Killing giant ragweed has become more difficult for some Wisconsin farmers

Some giant ragweed populations in Wisconsin have developed resistance to an important class of post-emergence herbicides

PUBLISHED ON 23 August 2024

URBANA, Ill. — When giant ragweed takes over a field, the towering weed reduces yields and sends clouds of its notoriously allergenic pollen into the air. There are few tools to stop the scourge, especially for farmers who grow non-GMO soybeans. Some Wisconsin farmers now have even fewer options.

New research from the University of Wisconsin-Madison and the University of Illinois Urbana-Champaign shows that some giant ragweed populations in Wisconsin have developed resistance to an important class of post-emergence herbicides known as protoporphyrinogen oxidase (PPO) inhibitors (Group 14 herbicides).

“Giant ragweed is difficult to control with pre-emergent herbicides, in part because the seeds are larger and can emerge from greater depths. So farmers rely on post-emergent products. For people growing non-GMO soybeans, those post-emergent products are ALS and PPO, and we already have pretty widespread ALS resistance in giant ragweed,” said Pat Tranel, study co-author and professor in the Department of Crop Sciences in the College of Agricultural, Consumer and Environmental Sciences (ACES) at Illinois.

“The loss of PPOs basically means you no longer have chemical options,” he added.

The results will not surprise some Wisconsin farmers. Rodrigo Werle, study co-author and associate professor and weed scientist at the University of Wisconsin-Madison, says that as early as 2018, farmers were mentioning that PPOs were no longer working as well.

“We thought they had problems with the timing of the application, they had missed the ideal window for application,” said Werle. “But the growers we worked with are very competent and did everything according to the instructions. Small plants grew back after spraying, which can be a sign of resistance.”

The research team asked farmers to collect and send seeds from plants in the affected fields.

“We tested fomesafen (a PPO inhibitor) at 1 and 3 times the dose, and many plants survived. Then we tested the dose-response relationship between fomesafen and lactofen (another PPO). We found that one population had almost 30-fold resistance to fomesafen and almost four-fold resistance to lactofen,” said lead study author Felipe Faleco, a doctoral student at UW-Madison.

Faleco allowed plants that had survived the single dose of fomesafen to grow to maturity, then collected seeds and handed them over to Tranel, who had previously determined the molecular basis of ALS and PPO resistance in ragweed, a close relative of giant ragweed.

“We sequenced the genes for the PPO target enzyme and found the same mutation that we had seen in ragweed,” Tranel said. “There were really no other mutations, so that's probably the basis of resistance in giant ragweed as well.”

Tranel's group went a step further and developed a molecular tool that allows diagnostic laboratories to detect PPO resistance and provide farmers with rapid answers.

The Wisconsin team also tested for resistance to acetolactate synthase inhibitors (ALS) and glyphosate and found four populations with resistance to ALS and two populations with resistance to glyphosate. These types of resistance had already been documented in giant ragweed, but the team also found one population with resistance to both.

“For us in Wisconsin, this is the first time we've documented two types of resistance in a single population of giant ragweed,” Werle said. “This shows that it's not just the waterhemp plant that's developing multiple resistances. We need to keep an eye on some other weeds as well.”

Resistance to glyphosate affects growers of genetically modified soybeans, who in these cases resort to PPO and ALS herbicides. Likewise, non-GMO growers who cannot use glyphosate rely on these chemicals. The authors say that with ALS and PPO resistance – essentially zero chemical options – more non-GMO growers could switch to genetically modified soybeans.

“Farmers plant non-GMO soybeans because of the premiums; there is a financial reason to go that route, even if weed control is more difficult,” Werle said. “But if a farmer knows he's dealing with this type of resistance, it could prevent him from growing non-GMO crops sustainably or profitably.”

In addition to the potential impact on business management and profits, the findings are also important for allergy sufferers.

“As farmers struggle to control ragweed, more ragweed species will escape and shed pollen,” Tranel said. “So if you live in a semi-rural area with corn and soybean fields nearby, it's likely that there will be more pollen in the air.”

The study “Resistance to protoporphyrinogen oxidase inhibitors in giant ragweed (Ambrosia trifida)” was published in Pest Management Science. [DOI: 10.1002/ps.8349]Authors include Felipe Faleco, Filipi Machado, Lucas Bobadilla, Pat Tranel, David Stoltenberg, and Rodrigo Werle. The Wisconsin Soybean Marketing Board supported Faleco's graduate studies.

— ACES of the University of Illinois