A recent study reveals that over half of the weedy rice collected from the rice-growing areas in the southeastern U.S. has developed resistance to herbicides.
Weedy rice, which closely resembles cultivated rice, is a troublesome invasive species that infests rice fields globally and significantly reduces crop yields. To tackle this challenge, rice farmers in the southeastern United States have been using specially engineered rice varieties that allow them to apply herbicides specifically targeting weedy rice without affecting the cultivated rice.
However, just a few years after the introduction of herbicide-resistant rice in the early 2000s, farmers in Arkansas started noticing that the weeds in their fields were also becoming resistant. Laboratory tests confirmed these observations; researchers discovered that the weeds were hybridizing with cultivated rice, resulting in new generations of weeds that displayed some degree of genetic resistance to herbicides.
According to a new study from Washington University in St. Louis, more than 50% of the sampled weedy rice from this region has now shown herbicide resistance.
The scientists at WashU found that 57% of the 201 samples of weedy rice taken from fields across nine counties in Missouri, Arkansas, and Louisiana in 2022 were resistant to herbicides from the imidazolinone (IMI) class. Furthermore, 3.5% of samples exhibited resistance to another newer type of herbicide, which has been applied in southeastern U.S. rice fields since 2018.
Marshall Wedger, a postdoctoral fellow in biology at Arts & Sciences, has been analyzing trends in herbicide resistance among weedy rice for several years. In his previous research, he discovered that 98% of the weedy rice he examined in Arkansas rice fields in 2018 carried genetic markers for herbicide resistance. The new study, published in the journal Molecular Ecology, expanded Wedger’s research to include samples from neighboring states and evaluated genetic differences among various populations of weedy rice within the study area.
Wedger stated, “Clearfield (a commercial rice seed) has been transformative for rice farmers dealing with weedy rice. It was a game changer for the industry.” He was referring to the herbicide-resistant rice varieties introduced in the early 2000s.
As IMI resistance grew more widespread in weedy rice, farmers quickly sought other herbicide-resistant rice options, he noted. “Provisia and Max-Ace rice (other commercial rice varieties) are resistant to QPE, which belongs to a different group of herbicides—therefore, there’s minimal cross-resistance. This serves as an additional tool for farmers.”
Wedger compared the situation to antibiotics in medicine. “When resistance becomes common, we switch to a different antibiotic. It’s not so much that Clearfield failed; rather, its effectiveness had a limited timeframe.”
Globally, weedy rice poses a major problem for cultivated rice production. The physical similarities between these agricultural weeds and crops create challenges for farmers, as the weeds can be hard to identify and remove. While competing for resources like water and nutrients, these imitative weeds reduce crop yields.
Up until the early 2000s, there was minimal interbreeding between weedy rice and the common rice varieties in the U.S.
The widespread adoption of herbicide-resistant rice happened alongside a growing dependence on hybrid rice varieties, Wedger explained. Hybrid rice typically yields more than traditional varieties but is also more likely to shed seeds in the field.
These seeds can survive through the winter, re-emerging in later years as “volunteer rice.” These volunteers bloom over a broader period, providing increased chances for them to interbreed with closely related weedy rice. This is one of the mechanisms by which traits can transfer from the cultivated crop to the weeds.
Wedger noted that, at least for the moment, individual fields exhibit different strains and levels of herbicide resistance among weedy rice, complicating management efforts.
Weedy rice outbreaks carry significant economic consequences, causing an estimated $45 million in losses annually in the United States alone, with hundreds of millions in further losses worldwide.
Kenneth Olsen, a George William and Irene Koechig Freiberg Professor of Biology in Arts & Sciences and the senior author of the study, commented, “These results highlight the extraordinary adaptability of weedy rice and other agricultural weeds, as well as their ability to survive despite our best control efforts. In this case, they have effectively taken the best method we had against them—herbicide resistance—from our own high-yielding rice varieties.”
This research was partially funded by the National Science Foundation (NSF) Plant Genome Research Program (IOS-1947609).
