Research tackles everevolving problem of weedy rice

Red rice in the field was a problem for rice growers, but at least it was easy to spot. But years of out-crossing with cultivated rice varieties has resulted in “weedy rice,” which appears in a spectrum of hues, some of which can blend in nicely with the crop.

But that camouflage is deceiving, and the result can be loss of both yield and rice quality, said Nilda Burgos, professor of weed physiology and molecular biology for the Arkansas Agricultural Experiment Station, the research arm of the University of Arkansas System Division of Agriculture.

Burgos has been working on the problem of red rice for many years. But now, she said, she has to think of it in terms of varying shades of weedy rice. “It’s not just red rice, anymore.”

Fraternization

The issue is that red rice is the same genus and species as cultivated rice, Burgos said. That leads to “gene flow,” when cultivated rice cross-pollinates with the weeds that survive from one year to the next. A problem may not manifest after a single rice season, but after repeated years of “fraternization” between weedy and domestic rice, the weeds present problems.

Burgos said she didn’t expect to find a big problem with red rice at first because rice is self-pollinating. That slows the rate of cross-pollination, especially since the reproductive window of individual varieties is relatively narrow.

“But we found that things begin to happen after multiple seasons,” Burgos said, “especially in fields where hybrids have been growing for many years.”

Rice grains that fall out and are left in the field will grow up volunteers in following years. Outcrossing with weedy rice results in numerous offspring that manifest many hues of off-color rice, and varying maturity dates. This leads to a wider window for cross-pollination, which leads to more varieties of weedy rice.

“The volunteers are bridges for outcrossing with weedy rice,” Burgos said.

Because weedy rice often matures later than conventional varieties, its development is often stunted before grain maturity when cooler weather comes on in the fall. “Second or third generations of weedy rice outcrosses only exist when it stays warm,” Burgos said.

That makes weedy rice a bigger problem in countries with tropical climates, Burgos said. But it’s also a problem as warm weather stretches longer into fall in the U.S.

“When it comes to global warming, weeds are going to love it,” Burgos said. “Outcrossing and herbicide resistance will become worse.”

The rise of weedy rice is not anyone’s fault, she said. “It’s a combination of factors, including plants, weather, climate, economics, available agricultural technology, available knowledge and farming practices.”

Problems in the fields

Hybrids have passed along herbicide resistance to weedy rice, Burgos said.

“Hybrid rice is more compatible with red rice and the outcrossing rate is higher,” Burgos said. “The outcrossing rate in hybrids is double that of conventional rice varieties.”

That’s still low, because of rice being self-pollinating, Burgos said. But it means that it takes fewer seasonal cycles before problems begin to mount up.

Besides causing headaches for weed control, the varying hues of weedy rice mar the consistent white color desired at the rice mills, Burgos said. That causes devaluation of the crop, and a discounted price paid to the farmers.

Being the same species as cultivated rice means that weedy rice is also competing with the crop for resources throughout the growing season, robbing the crop of nutrients and water, Burgos said.

The variability in maturity date also means the weedy rice may be overly mature or under-mature at harvest. Grains from overly mature weedy rice shatter in the field, leaving seed that will grow up as weeds in the following season, or during milling, damaging a crop’s milling yield.

Under-mature weedy rice at harvest means moisture content will be too high, complicating rice drying.

Plant height of weedy rice is consistent in the first generation, Burgos said. But it begins to vary in succeeding generations.

With all this going on, Burgos said, weedy rice wreaks havoc in the rice field. Burgos quantified yield loss for varying varieties and growing conditions. The weeds also result in lower rice quality and, in worst cases, can severely damage the whole crop.

Countermeasures

Avoiding damage from weedy rice begins with zero tolerance weed management, Burgos said. “Don’t leave anything in your field. And don’t forget the edges of the fields.”

Many growers clean up their fields thoroughly, but neglect the edges and ditches, Burgos said. The following year, weedy rice sprouts up at the peripheries of rice fields and spreads in from the edges.

Also, weedy rice seed that drops in water in the ditches gets carried to other areas and can sprout up anywhere.