Many farmers experience issues with herbicide efficacy in extreme weather years, such as the 2012 drought and 2019 spring floods.

And that trend could become more common as weeds not only adapt to herbicides, but changing weather conditions as well.

Aaron Hager, University of Illinois weed scientist, associate professor and faculty Extension specialist, and Marty Williams, USDA Agricultural Research Service ecologist and U of I affiliate professor in the Department of Crop Sciences, discussed the evolving situation during a recent farmdoc webinar.

“The climate on our planet is changing,” Williams said. “The annual mean temperature is increasing in the Corn Belt. We’re seeing more frost-free days and hot days year-round. We’re also seeing more extreme rainfall events.”

The amount of “heavy rainfall” events increased 37% in the past century across the Corn Belt. Meanwhile, timing of precipitation generally continues to change with the majority of increased precipitation events in the winter and spring while summers actually are on track to feature less rainfall over time, according to Williams.

So how does the weather variability affect weed management?

Application of a pre-emergence herbicide generally requires about a half-inch to an inch of rain, ideally within five to seven days of applications, to move the material into the solution to maximize its effectiveness.

Dry conditions, such as those experienced in 2012, subsequently reduce the level of herbicide efficacy, while heavy rain and flood events, similar to those in the spring of 2019, can simply wash some of it away.

“In both extremes we can get a good snapshot of how variability or extreme weather can have a significant impact on the efficacy of pre-emergence herbicides,” Hager said.

A joint study between the U of I and USDA-ARS found the future efficacy of pre-emergence herbicide treatments are threatened by more variable weather. Diminishing weed control in turn exacerbates crop yield loss potential to adverse weather.

“One thing we don’t talk much about is how can the climate affect weed management and what about weed species’ shifts,” Hager said. “Most weed species have tremendous genetic diversity that provide opportunities to take advantage of changing conditions.”

Farmers have already seen waterhemp control issues multiply. And a similar situation could develop with Palmer amaranth, a desert species that adapted to the climate in Arkansas, Missouri and made its way into Illinois, with a recent confirmation as far north as Canada.

Looking ahead, Hager believes farmers must diversify weed management strategies. Otherwise, if pre-emergence herbicides become less effective due to variable weather, farmers will be forced to rely more heavily on post-emergence herbicides or risk yield loss under many current systems.

“I think we’re going to continue to rely heavily on herbicides (for weed control),” Hager said. “But, with a changing climate and evolving (herbicide) resistance, we’ll be forced to ask, what else can we do?”

Hager and Williams believe use of cover crops and harvest weed seed control measures could play an even greater role in weed management systems.

Meanwhile, weeding robots currently used out west in high value crops could be adopted for use in the Corn Belt in the future, among other options.