Herbicide-Resistant Weedy Rice Spreads Across Southeastern U

[patricjfrenn]

Weedy rice, a close relative of cultivated rice, is a persistent pest that significantly reduces crop yields in rice fields worldwide. To combat this, rice farmers in the southeastern United States began planting herbicide-resistant rice cultivars in the early 2000s, enabling them to apply herbicides targeting weedy rice without harming the crops. However, just a few years later, farmers in Arkansas began reporting herbicide-resistant weeds. Laboratory analysis confirmed these concerns, revealing that weedy rice was interbreeding with cultivated rice, leading to hybrid weeds with genetic resistance to herbicides.

A new study from Washington University in St. Louis shows that more than half of the weedy rice sampled in the southeastern U.S. has developed herbicide resistance. The researchers found that 57% of 201 weedy rice samples collected from fields in Missouri, Arkansas, and Louisiana in 2022 were resistant to the imidazolinone (IMI) class of herbicides. Additionally, 3.5% of the samples showed resistance to a newer class of herbicides introduced to the region in 2018.

Marshall Wedger, a postdoctoral fellow in biology, has been tracking the rise of herbicide resistance in weedy rice for years. His previous research found that in 2018, 98% of weedy rice sampled in Arkansas exhibited some level of herbicide resistance. For the current study, Wedger expanded his research to include samples from neighboring states and analyzed genetic variations across different populations of weedy rice in the region.

"Clearfield rice has revolutionized the industry by helping farmers deal with weedy rice," Wedger explained, referring to the herbicide-resistant rice cultivar adopted in the early 2000s. "However, as resistance to IMI grew, farmers were eager for a new herbicide-resistant option." Wedger pointed to Provisia and Max-Ace rice, which are resistant to a different family of herbicides, offering farmers an additional tool in combating the weed problem.

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The situation with weedy rice parallels the challenges faced in medicine with antibiotic resistance. As resistance to one herbicide becomes widespread, new options must be developed to stay ahead of the problem. According to Wedger, the lifespan of Clearfield rice has simply run its course due to the rising resistance.

Weedy rice is a global problem, as these weeds are difficult to differentiate from the cultivated crop, making them challenging to manage. They compete with rice plants for essential resources like water, nutrients, and space, reducing overall yields.

Prior to the early 2000s, weedy rice rarely interbred with cultivated rice. However, the widespread adoption of herbicide-resistant rice coincided with the increased use of hybrid rice, which is higher yielding but more prone to seed shattering. These seeds can survive through the winter and germinate in subsequent seasons as "volunteer rice," which opens up more opportunities for crossbreeding with weedy rice.

Wedger noted that herbicide resistance varies between individual fields, making management even more challenging. "Each field has its own unique composition of weedy rice and herbicide resistance, which complicates efforts to control the problem," he said.