A few years ago, a team of scientists specializing in weed management discovered that herbicides applied to soil are becoming less effective against common agricultural weeds due to our changing climate. Now, this same research team, which includes experts from the USDA Agricultural Research Service and the University of Illinois Urbana-Champaign, has uncovered similar results regarding post-emergence (POST) herbicides.
A few years ago, a team of scientists specializing in weed management discovered that herbicides applied to soil are becoming less effective against common agricultural weeds due to our changing climate. Now, this same research team, which includes experts from the USDA Agricultural Research Service and the University of Illinois Urbana-Champaign, has uncovered similar results regarding post-emergence (POST) herbicides.
Researchers analyzed a 30-year database from 16 Extension weed science programs, including the Illinois Extension, throughout the U.S. Corn Belt. They found that varying weather conditions significantly decreased the effectiveness of three primary POST herbicides targeting major weeds that impact corn and soybean crops. Inadequate weed control could adversely affect both corn and soybean yields, posing a threat to global food security.
“Weather plays a crucial role, not just immediately after the POST application, as previous studies have indicated. Our analysis revealed that air temperature and rainfall were connected to herbicide effectiveness both days before and after application for the products and weeds we investigated,” stated Chris Landau, a postdoctoral researcher at USDA-ARS and the lead author of the study. “With the vast amount of data we examined — thousands of data points covering diverse weather conditions over three decades — we could assess the impact of weather on POST herbicide efficiency across a wider range of environments than earlier studies.”
The study indicated that average air temperatures below 66 or above 77 degrees Fahrenheit reduced the efficacy of fomesafen, glyphosate, and mesotrione — which are three herbicides with distinct mechanisms of action — against waterhemp, giant foxtail, and morning-glory species. Although the historical data does not clarify the reasons for reduced weed response to herbicides under these conditions, other studies offer insights.
“When it’s excessively warm prior to application — and if moisture is not a limiting factor — weed growth accelerates, resulting in larger plants that may withstand herbicides better,” explained co-author Marty Williams, an ecologist with USDA-ARS and an affiliate professor in the Department of Crop Sciences at Illinois. “Meanwhile, warm temperatures after application can boost plant metabolism, allowing them to neutralize the herbicide quicker, thus reducing its effectiveness.”
Conversely, colder temperatures slow down metabolism and the movement of herbicides within the plant, making them less effective.
Herbicide applicators must also consider precipitation. The analysis revealed that dry spells in the 10 days leading up to application or wet conditions within 10 days post-application significantly influenced herbicide effectiveness.
Landau again points to previous findings to shed light on why dry or rainy conditions might diminish weed responsiveness to herbicides. For instance, drought conditions cause plants to develop thicker waxy layers on their leaves, which helps conserve water but also impedes foliar herbicides from penetrating. Excess rainfall after application can wash herbicides off leaves shortly after application, while heavy rain over the following days may induce stress responses, curtailing plant growth and hindering herbicide distribution within the plant.
“Farmers are already facing formidable challenges due to climate change, such as spring floods and summer droughts. When you factor in that the tools for weed management are also impacted by these weather patterns, it complicates farming even further,” Williams commented.
Although much of the research concentrated on individual herbicides, the team also assessed the effects of combining glyphosate and fomesafen. This combination provided better weed control under varying weather conditions, but the researchers caution that relying solely on POST combinations does not guarantee effective control. Landau advocates for incorporating effective soil-applied residual herbicides and non-chemical methods to achieve more reliable weed management.
Williams emphasizes the need for advancements in weed science as a field.
“Weeds are evolving faster than our management strategies can keep up. The impact of climate change seems likely to accelerate this evolution,” he stated. “We hope there is an opportunity to proactively address the challenges of diminishing weed control through research and development, as the repercussions of waiting until complete failure could be severe.”
