- The decision of when to terminate irrigation will depend on the growth stage of the crop, available soil moisture, soil type and weather conditions.
- In general, a full soil water profile around the R6 growth stage should be sufficient to bring the crop through maturity without inducing water stress.
- The coarser the soil texture, the later the final irrigation should occur.
The optimal time to terminate irrigation in soybean will depend on the growth stage of the crop, available soil moisture, soil type and weather conditions. To minimize water stress and maximize yield potential, ensure that adequate water is available to the crop until the end of the R7 growth stage, when pods have matured. This is especially important from growth stages R3 (beginning pod) through R6 (full seed), which are the most sensitive to water stress and potential yield losses. Water stress before seed-fill is complete can result in yield penalties due to decreased seed size. In general, a full soil water profile during the R6 growth stage should be enough to bring the crop through maturity without inducing water stress.
Using Irrigation Scheduling
If an irrigation scheduling approach is used, ensure that available soil water remains above 50% depleted during the reproductive growth stages.1 Consider the predicted maturity date of the crop, the estimated water use to maturity (example given in Table 1) and available soil moisture to ensure that enough water will remain in the soil to bring the crop to maturity without inducing stress. By R8, water is no longer needed for seed enlargement and soil moisture can be allowed to reach 60% depleted.2
Using the Rule of Thumb Approach
If irrigation scheduling is not used, a practical approach is to determine the percentage of pods at filling capacity and estimate soil moisture using feel and appearance in order to determine when to terminate irrigation. In general, if soil moisture is adequate and 50% or more of the pods have seeds that are fully formed, then irrigation can be terminated. If the soil is beginning to dry, one additional irrigation should be applied to ensure that all seeds reach their maximum weight. If pods are not fully filled and soil is dry, one or more irrigations will be needed to achieve full yield potential. Recommendations on when in the R6 growth stage to terminate irrigation depend on soil type. For example, irrigation should be terminated later within R6 on soils with low water holding capacity or when root restricting layers are present in the soil profile.
1Helsel, D.G. and Helsel, Z.R. 1993. Irrigating soybeans. Publication G4420. University of Missouri Extension. http://extension.missouri.edu/p/G4420.
2Yonts, C.D., Melvin, S.R., and Eisenhauer, D.E. 2008. Predicting the last irrigation of the season. NebGuide G1871. University of Nebraska-Lincoln Extension. https://water.unl.edu/cropswater/managementpubs.
Other sources: Soybean irrigation and water use. University of Missouri Extension. http://extension.missouri.edu. Tacker, P. and Vories, E. Chapter 8: Irrigation. Arkansas Soybean Handbook. University of Arkansas Cooperative Extension Service. http://www.uaex.edu.
Web sources verified 6/1/15. 130708060105