Agronomy Talk


Herbicide and Weed Management Briefs

Published on Friday, November 5, 2021


The foundation to a successful herbicide program starts with the Power in the Pre™. A successful pre-emerge herbicide can help reduce the amount of weed pressure in the post-emergence application. Many post-emergence applications are sprayed in the earlier vegetative stage. However, data shows by delaying weed removal until the post-emergence trip, a significant yield reduction can occur. This highlights the negative impact of early weed competition and demonstrates the Power in the Pre.



Some residual herbicide labels allow you to spray “x” amount of days before planting in a pre-plant application. This may allow the burndown application (for early emerging weeds like giant ragweed) and the residual herbicide application to be accomplished in the same trip across the field. Residual herbicides typically only last 28 days before they begin to break. This means, in the scenario above, we may save a trip across the field, but the residual herbicide is also breaking down before it is needed. Some residual herbicides allow for pre-plant incorporated (PPI) applications and specify the depth in which the product can be incorporated. This application can be beneficial for the removal of existing weeds via tillage and is particularly important for herbicides which have limited reach-back capability. A PPI application can reduce splash-up injury from PPO and metribuzin-containing herbicides to the soybeans. However, if the herbicide is incorporated or moved too close to the seed, significant injury can occur.

Typically, the first place we see a residual herbicide break is any place where we have soil disturbance, such as tire tracks. The benefit of a pre-emerge application is we can have less soil disturbance when following the planter. However, if the seed trench is not properly closed, we can see significant injury. If we have cool, wet conditions or significant rainfall occurs when the cotyledons are pushing through the soil surface, we may see significant injury as well from the residual herbicide. Another thing to consider is once the crop is planted, the clock starts ticking, and many of these residual products MUST be applied before the crop emerges. Consult the label to see the time frame in which your herbicide can be safely applied.




This study was planted in early May of 2020 and experienced ver y cool, wet conditions. Applying the residual 21 days before planting (DBP) resulted in 87% waterhemp control by 45 days after planting. Though this application would allow growers to apply a burndown + the residual in one trip, there was an 11% reduction in control vs. applying the residual pre-emerge. The PPI application was applied after the pre-plant treatment but resulted in essentially the same level of waterhemp control at 88%. This showcases how soil disturbance can impact the e ffectiveness of the residual herbicide. Applying the residual herbicide pre-emerge right after the planter resulted in the best waterhemp control.



Applying the residual herbicide 21 DBP resulted in both the highest stand at 130,000 plants/A. and high est yield. Upon soybean germination and emergence, the herbicide was starting to break down, and therefore the plants had less product to metabolize, resulting in greater stands in the cool, wet conditions. The 21 DBP did, howev er, result in the poorest weed control because the blanket of protection was starting to break down. The pre-emerge application resulted in the poorest stand at 122,000 plants/A. but was within 1.6 Bu./A. of the 21 DBP application. This demonstrates soybeans’ ability to compensate for a reduced stand. PPI resulted in the same yield as the pre-emerge application but had a 10% reduction in weed control. However, this treatment can result in significant stand and yield reduction if the herbicide is moved too close to the seed. The i mpact to yield between 21 DBP vs. pre-emerge was not significant, however, the difference in waterhemp control was. Applying the herbicide closer to planting in a preemerge application resulted in the greatest waterhemp control.




Applying the residual in a pre-emerge application resulted in a lower stand but was within 1.6 Bu./A. of 21 DBP. The pre-emerge application resulted in the greatest waterhemp control because the residual blanket was not disturbed. Applying 21 DBP does allow the opportunity to save a trip across the field but resulted in poorer waterhemp control. BEWARE of injury with PPI applications, especially if the herbicide is moved too close to the seed. A PPI application resulted in a 10% reduction in control, showcasing how much soil disturbance can impact weed control.

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Jim Schwartz

Jim Schwartz

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