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Friday, September 1, 2006

Predicting the last irrigation for corn and soybeans in Central Minnesota*

Jerry Wright, Retired Associate Professor and Extension Engineer, Dale Hicks, Retired Professor and Extension Agronomist, Seth Naeve, Extension Soybean Agronomist
Revised 2006 (first issued July 1988)

Determining when one can discontinue irrigating for the season is an important water management decision. 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 paper is to present some guidelines for predicting the last irrigation for corn and soybeans when irrigation water supplies are adequate.

Corn and soybean plants require some moisture right up to the time of maturity. 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.

There are several crop and site-specific factors an operator needs to consider that will influence the optimum time to discontinue irrigating. Tables 1 and 2 show the potential yield losses at different stages of growth for corn and soybeans when under severe soil moisture stress.

Two basic irrigation water management strategies that an operator should fulfill when predicting the last irrigation near the end of the season are as follows:
  1. There should be adequate soil moisture available in the root zone to carry the crop to maturity to produce optimum yields.
  2. The soil moisture reservoir should be depleted farther than normal 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.
  • Predicted crop maturity date
  • Predicted rate of water use by the crop
  • Remaining usable soil moisture in the root zone
  • The probability of significant amounts of rainfall before crop maturity

Table 1. Effects of severe soil moisture stress on corn yield1.
Crop stage Final yield decrease
% per day % per 4 days
Early vegetative -- 5-10
10-Leaf 1-2 --
15-Leaf 2-4 --
Tassel emergence -- 10-25
Tassel - silk 5-15 --
Silk emergence - pollen shed -- 40-50
Blister 3-6 30-40
Milk 3-5 --
Dough 3-4 20-30
Dent 2-3 --
1Percent reduction from "severe" moisture stress when leaf rolling for several hours has occurred during the afternoon. Source: Classen and Shaw. 1970 Water Deficit Effects on Corn. Agr. J. 62:652-655

Table 2. Effect of 4 days of visible moisture stress on soybean yield1.
Crop stage Yield decrease
  Percent (%)
1st week flowering 8
1st week pod development 19
2nd week flowering
1st week of seed filling
3rd week pod development 36
4th week of flowering
2nd-4th week of seed filling 39-45
5th week of seed filling 12
1Source: Iowa State University

Information on the probability of rainfall will not be discussed in this paper. 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:

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 ¼ milk line. For soybeans, beginning maturity is generally identified when one normal pod on the mainstem has reached its mature yellow or brown color. Table 3 identifies the approximate number of days a corn or soybean plants has to maturity from different stages of growth.

The estimated water requirements between a given growth stage and maturity for corn and soybeans for central Minnesota under normal climatic conditions are presented in Table 3. 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. For current water usage estimations check out web site:

Remaining usable soil moisture in the root zone within a field is the difference between the current soil moisture deficit and the allowable soil moisture deficit at maturity. Research shows that 60 to 70 percent of the available soil moisture in the plant root zone can be depleted at crop maturity and not reduce grain yield. Therefore, the allowable soil moisture deficits can be calculated by the following equation:

ASMD = 0.65 x AWC x RZD
ASMD = allowable soil moisture deficit in inches
AWC = available water capacity of soil in inches per foot
RZD = root zone depth in feet

Table 4 lists available water capacities and the 60% allowable deficits for some typical irrigated soils. Available water capacity for other soil profiles can be obtained from a local county soil survey map or SCS office. 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.

The current soil moisture deficit is the difference between the soil's available water capacity and the actual soil moisture status in field. Current soil moisture deficit can be estimated by tensiometers, soil moisture blocks or the hand-free method (Table 5). More information to help in estimating current soil moisture deficit can be found on the University of Minnesota Extension publication, Irrigation Scheduling on the Irrigation web site:

Table 3. Estimated normal water requirements for corn and soybeans between various stages of growth and maturity in central Minnesota
Stage of Growth Approximate Number of Days to Maturity Water Use (ET) to Maturity
Blister kernel 50 7.0
Milk 40 4.7
Dough 28 2.50
Beginning Dent 24 2.00
Full Dent 20 1.50
1/2 milk line 13 0.80
1/4 milk line 7 0.30
Full flower (R2) 51 7.25
Full pod development (R4) 37 4.40
Beginning seed fill (R5) 29 2.90
Full seed fill (R6) 17 1.20
Beginning Maturity (R7) 10 .40
(R2) full flower - a flower at the node immediately below the upper most node with completely unrolled leaves
(R4) full pod development - a pod 3/4 inch long at one of the four upper most nodes with unrolled leaves
(R5) beginning seed fill - the presence of bean seeds in pod at one of the four upper most nodes
(R6) full seed fill - a pod with full-sized green beans at one of the four uppermost nodes
(R7) beginning maturity - one normal pod on the main stem has reached its mature yellow or brown color

The following form can be used in conjunction with Tables 3 and 4 as a guide to decide if one more irrigation is necessary at a given plant growth stage and how much more water (irrigation or rain) is needed to achieve full season yield.

Date: ExampleExample
Crop: CornSoybeans
Soil Type: EsthervilleDakota
  Sandy LoamLoam
1. Present stage of growthDoughBeginning to fill
2.Water required to crop maturity (Table 3)2.52.90
3.Allowable soil moisture deficit (60%) (ASMD), inches (See Table 4)1.503.45
4. Current soil moisture deficit, inches (Table 5)00.50
5.Remaining usable moisture (line 3 minus 4)1.502.95
6.Irrigation requirements assuming no rain, inches (line 2 minus 5)1.000*
*Note: If line 5 is greater than or equal to line 2, no more irrigation is needed.

If additional irrigation water is predicted, wait until at least 50 to 60 percent of the available water capacity in the active root zone is depleted before the next irrigation. By delaying irrigation until this point, the probability of receiving enough rainfall to replenish the soil deficit increases. For maximum water efficiency, the net water applied during the last irrigation should not exceed the calculated irrigation water requirement (step 6).

Table 4. Available soil water capacity and allowable soil moisture deficit at maturity for several irrigated soils

 Available WaterAllowable Deficit
Soil TypeCapacity*At 60% depletion
Becker (fine sandy loam)4.002.40
Dakota (loam)5.753.45
Estherville (sandy loam)2.501.50
Hubbard (loamy sandy)2.601.56
Renshaw (loam)3.752.25
Sioux (loamy sand)1.20.72

* Water capacity in the top 3 fee or less for soils having a root restrictive layer like coarse gravel.

Table 5. Guide for judging soil moisture deficit based on fee and appearance of soil

 Soil texture classification
Moisture deficiency in./ft.


(loamy sandy)


(sandy loam)




(clay loam)

 (field capacity)(field capacity)(field capacity)(field capacity)
0.0Leaves wet outline on hand when squeezedAppears very dark, leaves wet outline on hand, makes a short ribbonAppears very dark, leaves wet outline on hand, will ribbon out about one inchAppears very dark, leaves slight moisture on hands when squeezed, will ribbon out about two inches
0.4Appears moist, makes a weak ballQuite dark color, makes a hard ballDark color, forms a plastic ball, slicks when rubbedDark color, will slick and ribbons easily
0.6Appears slightly mosts, sticks together slightlyFairly dark color, makes a good ballQuite dark, forms a hard ballQuite dark, will make thick ribbon, may slick when rubbed
1.0Dry, loose, single-grained, flows through fingers (wilting point)Slightly dark color, makes a weak ballFairly dark, forms a good ballFailry dark, forms a good ball
1.2 Lightly colored by moisture, will not ballSlightly dark, forms weak ballWill ball, small clods will flatten out rather than crumble
1.4 Very slight color due to moisture, loose, flows through fingers (wilting point)Lightly colored, small clods, crumble fairly easilySlightly dark, clods crumble
1.8  Slight color due to moisture, powdery, dry, sometimes slightly crusted but easily broken down in powdery condition (wilting point)Some darkness due to unavailable moisture, hard, baked, cracked, sometimes has loose crumbs on surface (wilting point)

* Adapted from Bockstadter and Eisenhauer paper presented at Nebraska Irrigation Shortcourse, February, 1988. (Article first published in July 1988 by J. Wright, & Extension Agronomists, Leland Hardman & Michael Schmitt.)

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