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:

• 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. 

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.

 

Results (2022)

 

Treatments 6 (SuperWham® + Loyant®) and 7 (SuperWham® + Shark) show great control on watergrass. By the 28 DAS, treatment 6 controlled 87.5% and 7 controlled 83.3% Echinochloa spp. (Table 2). Treatment 5 (Regiment® followed by SuperWham®) caused significant stunting compared to other treatments. Treatments 3 and 5 had the lowest yields (Table 4).

 

Table 2. Percent watergrass control (%) (Treatments 2-8) compared to the untreated control in 2022 (7, 14, and 28 Days After Application). Treatment 1 (Untreated) is the percent watergrass cover per plot.

Table 3. Phytotoxicity (stunting) in 2022 field testing (7, 14, and 28 Days After Application).

Table 4. Rough rice yields (lb/A) in 2022 field testing.

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.

Table 5. Treatments applied in 2023 field testing (applied at 35-40 days after seeding) for watergrass control.

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:

• Using combinations of chemicals (granular) and tank-mixes (foliar)

• Rotating chemistries at the beginning of the season

• Rotating clean-up herbicides

• Crop rotation or fallow

• Winter flooding to maximize seed predation and decomposition over the winter

Herbicide recommendations include (at the beginning of the season):

• 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.

• Cerano® followed by Butte®, applied one week apart, which is effective even on tough grass.

• Implement a stale seedbed approach by applying glyphosate or Suppress® (capric/caprylic acid)

  pre-plant as a rotational tool.

• 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). 

Foliar options (best grass control) (see above tables for rates and adjuvants used):

• Abolish® + Regiment®

• Abolish® + SuperWham®/Stam®

• Regiment® followed by Superwham/Stam® (may cause injury on certain specialty varieties)

Foliar options (good grass control):

• SuperWham®/Stam® + Loyant®

• SuperWham®/Stam® + Shark H2O® (some phyto)

• Regiment® + Clincher®

• SuperWham®/Stam® + Clincher®

Not all of these treatments will work on all watergrass biotypes and fields. However, trying a new combination, even on one or two fields, will help growers and PCA's to evaluate the efficacy of these treatments and prevent selection for propanil resistance on your farm or ranch.

 

Continued Testing Planned for 2024:

 

In 2024, UC Advisors will continue testing herbicides for watergrass control across the valley, with new tank mix combinations.

 

Note: Remember to always check the label and with the local Agricultural Commissioner's office for relevant restrictions and permit conditions. The label is the law.

 

*Disclaimer: UC does not endorse any particular brand/product but uses brand names for audience familiarity with our research and content. Stam 80DF and SuperWham (a.i. propanil), Abolish (a.i. thiobencarb), Shark H2O (a.i. carfentrazone-ethyl), Loyant (a.i. florpyrauxifen-benzyl), Clincher (a.i. cyhalofop- butyl), Regiment (a.i. bispyribac-sodium), Sandea (a.i. halosulfuron-methyl), Cerano (a.i. clomazone), Suppress (a.i. capric/caprylic acid), Zembu (a.i. pyraclonil).