The decision of when to terminate irrigation can be difficult and will depend on crop growth stage, weather conditions, available soil moisture and soil type. To minimize water stress and maximize yield potential, ensure that adequate water is available to the crop until pods have matured (end of the R7 growth stage). 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.
Irrigation scheduling approach. If using an irrigation scheduling approach to monitor soil moisture, ensure that available soil water is not depleted below 50% during the reproductive growth stages.1 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. 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 (full maturity), water is no longer needed for seed enlargement and soil moisture can be allowed to reach 60% depleted.2
Rule of thumb approach. If soil moisture is not being monitored by an irrigation scheduling method, a practical approach is to determine the percentage of pods at filling capacity (various stages of R6) and estimate soil moisture using the “feel" method. In general, if there is adequate soil moisture remaining and 50% or more of the pods have seeds that are fully formed, then irrigation can be terminated. If the soil is drying at this point, one additional irrigation should be applied to ensure that all seeds reach their maximum weight. If the majority of pods have seeds that 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.
1 Helsel, D.G. and Helsel, Z.R. 1993. Irrigating soybeans. Publication G4420. University of Missouri Extension. http://extension.missouri.edu/p/G4420.
2 Yonts, 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.
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