When giant ragweed invades a crop field, it not only hampers crop yield but also releases considerable amounts of its well-known pollen, which causes allergies. Farmers, particularly those cultivating non-GMO soybeans, have limited options to tackle this issue. Unfortunately, some Wisconsin farmers now find themselves with even fewer alternatives.
When giant ragweed invades a crop field, it not only hampers crop yield but also releases significant amounts of its well-known pollen, which causes allergies. Farmers, especially those cultivating non-GMO soybeans, have limited methods to tackle this problem. Unfortunately, some Wisconsin farmers now find themselves with even fewer alternatives.
Recent studies conducted by the University of Wisconsin-Madison and the University of Illinois Urbana-Champaign have discovered that some populations of giant ragweed in Wisconsin have developed resistance to a key category of post-emergence herbicides called protoporphyrinogen oxidase (PPO) inhibitors (also known as Group 14 herbicides).
“Controlling giant ragweed with pre-emergence herbicides is challenging, partly because its seeds are larger and can sprout from deeper in the soil. Therefore, farmers rely on post-emergence herbicides. For non-GMO soybean farmers, the main choices are ALS and PPO herbicides, but we are already seeing a widespread resistance to ALS in giant ragweed,” stated Pat Tranel, a co-author of the study and a professor in the Department of Crop Sciences at Illinois.
“With the loss of PPO options, you’re left with virtually no chemical solutions,” he emphasized.
This information may not surprise many farmers in Wisconsin. According to co-author Rodrigo Werle, an associate professor and Extension weed scientist at UW-Madison, farmers began noting in 2018 that PPO herbicides were losing their effectiveness.
“Initially, we thought the issues were related to the timing of applications and that growers were missing the optimal application window,” Werle explained. “However, the farmers we worked with are very experienced and followed all the recommended practices. We observed that small plants were regrowing after being sprayed, which is often indicative of resistance.”
The research team asked farmers to collect and share seeds from affected plants in their fields.
“We tested fomesafen, a PPO inhibitor, at both the standard and triple the label rate, and many plants survived. We also assessed the dose response for fomesafen and lactofen (another PPO herbicide). We found that one population exhibited nearly 30 times more resistance to fomesafen and almost four times to lactofen,” said lead author Felipe Faleco, a doctoral student at UW-Madison.
Plants that survived the standard fomesafen dose were permitted to mature, and the seeds were collected and analyzed by Tranel, who had previously identified the molecular origins of ALS and PPO resistance in common ragweed, a close relative of giant ragweed.
“We sequenced the genes associated with the PPO target enzyme and identified the same mutation found in common ragweed,” Tranel stated. “There were hardly any other mutations, suggesting this is likely the mechanism behind the resistance in giant ragweed as well.”
Tranel’s team took further steps, creating a molecular diagnostic tool for labs to detect PPO resistance, providing farmers with quick insights.
The Wisconsin researchers also examined resistance to acetolactate synthase (ALS) inhibitors and glyphosate, identifying four populations resistant to ALS and two to glyphosate. While such resistances have been recorded in giant ragweed before, they discovered one population showing dual resistance.
“In Wisconsin, this is the first time we’ve documented two types of resistance within a single population of giant ragweed,” Werle remarked. “It demonstrates that it’s not solely waterhemp evolving multiple resistances. There are other weed species that require attention as well.”
Resistance to glyphosate poses challenges for GMO soybean farmers, who typically resort to PPO and ALS herbicides in such scenarios. Conversely, non-GMO farmers, who cannot apply glyphosate, also depend on these herbicides. The authors suggest that with both ALS and PPO resistance rendering chemical options nearly non-existent, more non-GMO farmers might switch to GMO soybeans.
“Farmers choose non-GMO soybeans due to the financial benefits they offer; there is a monetary incentive in this choice, even though controlling weeds can be more complex,” Werle noted. “However, if farmers realize they are facing this type of resistance, it could discourage them from sustainably or profitably growing non-GMO crops.”
Besides potential implications for agricultural practices and profitability, these findings are significant for allergy sufferers.
“As farmers struggle with weed control, more ragweeds are likely to escape and produce pollen,” Tranel warned. “If you live in a semi-rural area surrounded by corn and soybean fields, the amount of pollen in the air is likely to increase.”
