Scheduling Last Irrigation Cycle

Amy Timmerman, Extension Educator Author

11/16/2022 Added

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With pumping costs ranging from $6-15/acre-inch this year, any opportunity to save money by cutting back irrigation as early as possible sounds like a good strategy. Correctly timing the last few irrigations of the season offers an excellent opportunity to save some water and money.

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[00:00:00.812]This week’s Extension update is with Extension Educator Amy Timmerman.
[00:00:05.692]With pumping costs ranging from $6-15/acre-inch this year, an opportunity to save money
[00:00:12.352]by cutting back irrigation as early as possible sounds like a good strategy.
[00:00:16.898]Correctly timing the last few irrigations of the season offers an excellent
[00:00:20.898]opportunity to save some water and money.
[00:00:24.608]Factors such as the amount of water a soil can hold, the amount of water
[00:00:28.538]a crop will use until it reaches maturity, and the maximum allowable soil water depletion
[00:00:33.401]should be considered when deciding the last few irrigations of the season.
[00:00:37.511]In addition to water and dollar savings, another benefit of leaving the fields as
[00:00:42.251]dry as possible without lowering yields is the potential to reduce issues with
[00:00:46.786]nutrient leaching and increase the amount of precipitation stored
[00:00:50.696]during the offseason.
[00:00:52.726]The amount of water used by the plants during the tail end of the growing
[00:00:56.172]season changes from crop to crop.
[00:00:58.419]With different crops, it is important to know how
[00:01:00.772]weather conditions affect crop water use.
[00:01:03.462]For instance, weather conditions will affect the water use of the soybeans
[00:01:07.632]because they tend to mature based on daylength, as opposed to
[00:01:11.012]corn and sorghum that mature based on growing degree days.
[00:01:15.519]Thus, for corn and sorghum, hotter conditions will result
[00:01:19.373]in more water use per day but will also mature the crop sooner.
[00:01:23.801]On the other hand, soybeans may use more water during hotter weather conditions
[00:01:28.290]but won’t mature quicker, resulting in greater total water use.
[00:01:32.772]How much water corn and soybean will need until they reach maturity
[00:01:36.772]based on each growth stage is critical to know.
[00:01:40.122]This is the baseline information needed when it comes to deciding
[00:01:43.800]the last irrigation of the season.
[00:01:46.130]Corn at dough stage has approximately 34 days to maturity and
[00:01:50.420]will require 7.5 inches of water to maturity.
[00:01:54.420]Beginning dent has approximately 24 days to maturity
[00:01:58.420]and will required 5 inches of water.
[00:02:01.960]¼ milk line is approximately 19 days to maturity and will need an
[00:02:05.970]additional 3.75 inches of water.
[00:02:09.040]½ milk line is approximately 13 days to maturity and will need an
[00:02:12.997]additional 2.25 inches of water.
[00:02:16.127]And finally ¾ milk line is approximately 7 days to maturity and will need
[00:02:20.127]an additional 1 inch of water.
[00:02:22.557]Soybeans at growth stage R4 is approximately 37 days to
[00:02:26.131]maturity and will require 9 inches of water to reach that stage.
[00:02:31.833]R5 are approximately 29 days from maturity and will require 6.5 inches of water.
[00:02:38.269]R6 soybean are approximately 18 days from maturity
[00:02:41.799]and will require 3.5 inches of water.
[00:02:44.759]Soybeans at grow stage R6.5 where the leaves are beginning
[00:02:48.999]to turn yellow are approximately 10 days from maturity and will still require 1.9 inches of water.
[00:02:56.114]We should always keep adequate soil moisture levels to maximize
[00:02:59.842]yield on irrigated fields if we have the water.
[00:03:03.524]During peak water use, UNL recommends maintaining soil water storage levels
[00:03:07.524]above 50% of plant available water in the top three feet of soil.
[00:03:11.886]As plants approach the end of the cropping season,
[00:03:14.516]the days are getting shorter and cooler and their leaves begin to lose
[00:03:18.236]the ability to transpire water, which opens an opportunity
[00:03:22.196]to let the soil dry to a lower water content without affecting yield.
[00:03:26.574]The UNL recommendation is to lower the soil water content
[00:03:30.104]to 40% of plant available water to a four-foot depth after the dough
[00:03:34.140]stage in corn and R4 or end of pod elongation in soybean.
[00:03:39.443]Another factor besides knowing how much water the crops will use between now
[00:03:43.443]and maturity is determining the amount of water stored in the soil.
[00:03:47.143]A fine sand soil, for example, holds about one inch per foot of soil or
[00:03:52.173]four inches on the top four feet of soil.
[00:03:54.843]A silt loam soil, on the other hand, holds two inches per foot of soil or
[00:03:59.792]eight inches on the top four feet of soil.
[00:04:03.452]Assuming both soils are at field capacity,
[00:04:06.330]the maximum amount of water that can be used is 2.4 inches which is 60% of four inches
[00:04:12.643]for the fine sand soil and 4.8 inches (60% of eight inches) for the silt loam soil.
[00:04:19.983]The last piece of information needed is the expected rainfall amounts
[00:04:23.623]between today and the date the crop maturity.
[00:04:27.013]While many parts of the state are very dry this year,
[00:04:29.758]one should still keep in mind the long-term average rainfall in
[00:04:33.884]determining that last irrigation cycle.
[00:04:36.924]This has been Amy Timmerman with Nebraska Extension.

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