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PFR Report: Pre-Emergent Herbicide Application Timing

Published on Wednesday, March 3, 2021


  • Some residual herbicides can be applied with a burndown in a pre-plant application.  
  • Pre-plant + burndown applications can save a trip across the field.
  • However, residual herbicides typically begin to break around 28 days after treatment. Applying closer to planting in a pre-emerge application can provide greater weed control protection.
  • Pre-plant incorporation can move the herbicide close to the seed, causing significant injury.

The foundation of 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 stages. However, data shows by delaying weed removal until the POST trip, a significant yield reduction can occur. This highlights the negative impact of early weed competition and showcases the Power in the PRE.

When to Spray Residual Herbicides

Some residual herbicide labels allow you to spray “x” number of days before planting in a pre-plant application. This may enable the burndown application of early emerging weeds, like giant ragweed, and the residual herbicide application to be completed on the same trip across the field.

Residual herbicides typically only last 28 days before they begin to break. Although it can save a trip across the field, the residual herbicide is breaking down before it is needed. What about pre-plant incorporated (PPI)? Some residual herbicides allow for PPI applications and specify the depth at which the product should be incorporated. This can help remove weeds with tillage, which can be especially important for herbicides with limited reach-back capabilities or little burndown activity.

Sometimes growers ask, “Does a PPI application reduce splash up from PPO herbicides on the soybean cotyledons?” If the herbicide is moved too close to the seed from incorporation, significant injury can occur. Typically, the first place a residual herbicide breaks is anywhere that soil disturbance like tire tracks has occurred. The benefit of a pre-emerge application is reduced soil disturbance when following the planter. If the seed trench is not closed properly, significant injury can occur. If cool, wet conditions and significant rainfall persist when the cotyledons are pushing up through the soil, considerable damage can result from the residual herbicide. Once the crop is planted, the clock starts ticking, and many of these residual products MUST be applied before the crop emerges. Check the label to see the timeframe of when a herbicide can be applied.

PFR Results

This PFR study was planted in early May of 2020 and experienced very cool, wet conditions. By spraying the residual 21 days before planting (DBP), we saw an 87% waterhemp control by 45 days after planting. Though this application would allow farmers to spray a burndown and the residual in one trip, there was an 11% reduction in control vs. applying the residual as a pre-emerge. The PPI application was applied 21 DBP treatment but resulted in the same level of waterhemp control at 88%. This showcases how soil disturbance can impact the effectiveness of a residual herbicide. When applying the residual herbicide pre-emerge right after the planter, we saw the best waterhemp control.

Impact on Stand Counts and Yields

Applying the residual herbicide 21 DBP resulted in the highest stand at 130,000 plants/A. and the highest yield. The herbicide started to break down; therefore, the plant had less product to metabolize, resulting in greater stands despite the cool, wet conditions. However, the 21 DBP did result in the weakest weed control because the blanket of protection started to break down. The pre-emerge application resulted in the lowest stand at 122,000 plants/A. but was within 1.6 Bu./A. of the 21 DBP application. This showcases the ability of soybeans to compensate with a stand reduction. PPI resulted in the same yield as the pre-emerge application but had a 10% reduction in control. This treatment can result in a significant stand and yield reduction if the herbicide is moved too close to the seed. The impact on yield between 21 DBP vs. pre-emerge was not substantial; however, the difference in waterhemp control was. Applying the herbicides closer to planting in a pre-emerge 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 allows the opportunity to save a trip across the field but resulted in less 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.

What to learn more? Check out THIS VIDEO by Field Agronomist Luke Schulte.

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Author: Joe Bolte

Categories: PFR, PFR Reports


Joe Bolte
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