In the southeastern United States, weedy rice steals herbicide resistance from cultivated rice

Weedy rice is a close relative of cultivated rice that infests rice fields around the world and significantly reduces yields. To combat this agricultural pest, rice farmers in the southeastern United States have planted modified rice cultivars to allow them to apply herbicides targeting weedy rice without harming the crop.

But only a few years after the introduction of herbicide-resistant rice in the early 2000s, Arkansas farmers began reporting that weeds in their fields were also becoming resistant to herbicides. Laboratory analysis verified these stories: Scientists found that weeds reproduced with rice cultivation and that subsequent generations of hybrid weeds exhibited some level of genetic resistance to herbicides.

Now, a new study from Washington University in St. Louis shows that more than half of the weedy rice sampled in the southeastern U.S. rice growing region have become resistant to herbicides. The research is published in the journal Molecular ecology.

WashU scientists found that 57% of 201 weedy rice samples collected from fields in nine counties or parishes in Missouri, Arkansas and Louisiana in 2022 were resistant to the imidazolinone (IMI) family of herbicides. Additionally, 3.5% of samples were resistant to another, newer class of herbicides, used only in rice fields in the southeastern United States since 2018.

Marshall Wedger, a postdoctoral researcher in biology in Arts & Sciences, has been documenting trends in herbicide resistance in weedy rice for years. In previous research, Wedger found that 98% of the weedy rice he sampled from Arkansas rice fields in 2018 had genetic markers of some level of herbicide resistance.

For the new study, Wedger expanded his survey to include weedy rice sampled in neighboring states and also took into account genetic variations within different populations of weedy rice in his study area.

“Clearfield (a branded commercial rice seed) did wonders for rice farmers who were forced to live with weedy rice. It revolutionized the industry,” Wedger said, referring to the herbicide-resistant rice that growers began planting in the early 2000s.

“As IMI resistance became more prevalent in weeds, farmers were eager to discover the next herbicide-resistant cultivar,” he said. “Both Provisia and Max-Ace rice (other commercial rice seeds) are resistant to QPE, which is from a completely different family of herbicides, meaning there would be very little cross-resistance. As such, it is another tool in a farmer’s arsenal.

“It’s similar to antibiotics in medicine,” Wedger continued. “When resistance becomes widespread, we switch to another antibiotic. It’s less that Clearfield has failed and more that its shelf life has run its course.”

Weedy rice is a scourge for cultivated rice production worldwide. Agricultural weeds that are close relatives of crops pose a challenge to farmers due to their physical similarities to crops, making them difficult to detect and eradicate. Along the way, imitators compete with crops for water, nutrients and space, often depressing crop yields.

Until the early 2000s, weedy rice rarely interbred with types of rice commonly grown in the United States.

Widespread adoption of herbicide-resistant rice has coincided with an increased reliance on hybrid rice cultivars, Wedger said. Hybrid rice has a higher yield than traditional rice varieties, but it also tends to scatter more seeds across the field.

These seeds can overwinter and reappear in subsequent years in the form of “spontaneous rice”. Volunteers flower over a wider window of time, opening new opportunities for crossing with a weed closely related to rice. This is a way for genes to travel from the crop to the pest.

“We are seeing that, at least for now, individual fields have their own weedy rice compositions in terms of strain composition and herbicide resistance, which makes management more difficult,” Wedger said.

Weed infestations on rice are important because they have a significant economic impact. Weedy rice causes an estimated $45 million in economic losses in the United States each year, and hundreds of millions of dollars in additional losses worldwide.

“These results are yet another indicator of the incredible adaptability of rice weeds and other agricultural weeds – and their ability to thrive despite our best efforts to control them,” said Kenneth Olsen, George William Professor and Irene Koechig Freiberg of biology in arts and arts. Sciences and lead author of the new study. “In this case, they took the best defense we have against them – herbicide resistance – directly from our own high-yielding rice cultivars.”