How a rice-mimicking weed has taken over the planet

The domestication of plants proved a pivotal point in human history, propelling our species to ever newer heights of sophistication and control over the environment, for better or worse. Right from the get-go, though, farmers made mistakes with long-lasting consequences.

Take the wild grass Echinochloa crus-galli, which originated in tropical Asia and has turned into one of the world’s worst agricultural weeds. The grass has long caused people plenty of trouble as the fast-growing grass needs to be weeded constantly, usually by hand. Barnyard grass is an invasive weed that has conquered the planet.

The reason for its spread? A mistake by early rice growers in the Yangtze River region a millennium ago. These farmers mistook the grass for rice because the plant has come to imitate rice stalks, explain the authors of a new study published in the journal Nature Ecology & Evolution.

The researchers behind the study postulate that the mimic version of the grass emerged during the Song Dynasty in China when local populations were growing fast and demand for rice as the region’s staple grain was very high. A quick-maturing rice called Champa rice, which is better able to resist droughts, was introduced to the Yangtze basin from Vietnam at this time. Its cultivation allowed farmers to have two harvests in a year instead of just one. On the downside, the weedy grass that mimics rice likely gained a foothold in and around paddies.

“In Asia, rice farmers have traditionally planted and weeded their paddies by hand. Any weeds that stick out are easily detected and removed,” said Kenneth Olsen, a professor of biology. “Over hundreds of generations, this has selected for some strains of barnyard grass that specialize on rice fields and very closely mimic rice plants. This allows them to escape detection.”

This form of crop mimicry, also known as Vavilovian mimicry, is an evolutionary adaptation whereby weeds come to resemble domesticated plants so as to avoid detection by people. Echinochloa crus-galli has come to imitate rice stalks with nearly identical green stems.

“With the advent of agriculture about 10,000 years ago, humans all over the planet began creating a wonderful habitat for naturally weedy plant species to exploit,” Olsen explained. “The most successful and aggressive agricultural weeds were those that evolved traits allowing them to escape detection and proliferate in this fertile new environment,” he added.