Weedy rice control remains elusive for California growers, but researchers continue to search for potential herbicides.
By Vicky Boyd
Whitney Brim-DeForest displayed the five different biotypes of weedy rice found in California at the Rice Experiment Station’s annual Rice Field Day.
The California rice industry continues work to contain small, spotty infestations of red rice before the pesky weed can become endemic as it has in the South.
At the same time, researchers are working to identify traits that might aid management as well as genetic fingerprinting that might shed light on the weed’s origins, says Whitney Brim-DeForest, a University of California Cooperative Extension farm adviser for Yuba and Sutter counties. She provided research updates at a recent UCCE winter grower meeting in Colusa.
The battle against weedy rice in the state dates back to at least 2006, when farm advisers identified four infested fields in two counties. Between 2008 and 2015, several suspect samples were submitted to the Rice Experiment Station for testing.
In 2016, the number of infested fields exploded, totaling 60 and covering about 10,000 acres. Among the positives were eight seed fields, she says. All were rejected because of the presence of weedy rice.
Laboratory technicians continue to process samples from the 2017 growing season, and the number of infested acres is likely to change, Brim-DeForest says.
Based on non-scientific electronic polls of those attending UCCE’s 2016 winter grower meetings, 49 percent said they saw weedy rice before 2016.
In addition, 57 percent said they hadn’t reported a suspected infestation to UCCE, and 40 percent said they suspected they might have weedy rice in a field.
Seed survival in the soil
Researchers also have identified five different biotypes that have different visual identifying characteristics. But all of them have red bran, are lighter green in color and are taller than the medium-grain Calrose varieties grown in California.
During the 2016 fall, researchers sampled 10 infested fields to determine weedy rice seedbank populations, since all of the biotypes shatter easily. They collected 34 soil cores every 20 feet along a transect.
In a Sutter County field with biotype 1, 6 percent of the samples contained weedy rice seeds, Brim-DeForest says. Based on 2.3 seeds per square foot, that equates to 9,500 seeds per acre.
In another Sutter County field with biotype 5, 42 percent of the samples had weed rice seeds. That’s 39.5 seeds per square foot or 165,000 seeds per acre.
But these numbers don’t tell the entire picture. Biotypes 2 and 5 have low dormancy, meaning a large portion of the seeds will germinate once growers flush the field. They can then apply a burndown herbicide or use tillage to kill the plants.
Biotypes 1, 3 and 4, on the other hand, have high dormancy. Even with flushing, Brim-DeForest says only about 5 percent of the seeds will germinate, and seeds may remain viable in the soil for at least 10 years.
In search of a control
Researchers continue to look for herbicides that will kill the weedy rice without damaging the cultivated rice. Although growers in the South have the Clearfield and the newly launched Provisia systems to manage weedy rice, they are not registered for use in California.
As a result, researchers in California looked at treatments of already registered herbicides one day and 10 days after planting to see if they had an effect on weedy rice. None provided control, Brim-DeForest says.
GoalTender herbicide, part of the experimental ROXY herbicide rice system, applied to two-leaf rice controlled biotypes 1-4 but was ineffective against biotype 5, she says. Oxyfluorfen, the active ingredient in GoalTender, is not registered for use on rice.
The researchers also examined spot spraying five other herbicides alone and with crop oil concentrate at rice tillering. Of those, paraquat, glufosinate and clethodim controlled all biotypes.
“Currently, none of these herbicides are registered (for use on rice), but there are several spot-spray herbicides that the California Rice Commission will be pursuing,” Brim-DeForest says.