if we're above that up in that blue area,
[00:04:19.510]that means that we are gonna be above 100%.
[00:04:22.200]In other words, we got more than a what we would call
[00:04:24.893]100% plant available water.
[00:04:27.410]It gets higher than that.
[00:04:28.460]And then we go below that.
[00:04:30.020]That's when we're down somewhere less than field capacity.
[00:04:32.660]So we might have 70% or 50% or something.
[00:04:36.060]The next column then gets at the actual amount of water
[00:04:39.423]that is in the soil.
[00:04:41.230]And again, it's based off a field capacity.
[00:04:43.690]So it's amounts above or below field capacity.
[00:04:46.110]So right at field capacity, we're at zero.
[00:04:48.550]And then if we're above that, it's a positive number.
[00:04:51.300]And we can have a number that maybe represents 15,
[00:04:55.340]which would be like a half inch above field capacity.
[00:04:58.560]And then if we go below, and of course it's the amount
[00:05:01.040]of water that's been used and it would be need to be
[00:05:03.650]put back in that one foot layer soil to bring it back
[00:05:06.170]up to field capacity.
[00:05:08.300]So let's take a look at an example.
[00:05:10.210]If we take a look at watermark sensor readings,
[00:05:13.240]if you've worked with those, you know you can't just
[00:05:15.060]add them up and average them together.
[00:05:17.280]But if we use this column and fill these in, we can do that.
[00:05:20.450]We can see 35 on the first one.
[00:05:23.780]So we come in to as close as we can on the chart.
[00:05:26.680]Next one's 120 and then we've got a 90 and 60.
[00:05:31.710]So if we move those over to our example,
[00:05:34.739]you can see we put the numbers that corresponded
[00:05:37.200]with the percent of plant available water,
[00:05:40.530]93, 51, 60 and 73.
[00:05:42.690]As you can see from the second column there in the chart,
[00:05:45.580]we add those together, we get 277.
[00:05:48.140]And if we divide that by four, so we can get the average
[00:05:50.496]of what those four feet zone would be,
[00:05:54.480]you can see that it's about 70% or 69.25.
[00:05:57.750]So it gives us a very good usable number,
[00:05:59.950]an intuitive number, of what's the percent
[00:06:02.070]of plant available water left in the soil.
[00:06:05.960]If we move on to the next one, we can see
[00:06:08.880]that there are two boxes that showed up there.
[00:06:12.170]The first one is at 70% or as close
[00:06:14.480]as we can get to that, 68%.
[00:06:16.740]And then the next ones down at 40%, remember again,
[00:06:19.160]we want to dry our soil down to only 40%
[00:06:21.440]of plant available water.
[00:06:23.080]So if we look in the difference of those two,
[00:06:24.890]which we've got over in our example, we've got the 68%.
[00:06:29.710]That would mean that soil, that one foot zone of soil
[00:06:32.500]would be down .64.
[00:06:34.670]And if we move down to 40%
[00:06:36.358]we could take it down to whether it was down 1.2 inches.
[00:06:39.740]In other words, take 1.2 inches of water
[00:06:41.690]to refill that profile.
[00:06:43.760]If we do the math on that, we can see that each foot of soil
[00:06:46.530]we could take out another .56 inches of water
[00:06:50.080]to take it down to that 40% level.
[00:06:52.070]So that's how much we've got left to use on our profile.
[00:06:54.730]And since that's for four feet, we can multiply it
[00:06:57.280]times four, I guess I kind of jumped ahead there,
[00:06:59.850]but we can see that it would be 2.24 inches
[00:07:02.820]of water left that we could use out of our profiles.
[00:07:05.270]So that gives us a pretty good feel about what's there.
[00:07:07.920]So now let's think about how many days that might last.
[00:07:10.420]So we've got soybeans on this chart at the top.
[00:07:13.920]We moved down to seed fill and we go over
[00:07:16.420]and that's in Nebraska on average, that's about .24 inches
[00:07:19.940]of water per day.
[00:07:20.790]And of course that's an average number.
[00:07:22.040]If it's a cooler time, it would be less.
[00:07:24.640]If it's hot, it'd be a little bit more,
[00:07:26.448]but we can see that about .24 on average per day.
[00:07:30.190]And if we go over here and do the math again,
[00:07:31.900]divide the .24 into the 2.24, we can see that'd last
[00:07:35.600]about nine days before we would be completely out of water.
[00:07:38.950]So it gives us a pretty good feel about how quickly
[00:07:41.060]we need to get out to the field
[00:07:42.420]and get some irrigation going.
[00:07:43.660]And with this many days, you know, we're kind of winding
[00:07:46.290]down to the end of the season.
[00:07:47.190]We might want to delay just a little bit
[00:07:49.040]and use up a little bit of this water,
[00:07:51.057]because of course we don't know
[00:07:52.500]how much it's gonna rain, right?
[00:07:53.690]So we'd kinda like to take advantage
[00:07:55.350]of that rain and still have things fairly dry.
[00:07:57.300]So I might hold up here a couple of days anyway,
[00:07:59.880]before I'd put any water on, kind of look at the forecast
[00:08:02.370]and again, the end of the season's
[00:08:03.760]when we want to do more management
[00:08:05.900]because over the winter we get a free refill, right?
[00:08:08.730]We get our off season precipitation refilling that profile.
[00:08:12.770]So if we take a look at this chart
[00:08:14.280]that's out of a NebGuide G81 or 1871, and take a look
[00:08:19.610]at the soybeans down at beginning seed fill,
[00:08:22.720]we can see a couple of numbers on there.
[00:08:24.540]The first one is the number of days approximate to maturity,
[00:08:27.630]and that's not too, that's typically
[00:08:29.440]pretty close for soybean.
[00:08:30.670]So if we take a look, we can see that today's
[00:08:33.660]the 11th of August, we'd be at about the 9th of September,
[00:08:37.000]is when our beans should be hitting maturity.
[00:08:39.450]And we wouldn't have to worry about irrigating anymore,
[00:08:41.550]but also there's a number on there that we use
[00:08:44.130]down in the next box.
[00:08:45.250]It shows how much water it will take to move
[00:08:47.740]to that maturity stage.
[00:08:50.220]Now again, if it's cooler, it's gonna be a little bit less.
[00:08:52.750]If it's hot, it's gonna be a little bit more,
[00:08:54.270]but it gives us an ability to kind of predict
[00:08:56.700]into the future how much water this crop may need.
[00:08:59.440]So if we take a look with this one, we do the math on that.
[00:09:02.350]We can see that we're gonna need about four and a quarter
[00:09:04.350]inches of water yet to mature this crop.
[00:09:07.470]And so it gives us an idea how much we would need,
[00:09:10.220]and we could just go pump that much water
[00:09:11.850]on the field and call it good, and say we're done
[00:09:14.610]irrigating, but again with the rainfall,
[00:09:16.420]we really need to take that into account.
[00:09:18.480]So let's take a look at what we might have.
[00:09:20.730]Again, this is a chart in Nebraska
[00:09:22.310]for the average precipitation in August.
[00:09:24.650]And we can go over to Eastern to about where
[00:09:27.250]this area is at in Polk County.
[00:09:28.960]And it's about .75 inches of water per week
[00:09:31.930]in August is what we can expect.
[00:09:34.080]Of course, everybody knows some weeks
[00:09:35.560]it does way more than that.
[00:09:36.660]And, and a lot of weeks it doesn't rain any,
[00:09:38.410]but that gives us a planning tool.
[00:09:40.520]If we take a look at September in the same area,
[00:09:42.670]it's about .65, we've got about two weeks
[00:09:45.620]left in August yet.
[00:09:48.240]So if we come down here, our 4.265.
[00:09:51.423]And the next box down, we're adding in one and a half inches
[00:09:54.500]because we've got about three quarters an inch of rain
[00:09:56.790]on average, we expect the first, each of the first two weeks
[00:10:00.150]or the last two weeks of August.
[00:10:01.940]Next box, and down as the first two weeks of September
[00:10:04.480]we'd expect about an inch 30 hundreds in those two weeks.
[00:10:07.930]So if we do the math, we can see that we need to apply
[00:10:10.940]about an inch 46, if we get average rainfall.
[00:10:14.550]And so again, this is a planning tool
[00:10:16.610]that helps us take a look.
[00:10:17.850]Of course, we need to respond to how hot and dry
[00:10:20.200]it is versus how cool it is.
[00:10:21.700]We also need to respond on if we get average moisture
[00:10:24.015]more or less, but it gives us an ability to really predict
[00:10:27.720]out to the end of the season and try and use,
[00:10:30.206]you know, as little water as we can,
[00:10:32.490]but yet end up with as our maximum potential yield.
[00:10:37.350]So hopefully this gives you a little bit
[00:10:38.790]of an idea of how some of these things could play out.
[00:10:41.520]Couple of resources that I would reference you to,
[00:10:44.230]the first one there on the left is where these charts
[00:10:47.810]can be found.
[00:10:48.770]There's a website or a just a link there that you can go,
[00:10:52.719]unl.edu/ec3036, and it has these charts
[00:10:57.970]for the 11 different soil types
[00:10:59.230]plus a description of how to use those.
[00:11:01.840]Before that, the middle one is some videos
[00:11:04.490]that goes into much more detail on how to use these charts,
[00:11:08.210]but more importantly, some other irrigation scheduling
[00:11:10.550]strategies that you might use earlier in the year.
[00:11:13.180]You can see that it's the
[00:11:17.895]And then on the right is where the NebGuide
[00:11:20.157]that we referenced for the last irrigation.
[00:11:23.460]So I think some take home points from this is the fact
[00:11:25.593]that we always come up against this the end of the year.
[00:11:28.690]We're trying to play that game of using all the water
[00:11:31.460]that we can while we're still getting maximum yields
[00:11:33.660]because we want the soil as dry as possible
[00:11:36.240]at come end of our season because we know
[00:11:38.920]we're gonna get a free refill over the winter.
[00:11:41.250]And also if it gets wet during harvest,
[00:11:43.110]a dry field is gonna take more rain
[00:11:45.300]to get it to where it's too muddy to go out and harvest.
[00:11:47.420]So a couple of very good incentives to you know,
[00:11:51.220]take some time, do a little bit of calculating,
[00:11:53.170]is probably the time when you need to be the most accurate
[00:11:55.450]about how you're doing irrigation scheduling.
[00:11:58.410]So hopefully this was helpful, and take a look
[00:12:00.760]at these resources for more assistance.
[00:12:03.300]So producers, a question received was, you know,
[00:12:06.700]what about planting a fall cover crop in this situation?
[00:12:10.200]And of course, if we used up all the water,
[00:12:12.717]the top of the soil is gonna be very dry,
[00:12:15.010]but keep in mind it does rain some in the fall.
[00:12:16.910]And also we're talking irrigated cropping systems here.
[00:12:19.720]So, you know, we may very well want to spend an inch
[00:12:22.610]of water after we plant the cover crop,
[00:12:24.580]and get some, get that crop off to a very good start
[00:12:27.320]because it may very well be a very you know,
[00:12:29.920]very well invested inch of water.
[00:12:32.800]Of course there's a cost associated with that,
[00:12:34.870]but it may get the crop off to a good start
[00:12:36.848]and get some very good benefits.
[00:12:39.400]And it just makes for a better situation some years.
[00:12:42.860]Some years we don't need to do that.
[00:12:44.310]What was your second question?
[00:12:47.060]So from an irrigation, another question
[00:12:49.610]that we might think about is from an economic standpoint,
[00:12:53.280]how do cover crops impact what we're doing
[00:12:55.270]with irrigation scheduling and managing the amount of water
[00:12:57.970]that we have out there.
[00:12:59.350]And I think probably the spring time's the main time
[00:13:01.310]to think about that, can we use some of the water
[00:13:03.309]that's probably gonna deep percolate
[00:13:05.710]on our irrigated land with a cover crop.
[00:13:08.340]And that's probably not gonna impact a lot
[00:13:10.420]on what we do with irrigation scheduling
[00:13:12.100]but it certainly a fact that most years in Nebraska,
[00:13:15.270]probably across most of the state,
[00:13:16.760]but for sure the Eastern half of the state,
[00:13:18.430]we can use some water in the spring
[00:13:20.020]and still have a full profile.
[00:13:21.690]And so again, just really emphasize
[00:13:23.934]the point we want to do a late season irrigation
[00:13:27.010]scheduling and use as much water as we can.
[00:13:29.460]And then we can still plant cover crops
[00:13:31.280]and still have enough water to, you know,
[00:13:34.650]use that in the spring and still have a full profile.
[00:13:37.310]Not really have to increase our pumping costs any.
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