The researchers analyzed thousands of data points, including a broad range of weather conditions over 30 years, which allowed them to characterize the effects of weather on post-emergent herbicide efficacy on a much broader range of environments than previous studies.
The analysis showed average air temperatures below 18.9 C or above 25 C made fomesafen, glyphosate and mesotrione less effective on waterhemp, giant foxtail and morning glory species. The historical data set 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 Marty Williams, an ecologist with USDA-ARS and affiliate professor at the University of Illinois.
“Likewise, a hot period after application could supercharge plant metabolism and neutralize the herbicide faster, rendering it less effective.”
Cold temperatures have the opposite effect, slowing metabolism and movement of herbicides within the plant and rendering them less effective.
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.
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 them avoid water loss, but it also makes it harder for foliar-applied herbicides to get in.
Wet conditions could wash herbicides off leaf surfaces in the hours after application and excess precipitation in the days after application could trigger stress responses, leading to slower growth and reduced movement of herbicides within the plant.
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 combinations alone won’t guarantee full control.
Landau recommends layering 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.”