Terminating Soybean Irrigation

At this point in the growing season, those with irrigation capabilities are looking at their crop to decide how much additional water it will take to finish things off. Scattered rains over the last few days have helped make this decision easier for some. However, additional moisture is still needed in many places. Mississippi’s soybean crop is approaching the end of reproduction in many areas and, according to the USDA Agricultural Statistics Service in Mississippi, 31% of the soybean acres are turning color. I have received several calls over the last few days on terminating irrigation. So, here are some things to consider.

The first thing to think about is whether or not you are growing indeterminate or determinate varieties. Indeterminate soybeans begin maturing at the bottom of the plant with maturity progressing upwards. Determinate soybeans mature uniformly up and down the plant. Therefore, indeterminate soybeans may have mature pods in the bottom of the plant with immature pods in the top.

Next, focus on the pods to determine the growth stage (average of upper four nodes on indeterminate soybeans & anywhere on determinate soybeans). The following points will help in identifying the late reproductive growth stages:

• R5.5 – Pods contain soybeans which fill half the space inside the pod cavity
• R6 – Soybeans completely filling the inside of the pod cavity
• R6.5 – Pod and pod wall beginning to turn mature color. Soybeans inside the pod begin to pull away from the protective membrane within the pod
• R7 – Pod mature in color

The decision for terminating soybean irrigation will likely depend on the field in question. Factors such as irrigation method, soil type, soybean maturity group, and the environment at the time should be considered when making this decision. Generally speaking, adequate soil moisture is needed to finish filling all the pods. Therefore, if adequate soil moisture is present and the crop is at or around the R6 growth stage, it is likely that irrigation can be terminated. In the case of indeterminate soybeans, be mindful of the fact that there may be some pods in the lower part of the plant that are approaching R7 while pods in the upper four nodes still need water to finish filling. Terminating irrigation too soon can result in smaller seed and therefore reduce the overall yield potential. If soil moisture conditions are questionable with little to no rain in the forecast when your crop reaches R6, one last irrigation will likely provide a benefit to your overall yield.