Weedy Red Rice
Control in Rice, 2022


Whitney Brim-DeForest, UCCE farm advisor, Sutter, Yuba, Placer, and Sacramento counties

The overall goal of this research project is to continue to provide updated information on the spread, management, and identification of weedy rice in California.

Survey and Grower Interviews

Since the beginning of the 2016 season, the UCCE Rice Team has been working together with growers, PCA’s, and County Agricultural Commissioner’s offices to identify weedy rice infestations. Out of California’s approximately 500,000 rice acres, weedy rice infestations have been found on only 0.4% of the acreage. See the weedy rice-specific website, www.caweedyrice.com for pictures.

In 2022, we had 6 samples submitted, none of which were weedy. We encouraged many growers to take out fields that had been previously infested with weedy rice, as a control measure.

In Season Field Trials

The project looked at fallow systems to manage weedy rice. Since many growers are using fallow or managed fallow to control weedy rice, this may yield more applicable and useful data for California growers than crop rotation. The treatments were as follows:

• Treatment 1: Fallow (no irrigation)
• Treatment 2: Managed fallow (1 irrigation flush, followed by spray of a non-selective herbicide)
• Treatment 3: Managed fallow (2 irrigation flushes, each followed by spray of a non-selective herbicide)

In the fallow treatment, no weeds of rice emerged. Under the managed fallow treatments (2x flush and 1x flush), no weeds of rice emerged except for weedy rice. The field does have a known population of barnyardgrass, but it did not emerge. Weedy rice emerged under the two managed fallow treatments. In the sprayed plots, most of the weedy rice was cleaned up in both fields. At a timing near heading (10/6/21-10/15/21), no weedy rice plants survived to produce seed. It indicates that the use of the managed fallow can effectively reduce weed seeds in the soil, without contributing weed seeds bank into the seedbank for the following season.

The treatment that was most effective at longterm control throughout the first season was Treatment 3 (2 flushes and 2 sprays). In 2022, no weedy rice emerged, and percent weed cover for non-rice weeds was much higher on average than in 2021.

Overwintering Experiment

Researchers are seeking to know what happens to the Weedy Rice seeds over the winter. Do they survive or do they die? To find out, Weedy Rice seeds of the four major biotypes are placed in mesh bags, then buried outdoors over the winter months. Each of the Weedy Rice types has a different best practice for control, so know what you have before you act.

One of the most interesting data points was the pre-emergence of weedy rice biotype 5, before removal from the soil in the control treatments. Because biotype 5 has little to no dormancy, this may indicate a non-flooded winter field may be the best treatment option, as opposed to flooding. In this scenario, since growers are doing field preparation in April/May using tillage, they would likely till under all of the pregerminated weedy rice biotype 5. However, it is the only biotype that pre-germinated in large numbers, so it would be the only biotype for which this would be an effective control method.

It appears that shallowly buried seeds (seeds near the soil surface), had greater mortality than seeds that were buried. This applies across flooded and non-flooded treatments as well as across biotypes 1,2,3, and 5. Biotype 3 had the lowest mortality rate, which means that most of the seed was either viable or dormant. This correlates well with what we have seen of Biotype 3’s persistence in the field (some fields have been infested for 10 or more years). For Biotype 1 and Biotype 5, non-flooded (ambient) conditions proved to be most effective at causing mortality, whereas, for the other types (Biotype 2 and 3), there were no large differences between the flooded and ambient treatments.

New Biotypes

In 2018, we found Type 6, and in 2019, we found Type 7. In 2021, we found at minimum 3-4 other possible types (at least one of which appears to NOT have a red pericarp). In 2022, we tested 17 suspicious samples. Most samples were from one location only, meaning that they are likely lowacreage infestations at this time. All were tested against known weedy rice samples, California japonica varieties, and some specialty varieties. At this point, it appears that we have several likely new biotypes (8 out of the 17 tested), two of which are white-pericarped. In the next year, once we have completed the third replication, we will begin to disseminate information to growers, PCAs and other stakeholders regarding the new types, their identification, and distribution.