California Rice: Preliminary Herbicide Screening on Watergrass Species

New/Unknown watergrass biotype in the field. Photo: University of California

In 2017, I started getting reports of a watergrass biotype/species (Echinochloa spp.) that was difficult to control using our suite of herbicides registered in rice. At the time, I knew we had multiple herbicide-resistance in late watergrass (Echinochloa phyllopogon), so I initially just thought the resistant biotype was spreading, and had maybe gained resistance to additional herbicide modes of action.

However, once I started visiting fields, it quickly became apparent that this was not late watergrass (phenotypically-speaking). It also did not appear to be barnyardgrass (Echinochloa crus-galli), although the Echinochloa species are notoriously difficult to identify, and phenotypically quite variable in nature.

We came up with a preliminary set of characteristics to distinguish this unknown biotype or species (we are unsure if it is a distinct species) from the typical barnyardgrass and late watergrass found in California rice fields. All were characterized by their seed size and awns (Table 1).

Table 1. Preliminary distinguishing characteristics of watergrass species in California rice fields, based on seed characteristics. 
Preliminary Identification Seed Description
Late watergrass Large size, no awns
Early watergrass Large size, awned (all seeds)
Barnyardgrass Small size, variably awned (some seeds have awns, some do not)
New biotype/species (unknown) Small size, awned (all seeds)

In 2018, we collected 8 samples from the field, and used two late watergrass samples from known susceptible populations to use as controls. We conducted a screening in the greenhouse, to see if we could replicate what we were seeing in the field. Field rates of Cerano (clomazone), Butte (benzobicyclon+halosulfuron), Granite GR (penoxsulam), and Bolero (thiobencarb) were used as the early-season granular applications.

Field rates of SuperWham (propanil), Regiment (bispyribac-sodium) and Clincher (cyhalofop) were used to test for the late-season cleanup applications. In the greenhouse, all applications were made at the 1.5 leaf stage of the grass.

Results indicate that 8 of the 8 samples were not controlled (less than 50% by biomass, in comparison to the untreated controls) by Granite GR or Butte. 7 of the 8 samples were not controlled by Bolero®, and 6 of the 8 were not controlled by Cerano.

This closely follows what growers were stating had occurred in the field: the watergrass was escaping early-season control, and was then difficult or impossible to control with later-season herbicide applications. SuperWham, Regiment, and Clincher controlled 8 of 8 samples (at least 60% control).

However, since the greenhouse application was conducted at an early timing (1.5 leaf stage), it is possible that later applications in the field may be less effective.

For growers, the implications of this preliminary screening are that control of this new biotype/species will need to be prioritized early in the season.

Possible treatments (keep in mind that these have not been field-tested and could cause phytotoxicity) could be: a stale seedbed using a non-selective herbicide; pre-plant Abolish (thiobencarb) followed by Cerano or Butte or Granite GR; Cerano followed by Butte or Bolero or Granite GR; or Butte followed by Granite GR or Bolero.

There is still a strong likelihood that a follow-up application may still be required later in the season, even with these early-season applications.

In 2020, more than 60 watergrass samples were collected from all over the rice-growing region. We will continue working on identification and conduct further herbicide screening this year.

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