Farmers Increasingly Plagued By 'Weedy' Strains Of Rice
July 18, 2013

Farmers Increasingly Plagued By ‘Weedy’ Strains Of Rice

Brett Smith for - Your Universe Online

Over the past decade or so the de-domestication of rice has increasingly plagued farmer's fields, with "weedy" strains of the grain cutting agricultural yields by as much as 80 percent.

Being domesticated thousands of years ago in both Africa and Asia, the de-domestication of rice also provided an interesting case study for two biologists at Washington University in St. Louis who recently published a genetic analysis of both types of rice hull color in the Journal of Evolutionary Biology.

"In some parts of the world farmers have given up trying to grow rice and just market the weedy stuff that's infested the fields as a health food," said Kenneth Olsen, an associate professor of biology at WUSTL.

Referring to red rice growing in a delta region in southern France, Olsen said, "Red rice is full of antioxidants, which tend also to be plant defense chemicals, but it is basically a weed."

Less desirable, weedy forms of rice have evolved from crop forms, with the plants relapsing into traits seen in their wild ancestors.

"They're very aggressive competitors," Olsen said, "and they've become a huge problem both here in the US and all over the world."

The most cultivated rice around the world is Asian rice, Oryza sativa, which was domesticated within the past 10,000 years. Another similar domestication took place about 3,500 years ago in Africa along the Niger River delta.

When these crops were domesticated, they began to express a series of desirable traits referred to as domestication syndrome. One trait was the loss of shattering, or the breaking off of seeds, before they can be harvested. Other traits included an increase in seed size and the seeds all germinating simultaneously for convenient harvest.

According to Olsen and co-author Cindy Vigueira, a postdoctoral researcher at the university, different mutations of the same genes result in both the loss of shattering and the hull colors of Asian and African cultivated rice, meaning the early farmers in both continents tamed wild rice in the same way -- genetically speaking.

Similar to domestication, the emergence of weedy forms of rice has also happened twice, once in a part of India and once in the tropics, according to the research team. These less palatable species have regained many wild-like traits, such as shattering -- even though they still have the loss-of-shattering gene that early farmers selected for. Olsen said the plants have re-evolved this ability through different genetic means.

Olsen noted that the crop and weed varieties of rice are closely intertwined, meaning the weedy varieties can draw from both crop and wild genes. The biologist warned about hybridization among plants that could severely impact US farmers.

"We're already seeing more and more hybridization occurring," Olsen said. "It's going to change the overall composition of the weeds in U.S. rice fields and presumably elsewhere in the world as well."

Because of industrialized farming methods, there is little opportunity to remove weedy rice. Both types of seedlings look similar so farmers could potentially not realize they have a problem until their field is overrun. If a field is too heavily infested, abandoning it may be a farmer's only option.