Soybean Irrigation Initiation

Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist June 23, 2014 10:00

Although we have received lots of rainfall this season, it is time to initiate irrigation in soybean. Keep in mind that irrigation during the early vegetative phase provides little to no yield benefit. The reason to apply irrigation during the vegetative stages is to promote adequate vegetative growth and node development. It is during the reproductive phase of soybean that yield loss due to drought stress is most severe. Specifically, stress associated with lack of available soil moisture during the pod development and pod fill stages of reproduction has the greatest impact on soybean yield.

With this in mind, the first step in determining when to initiate irrigation in soybean is to properly identify the growth stage of the crop. The commonly recognized soybean reproductive growth stages are as follows:

R1: First flower anywhere on plant R5.5: Soybeans filling half the space in the pod in upper four nodes
R2: Flower in the upper two nodes R6: Soybeans touching inside the pod in upper four nodes
R3: 3/16” long pod in upper four nodes R6.5: Pod and pod wall beginning to turn mature color
R3.5: 1/2” long pod in upper four nodes R7: Pod mature anywhere on plant
R4: 3/4” long pod in upper four nodes R8: 50% of the pods contain mature seed and are mature in color
R5: Visible seed in pod in upper four nodes    

Generally speaking, soybean can use 0.25 inch of water per day during reproductive development. With this in mind, stress from lack of moisture between R3 and R4 may result in a decreased number of pods that develop on the plant. This scenario may result in decreased soybean yield.

The next step is to determine the available soil moisture as the crop moves into these reproductive stages. A recommended method of determining available soil moisture is through the use of soil moisture sensors. Dr. Jason Krutz posted an article describing the use of sensors and how to use that information to trigger an irrigation event. This article can be found at: How to Use Watermark Sensors to Assess Soil Moisture and Schedule Irrigation.

If the soybean crop is at R2 and the moisture sensors indicate that the soil profile is not fully charged, it is best to initiate irrigation to ensure adequate soil moisture is available as the crop moves into the pod development stages.

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Trent Irby, Extension Soybean Specialist
By Trent Irby, Extension Soybean Specialist June 23, 2014 10:00
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