Minnesota Crop News > 2001-2008 Archives
August 1, 2006
Predicting the Last Irrigation for Corn and Soybeans
in Central Minnesota1
Jerry Wright, Associate Professor and Extension Engineer: firstname.lastname@example.org
Dale Hicks, Professor and Extension Agronomist: email@example.com
or 612 625-1796
Seth Naeve, Assistant Professor and Extension Agronomist: firstname.lastname@example.org
University of Minnesota Extension Service
- Revised July 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. Table 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:
- There should be adequate soil moisture available in the root zone to carry the crop to maturity to produce optimum yields.
- 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 Yield
* percent 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 Yield
* Source: Iowa State
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: www.climate.umn.edu
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: www.soils.wisc.edu/wimnext/et/wimnet.html
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 soilís 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 in the University of Minnesota Extension Service Bulletin #AG-FO-1322 "Irrigation Scheduling" at web site: www.extension.umn.edu/distribution/cropsystems/DC1322.html
Table 3. Estimated normal water requirements for corn and soybeans between various stages of growth and maturity in central Minnesota
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.
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
* 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
1 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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