Will agricultural weeds finally claim the upper hand in a changing climate?

Mining a 30-year database from 16 Extension weed science programs (including Illinois Extension) across the U.S. Corn Belt, the researchers found variable weather significantly reduces the effectiveness of three leading POST herbicides against major weeds affecting corn and soybean. And if farmers can't adequately control weeds with PRE or POST herbicides, corn and soybean yields -- and global food security -- will suffer.

"Weather doesn't just matter in the hours after POST application, as other studies have shown. Our analysis showed air temperature and precipitation were linked with herbicide effectiveness days before and after application for the products and weeds we studied," said Chris Landau, postdoctoral researcher for USDA-ARS and first author on the paper. "With the sheer amount of data we analyzed -- thousands and thousands of data points, including a broad range of weather conditions over 30 years -- we were able to characterize the effects of weather on POST herbicide efficacy on a much broader range of environments than previous studies."

The analysis showed average air temperatures below 66 or above 77 degrees Fahrenheit made fomesafen, glyphosate, and mesotrione -- representing three herbicide sites of action -- less effective on waterhemp, giant foxtail, and morning-glory species. The historical dataset couldn't explain why weeds were less responsive to the herbicides under these conditions, but other studies provide clues.

"If it's excessively warm before application -- and water is not limiting -- weed growth speeds up, resulting in larger plants that may have a better chance of surviving the herbicide," said co-author Marty Williams, an ecologist with USDA-ARS and affiliate professor in the Department of Crop Sciences, part of the College of Agricultural, Consumer and Environmental Sciences at Illinois. "Likewise, a hot period after application could supercharge plant metabolism and neutralize the herbicide faster, rendering it less effective."

Cold temperatures have essentially the opposite effect, slowing metabolism and movement of herbicides within the plant and rendering them less effective.

Herbicide applicators don't just need to pay attention to air temperature, however. The analysis found precipitation patterns -- particularly dry conditions in the 10 days before application or wet conditions 10 days after application -- also affected herbicide effectiveness.

Again, Landau points to previous research to explain why dry or rainy conditions might prime weeds to be less responsive to herbicides. For example, drought prompts plants to thicken the waxy cuticle layer on leaf surfaces. That helps plants avoid water loss, but it also makes it harder for foliar-applied herbicides to get in. Wet conditions could not only wash herbicides off leaf surfaces in the hours after application; excess precipitation in the days after application could trigger stress responses, leading to slower growth and reduced movement of herbicides to target sites within the plant.

"Farmers are already dealing with serious challenges due to climate change, such as spring flooding and summer droughts. When you consider that the tools used to combat weeds are also affected by these weather patterns, farming becomes that much more difficult," Williams said.

While much of the analysis focused on the effects of single herbicides, the team also analyzed the combination of glyphosate and fomesafen. Weed control was more robust with the combined application across variable weather conditions, but the researchers warn that POST combinations alone won't guarantee full control. Landau recommends layering in effective soil-applied residual herbicides and non-chemical tactics to provide more consistent weed control.

Williams adds that weed science as a discipline needs to invest in new tools.

"Collectively, weeds are adapting quicker than our ways to manage them. Climate change appears poised to put this evolution on a faster track," he said. "We hope it's possible to get ahead of the issue of failing weed control on the research and development side, because the cost of waiting until everything fails could be painful."