The recent stretch of cool, weather has placed corn under stress, meaning it's best to avoid applying herbicides for the moment, if possible, says Iowa State University agronomist Bob Hartzler.
"It's important to note that all herbicides place stress on the crop, regardless of whether a hybrid is resistant to the particular active ingredient, such as with glyphosate or Ignite, or whether the herbicide contains a safener, like with Corvus or Status," Hartzler says. "Corn that lost significant leaf area to frost will be particularly prone to injury since it will deplete much of its energy reserves replacing the lost foliage."
Hartzler says there are no clear-cut criteria to determine how long to wait before resuming herbicide applications.
"At a minimum, it probably takes 3 to 4 days of favorable growing conditions for the crop to approach normal levels of tolerance to herbicides," he says.
Factors to consider when deciding how soon to spray a struggling corn field include the severity of the weed infestation and the margin of crop safety to the specific herbicide, Hartzler adds. A light weed infestation probably poses less risk to yield potential due to early season competition than herbicide injury, thus the application could be delayed until the crop regains much of its vigor.
"This assumes that delaying application will not allow weeds to reach a size that diminishes the likelihood of effective control, such as with giant ragweed or horseweed/marestail," Hartzler says. "On the other hand, if a field has a heavy weed infestation, the weeds may pose a bigger threat to the yield potential than herbicide injury, so applications made before the crop has fully recovered from the weather-induced stress may be warranted."
Hartzler says another reason to delay applications when possible is that weeds under stress are often less likely to be killed by herbicides than healthy plants.
"Applicators are frequently forced to make compromises due to limited application windows, but be sure to weigh the benefits and risks of herbicide applications to crops that are struggling due to weather or other factors," he says.