Predicting the Last Irrigation for Corn and Soybeans in Central Minnesota

 

 

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.

 

 

The two basic irrigation water management strategies that an operator should keep in mind when predicting the last irrigation are:

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.

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: www.climate.umn.edu

 

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

 

 

 

 

 

 

 

 

 

 

 

 

 

 

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.

 

 

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.

 

 

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). 

 

TAW = AWHC x 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 https://websoilsurvey.sc.egov.usda.gov/App/HomePage.htm

 

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

 

 

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: 

 

 

 

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.

 

RUSM = ASMD - CSMD

 

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

 

IWR = WUCM – RUSM

 

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