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Weedy red rice threatens to reduce yield and quality for California producers

California’s 2,500 rice growers should closely examine questionable weeds in fields this year to help identify and prevent the spread of weedy red rice. If spread over a large area, the weed can substantially reduce rice yield and quality.

“Some California rice growers may scout a field, see a plant that resembles watergrass, and then keep on driving,” said Chris Greer, University of California (UC) Cooperative Extension (UCCE) rice farming systems advisor in Sutter, Yuba, Placer, and Sacramento counties.

“The grower should stop the truck and closely examine the plant. Red rice may easily be mistaken as watergrass from a distance early in the season,” Greer said. “If ligules and auricles are present at the leaf collar, a pendage that wraps around the tiller, the plant is not watergrass.”

The perennial weedy red rice plant was positively identified by researchers in Glenn County in 2003 and was subsequently found in six rice fields in Glenn and Colusa counties. Weedy red rice is taller and lighter in color than standard rice.

Rice in California is grown on about 500,000 acres mostly around the Sacramento Valley. Yields average about 8,000 pounds per acre annually.

Generally a rice plant has three to four tillers, each with roots, culm, and leaves. Rice growers in the Southern U.S. are experiencing the wrath of large weedy red rice infestations. Twenty-five thicker tillers are found on each plant. Weedy red rice becomes a fiercer competitor for field space and nutrients than the cultivated rice varieties. Rice yield losses in the South can be a whopping 60 percent to 70 percent.

Weedy red rice heading can occur over a prolonged period with seeds that shatter easily. Seeds that fall to the ground can germinate in favorable conditions or remain dormant for several years.

Red rice has red bran that lowers the grade of regular rice at the mill. A maximum limit of 0.5 percent red rice and damaged kernels (single or combined) is allowed to achieve Grade U.S. 1 rice quality. Millers face increased costs to remove extra red kernels — costs likely to be absorbed by growers through reduced prices.

Greer discussed weedy red rice and other problematic weeds during the 61st California Weed Science Society annual conference in Sacramento, Calif.

According to UCCE Colusa County, weedy red rice is a generic term used to describe any number of wild-rice types that have weedy characteristics and red bran. The close relationship to cultivated rice grown in California makes management difficult. The term ‘red rice’ refers to the red bran that covers red rice grain kernels. Red rice is problematic because of the weedy characteristics it exhibits.

“No effective herbicides exist for weedy red rice control in California since the weed is so closely related to cultivated rice,” Greer said.

A single weedy red rice plant left in a field can result in several hundred plants the next year, and thousands of plants the year after if not removed. The only way to effectively remove weedy red rice is by rouging (hand removing) plants prior to seed production for five or more consecutive years.

“Rouging is a long-term strategy to rid fields of weedy red rice,” Greer says. “If you miss one year then you’re back to where you started.”

The Weedy Red Rice Workgroup was formed in part to work on weedy red rice containment and eradication. The group includes the California Rice Commission, UC, County Ag Commissioners, California Rice Research Board, California Crop Improvement Association, and the California Rice Experiment Station. The group’s goals include maintaining weedy red rice-free fields and eradication in infested fields.

“Both avenues are dependent on proper identification,” Greer said. “That means getting out of the truck for an up-close examination of the plant.”

A good precaution to keep fields weedy red rice-free is only planting certified rice seed that has a zero tolerance for red rice. Greer shared an instance where a farmer with one weedy red rice plant in a field saved his own seed. The seed was planted the next season and the weedy red rice spread over a much larger area. Minimizing movement of equipment and harvesters can also reduce the spread.

Greer said other weeds that growers should keep on the radar screen include ducksalad, Heteranthera limosa, an annual plant immersed in water. While the white-flowered ducksalad was found in California fields in the 1990s, a more aggressive purple variety, Hederanthera rotunda folia, is now displacing most white-flowered ducksalad. Like weedy red rice, ducksalad competes with rice for space and nutrients.

Greer says rice cut grass, a California native, is traditionally found in wetland areas including the edges of rice fields. Rice cut grass is now creeping into the fields.

California rice fields are traditionally kept under water during the growing season. The crop is seeded by airplane. Fields are drained several weeks before harvest. Greer calls water “the original herbicide for grassy weeds in rice.”

Flooding was instituted to combat barnyardgrass and some widespread red rice populations in the 1930s throughout California.

Rice production practices are improving thanks to technology. Laser field leveling has allowed the transition from contoured field levees to straight levees which provide improved water and weed management. More draining is occurring after air seeding to improve root development and seedling growth.

Herbicide resistance in rice weeds has increased over the last 15 to 20 years, Greer says. New contact herbicides require foliar application with a ground rig which requires improved precision, timing, and scouting. Increasing production costs have led to more drill or dry seeding.

Herbicide resistance is also the result of few herbicide modes of action (MOA) on the market. “Growers must use the same types of MOA herbicides year after year,” Greer says. “Another issue is some growers find a herbicide that works well and then use the same product year after year which can result in weed resistance. They should be rotating the MOAs.”

This has led to additional and larger flushes of weed populations.

The California rice industry needs to be more creative in dealing with conventional and herbicide-resistant weed populations, Greer said.

“We must be more vigilant in keeping tabs on new problems and develop management strategies before problems become a crisis.”

Albert Fischer, UC Davis associate professor of weed ecophysiology, is researching herbicide-resistant weeds using stale seedbed methods, pre-irrigating to germinate weeds, and then spraying with glyphosate to eliminate a large portion of the seed bank prior to planting.


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