Post-emergent weeds in corn and soybeans can rob your crops of essential nutrients, moisture, and sunlight, leading to stunted plants and reduced yield.
However, managing post-emergent weeds can be challenging, and properly timing post-emergence herbicides is a critical way to gain control over costly weed populations.
Ask yourself these questions to help determine when to apply post-emergent herbicide for maximum weed management.
1. What has your weed control program looked like to date this season?
If your burndown or pre-plant herbicide applications were interrupted or impeded by poor weather, scouting every field before post application is recommended. There are only so many hours in a day, and while it’s tempting to skip ahead and get started spraying, knowing what weeds you’re up against allows you to manage them more effectively.
2. What weeds are you dealing with?
Weed germination times vary, meaning that the ideal time to apply post-emergence herbicides will also vary. For example, giant ragweed and waterhemp germinate rapidly, necessitating a quick post-emergence application to hit weeds before they reach 4” in height.
Fast-growing weeds leave a very short application window for post-emergence herbicides to be most effective. Waterhemp, for example, can grow 1” per day, reach 4” very quickly, and without careful monitoring, your herbicide application window has closed.¹
3. Are weather conditions conducive to rapid weed growth?
Weeds can grow rapidly when temperatures are warm and soil moisture is adequate for germination and emergence. If these conditions occur after a cold, late spring, post-emergence herbicides can be especially important because very few weeds would have been controlled by pre-plant tillage.
4. What trait package and growth stage is your crop to determine what herbicides to use?
Always read and follow post-emergence herbicide label instructions. It is especially important to be conscious of the variable cutoff times for post-emergence herbicides as they are commonly determined by both crop growth stage or pre-harvest (PHI).
Note, many pre-emergence herbicides cannot be used post-emergence on corn and soybeans. For example, pre-emergence herbicides containing sulfentrazone, metribuzin, or flumioxazin have explicit statements warning against applying these herbicides to emerging soybeans.²
In soybeans, traits enabling Liberty® herbicide, approved 2,4-D formulations, and glyphosate allow for additional post herbicide options and herbicide combinations. In corn, glyphosate and Liberty Link® traits are common allowing more options as well.
Experts are available to help you make your post-emergence herbicide application decisions. Reach out to your seed retailer, a nearby extension office agent, or a seed company professional like your regional BASF representative.
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Endnotes
- Stahl, Liz, et al. “Things to Consider for a Successful Postemergence Herbicide Application.” Minnesota Crop News, University of Minnesota Extension, 31 May 2022, blog-crop-news.extension.umn.edu/2022/05/things-to-consider-for-successful.html. Accessed 16 Feb. 2025.
- Anderson, Meaghan. “So Your Crop Emerged Before It Got a Herbicide.” Integrated Crop Management, Iowa State University, 9 May 2024, crops.extension.iastate.edu/blog/meaghan-anderson/so-your-crop-emerged-it-got-herbicide. Accessed 16 Feb. 2025.
