Aggressive watergrass challenges California growers and researchers who seek effective control programs.
• By Vicky Boyd,
A new weed in the watergrass genus continues to bedevil California rice producers and researchers as they try to identify it and develop herbicide programs to control it.
Kurt Richter, who farms with his family near Colusa, is among those who have faced the newcomer. He probably first saw a bit of the new grassy weed in one field in 2017 but didn’t think much of it.
“In 2018, it just exploded and took over a field,” Richter said. “We found a chemical program, while expensive, that worked.
Last year we didn’t have a single living watergrass plant in the field before harvest.”
Jim F. Cook, who heads research for Colusa County Farm Supply in Williams, also has been looking at control programs for the new weed with mixed results. He plans to conduct additional trials in 2021.
“That tillered barnyardgrass is a monster to control,” he said. “The other issue is the weed is so aggressive, it’s pushing out the rice stand.”
Whitney Brim-DeForest, University of California Cooperative Extension rice farm advisor for Yuba and Sutter counties, has narrowed the newcomer down to a member of the Echinochloa genus, which includes barnyardgrass, early watergrass and late watergrass. Whether it is a species not before seen in California or a new biotype of a resident species remains unknown.
Knowing exactly what they’re dealing with could help with developing control programs, she said.
The new biotype/species is small seeded and completely awned with large, bushy purple structures. Based on phenotypical characteristics, it differs from late watergrass, which has large seeds and no awns.
While early watergrass is awned, it has large seeds. Barnyardgrass, on the other hand, has small seeds that may or may not be awned.
“Barnyardgrass can be very plastic in appearance,” Brim-DeForest said. “Barnyardgrass in the Midwest looks different than what we have here even though they are the same species.”
Genetic testing to determine whether the weed is a new biotype or a species related to other Echinochloa worldwide is the next step, but she said she needs additional funding before proceeding.
First reports in 2017
Brim-DeForest said she first became aware of the newcomer in 2017. A pest control advisor had contacted her about a grassy weed he had sprayed with several herbicides, resulting in poor control. She already was aware of late watergrass populations that were resistant to multiple herbicides, but this was not late watergrass.
Brim-DeForest ruled out possible causes for herbicide failure, such as a water level drop or incorrect application methods.
During 2018, she received a few more calls about the troublesome weed, prompting her and Butte County farm advisor Luis Espino to start collecting samples.
“In 2019, we kept getting other calls and realized this was more serious,” Brim-DeForest said. “Now it’s looking like it’s pretty widespread.”
With funding from the California Rice Research Board, she and Espino issued a request in August 2020 for samples suspected of being the new Echinochloa. They received about 60 that Brim-DeForest is still trying to sort through and work to identify.
During 2018, Brim-DeForest conducted a greenhouse trial in Davis that compared the efficacy of different herbicides on eight samples of the new biotype and two samples of late watergrass. The project also was funded by the Rice Research Board.
The goal was to look at representative species in rice fields to determine whether there were similarities or differences in herbicide response.
Treatments included field rates of Cerano (clomazone), Butte (benzobicyclon + halosulfuron), Granite GR (penoxsulam) and Bolero (thiobencarb) used as early season granular applications.
Field rates of SuperWham (propanil), Regiment (bispyribac-sodium) and Clincher (cyhalofop) were tested as late-season clean-up applications. In the greenhouse, all applications were made at the 1.5 leaf stage of the grass.
None of the eight samples of the new biotype were controlled by Granite GR or Butte, seven of eight were not control by Bolero and six of eight were not controlled by Cerano.
“It pretty much matches up with what I was hearing in the field,” she said. “Whatever they put on up front wasn’t really working.”
The biotype had escaped early season control in the field and was then difficult if not impossible to control with later-season herbicide applications, Brim-DeForest said.
In the greenhouse tests, three products — Super Wham, Regiment and Clincher — each provided at least 60% control of all eight samples. But her results don’t necessarily match up with what growers have seen in the field.
Since the greenhouse application was made at an early timing — 1.5-leaf stage — she said it’s possible that later applications in the field may be less effective. She plans further herbicide screenings this season.
Based on preliminary greenhouse results, she said growers may need to prioritize control of the new biotype/species early in the season. Although Brim-DeForest provided a number of possible treatments, she emphasized they had not been tested in the field and could be phytotoxic to rice plants.
Among those are a stale seedbed using a non-selective herbicide such as glyphosate. The field is prepared and flushed to cause weed-seed germination. A non-selective herbicide then is used as a burndown.
This is program is similar to one that growers have used to control weedy rice. One large drawback is it may delay planting.
Other possible treatments include preplant 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.
Even so, Brim-DeForest said there’s a good chance that growers will have to make a follow-up application later in the season.
Although a program with two sequential propanil applications has been widely used by growers, she said the industry needs to be very careful because the practice can select for and accelerate herbicide resistance.
And much like fields with weedy rice, she recommended growers harvest fields infested with the new grassy weed last so they don’t spread weed seeds to other fields.
Trial and error
On his farm, Richter had one 70-acre field divided into three checks that had varying degrees of infestation. The middle check was completely infested from corner to corner. The upper check had little to none of the new biotype, whereas the bottom check was about 50% infested.
Initially they tried Regiment, which is labeled for early season control of barnyardgrass, with poor results. In fact, all ALS herbicides proved useless against the newcomer, Richter said.
But he said they found an early application of SuperWham, followed five days later by a second application, provided excellent control of all grasses. The first application weakened the grass, and the second one “smoked them,” Richter said. “It picked up everything. It was really a clean field.”
He said they will probably use the same program in the field in 2021 to pick up any seeds in the seed bank that may germinate.
Watergrass a major focus
CCFS’ Cook also is in the hunt for a control program and in 2020 conducted a small-plot field trial in a Colusa County rice field infested with the new biotype/species. He plans to continue his work this season, adding in different application timings.
Treatment 1 consisted of a tankmix of standard post-emergence herbicides plus multiple adjuvants. Although the treatment gave the best performance — about 60% control — Cook said it needed to be simplified to be commercially viable.
Treatment 2 was liquid benzobicyclon and methylated seed oil, or MSO. Treatment 3 was a tankmix of Loyant and RebelEx with crop oil concentrate. Loyant and liquid benzobicyclon are not registered for use in California rice.
As these treatments showed promise on watergrass in other areas, he believed the applications were too late in the study to achieve commercial performance. He plans to look at different timings this season.
“Watergrass is going to be a major focus for us,” he said, adding he also planned to test some experimental products against the new biotype/species this season. “We’ll try an earlier timing before this thing really gets revved up and maybe a later timing after it heads to see if we can get better performance. When these things get into the very aggressively growing tillering phase, it’s very hard for them to be controlled because they’re growing too fast.”