Terminating Irrigation in Corn and Soybean

Irrigation should be carefully managed because water stress during critical crop growth stages can reduce yield potential. A moisture deficit accounting system and the current growth stage can be used as an aid in predicting water needs to reach physiological maturity. Continuing irrigation after physiological maturity is an added expense resulting in no additional yield.

Corn Irrigation Termination. Corn plants require about 5.0 to 5.5 inches of water from beginning dent growth stage to reach physiological maturity.1,2 Physiological maturity (R6) occurs when the abscission layer or “black layer” forms at the base of each kernel. Corn kernels continue to accumulate seed weight until black layer formation. At this point the kernels have attained their maximum dry weight with moisture content at about 28 to 35%.

Water requirements decrease as the plant gets closer to maturity; however, the plant still requires about 3.0 inches of total water from full dent growth stage (R5) to maturity (Table 1).1 This amount doesn't have to all come from the irrigation system as some will come from available soil water in the root zone supplied by previous irrigations and precipitation.

Table 1. Average Water Use for Given Growth Stages to Maturity. Colorado State University (may vary by region).
Corn Growth Stage Approximate Days to Maturity Approximate ET to Maturity (inches)*
Blister (R2) 45 11.5
Dough (R4) 35 8.5
Beginning Dent 24 5.5
Full Dent (R5) 14 3.0
Maturity (R6) 0 0.0
* Varies according to areas of the state. Source: 1Bauder, T., Waskom, R., Schneekloth, J., and Alldredge, J. 2003. Best management practices for Colorado corn. Extension bulletin XCM574A. Colorado State University Cooperative Extension.

Farmers can estimate the final irrigation by determining water use needed to reach maturity from the present growth stage (Table 1 and 2). Colorado State University (Table 1) and University of Nebraska-Lincoln (Table 2) water requirements from various corn growth stages to maturity differ slightly when compared to each other but are based on their respective land grant university research. These differences may be influenced by various environments and seed products.

Table 2. Normal water requirements for corn between various stages of growth and maturity in Nebraska (may vary by region).
Corn Growth Stage Approximate Days to Maturity Water Use to Maturity (inches)
Dough (R4) 34 7.5
Beginning Dent (R4.7) 24 5.0
1/4 Milk Line (R5) 19 3.75
1/2 Milk Line (R5) 13 2.25
3/4 Milk Line (R5) 7 1.0
Maturity (R6) 0 0.0
Source: 2Younts, C.D., Melvin, S.R., and Eisenhauer, D.E. 2008. Predicting the last irrigation of the season. NebGuide. G1871. University of Nebraska-Lincoln Extension.

Soybean Irrigation Termination. If using an irrigation scheduling approach to monitor soil moisture, ensure that available soil water is not depleted below 50% during the soybean reproductive growth stages.4 This is especially important from growth stages R3 (beginning pod) through R6 (full seed), which are the most sensitive growth stages to water stress and potential yield losses. Soybean require about 5.5 inches of water from beginning seed (R5) growth stage to reach full maturity.4 Consider the predicted maturity date of the crop, the estimated water use to maturity and available soil moisture to ensure that enough water will remain in the soil to bring the crop to maturity without inducing stress (Table 3). By R8 (full maturity), water is no longer needed for seed enlargement.

Table 3. Example of water requirements for soybean during late reproductive stages through maturity (may vary by region).
Growth Stage Days to Maturity Water Use to Maturity (inches)
R4 Full Pod. Pods are 3/4-inch long on at least one of the four uppermost nodes. 50 8.5
R5 Beginning Seed. Seed is 1/8-inch long in a pod located on one of the four uppermost nodes. 40 5.5
R6 Full Seed. Pod contains at least one green seed that fills the pod cavity located on one of the four uppermost nodes. 30 3.5
R7 Beginning Maturity. One pod located on one of the four uppermost nodes has reached mature pod color. 10 1.0
R8 Full Maturity. 95% of the pods have reached mature pod color. 0 0.0
Source: Table modified from 4 Soybean irrigation and water use. University of Missouri Extension. http://extension.missouri.edu.

Irrigation Management. When using a soil moisture accounting system, it is important that a farmer use their calculations when determining when to terminate irrigation. An accounting system can be used to determine application timing as it accounts for applied water, crop usage, rainfall, temperature, and evaporation. Utilization of these factors can help determine the allowable soil moisture deficit.

Several universities have interactive computer models to help determine when and how much to irrigate. The models are based on parameters such as soil type, temperature, evaporation, humidity, wind speed, solar radiation, and cloud cover. Users may have to register to use the models. Examples of online irrigation tools are as follows:

Kansas: http://www.bae.ksu.edu/mobileirrigationlab/

Missouri: http://agebb.missouri.edu/weather/reports/cwu/

Nebraska: http://hprcc- agron0.unl.edu/soywater/

Oklahoma: https://www.mesonet.org/index.ph p/agriculture/irrigation_planner

University of Nebraska-Lincoln suggests to start drying soil down four to six weeks prior to maturity with the target of having the soil dried down to about 40% of available water by crop maturity.2 In addition to saving water and irrigation expenses, leaving the soil dry in the fall may have other advantages such as resisting compaction from heavy harvesting equipment better than wetter soils and dry soil allows more room for storing off-season precipitation.

Summary. If available soil moisture equals or exceeds the water requirement to reach maturity, additional irrigation is not required. Also, maintaining proper soil moisture until plants reach physiological maturity is important to minimize stress and maximize yield potential.