Herbicide Susceptibility Survey of Watergrass (Echinochloa spp.) in California Rice

From the CAPCA Adviser magazine :: June 2022

Whitney Brim-DeForest is a UCCE Rice and Wild Rice Advisor for Sutter, Yuba, Sacramento, and Placer Counties; Taiyu Guan is an Assistant Specialist in UCCE Sutter-Yuba Counties; Troy Clark is a Rice Junior Specialist for UCCE Butte County.


In California rice, herbicide resistance has been documented in Echinochloa spp. since the early 2000's. Recent reports of uncontrolled grasses, as well as possible new species or biotypes have precipitated renewed research on this genus. Sixty-four watergrass samples were collected from a survey conducted in 2020, with grower and PCA-submitted samples from across the Sacramento Valley, as well as samples collected from University of California and Rice Experiment Station fields. Those samples were representative of all the watergrass species/biotypes: late watergrass, junglerice, barnyardgrass, and the new biotype/species. This experiment was a follow-up to our 2018 screening of watergrass (CAPCA Adviser, 2021).

The overall objective of this study was to determine the distribution and status of resistance to currently-registered herbicides in these species (cyhalofop, propanil, bispyribac-sodium, penoxsulam, benzobicyclon+halosulfuron, clomazone, and thiobencarb).


In August and September of 2020, 64 watergrass samples were collected from rice fields across the rice-growing region of California (Figure 1). The samples were representative of the Echinochloa spp. present in California rice, but were likely resistant, as they were self-reported by growers and PCAs: late watergrass (Echinochloa phyllopogon), junglerice (E. colona), barnyardgrass (E. crus-galli), and a currently unknown new biotype which is being characterized in a complementary study (Table 1). The overall objective was to determine the distribution and status of resistance to currently-registered herbicides in these species (cyhalofop, propanil, bispyribac-sodium, penoxsulam, benzobicyclon+halosulfuron, clomazone, and thiobencarb). Two known susceptible controls of late watergrass (E. phyllopogon) were added to the screenings as controls.

Screenings took place at the Rice Experiment Station greenhouse in Biggs, CA, in the summer and fall of 2021. All formulations were tested at the 1.5 leaf stage of watergrass. Dormancy was broken for the watergrass by wet-chilling in the fridge for approximately two weeks before planting. Seeds were pre-germinated in the incubator. Pots were seeded and then thinned down to 4 plants per pot.

All foliar-applied formulations (cyhalofop, propanil, and bispyribac-sodium) were applied with the label-recommended surfactants. Applications for into-the-water herbicides (granular formulations of penoxsulam, benzobicyclon+halosulfuron, clomazone, and thiobencarb) were made onto the water surface of bins that were flooded to 10 cm above the soil surface of the pots (where the watergrass was planted). All liquid herbicide treatments were applied with a cabinet track sprayer with an 8001-EVS nozzle delivering 40 gallons of spray solution per acre (at a pressure of approximately 20 psi). A flood was applied at 10 cm above the soil surface 48 hrs after the foliar applications. All herbicides were applied at standard field rates for California rice, though not at the maximum label rate for all herbicides (Table 2).

At 14 days after treatment, the number of living plants per pot was counted, and fresh biomass was measured (per pot) by cutting plants at the soil surface and taking the weight (per pot). Dry biomass was measured after drying the fresh weight samples down to a constant weight. Samples were classified as resistant to an herbicide if the average percent (%) dry weight control was less than that of the susceptible controls.


Out of the barnyardgrass samples (31), 23 were resistant to cyhalofop (CY), 3 were resistant to propanil (PR), and 26 were resistant to bispyribac-sodium (BS). Out of the late watergrass samples (9), there were 9 CY-resistant, 5 PR-resistant, and 9 BS-resistant. For the new unknown biotype samples (22), there were 17 CY-resistant, 3 PR-resistant, and 20 BS-resistant. For the granular formulations, barnyardgrass (31 samples) had 27 that were thiobencarb resistant (TH), 24 that were benzobicyclon+halosulfuron resistant (BH), 17 that were clomazone resistant (CL), and 26 that were penoxsulam resistant (PE). Out of the late watergrass samples (9), 9 were TH-resistant, 9 were BH-resistant, 6 were CL-resistant, and 9 were PE-resistant. For the new unknown biotype samples (22), there were 20 TH-resistant, 18 BH-resistant, 11 CL-resistant, and 20 PE-resistant.

The majority of the samples of all species are resistant to all of the tested herbicides, with only propanil and clomazone showing control of approximately 50% (or more) of the samples (Tables 3 and 4). Late watergrass is widely resistant to all of the herbicides tested, with only propanil showing some degree of control in roughly 50% of the samples. Surprisingly, 100% of samples tested were resistant to thiobencarb, benzobicyclon+halosulfuron, cyhalofop, bispyribac-sodium, and penoxsulam.

The new biotype is best controlled with clomazone (50% of samples) or propanil (76% of samples), while a smaller proportion of samples were controlled by the other herbicides tested. Barnyardgrass is best controlled by propanil (90% of samples), and clomazone (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.


The implications of this study reflect anecdotal evidence relayed by growers. Echinochloa spp. are becoming increasingly difficult to manage using our currently registered herbicides. For growers, this means it is increasingly difficult to plan an effective program that both controls grasses and prevents further selection for resistance. Aside from rotations with the above-utilized herbicides, some other alternative management strategies include: deep water, utilizing a stale seedbed, and rotating to a dry-seeded or drill-seeded system.

Deep Water:

Maintaining a deep flood (of at least 4–6 inches) can suppress some grass emergence. Deeper water will provide more suppression. Deep water also improves herbicide efficacy for granular herbicide applications, and the deep water may also improve efficacy of pre-emergent herbicides. Keeping the water on the field as long as possible will improve control. Watergrass typically emerges in the first 30 days after water is put on the field, so longer flood duration is better.

Stale Seedbed:

A stale seedbed has been shown to provide good control of watergrass in heavily infested fields. To implement a stale seedbed, prepare the field as normal (in spring). The field can be tilled or untilled. If untilled, please keep in mind that watergrass seeds typically only emerge from the top 6 cm (3–4 inches) of soil.

Once the seedbed is prepared, flood the field until water is 3 to 4 inches deep, then turn off water and let it sink into the soil. This will increase watergrass germination. Roughly 1 to 2 weeks later, spray a nonselective herbicide (make sure the field is fully drained to ensure coverage). Tillage can also be utilized in place of an herbicide, but avoid deep tillage, as it will bring up additional grass seeds. Timing of the herbicide application or tillage will depend on temperature. Warmer temperatures cause faster emergence of grass. Two weeks should be more than enough time to bring up most of the grass population that will be germinable (able-to-germinate), regardless of temperature.

If not planting rice, this process (flushing/flooding, followed by tillage or herbicide application) can be repeated multiple times throughout the season. If planting rice, flood up the field after the application of the nonselective herbicide (follow label for instructions on flood timing).

Rotation to Drill- or Dry-Seeded System:

Drill-seeding or dry-seeding rice allows for the use of pendimethalin, which is a different mode of action from all other currently-registered rice herbicides. Depending on the actual product used, pendimethalin may be best used in a drill-seeded system, due to the possible injury to emerging rice plants. Or it can be used in a dry-seeded system, where seed is flown on instead of drilled. For more information on application methodology, refer to the product herbicide label.


Original source: CAPCA Adviser magazine :: June 2022


By Whitney Brim-DeForest
Author - County Director, Sutter and Yuba Counties and CE Rice and Wild Rice Advisor
By Taiyu Guan
Author - Assistant Specialist
By Troy Clark
Author - Rice Junior Specialist
By Gale Perez
Posted by - Program Representative