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Predicting the last irrigation for corn and soybeans

Updated by: Vasudha Sharma, Assistant Extension Professor-Irrigation Specialist
*Article was first published in July 1988 by Jerry Wright and Extension Agronomists, Leland Hardman & Michael Schmitt.
*Article was revised in 2006 by Jerry Wright, Retired Extension Engineer, Dale Hicks, Retired Extension Agronomist, Seth Naeve, Extension Soybean Agronomist

Determining the amount and timing of the last few irrigations of the season is one of the most critical water management decisions. Discontinuing too early in the season to save water or reduce pumping cost could mean a much greater reduction in yield returns than the cost of pumping. On the other hand, irrigating right up to crop maturity may mean using 1 to 3 inches more irrigation water than necessary and increasing operating costs $3 to $15 per acre depending on power source. The purpose of this article is to present some guidelines for predicting the last irrigation for corn and soybeans when irrigation water supplies are adequate.

Basic requirements for estimating the last irrigation of the season

The two basic irrigation water management strategies that an operator should keep in mind when predicting the last irrigation are:
  1. To provide adequate soil moisture in the root zone to carry the crop to maturity without reducing yields.
  2. To deplete the soil moisture farther than normal (i.e., 60-70% of available water can be depleted at maturity) when nearing maturity. This will minimize irrigation water supply needs, fuel and labor for the season and allow the off-season precipitation to recharge the soil profile.

These requirements may appear to be conflicting, but the problem can be solved rather easily if adequate field information is available or is predictable. The following field information is necessary to predict the date of the last irrigation.
  • Current crop growth stage and predicted crop maturity date
  • Predicted rate of water use by the crop to maturity
  • Remaining useable water in the root zone
  • The probability of significant amounts of rainfall before crop maturity

Information on the probability of rainfall will not be discussed in this article. But, the latest weather forecast at time of predicting the last irrigation should be considered in the decision. Also, if you are located in an area having some level of drought, the ongoing drought situation can be monitored at the University of Minnesota climate web site:

Steps to schedule the last irrigation

  1. Record the date, field, crop, soil type, and the crop growth stage. The soil type information can be obtained from the NRCS Web Soil Survey and Tables 2–3 can be used to determine the crop growth stage.
  2. Determine the water use by the crop to reach maturity (WUCM). Table 4 presents the approximate water use at particular growth stage to reach maturity.
  3. Determine the allowable soil moisture deficit (ASMD) for the soil listed in step 1. The ASMD values for different soil types are presented in table 5.
  4. Measure the current soil moisture deficit (CSMD).
  5. Calculate the remaining useable soil moisture (RUSM) in the rooting zone by subtracting CSMD from step 4 from ASMD found in step 3.
  6. Calculate the irrigation water requirement (IWR) by subtracting the remaining useable soil moisture found in step 5 from water use to crop maturity found in step 2. If the remaining useable water (step 5) is greater than the water use to crop maturity (step 2), then no irrigation is required.

Detailed description of how to calculate various parameters in table 1 is described below.

Table 1. Estimation of the last irrigation requirement form.
Steps Example 1 Example 2 Your field
1. Date
Soil type
Crop growth stage
(Table 3 and 4)
Test 1
Esterville sandy loam
Test 2
Dakota loam
Beginning seed
2. Water use to crop maturity (WUCM)
(Table 4)
2.5 2.90
3. Allowable soil moisture deficit (ASMD)
(Table 5)
1.5 3.45
4. Current soil moisture deficit (CSMD)
0 0.50
5. Remaining usable water (RUSM)
(Step 3 minus Step 4)
1.5 2.95
6. Irrigation water requirements in inches (IWR). Assumes no rain.
(Step 2 minus Step 5)
1 0*
*If line 5 is greater than or equal to line 2, no more irrigation is needed.

Table 2. Corn reproductive growth stages.
Growth stage Description
Blister (R2) 10-12 days after silking. Kernel is visible and resembles a “blister” filled with clear fluid and embryo is barely visible. Approximately 85% moisture content.
Milk (R3) 18-20 days after silking. Kernel is colored yellow with the inside containing “milky” white fluid. Kernel moisture content is approximately 80%.
Dough (R4) 24-26 days after silking. Interior of kernel has thickened to a dough or paste-like substance. Kernel moisture content is approximately 70%.
Beginning dent (R4.7) Kernels begin to dent at the base of the ear.
Full dent (R5)
1/2-milk line (R5.5)
3/4-milk line (R5.75)
31-33 days after silking. Kernels dented at kernel top with the “milk line” separating the liquid and solid (starch) portions. Within R5 stage, kernels are often staged according to the progression of the milk line; i.e. ¼, ½, and ¾.
Source: Corn Growth and Development, PMR 1009, Iowa State University, 2011

Table 3. Soybean reproductive growth stages.
Growth stage Description
Full flowering (R2) Open flower at one of the two uppermost nodes
Full pod (R5) Pod is ¾” long at one of the four uppermost nodes
Beginning seed (R5) Seed is 1/8” long in pod at one of the four uppermost nodes.
Full seed (R6) Pod containing a green seed that fills the pod cavity at one of the four uppermost nodes
Beginning maturity (R7) At least one pod with its final mature color is present anywhere

Table 4. Estimated normal water requirements for corn and soybeans between various growth stages and maturity in central Minnesota.
Growth stage Approximate number of days to maturity Water use (ET) to maturity
Blister (R2) 50 7.0
Milk (R3) 40 4.7
Dough (R4) 28 2.50
Beginning dent (R4.7) 24 2.00
Full dent (R5) 20 1.50
1/2 milk line (R5.5) 13 0.80
3/4 milk line (R5.75) 7 0.30
Full flowering (R2) 51 7.25
Full pod (R4) 37 4.40
Beginning seed (R5) 29 2.90
Full seed (R6) 17 1.20
Beginning maturity (R7) 10 0.40

Table 5. Available soil water capacity and allowable soil moisture deficit at maturity for several irrigated soil in Minnesota.
Soil type Total available water* (TAW)
Allowable soil moisture deficit (ASMD) (60% depletion)
Becker (fine sandy loam) 4.00 2.40
Dakota (loam) 5.75 3.45
Esterville (sandy loam) 2.50 1.50
Hubbard (loamy sand) 2.60 1.56
Renshaw (loam) 3.75 2.25
Sioux (loamy sand) 1.20 0.72
*Water capacity in the top 3 feet or less for soils having root restrictive layer like coarse gravel

Step 1: Crop Growth Stage

Corn and soybean plants require some moisture right up to the time of maturity. However, with shorter and cooler days towards the end of the season, crop is using less water thus required less water than rest of the season per day. Since some of the required moisture near the end of the season can be obtained from the soil moisture reservoir, the last irrigation can usually be applied two to three weeks prior to physiological maturity depending on the soil's water holding capacity. To estimate the number of days left to reach the maturity and water use by crop until maturity, knowledge of crop growth stage is very important. Maturity of a crop is defined as the time when the kernels or seeds have reached maximum dry weight. For corn, a black layer formation at the tip of the kernel is the normal indication of physiological maturity. This occurs approximately 7 days after the kernel has reached the 1/4 milk line. For soybeans, beginning maturity is generally identified when one normal pod on the main stem has reached its mature yellow or brown color. Tables 2 and 3 of this article describes the reproductive growth stages of corn and soybeans. A more detailed information about the crop growth can be found in the source provided at the end of each table.

Step 2: Water use to crop maturity (WUCM)

Estimation of crop water use until maturity involves the estimation of daily crop evapotranspiration (ET) and then summing it up from the growth stage of interest until maturity. ET is the combination of evaporation from the soil surface and transpiration from the plants. It is the total amount of water used by the crop. Evapotranspiration is affected by many factors including weather (radiation, air temperature, humidity and winds speed), crop factors such as crop type, variety and development stage, and management conditions. The evapotranspiration from a reference surface (grass and alfalfa) not short of water is called reference evapotranspiration (ETref). To estimate the crop ET for a particular crop at a particular growing stage, reference ET must be multiplied by a crop coefficient (Kc):

Crop ET = ETref x Kc

Table 4 shows the estimated water requirements (ET) between a given growth stage and maturity for corn and soybeans for central Minnesota under normal climatic conditions. These estimates were calculated by using normal crop development rates for 95 RM corn and central soybean maturity zone and normal water use patterns for central Minnesota.

Step 3: Determining available soil moisture deficit (ASMD)

The available water holding capacity (AWHC) of different soils is different. It is a function of soil texture, soil structure and organic matter of the soil. It is the soil moisture (water) that can be extracted and used by plants. Available water holding capacity (AWHC) is the amount of water the soil holds between the upper limit, i.e., field capacity, and the lower limit, i.e., permanent wilting point. To determine the total available water (TAW) in the root zone depth, the AWHC is multiplied by root zone depth (RZD).


Table 5 presents the TAW of different soil types in top 3 feet or less. The AWHC for other soil types can be obtained from NRCS web soil survey

Since most fields have several types of soils, the lowest water holding capacity soil covering at least 25 percent of the field should be used in the above calculations. Lower water holding soils found on ridges or hill tops should not be used to plan the next irrigation. Research shows that 60 to 70 percent of the TAW can be depleted at crop maturity without reducing the grain yield. Therefore, the ASMD in top 3 feet or less can be calculated by the following equation:

ASMD = 0.60 x TAW

Step 4: Measuring current soil moisture deficit (CSMD)

The current soil moisture deficit is the difference between the TAW and the actual soil moisture status in the root zone depth in field. There are many methods available to measure the current soil moisture deficit. These method include measuring soil moisture electronically using neutron gauge or resistance blocks, measuring by physical methods like tensiometers, estimating using traditional hand feel or appearance method, and using irrigation scheduling methods such as checkbook method which uses ET data. More information to help in estimating current soil moisture deficit can be found on the University of Minnesota Extension publication:

Steps 5 and 6: Useable water (RUSM) and irrigation water requirement (IWR)

Once we determined the ASMD and the CSMD, the remaining usable soil moisture (RUSM) in the root zone can be calculated by subtracting the current soil moisture deficit (CSMD) from the allowable soil moisture deficit (ASMD) at maturity.


Irrigation water requirement (IWR) can be calculated by subtracting the RUSM from the WUCM


Note: If IWR is negative that means no irrigation is needed.

*Adapted from Bockstadter and Eisenhauer paper presented at Nebraska Irrigation Shortcourse, February, 1988.
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