In the tests published in 2016, Matzrafi found that at high temperatures four different weed grass species were resistant to diclofop-methyl, an ingredient in a Bayer-made herbicide, significantly better than at lower temperatures. Matzrafi also found that the high temperatures made another herbicide, pinoxaden, less able to suppress the growth of invasive false bromine. Additionally, the grass thrived even when it moved from cooler conditions to a warmer environment for up to two days after the herbicide treatment. (The research was partially funded by ADAMA Agricultural Solutions, an Israel-based agrochemicals company.)
“Our results, and many other studies since the 90s, suggest that post-application environmental conditions may also affect herbicide sensitivity,” Matzrafi explained in an email. Even if farmers spray in cooler temperatures, this might not be enough to avoid the effects of the heat.
These conditions, experts fear, will worsen with climate change. Already, many agricultural states in the United States, as well as other major food-producing regions around the world, regularly experience temperatures exceeding 90 degrees Fahrenheit during growing seasons. Some researchers say that problems with heat and herbicide performance are now manifesting in part because of more frequent episodes of extreme heat in recent decades.
However, it is difficult to pin down the effects seen today on recent climate change, wrote Lewis Ziska, a plant physiologist at Columbia University in New York City, in an email to Undark. But noting that weeds are “the biggest constraint on food production,” Ziska warns that “they will pose a formidable challenge for farmers in a more extreme environment.”
In the Midwest, for example, temperatures could rise an average of 8.5 degrees Fahrenheit by the turn of the century, with longer and more frequent stretches of extreme heat, according to federal government projections. And in South Asia, including India, a region of global importance for the production of rice, pulses, nuts and cotton, the Intergovernmental Panel on Climate Change predicts that annual average temperatures will increase by almost 6 degrees Fahrenheit by 2100.
In the first such experiment, the results of which were reported last year, Matzrafi studied the joint effect of heat and high levels of carbon dioxide on two different weed species and found that the combination strengthens the herbicidal defenses of weeds beyond that of either one or the other factor alone.
It is not clear whether herbicide manufacturers are prepared for the challenges ahead of a warming planet. Many do not recommend optimal spray temperatures to ensure the effectiveness of the guidelines they distribute to farmers.
In a written statement, Clark Ouzts, a spokesperson for Sygenta, the maker of pinoxaden, says the company has not studied the potential effects of climate change on the activity of the herbicide, but that “research field and commercial applications have not shown that temperature has an impact on the activity of pinoxaden. “
Charla Lord, spokesperson for Bayer, wrote in a statement that the company’s herbicides are “extensively tested to meet the requirements of all regulatory bodies” and “labeled so that applicators know how to apply them for control and optimal success. ” The company did not respond to specific questions regarding the effectiveness of its products under high temperatures, although the company did posted on 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 herbicide performance.
Not everyone is convinced that these experimental results wreak havoc on farmers. Some researchers and weed experts say laboratory conditions differ dramatically from the field, making the results less relevant. “I don’t think we can say for sure that this has an impact on a real world scale,” wrote Brad Hanson, a weed expert at the University of California at Davis, in an email to Undark. Hanson worked with Matzrafi on research published last year.