- Author: Whitney Brim-DeForest
- Author: Taiyu Guan
Watergrass (Echinochloa spp.) in California rice is the most competitive weed complex. Plants can emerge under both continuously flooded conditions and flushed conditions, causing huge yield losses (up to 100% in dry- or drill-seeded systems). Watergrass is one of the first weed groups in which herbicide resistance was found (in the early 2000s). It has developed multiple herbicide-resistance and the resistance is metabolic, meaning that plants can essentially “consume” the herbicide, breaking it down so it does not kill the plant.
Currently, there are 4 main watergrass species in the California rice system: barnyardgrass (E. crus-galli), early watergrass (E. oryzoides), late watergrass (E. phyllopogon), and coast cockspur (E. walteri). Coast cockspur is a new species to California rice. We first found coast cockspur in California rice fields in 2017. It is robust and large-stemmed and can reach heights of over 6 ft tall when uncontrolled. All species have some level of resistance, and resistant biotypes are found throughout the Sacramento Valley (all counties).
In the past few years, we have been having increasing issues controlling watergrass, and many of us have resorted to using a double application of propanil to control it. The issue with this is that we are already seeing propanil resistance, and this practice will select for grasses that are propanil-resistant, causing us to eventually lose the product.
The best means to prevent the selection for propanil resistance are:
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Rotating modes of action (not using propanil as a clean-up spray year after year)
- Using tank mixes as clean-up sprays (in combination with propanil)
We have been researching possible cleanup tank mix options for the last few years (alternatives to the double propanil spray), and will continue to do so in 2024, to provide growers and Pest Control Advisors with feasible watergrass control options.
Alternatives to the Double-Propanil Application (2022)In 2022, we conducted one trial in a sweet rice field in Yuba County. Treatments tested are listed in Table 1. Applications were made at tillering (approximately 35-40 days after seeding), at 20 gallons per acre spray volume. Weed control (%) and phytotoxicity data were collected on 7, 14, and 28 days after spray (DAS) (Tables 2 and 3).
Table 1. Treatments applied in 2022 field testing (applied at 35-40 days after seeding) for watergrass control.
Table 4. Rough rice yields (lb/A) in 2022 field testing.
![Screenshot 2024-06-09 at 6.46.02 PM Screenshot 2024-06-09 at 6.46.02 PM](http://ucanr.edu/blogs/riceblog/blogfiles/107232.png)
Alternatives to the Double-Propanil Application (2023)
Weed control (% control, watergrass only) and phytotoxicity (% Stunting, % Stand reduction, % Tip Burn) evaluations were made 7 Days After Application (DAA), 14 DAA, and 21 DAA. Fields were harvested in September 2023. Yields were lower than normal due to hand-harvesting as well as rice laying down flat in the water at harvest in a couple of the fields.
Results (2023)
Figure 1. Phytotoxicity (Tip Burn, Stunting, and Stand Loss) in 2023 field testing (7, 14, and 21 Days After Application) averaged across all 5 sites.
Figure 2. Percent watergrass control (%) (Treatments 2-12) compared to the untreated control (Treatment 1) in 2023 (21 Days After Application). Treatment 1 (Untreated) is the percent watergrass cover per plot, not the percent control. Averages are across 4 sites (Rice Experiment Station was not included due to low watergrass populations).
Figure 3. Rough rice yields (lbs/acre) for 2023 watergrass field testing averaged over the 5 locations.
Recommendations:
To effectively manage tough watergrass, growers should use integrated weed management where possible. This includes:
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Using combinations of chemicals (granular) and tank-mixes (foliar)
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Rotating chemistries at the beginning of the season
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Rotating clean-up herbicides
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Crop rotation or fallow
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Winter flooding to maximize seed predation and decomposition over the winter
Herbicide recommendations include (at the beginning of the season):
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Zembu® (pyraclonil) if other granular options are ineffective, to give other chemistries a break. Zembu® suppresses grass (does not control) but will help prevent the selection of resistance as it is a new mode of action for watergrass.
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Cerano® followed by Butte®, applied one week apart, which is effective even on tough grass.
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Implement a stale seedbed approach by applying glyphosate or Suppress® (capric/caprylic acid)
pre-plant as a rotational tool.
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Pendimethalin (Harbinger®, Prowl H2O®, and others) to rotate MOA (please see additional
recommendations about the use of pendimethalin in the other article in this newsletter).
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Abolish® + Regiment®
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Abolish® + SuperWham®/Stam®
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Regiment® followed by Superwham/Stam® (may cause injury on certain specialty varieties)
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SuperWham®/Stam® + Loyant®
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SuperWham®/Stam® + Shark H2O® (some phyto)
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Regiment® + Clincher®
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SuperWham®/Stam® + Clincher®