In tests published in 2016, Matzrafi found that at high temperatures four different species of grass weeds stood up against diclofop-methyl, an ingredient in an herbicide manufactured by Bayer, significantly better than they did at lower temperatures. Matzrafi also found that high temperatures made another herbicide, pinoxaden, less able to curb growth of the invasive grass false brome. Moreover, the grass thrived even when it was switched from cooler conditions to a hotter environment up to two days after the herbicide treatment. (The research was partially funded by ADAMA Agricultural Solutions, an agrochemical company based in Israel.)
“Our findings, and many other studies since the ’90s, suggest that post-application environmental conditions may also affect herbicide sensitivity,” explained Matzrafi in an email. Even if farmers spray during cooler temperatures, that might not be enough to avoid the effects of heat.
Those conditions, experts fear, will worsen under climate change. Already, many US states important for agriculture, as well as other major food-producing regions around the world, regularly experience temperatures topping 90 degrees Fahrenheit during growing seasons. Some researchers say that problems with heat and herbicide performance are coming to the fore now partly because of more frequent episodes of extreme heat over the past few decades.
However, it is hard to pin the effects seen today on recent climatic changes, Lewis Ziska, a plant physiologist at Columbia University in New York, wrote in an email to Undark. But noting that weeds are “the greatest constraint for food production,” Ziska warns that “they will be a formidable challenge for farmers in a more extreme environment.”
In the Midwest, for example, temperatures could rise by an average of 8.5 degrees Fahrenheit by the end of the century, with longer and more frequent stretches of extreme heat, according to federal government projections. And in South Asia, including India—a globally important region for producing rice, pulses, nuts, and cotton—the Intergovernmental Panel on Climate Change projects that average annual temperatures will rise by nearly 6 degrees Fahrenheit by 2100.
In the first experiment of its kind, the results of which were reported last year, Matzrafi studied the joint effect of heat and raised carbon dioxide levels on two different weed species and found that the combination boosts weeds’ herbicide defenses beyond that of either factor alone.
It’s not clear whether herbicide manufacturers are prepared for the coming challenges of a warming planet. Many do not recommend optimal spraying temperatures to ensure efficacy in the guidelines they distribute to farmers.
In a written statement Clark Ouzts, a spokesperson for Sygenta, the manufacturer of pinoxaden, says the company has not studied the potential effects of climate change on the herbicide’s activity, but that “field research and commercial applications have not shown temperature to have a significant impact on the activity of pinoxaden.”
Charla Lord, a spokesperson for Bayer, wrote in a statement that the company’s herbicides are “extensively tested to meet all regulators’ requirements” and “labeled so applicators know how to apply them for optimal control and success.” The company did not respond to specific questions regarding the efficacy of their products under high temperatures, although the company has posted about the challenges of high-temperature spraying on its website.
Corteva, which makes herbicides incorporating 2,4-D, did not respond to requests for comment on how high temperatures affect the herbicides’ performance.
Not everyone is convinced that these experimental findings spell trouble for farmers. Some researchers and weed experts say that laboratory conditions differ radically from the field, making the results less pertinent. “I don’t think we could say for sure that this is having an impact at the real-world scale,” wrote Brad Hanson, a weed expert at the University of California, Davis, in an email to Undark. Hanson worked with Matzrafi on the research published last year.