Irrigation Termination on Soybean

Irrigating crops has gone from a luxury to a necessity in recent years due to the extreme heat and droughts we have been experiencing. Having the ability to irrigate your soybean crop could mean the difference between making a crop and making an outstanding crop. As we close out another growing season a reoccurring question continues to be asked, “When can I terminate irrigation in my soybeans?”

Before this question can be answered properly a determination of soybean growth stage should be established. 

First, determine whether you are growing determinate or indeterminate soybeans. Determinate soybeans, mainly maturity group V and VI, flower uniformly up and down the main stem branch. This means that the pods should mature evenly up and down the stalk regardless of the location on the plant. Indeterminate soybeans, typically group III and IV, mature from the bottom of the plant upward so there may be some mature pods toward the bottom of the plant whereas the pods in the top portion of the plant may be immature.  

Second, look for pods in the upper four nodes of indeterminate soybeans or anywhere on determinate varieties. Feel for the presence of beans within the pod. You should be able to hold the pod up against the sky to highlight the presence of beans. If the beans are touching each other within the pod this is considered R6 growth stage (Photo 1). It is at this point when beans are considered to have reached maximum size and yield potential but may still be susceptible to drought stress. Another stage to be familiar with is R6.5 which is when the beans begin to recede in size and pull away from the protective membrane within the pod (Photo 2).

Photo 1

Photo 2

Now back to irrigation termination. While there are always exceptions to every rule the general rule of thumb is when soybeans reach the R6 growth stage and good soil moisture is present then irrigation may be terminated. Some experts contend that irrigation should not be terminated until the R6.5 growth stage, as the bean is at that point beginning to lose moisture and mature. Either way if you feel one more irrigation or rainfall event would be beneficial then by all means give it one last drink. Each field and scenario will likely be dependent on many different factors including soil type, irrigation methods, soybean maturity group, environment, etc. but the best rule of thumb is don’t skimp on water.

For more information about soybean growth stages see the MSU publication Guide to Soybean Growth Stages: http://msucares.com/pubs/publications/p2588.pdf