Rice is the world's second-biggest cereal crop, and it's the most important for human consumption. Yet in a vast majority of rice operations, a harmful copycat sits right in plain sight, sucking up nutrients and infuriating farmers.
This copycat is a weed that’s extremely difficult to find, has lots of evolutionary protections to ensure its ongoing survival, and has a significant negative effect on rice crops around the world. Even in the US, where it’s less of a problem than it is elsewhere, it’s estimated to cause crop losses of more than $50 million per year. This weed is called weedy rice, and scientists are trying to figure out where it comes from—and how to fight it.
Weedy rice is in fact a form of rice. It comes in a bunch of different species, but they’re all in the same Oryza genus and therefore authentically rice plants. You can sometimes tell them apart from regular rice by its longer awns (awns are those hairs you see on grasses sometimes), or because it’s more red in color. But while technically rice, they’re no good to us: they produce very few seeds (otherwise known as grains of rice), and what they do produce is hard and unpalatable.
But weedy rice is also super frustrating. It grows comfortably amongst regular rice, and it self-propagates easily—it releases its seeds before regular rice is ready for harvest, which ensures it’ll be present the following year. It can also stay dormant for years, unlike regular rice, and the combination of that seed-release schedule and its dormancy means that weedy rice lingers in the soil of rice farms for quite a long time.
What’s worse, because weedy rice is such a close relative of regular rice, there are no available pesticides that will kill just weedy rice while leaving regular rice alone. Because it’s such a worthy adversary, it’s found in huge percentages of rice fields worldwide: it’s in up to 75 percent of fields in Europe, more than half in parts of West Africa and Latin America, and up to 80 percent in Cuba.
“Control of weedy rice plants is much more difficult than that carried out on other weeds because of the great morphological variability, particular growth behaviour, and high biological affinity with cultivated varieties,” writes Aldo Ferrero for the United Nations’ Food and Agriculture Organization.
A new study from Washington University in St. Louis attempts to find out more about these pests by conducting what they call “an ancestry.com-type adventure” to see where they come from. What they found out is that two known strains of weedy rice, though they evolved independently, evolved in a similar way.
Weedy rice, the study finds, is a natural and frustrating evolution of regular, domesticated rice. After looking into the precise changes in the plant’s DNA, researchers found that it takes comparatively few changes for rice to evolve into weedy rice. “It’s different genomic islands in each weed type,” said Kenneth Olsen, the lead author of the paper, in a press release. “So changing a crop into a weed doesn’t take many genetic changes and it can occur through different genetic mechanisms.” In other words, rice just sort of…does this. It has a natural proclivity to becoming an incredibly frustrating weed.
This particular study was aimed at learning more about weedy rice, but that’s the first step to figuring out how to stop it because at the moment, there’s really no effective way to fight it.