Knowing the ideal time for terminating soybean irrigation is very important. Stopping irrigation too soon decreases kernel size and grain yield while stopping it too late incurs unnecessary extra cost. Using information on “crop development stage” and “soil moisture levels in root zone” is helpful in determining ideal termination time for irrigation.

The right pods to check

First step is to check with your seed dealer to see what variety (indeterminate or determinate) of soybean you are growing. For indeterminate variety, the pods at the top four nodes of the main stem, having fully developed leaves, should be examined to determine the growth stage. In case of determinate varieties, growth stage identification can be performed anywhere throughout the plant as pods develop uniformly along the stem. For growth stage assessment, include plants from all representative field sections (possibly known), with particular attention to later-developing areas.

Identify the growth stages

• R5 is a “beginning seed” stage when seed starts to develop, attaining 1/8 inch of length.
• R6 is a “full seed” stage when pods have full seed (green) that completely fills the pod cavity.
• R6.5 is a “late seed fill” stage when seed (contained in the pod) gets fully separated from the protective membrane of the pod wall.
• To look for photos of different soybean growth stages (including R5, R6, and R6.5), please visit this link- https://extension.msstate.edu/publications/guide-soybean-growth-stages

How to make a final decision on irrigation

• Once your soybean plants are at R6.5, it’s generally the right time to terminate irrigation if your field’s root zone has sufficient moisture to sustain the rest of seed filling. If your soybean plants are at R6, they still need irrigation.
• To guide irrigation termination timing, it is helpful to i) Keep in mind that the sandy soils in Southern New Jersey have limited soil-water reserves, ii) Check moisture in root-zone with sensors or soil sampling at or just before soybean reaches R6.5 stage, iii) Stay up to date with the weather forecasts, and iv) Reassess the decision in case expected rainfall does not come.

Summary/ Takeaway Message

Use R6.5 growth stage identification method and soil moisture information to decide your irrigation termination time. Do not rely only on the calendar, leaf color, or mature-looking pods on lower part of the plant.

References

• Gholson, D., & Irby, T. (2024, August 7). Soybean Irrigation Termination. Mississippi State University Extension, Mississippi Crop Situation. https://www.mississippi-crops.com/2024/08/07/soybean-irrigation-termination-5/

• Irby, T., Elliott, B., Oswalt, G., & O’Neal, P. (2024). Guide to soybean growth stages (Publication 4028). Mississippi State University Extension. https://extension.msstate.edu/publications/guide-soybean-growth-stages
• Irby, T., & Roach, D. (2014, August 25). Soybean irrigation termination. Mississippi State University Extension, Mississippi Crop Situation. https://www.mississippi-crops.com/2014/08/25/soybean-irrigation-termination-2/

Questions? Contact Ramandeep Kumar Sharma at rs2794@njaes.rutgers.edu.