Emerging Weed Issues in Rice, 2022


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

Over the past several years, there have been several new weed species identified. In 2017, at least 2 fields were identified with an unknown watergrass biotype. After extensive attempts at identification, we were unable to conclusively identify the species. Control methods have become the focus in the meantime.

The overall goal of this research is to continue to address emerging weed issues promptly. In 2019, we collected a survey of soil samples from across the rice-growing region, to establish a baseline of the weed species and biotypes present across the California rice region. In 2020 we finished the baseline survey, concluded the herbicide screening for the 2018 watergrass samples, and conducted a larger watergrass survey. In 2021, we conducted the herbicide screening for the 2020 watergrass samples, and the phenotyping of the same samples.

In 2022, we reanalyzed data from the herbicide screenings, sent reports to all 64 growers and PCAs, made contacts and connections to further our watergrass identification and studies, and finished the website hosting the ArcGIS maps for the 2019 rice weed survey.

Watergrass Identification

Brim-DeForest has been reaching out to scientists worldwide for assistance with the identification of the unknown watergrass species. Researchers in Texas, Japan, Turkey and Colorado are helping.

Since there has been no success identifying the new Echinochloa species, the team is working to refine the list of obvious and measurable traits of the watergrass species. These key characteristics make an identification key possible. Once finalized, the key will be published through UCANR and made widely available to the rice industry.

Online Mapping

In the fall of 2019, a comprehensive survey took place by soil sampling from fields in each major rice-growing county. The samples were collected after the rice harvest, from October to November. Each field was sampled four times, randomly throughout the field. Samples were then placed in pots and grown out in the Department of Plant Biology greenhouses in Davis using different types of irrigation.

Each soil sample was marked simply for the presence or absence of known weed species. Any unidentifiable or unknown species were grown to flowering and were identified by Advisor Brim-De- Forest. Samples were averaged per field, and averages for all samples in each county are summarized and all maps have been loaded into ArcGIS. All of the data is available at: https://sites.google.com/ucdavis.edu/californiariceweedssurvey2019

Herbicide Testing

The majority of the watergrass samples of all species are resistant to all of the tested herbicides, with only SuperWham®/Stam® and Cerano® showing control of approximately 50% (or more) of the samples. Late watergrass is widely resistant to all of the herbicides tested, with only SuperWham®/Stam® showing some degree of control of roughly 50% of the samples. Surprisingly, 100% of samples tested were resistant to Bolero®, Butte®, Clincher®, Regiment®, and Granite GR®.

The new watergrass biotype is best controlled with Cerano® (50% of samples) or SuperWham®/Stam® (76% of samples). Barnyardgrass is best controlled by SuperWham®/Stam® (90% of samples), and Cerano® (45% of samples).

Although the new biotype shows widespread resistance, its impact on yields is likely explained by more than just herbicide resistance and is likely due to its competitive ability as well.