Identifying and Reporting Weedy Rice
What is weedy rice?
Weedy rice is the most important weed of rice worldwide. It is also known as red rice because the kernels' pericarp is red. Weedy rice infestations can result in yield and quality reduction.
Weedy rice belongs to the same genus and species as rice, but it is considered weedy because:
- Panicles shatter easily, resulting in seeds that go into the soil and are not picked up with harvest
- Seeds have dormancy, which allows them to stay in the soil for several years before germinating
Weedy rice plants compete with cultivated rice, reducing rice yield. When weedy rice densities are high, they will contaminate the harvested grain, reducing its quality.
Red rice kernels and non-colored kernels
Weedy Rice in California
In California, weedy rice was reported as present as early as 1917; it was most likely brought into the state with contaminated rice seed. During the first decades of the rice industry, weedy rice was an important problem in California. Growers were recommended to use seed free of weedy rice and to rogue plants by hand from fields.
After the 1950s, weedy rice importance as a pest started to wane. It is believed that the wide adoption of certified seed was the reason. Below is an excerpt from the book Rice in California published by BUCRA in 1979.
In 2003, weedy rice was identified is six fields in Glenn and Colusa counties. Affected growers worked diligently to eliminate weedy rice. In one case, a grower decided to turn the infested part of a basin into an air strip.
After the initial efforts to alert the industry about weedy rice, it seemed that the thread was contained. Only one type of weedy rice was found, what we know now as type 3.
Then in 2015, several more fields were identified infested with several different weedy rice types. Five types were identified. For most fields, it is unclear how the infestation began, but in several cases, it was clear that using saved seed was the cause of the infestation.
By 2018, the acreage of all fields infested with weedy rice was estimated to be 14,000. However, most fields did not have uniform infestations and only a few basins per field had weedy rice plants in them. A more detailed survey conducted in 2020 and 2021 showed that, when considering only infested basins, 3,500 acres of rice were infested with weedy rice.
To prevent the spread of weedy rice, in 2017 the rice industry decided to develop an inspection system for all rice seed production, not only for certified seed. This lead to the institution of a Rice Seed Quality Assurance Program. Since 2019, the rice industry requires that all rice fields in California be planted with certified seed or seed approved by the Seed Quality Assurance Program.
Weedy Rice Identification
Weedy rice plants:
- Tend to be taller than cultivated rice.
- Are usually lighter green than cultivated rice.
- To distinguish weedy rice from water grass and other grassy weeds, look for a ligule and auricule.
- Weedy rice leafs feel rough when you run your fingers from the tip to the base of the plant.
- Some older and specialty rice varieties can be taller and lighter green than cultivated rice, and may have rough leaves. Knowing what varieties are planted in neighboring fields and the history of the field may help in deciding if the suspect plant is weedy.
Identifying Weedy Rice in the field
Later in the season, during heading, weedy rice panicles usually look different than the cultivated variety. In California, six weedy rice types have been identified. Type I is the most widespread, types II, III, and V have been found in multiple locations, and types IV and VI have only been found in a single location each.
Weedy Rice Types
Reporting Weedy Rice
Because weedy rice is the same genus and species as cultivated rice, it cannot be considered a noxious weed; therefore, there is no quarantine or other regulatory requirements on infested commercial rice fields. The exact location of any weedy rice suspect report or confirmed infestation is not shared; only County level data is shared in research and extension reports.
However, for rice seed growers, there is zero tolerance for weedy rice. That means, that if weedy rice is found on a rice seed field, it may not be approved for seed.
If you find a suspect plant, notify one of the UCCE Rice Farm Advisors. You can take a picture and e-mail or text it with the location. One of the Farm Advisors will inspect the suspect plant. In many cases, the Farm Advisors can rule out if the plant is weedy rice. If not, the plant may be taken to the greenhouse to be grown until heading when the grain can be tested for red pigmentation, shattering and dormancy.
Currently, there are no genetic tests that can be used to determine if a rice plant is weedy. There are some colored-bran varieties being grown in California, so just having colored pericarp does not classify a rice plant as weedy.
Why report weedy rice?
UCCE is tracking the spread of weedy rice in California. It is important to know where infested fields are located, so that the seriousness of this weed can be truly assessed if it continues to expand. Also, knowing the acreage affected will aid in efforts to obtain herbicide registrations.
Reporting contacts
Sacramento Valley:
- Whitney Brim-DeForest, 530-822-7515, wbrimdeforest@ucanr.edu
- Luis Espino, 530-635-6234, laespino@ucanr.edu
San Joanquin Valley:
- Michelle Leinfelder-Miles, 209-953, 6100, mmleinfeldermiles@ucanr.edu
