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Preemergence Herbicide Application Timings: Pros and Cons

April 9, 2020
Content Author: Bob Hartzler, Meaghan Anderson, Prashant Jha

Preemergence herbicides are the foundation of herbicide-based weed management systems, and effective use of these products is essential to protect crop yields and reduce selection pressure for herbicide resistant weeds. In a perfect world, applying preemergence herbicides immediately after planting would provide the greatest likelihood of maximum performance, but equipment and labor availability limit many farms from using this approach. This article will provide a brief overview of the pros and cons of different application strategies.

Early preplant: Applications made more than 7 to 10 days prior to planting.

Pros

Cons
  • Applications typically completed before summer annual weeds begin to germinate, thus increasing the likelihood of timely activation by rainfall.
  • May provide a weed-free seedbed at planting. In no-till systems, include a burndown product to control emerged winter annual and other weeds.
  • Application completed before planting, spreading workload.
  • Residual activity into the growing season is shorter than when product is applied near planting. Use of layered residual (including a residual with postemergence application) approach reduces this risk.
  • Final seedbed preparation tillage may ‘dilute’ the herbicide within the soil profile if tool is run too deep, and may result in uneven distribution of the herbicide.
  • Planter units may move herbicide out of the crop row, allowing weed escapes
  • If planting is delayed, much of the value of the herbicide may be lost.

 

Preplant/Preemergence: Applications made within a week before or after planting.

Pros

Cons
  • Product applied near time that summer annuals initiate emergence.
  • Residual control extended later into growing season than early preplant applications.
  • If rain doesn’t occur within a week of planting, early-emerging weeds may escape  control due to lack of herbicide activation.
  • Planter units may move herbicide out of the crop row if applied preplant, allowing weed escapes.

 

Delayed preemergence: Applications made more than a week after planting.

Pros

Cons
  • May spread workload.
  • Residual control is extended later into growing season.
  • Herbicide options may be reduced if crop has emerged before application.
  • Summer annual weeds likely will have emerged at application, requiring additional postemergence product to control these weeds.
  • Rainfall is needed within a few days of the application to activate product.
  • Application delays can result in early-season competition between crops and weeds, and may allow weeds to exceed optimum size for postemergence control.

Herbicide resistant weeds have limited our ability to ‘rescue’ fields when weather delays preemergence applications following planting.  Thus, consider how you can ensure that all acres are protected with appropriate preemergence herbicide applications when prolonged wet periods or other factors interfere with field operations.  

 

Waterhemp escapes in crop row due to preemergence herbicide being moved by row cleaners.  

Links to this article are strongly encouraged, and this article may be republished without further permission if published as written and if credit is given to the author, Integrated Crop Management News, and Iowa State University Extension and Outreach. If this article is to be used in any other manner, permission from the author is required. This article was originally published on April 9, 2020. The information contained within may not be the most current and accurate depending on when it is accessed.

Authored by
Bob Hartzler
Bob Hartzler
  • Retired Weed Scientist and Professor of Agronomy
Dr. Bob Hartzler is a former professor of agronomy and a former extension weed specialist with a Ph.D. in Crop Physiology from Iowa State University. He conducted research on weed biology and how it impacts the efficacy of weed management programs in corn and soybean. Dr. Hartzler also taught undergraduate classes in weed science and weed identification.
Meaghan Anderson
Meaghan Anderson
  • Field Agronomist in Central Iowa
Meaghan Anderson is a field agronomist in central Iowa and an extension specialist at Iowa State University Extension and Outreach. Areas of expertise include weed management, weed biology, cover crops, corn and soybean management, and integrated pest management.
Prashant Jha
  • Former Weed Scientist and Professor
Dr. Jha earned a Ph.D. in Plant and Environmental Science (Weed Science) from Clemson University, SC. His research program focused on improved understanding of weed biology and ecology to develop effective, integrated weed management strategies in corn and soybean production systems of Iowa and Midwest, optimization and stewardship of herbicide application technology and herbicide use, and understanding the evolution of herbicide resistance including genetic/molecular and physiological mechanisms, and other factors influencing weed population dynamics and management in agroecosystems.

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