In a paper published Sept. 8 in the journal Communications Biology, scientists from Washington University in St. Louis and the University of Arkansas report that a crop pest called weedy rice has become widely herbicide resistant in regions where herbicide-resistant rice is planted. The study highlights challenges facing U.S. rice farmers when they battle a weedy enemy that is closely related to a desirable crop plant.

Weedy rice is a closely related cousin of crop rice. It aggressively competes with cultivated rice in the field, leading to loss of yield and reductions in harvest quality that compromise market value. Weedy rice infestations cause an estimated $45 million in economic losses in the United States each year and hundreds of millions of dollars worldwide.

Biologists used whole-genome sequences of 48 contemporary weedy rice plants to show how herbicide resistance evolved by gene flow from crop rice. Almost all other cases of herbicide resistance in agricultural weeds result from selection of tolerant genotypes in the weed species. Just 20 years after herbicide-resistant rice was first adopted in the southern United States, the majority of fields with a history of herbicide-resistant rice cultivation have weedy rice plants that are also herbicide resistant.

"Throughout its nearly 200-year history in the United States, weedy rice had a very low rate of outcrossing with cultivated rice," said Marshall Wedger, a postdoctoral research associate in biology in Arts & Sciences at Washington University and first author of the study. "We found that U.S. weedy rice has persisted through herbicide pressure with the survival of those few plants that outcross, consequently acquiring the herbicide- resistance trait."

"Technological changes in U.S. rice farming since the 2000s have led to a complete genetic revolution in the makeup of the weedy rice that infests U.S. fields," said Kenneth Olsen, professor of biology at Washington University and senior author on the study.

"In the last 20 years, weedy rice has gone from being very genetically distinct from U.S. crop varieties to nowadays mostly being derived from crop-weed hybridization," Olsen said. "The weeds are grabbing certain traits from the crop that are beneficial to them, including herbicide resistance."