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Continue to keep eyes peeled for weedy rice in California fields.

• By Vicky Boyd,
Editor •

weedy rice
Photo courtesy University of California Cooperative Extension

The amount of California rice ground infested with weedy rice hovers around 14,000 acres, leading experts to believe that pest control advisers and growers are carefully scouting for the pest. And they hope the diligence continues.

“We had three new sites (in 2019), but the number of people who called us was way higher — maybe 20,” says Dr. Whitney Brim-DeForest, University of California Cooperative Extension rice and wild rice adviser for Yuba and Sutter counties. “We had a lot of submissions for testing, but most of them turned out to not be weedy rice, so I know they’re definitely looking for it. What we’ve told them is if they see anything that looks suspicious, they should give us a call.”

In one field she visited, the PCA had found only a few plants that weren’t even visible from the field edge.

“So folks are really, really looking for it,” Brim-DeForest says. “I’ve been surprised at how they found that weedy rice because I wouldn’t have been able to.”

To help answer questions about potential yield losses tied to weedy rice, seed longevity in the soil and possible controls, she and UC colleagues have embarked on field and greenhouse studies.

A new biotype

Although infested acres have remained relatively stable over the past few years, UC researchers did confirm three new infested sites and one new biotype or strain, dubbed Biotype 7, during 2019.

march 2020 cover
Dr. Whitney Brim-DeForest, a University of California Cooperative Extension rice adviser, is conducting trials examining weed control and differences in biotypes — photo by Vicky Boyd

Type 7 is straw hulled; has long, reddish awns; is about the same height as most other weedy rice types; and has no color on the nodes. For descriptions and images of all the biotypes, visit

That brings the total weedy rice biotypes to seven in California, Brim-DeForest says. The finding was based on lab testing of seed samples sent by growers and PCAs suspecting weedy rice.

Biotype 7, along with another biotype, was discovered in a field previously infested with two other biotypes. In addition, weedy rice was confirmed on three new sites in San Joaquin, Sutter and Placer counties during 2019.

“The more we have weedy rice in the field and the longer we have it in the field, we’ll find more weedy rice types crossing between weedy rice types and with our varieties,” she told growers at a recent UCCE winter rice meeting. “That’s why we’re encouraging people to get rid of weedy rice if they have it or report it if they just found it.”

Brim-DeForest bases her concerns on genetic fingerprint studies of the different biotypes. Type 1, for example, has genetic material from Type 5, meaning the two types have hybridized.

Having more biotypes also may mean having additional visual characteristics to watch for when scouting fields.

Identifying the biotype or biotypes in an infested field is important because seed dormancy varies. Types 1, 3 and 4 have high dormancy, whereas Types 2 and 5 have low dormancy. Biotypes with low dormancy tend to germinate in large numbers over a short time period. These biotypes lend themselves to management that involves a stale seedbed, flushing and applying a broad-spectrum herbicide to the young seedlings.

Seeds from types with high dormancy, on the other hand, may remain dormant in the soil for several years before germinating, complicating management and lengthening eradication efforts.

Cause for concern

hand roguing weedy rice
Hand roguing removes the weedy rice plants before they can go to seed — photo by Jim Cook, Colusa County Farm Supply

One of the main concerns with weedy rice is it competes for resources, such as sunlight, space and nutrients, with commercial rice cultivars.

Based on preliminary research results, Brim-DeForest says it appears several of the biotypes are aggressive competitors, with two producing over twice the number of panicles and tillers than the widely grown commercial medium-grain variety, M-206.

In addition, three weedy rice biotypes produced significantly greater fresh biomass than M-206, and two biotypes produced a dried root biomass more than twice that of M-206.

As few as eight weedy rice plants per meter squared (roughly a yard squared) can reduce rice yields by up to 42%, according to preliminary trial results.

Most of the weedy rice biotypes mature earlier than commercial rice, with their heads easily shattering and leaving seed on the ground. In addition, many of the biotypes have black or red kernels that can discolor white rice during milling.

To add to the challenge, California producers are handicapped in their weedy rice battle because there are no Clearfield or Provisia cultivars bred for the state. The herbicide partners for those systems also are not registered for use in California.

Effects on water-seeded production

Much of the data surrounding weedy rice is based on research conducted in the Mid-South.

To determine how the biotypes found in California affect water-seeded production systems used in the state as well as find possible controls, Brim-DeForest began a number of research projects in 2018. They are being conducted in a secure location on the UC Davis campus away from commercial rice fields.

weedy rice seed trials
UCCE rice adviser Whitney Brim-DeForest is conducting trials to determine seed dormancy and overwintering potential of different weedy rice biotypes using plastic wading pools — photo courtesy Whitney Brim-DeForest, UCCE

One series of trials is examining three cultural systems to determine whether they help reduce weedy rice populations. They are continuous rice under flood, a rice-sorghum rotation and a stale seedbed. The stale seedbed treatment involves two flushes followed by a glyphosate application before flooding.

In 2018, Brim-DeForest established weedy rice plots using seed harvested from biotypes 1, 2, 3 and 5, since she didn’t have enough seed to plant biotypes 4 and 6. Type 7 hadn’t been discovered yet.

In 2019, she began the treatments planting the short-maturity medium-grain, M-105, at 150 pounds per acre. Brim-DeForest plans to continue the trials this season but offers a few observations.

“We had weedy rice come up through the flood,” she says. “We used to say that flood suppresses weedy rice, but that’s not so. Flood alone is not useful to suppress weedy rice.”

Possible chemical controls

Brim-DeForest also looked at a handful of herbicides to determine their potential in controlling weedy rice. The California Department of Pesticide Regulation denied a Section 18 emergency-use exemption for clethodim used as a spot-spray material, saying it didn’t agree the situation was an emergency.

Glyphosate, which researchers had tested using rope wick application in the past, is not labeled for spot applications on weedy rice. The registrant is not interested in a Section 18.

The Environmental Protection Agency also eliminated tolerances for glufosinate on rice, nixing any chance of obtaining a Section 24(c) special local needs registration for that herbicide.

A quick field trial in Colusa County last season with an organic herbicide on Type 3 weedy rice found it burned the plants.

Because the material is not a systemic, several panicles came through afterward and looked fine, Brim-DeForest says.

Jim Cook, a researcher with Colusa County Farm Supply, conducted the trial last season, and he theorizes the application was made too late. This season, he plans to look at earlier timings.

“If you have a seed head forming, it’s not going to stop it,” he says.

To optimize efficacy, Cook says you need to make the application with sufficient water. Using a hand-pump backpack sprayer, he says he typically would spot spray weeds with glyphosate for 3-5 seconds. With the organic herbicide, Cook says you need to drench the weeds by spot spraying 5-10 seconds.

The spray tip also needs to be pointed down at a 90-degree angle so it shoots straight on the weedy rice clump.

“You need to get a good pattern with a 15-inch diameter on top of the rice canopy so you’re making sure you’re covering all of the foliage of the weedy rice,” Cook says.

The organic herbicide also needs to be paired with an organic surfactant, he says. The herbicide is not yet labeled for use on rice.

Brim-DeForest also is looking at a handful of other herbicides not registered for rice to determine whether they might have a fit for weedy rice management.

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