Weedy Rice Control, 2019


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

Weedy red rice, or simply weedy rice, was rediscovered in California in 2016. Since then, UC Cooperative Extension advisors and specialists implemented a multifaceted research program covering genetics, phenotyping, surveying and sampling, as well as in-the-field management advice. Objectives of 2019 research were:

  • Assess the current distribution of California weedy rice by conducting a survey, along with grower interviews.
  • Expand knowledge of weedy rice ecology, including seedbank longevity, burial in the soil, and germination and emergence patterns.
  • Disseminate pertinent results and best management practices to rice growers and other stakeholders.
  • The UC rice team has been working with growers, pest control advisers, and county agricultural commissioners to identify weedy rice infestations. By the end of the 2016 season, five distinct populations covering more than 10,000 acres throughout the rice growing areas were documented. Infested acreage increased in 2018 to 13,866 acres. The number of new fields with weedy rice was relatively low in 2019.

    Acreage by weedy rice type is important to note. Type 1 is the most widespread, and it is most similar to Calrose medium grain varieties, making it more difficult to identify and allowing it to spread more easily.

    In 2017, 53 samples were submitted for testing, and 15 were confirmed to be weedy rice. The California Crop Improvement Association found eight seed fields with weedy rice—all were rejected for certification. In 2018, 25 samples were submitted. Five were confirmed weedy rice, and one new biotype was identified, Type 6. It is described as black-hulled, awned with red color, and tall. It was found at only one location in Butte County.

    In 2019, nine samples were submitted. Five were weedy rice, mostly Type 1, but a new population, Type 7, was identified in Butte County. It is described as straw-hulled, long reddish awns, no color on the stem nodes, and a height similar to the other biotypes.

    Nine grower fields with large weedy rice infestations were selected for soil sampling. Samples were taken after harvest in 2016, 2017, and 2018. The number of weedy rice seeds was averaged across soil samples per field, and the percentage of samples containing seeds was calculated. The number of seeds 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 rice seeds were found in the soil. Growers who were taking steps to reduce the infestation tended to have lower seed counts, as well.

    2018 weedy rice acreage by county, and total acreage across all nine major rice growing counties
    County 2018 Acreage
    Butte 3,365
    Colusa 795
    Glenn 926
    Placer 873
    Sacramento 354
    San Joaquin 659
    Sutter 2,688
    Yolo 1,836
    Yuba 2,370
    Total 13,866

    Over all locations, the number of seeds and percentage of soil samples containing seed increased from 2016 to 2018. One field in Butte County and one field in Sutter County showed decreases over the three years of sampling. Both fields were fallowed. However, it is difficult at this point to determine whether fallowing reduces the weed seedbank. Follow-up sampling of these fields will be repeated in five years where possible to determine whether there has been any long-term change.

    A greenhouse experiment was conducted to determine the relative competitive ability of the weedy rice biotypes against M-206. In terms of early growth, height and tiller count, weedy rice biotypes and M-206 did not vary much initially. By week 12, however, all the biotypes had significantly more height and tillers than M-206. There were some differences between biotypes. Type 2 was the tallest. Types 1, 3, and 4 had the most tillers, significantly more than M-206.

    In terms of competitive effects of weedy rice on M-206, all weedy biotypes had a significant effect on final plant height, tiller number, yield per panicle, panicle number, panicle weight, plant yield, fresh biomass, and dry biomass. However, there were no differences between biotypes and their effect on M-206 plant height reduction, panicle reduction, panicle weight loss, plant yield loss, fresh biomass, and dry biomass. There were differences between biotypes and their effect on M-206 tiller number and yield loss per panicle. Type 4, the smallest of the biotypes, had the least effect on plant height and biomass.

    Work also continued on an in-season field trial at UC Davis comparing conventional flooded rice, stale seedbed (flushed) management and crop rotation (to be seeded with sorghum in 2020). There was a slight difference in weedy rice emergence between the conventional and stale seedbed. Also noted were differences between the number of plants that emerged in the two treatments. In the flushed field, Type 3 had as many as five plants per square foot, compared to approximately two plants per square foot in the flooded field. Similar patterns were found with other biotypes. This reinforces data that show weedy rice cannot emerge from deep in the soil under flooded conditions.

    Over-wintering research is taking place outdoors at a site in Colusa County. The first year of the experiment began in the fall of 2019 and will finish in spring 2020. Weedy rice types 1,2,3, and 5 were buried in mesh bags at two depths—near the soil surface and 6 inches deep. Bags were to be removed in December, January, and February. Removed seed will be tested for dormancy and viability. This experiment will be repeated after the next growing season.

    A number of extension publications were disseminated through several different channels, including an identification brochure and poster. A weedy rice-specific website has been created: www.caweedyrice.com. Two videos were published on YouTube with the assistance of the California Rice Commission. Other outreach efforts include an email listserv, a weedy red rice app, and blog posts. Research and information about best management practices also were communicated during winter grower meetings, the Rice Experiment Station’s annual field day, and weedy rice specific workshops.