Agronomy Talk


Published on Tuesday, November 10, 2020


  1. Many post-emergence herbicide application cut-offs occur before the R1 growth stage or through the R2 growth stage.

  2. For post-emergence applications past the R2 growth stage, use clethodim in place of glyphosate for grass control.

  3. Once the plant reaches the R1 growth stage, Liberty® can no longer be applied, making post-emerge options more limited depending on the trait platform.


There are many effective post-emerge herbicide options available, but what happens if the soybeans are beyond the growth stage at which Liberty® can be applied? Depending on the trait platform, once we reach the R1 growth stage, soybeans must be managed like a glyphosate-tolerant soybean. This means relying heavily on PPO post-emergence herbicides like Cobra® and Flexstar®, as well as Group 5 herbicides such as Basagran®. The waterhemp population was suspected to be resistant to glyphosate, but glyphosate was still added for grass control. In a non-GMO scenario, glyphosate must be substituted with clethodim for grass control. Once past the R2 growth stage, Cobra, Flexstar, Basagran, and clethodim can be applied later in the season but the pre-harvest interval must be followed. Therefore, we would need to use clethodim in place of glyphosate for applications later in the season. 

ABOVE: The Power in the PRE™ — No pre-emerge was applied to stimulate heavy weed pressure. No pre-emerge resulted in new flushes of weeds and some escapes. 


Beck’s PFR looked at various post-emerge options in glyphosate-tolerant soybeans. Applications were applied at the V3 growth stage when waterhemp plants were less than 4 in. in height. By 14 DAT, many of the waterhemp plants were partially controlled and averaged 68% control across treatments. However, by 28 DAT regrowth had occurred, lowering the waterhemp average to 62% control. Products such as Anthem® MAXX contain a PPO post-emerge herbicide as well as a Group 15 for in-season residual control of waterhemp and grasses.

Anthem MAXX will have a shorter application interval compared to other products and can be applied through the V6 growth stage. Warrant® Ultra contains a Group 14 post-emerge herbicide and a Group 15 herbicide for the in-season residual. Just like Anthem MAXX, Warrant Ultra will have a shorter application window compared to some of the other products tested. Anthem MAXX plus the addition of Cobra® resulted in the greatest control. However, the mixture of two PPO herbicides resulted in the greatest crop injury. Basagran® did not increase waterhemp control when added to PPO herbicides like Cobra or Flexstar®. When applications were made, the waterhemp was less than 4 in. in height. Therefore, success will be greatly impacted by weed size. Once past the R2 growth stage, we must rely on Cobra, Flexstar, Basagran, and clethodim. These products never resulted in greater than 63% control on waterhemp that was less than 4 in. tall Control options become more limited later in the season, showcasing the power is in the pre-emerge and in-season residuals. 


The power is in the pre-emerge and in-season residuals. Group 5 or 14 post-emergence herbicides never resulted in greater than 73% control. Group 14 + 15 premix application timings will be shorter compared to other PPO post-emergence only options. For later season applications, pre-harvest intervals must be followed.
Therefore, use clethodim in place of glyphosate. 

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Author: Joe Bolte

Categories: Agronomy, Agronomy Talk


Joe Bolte
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