Weedy Red Rice
Control, 2017


Project Leader

Whitney Brim-DeForest, farm advisor, UC Cooperative Extension Sutter, Yuba, Placer, and Sacramento counties

Since the beginning of the 2016 rice growing season, the UC Cooperative Extension rice team has been working with growers, pest control advisers, and county agricultural commissioners to identify weedy red rice infestations. By the end of the season, five distinct populations had been identified on more than 10,000 acres in every rice growing area except Sacramento County.

The team conducted an informal, anonymous survey of 160 growers and pest control advisers during five 2017 winter meetings in Butte, Glenn, Colusa, and Sutter counties. About 40% of the respondents suspected that a field had been infested, while 57% indicated that they had seen weedy red rice but did not report the infestation. The survey results indicate that the number of infested fields is likely higher than reported in 2016.

Surveys and submissions continued through December 2017. From 53 samples submitted for testing, 22 were confirmed weedy red rice while 31 were not weedy red rice.

The California Crop Improvement Association found eight seed fields infested with weedy red rice—all were rejected for seed. Three of the fields were new medium grain seed fields that had never been in the seed certification program before. One field was a previously certified medium grain seed field. Four fields were specialty variety seed fields newly submitted to the quality assurance program.

Soil sampling and nine grower interviews were conducted in the fall of 2016. The number of seeds per soil volume varied considerably between fields, as did the percentage of soil cores containing seeds. In general, the longer the length of time that the field was infested, the more weedy red rice seeds were found in the soil. Growers who were taking steps to reduce the infestation tended to have lower seed counts. Once data from 2017 has been analyzed, the rice team will advise growers on best practices to reduce weedy red rice seeds in the soil bank.

Genetic study

Genotyping of 2016 weedy red rice samples and those collected previously is ongoing. The goal is to create a genome-wide survey of the DNA profile for each weedy red rice population or “ecotype.” Plants have been grown in a UC Davis greenhouse and then characterized for morphological and physiological attributes to assist in early differentiation between weedy red rice and cultivated rice varieties. Seed shattering and dormancy also were analyzed.

Weedy red rice characteristics and control

California weedy red rice collections were evaluated on the basis of 14 morphological traits. Significant differences were observed between rice varieties and the five ecotypes—pubescence, hull, bran auricles, node colors, and plant height. Significant differences were also noted in physiological traits—chlorophyll content, heading date, seed shattering, and dormancy. All ecotypes have red pericarp and pubescent leaves that are pale green, an indication of lower chlorophyll content. All are taller than cultivated rice. Flowering time also is varied.

Weedy red rice types exhibit high degrees of shattering and dormancy. The variation in the physiological characteristics of these weedy red rice ecotypes has significant implications for management. If allowed to shatter onto the soil surface and tilled under, the seeds can remain viable for many years. Studies in the South have shown that may be as long as 10 years. Further research on dormancy and shattering is planned.

To expand current knowledge of weedy red rice ecology, a greenhouse study was begun in summer 2017 focusing on competition with M-206. Herbicide susceptibility experiments also were begun in 2017. There was no effect of any of the pre-emergent herbicides on any of the weedy red rice ecotypes. In post-emergent granular herbicide testing, Bolero® had some effect on four of the five ecotypes. In post-emergent foliar herbicide testing, Goal®2XL (oxyfluorfen) provided complete control of four of the five ecotypes. In spot-spraying foliar herbicide testing, clethodim, paraquat, and glufosinate provided complete control of all weedy red rice types.

Initial research on weedy red rice provides a strong foundation from which to continue research and extension work on this important pest of California rice.


From this study, at least five genetically distinct ecotypes of weedy rice have been identified. Four of these ecotypes are related to weedy red rice from the South. These findings support earlier reports of weedy red rice introduction to California by contamination of imported seeds from the South. The introduced weedy red rice likely became adapted to California and became genetically distinct from Southern weedy red rice. The phenotypic variability of the fifth ecotype suggests possible outcrosses or natural hybridization of existing weedy red rice with cultivated California varieties.

Overall, the diversity of California weedy red rice ecotypes reflects multiple possible origins, thus indicating the complexity of control measures to implement. The movement and sharing of seeds between growers, between counties, and rice seed imports into California should be monitored closely to prevent the spread of weedy red rice. Strict implementation of “certified seed only” usage should be practiced to avoid further hybridization of weedy red rice with cultivated California varieties.