Soybean South
 - Soybean Irrigation -

Visual Technique Predicts
Irrigation Termination



The decision to stop irrigating soybeans and corn is never black and white – or easy. Turn off the tap too soon, crop yields suffer. Turn it off too late, profitability is reduced. Either way, it’s money down the drain.

How far back from maturity to put the last irrigation is a question many have tried to answer in the past, says Joe Henggeler, MU Extension irrigation specialist at the Delta Research Center in Portageville, Mo.

“Other Midwest universities have developed guides for timing last irrigation using a series of calculations where farmers are asked to input everything from the crop’s water requirement to the depth of the root zone to the water-holding capacity of the soil,” he explains. “The problem is these guides are complicated, and you can’t always count on a mathematical back-calculation because things are always happening in the system.”

Farmers want a a good rule of thumb for terminating irrigation, not a complex math equation, Henggeler says.

“What we’re trying to develop is a set of photos that show what the crop should look like before terminating irrigation on different soils,” he says. “It simplifies the decision by saying, ‘If it looks like this, cut off irrigation.’ You don’t have to worry about dates or anything else.”

‘Five-bushel bump’ noted
During the past three seasons, Henggeler has experimented with cutting off irrigation at different dates in both soybeans and corn on three different soil types – sand silt and clay. The study has used several varieties of 114-day corn and maturity Group V soybeans under overhead sprinklers.

“We’ve found that overall, folks have been cutting off irrigation way too early in soybeans and just a little early in corn,” he says. “Rather than terminating when beans are beginning to touch the pods in the R6 growth stage, irrigation should be maintained until the pods reach beginning maturity. We’ve seen about a five-bushel bump per acre by extending irrigation from R6 to R7.”

Once Henggeler gathers all of the data, he is going to work on developing a flashcard-like guide that farmers can use. For each soil type, pictures of a cross-section of an ear, a husked ear, an unhusked ear and the overall canopy would be included for corn. Pictures of pods, the last four nodes on a plant and the overall canopy would be included for soybeans.

“We engineers don’t mind all of those formulas, but producers want a guide that they can use without all of those calculations,” Henggeler adds. They will be able to just pull this guide out of the glove box when they need it and leave the calculator at home.”

Jason L. Jenkins is with Extension and Ag Information at the University of Missouri. Contact Jenkins at (573) 882-2980 or Contact Henggeler at (573) 379-5431 or

Coming Soon: A Pictorial Guide To Cutting Off The Water

Missouri irrigation specialist Joe Henggeler plans to test his visual technique for determining when to terminate irrigation for soybeans and corn one more year before making an official recommendation.

“Soybeans are a lot easier to identify and communicate the exact stage they are in vs. corn where you go from beginning dent to full dent and there is a big time frame in between,” Henggeler says.

“Also, everyone may not agree on what full dent is.

“On the other hand, stages R6 or R7 happen pretty quickly, and you can definitely identify R6 where we see some yellowing of the leaves. We’re coming up with a recommendation for terminating irrigation, but I want to go through our pictures again to make sure we can say it photographically.”

In Henggeler’s opinion, some of the recommendations found on the Internet from Extension publications recommend cutting off the water too early on beans and corn. Before cutting off irrigation water, he says the leaves need to be touching really well and there should be some yellowing in the foliage.

“Clear up until that time, you can still realize a yield increase,” Henggeler explains.

– Carroll Smith