Is Herbicide Burn a Concern?

Yield loss from crop injury can depend on a number of factors

With the potential to reduce wheat yields up to 40%, wild oat populations must be controlled.  However, because the biology and growth cycles of wheat and wild oats are very similar, grass herbicides used to control wild oats and other grassy weeds can impact the crop as well. Indeed, many seasoned growers can well recall wincing at the sight of a green, developing wheat field turning yellow after a grass herbicide application, and cringing at the sight of herbicide-stunted crop maturing inches shorter than other fields.

Herbicide injury or “burn” is generally caused by the plant’s failure to quickly metabolize and detoxify the herbicide before it accumulates to lethal levels that result in injury.  Although chemical stress doesn’t damage yields as much as wild oats do, growers wonder if those fields can still achieve their full yield potential.

Brian Jenks, weed scientist at the North Dakota State University research center in Minot, N.D., acknowledges grower concerns about crop injury. “Economically, they are already scraping the bottom, so if there is any chance of crop injury or yield reduction, they are more hesitant to use a product,” he explains. “Plus, injury looks bad, and they don’t want the neighbors to see a bad field.”

However, Jenks notes that it is difficult to determine how much yield loss any crop injury will cause.  “Usually in our research, we do not see a yield loss in test plots that were injured,” he says. 

University of Minnesota research documents the unpredictability of injury-induced yield reduction.  For example, a series of wheat tolerance trials in Rosemount and Crookston, Minn., in 2000 and 2001 evaluated injury and yield after treatment by several post-emergence grass herbicides.

Average injury ratings and yields for each herbicide application across all varieties showed that more significant injury usually resulted in slightly lower yields.  For example, at Crookston in 2001, herbicide applications that caused 13 to 14% initial injury yielded three to five bushels per acre less than the weed-free check plot average, and up to eight bushels less than treatments causing virtually no injury. 

Factors affecting yield loss due to crop injury

•  Weather patterns: Weather extremes can worsen herbicide injury. According to U of M researchers, a low temperature of 33 degrees F on the day of application at Rose-mount in 2000 appeared to have caused greater injury and subsequent yield loss across varieties and herbicides compared to other locations and years.

•  Variety: Trials also showed that injury-related yield loss varies greatly depending on variety and soil type.  Some varieties are more susceptible to injury from certain modes-of-action than others, as specified on herbicide labels.

•  Application methods: Failing to follow the herbicide label accurately often causes injury problems as well, Jenks points out.  Growers need to check for requirements, especially when tank mixing grass and broad-leaf herbicides.

•  Herbicide: Some herbicides are more prone to cause injury than others. To prevent injury, certain herbicides like Discover have a built-in safener that makes it easier for spring wheat to break down, or detoxify, the active ingredient.

•  Application timing: Some herbicides have a wider window of application than others. However, most extension specialists like Jenks recommend spraying wild oats and foxtail when wheat is in the two- to three-leaf stage, to prevent yield loss from weed competition. Jenks notes that younger crops recover more easily.  “When the injury occurs early in the vegetative growth stage – before the crop is into the jointing stage – good moisture conditions will allow the crop to grow out of the injury,” he says.  “But if you get crop injury in a dry year, there may not be enough moisture available for the wheat to recuperate.”

Herbicide Injury on wheat in Rosemount, MN.