Too many weeds in your soybean fields? Or maybe you don’t like green stems at harvest while beans are dry and ready. For either problem, is desiccating soybeans to prepare them for harvest a good option?
These questions are asked frequently enough that several researchers set out to find answers. Shaun Casteel, Purdue Extension soybean specialist, compared different timings and products for desiccation. Chad Lee, Extension crops specialist at the University of Kentucky, dove into the topic as well, and then summarized research from around the country.
The jury is still out as to whether desiccation is a viable alternative, especially for those trying to combat green stem syndrome.
“When you do it is critical,” Casteel says. “If you spray and desiccate too early, you can lower yield. Spraying at growth stage 6.5 shaved off bushels compared to waiting until growth stage 7.0.”
Casteel applied 4.8 quarts per acre of Defol 5 with 1 pint per acre of methylated seed oil and 20 gallons of water. Active ingredient in Defol 5 is sodium chlorate.
Mark Licht, Iowa State University cropping systems specialist, says one of the best sources for staging soybean growth is “Soybean Growth and Development,” ISU PM 1945. The publication states: “As the soybean plant matures past R6, the potential degree of yield reduction by stress gradually declines. After R6, stress may cause large yield reductions, mostly by reducing seed size, but also by reducing pods per plant and beans per pod.”
Compare that to what the publication says about R7: “Stress occurring at R7 or thereafter essentially has no effect on yield.”
Testing in 12 states
Lee desiccated soybeans in 2024 trials at Lexington, Ky., with 2 fluid ounces of Sharpen at R6, R6.5 and R7 timings and didn’t see any yield differences. However, researchers over a 12-state area who conducted trials, including Lee, agree that R7 is the preferred timing. By comparison, studies in Arkansas saw yield loss even at R7.
Here are key tips if you decide to desiccate soybeans:
Green stem impact not clear. Wisconsin researchers measured a significant reduction in green stem issues after desiccating. However, this effect wasn’t as clear-cut everywhere.
Distinguish growth stages correctly. At R6, seed margins in the pod are still not clearly defined, and the white membrane is attached. By R6.5, margins are defined, and the membrane is detached. At R7, there is at least one mature-colored pod somewhere on the stem. Seed moisture at early R7 is about 60%. Research at Louisiana State University still showed yield loss to desiccants. By 30% moisture, still in R7, there was no yield loss.
Products and labels vary. Common choices include paraquat, saflufenacil, sodium chlorate, carfentrazone, glyphosate and dicamba. Brand names for these, respectively, are Gramoxone, Sharpen, Defol 5, Aim, Roundup and Clarity. Labels vary in describing when to apply. Follow preharvest intervals in labels.
Harvest timely once applied. Shatter can become a significant problem if harvest is delayed, especially with multiple rain cycles.
Weather conditions matter. Optimally, apply when it is hot and humid. Don’t apply if rain is expected within the next seven days. “If weeds are minimal, the best time to apply a harvest aid is when it is warm with dry weather forecast for seven days,” Lee says. “Of course, that is also when a harvest aid is least needed.”