Weed Management
in Rice, 2017


Project Leader

Kassim Al-Khatib, professor and UCCE specialist, Weed Science Program, Dept. of Plant Sciences, UC Davis

The weed control project seeks to assist California rice growers in developing economic and timely broad-spectrum weed control, the prevention and management of herbicide resistant weeds, application of new tools and technology to manage weeds in rice, and to comply with personal and environmental safety requirements.

Research in 2017 focused on the efficacy of new and existing herbicides, alternative crop establishment methods, and herbicide resistance strategies to deal with this problem in California rice fields.

Herbicide programs

Herbicide research was conducted in continuous flood, pinpoint flood, and drill-seeded rice at the Rice Experiment Station. Several herbicide combinations delivered near-perfect weed control with great crop safety. Following are highlights of this work.

Continuous flood systems promote the suppression of weeds such as barnyardgrass, watergrass, and sprangletop. Predominant weeds in the 2017 research plots included late watergrass, ducksalad, and ricefield bulrush, followed by barnyardgrass, monochoria, water hyssop, redstem, and sprangletop. All of these weeds are susceptible to herbicides registered for use in California rice.

Several granular into-the-water herbicides are available for controlling weeds in continuously flooded rice, including Bolero®, Butte®, Cerano®, Granite® GR, League® MVP, Shark® H20, and Strada®.  Butte® is a new addition to the California rice herbicide portfolio. These herbicides can be applied early to provide good to excellent control of target weeds. It is useful at times to combine herbicides in a program to expand the spectrum of weed control.

Optimizing Butte® programs

Newly available Butte® is a granular mixture of benzobicyclon and halosulfuron developed by Gowan® for use in water-seeded rice. Previous studies suggest that Butte® provides good broad-spectrum weed control. However, there is a need to examine its use with other herbicides to improve grass and broadleaf weed control.  Research on Butte® was conducted in several studies.

In the first study, Butte® was tested in a continuous flood system at two rates—both alone and in combination with other herbicides. Applied at day of seeding, Butte® afforded excellent control of sedges, ducksalad, monochoria, and sprangletop. Additionally, it provided good control (90%) of watergrass and barnyardgrass and fair control of redstem. Overall, Butte® alone provided a broad spectrum of weed control and offered an exceptional level of crop safety.

Butte® with other into-the-water herbicides such as Cerano® or Granite® GR provided greater control of grass species compared to Butte® alone. A follow-up application of Clincher® or Regiment® at 1-tiller stage improved barnyardgrass and watergrass control. A tank mix of Stam® and Grandstand® or Granite® SC following Butte® provided exceptional control of all weeds, including watergrass and redstem.

In the second study, a single rate and timing of Butte® was tested in a continuous flood system with a follow-up, into-the-water application of Granite® GR or a foliar application of Granite® SC (with and without an additional tank-mix application of Stam® and Grandstand®). Cerano® and Granite® SC at 1-tiller stage (with and without the tank-mix application of Stam® and Grandstand®) also was evaluated.

Butte® followed by all timings and/or formulations of Granite® provided excellent control of grasses, sedges, and broadleaf weeds without the need for an additional tank-mix application of Stam® and Grandstand®. Cerano® with Butte® was as good as Butte® with Granite® in controlling weeds. However, Cerano® or Granite® applications caused some rice injury early in the season, which mostly disappeared by 50 days after seeding. In conclusion, Butte® followed by Granite® herbicides offered a broad spectrum of weed control and excellent crop safety in continuously flooded rice.

In a third study, Butte®, with a follow-up, tank-mix application of Stam® and Grandstand®, or a tank-mix application of Stam® and Londax® were both very effective in controlling all weeds.

A fourth study compared the efficacy of weed control between Butte® and League® MVP applied at 2-leaf stage alone or followed by Regiment® applied at 1-tiller stage of rice. Butte® alone provided at least 80% control of late watergrass and barnyardgrass and excellent control of other weed species. League® MVP alone was better than Butte® alone in controlling late watergrass and barnyardgrass. However, the control of most other weeds except ricefield bulrush was similar.  Overall, control of weeds increased to at least 98% with the follow-up application of Regiment® in both situations.

Shark® H2O for sedge control

A field study examined the efficacy of Shark® to control smallflower umbrella sedge and ricefield bulrush. Shark® can be a good option to control ALS inhibitor- or propanil-resistant sedges and ALS resistant redstem.

Shark® H2O alone at 1 tiller provided more than 90% control of ricefield bulrush and smallflower umbrella sedge. A tank-mix application of Grandstand® and propanil at 1 tiller also provided more than 90% control of late watergrass and barnyardgrass, more than 98% control of the sedges, and good control of other broadleaf weeds.  The inclusion of Cerano® or Granite® GR significantly reduced early weed pressure, providing superior weed control.

New adjuvants for weed control

The efficacy of four adjuvants applied with Regiment®, Clincher®, and Stam® was studied in a continuous flood system. Adjuvants included InterLock®, MasterLock® and DownDwarf. Regiment® and Clincher® were tested in standalone applications, while Stam® was applied following Cerano® at day of seeding.

Two of the Stam® applications provided more than 90% control of sedges and major broadleaf weeds. In the application with DownDwarf, control was less than 70%. The Regiment® application, regardless of the adjuvant included, provided excellent control of all weed species except sprangletop.

A second study in a pinpoint system examined two experimental adjuvants—OR 108G and OR 009—with Granite® SC and with Clincher® CA, as well as in a tank mix of Abolish® and Regiment®. Granite® SC and Clincher® with the adjuvants provided more than 90% control of grasses, sedges, and broadleaf weeds (except redstem~25%). Applications of herbicides with higher rates of these adjuvants provided slightly higher control (above 95%). The tank mix of Abolish® and Regiment® provided good overall control (85%)—including redstem. However, barnyardgrass and late watergrass control was not as good.

Herbicide research

Nine separate studies evaluated different formulations and new active ingredients in continuously flooded rice systems.

Evaluation of ROXY™

The Rice Experiment Station discovered a trait (Roxy™) that provides resistance to the herbicide oxyfluorfen. An experimental line containing the Roxy™ trait was evaluated in 2017 for its crop performance and tolerance when exposed to oxyfluorfen (Goal® 2XL) alone and in combination with other herbicides. This herbicide has the same mode of action as Shark® H20 and has good activity on a broad spectrum of broadleaf, grass, and sedge weeds.

Testing with Goal® 2XL applied preflood took place in a continuously flooded system. The experimental line showed minimal injury early in the season. The standalone application provided excellent control of broadleaf weeds such as ducksalad, monochoria, redstem, and water hyssop. Control of late watergrass and barnyardgrass was at least 90%. Control of smallflower umbrella sedge was excellent. Control of ricefield bulrush was fair. Overall, Goal® 2XL provided a broad spectrum of weed control and offered an exceptional level of crop safety.

Inclusion of into-the-water herbicides such as Bolero®, Butte®, Cerano®, and Granite® GR increased control of grasses and ricefield bulrush to more than 98%. Similarly, an early follow-up application of Clincher® improved grass control to more than 99%. Regiment® or Granite® SC applied at 1 tiller improved control of both grasses and ricefield bulrush to more than 99%. A late tank-mix application of Stam® and Grandstand® or RiceEdge® following the preflood application of Goal® 2XL provided excellent control of all major weeds.

This research suggests that the majority of initial flush weeds early in the growing season can be controlled with Goal® 2XL alone without causing significant injury to rice containing the ROXY™ trait. Preliminary results are encouraging. This new approach should fit well into various weed control programs and should prove cost effective.

NAI-1777 control and safety

NAI-1777 is a granular formulation of pyraclonil, which is currently under development by Nichino America Inc. This herbicide has a similar mode of action as Shark® H2O.

In 2017 this experimental herbicide was evaluated alone and in combination with other herbicides, including Cerano®, Butte®, or Bolero®, followed by propanil. Two other combinations included NAI-1777 followed by Regiment® or Strada®.

All NAI-1777 programs were exceptionally effective in controlling weeds. However, NAI-1777 in combination with Bolero® reduced rice stands as much as 30% and resulted in 40% stunting 60 days after seeding. Any rice injury with the other programs recovered by 60 days after seeding.

Ten different formulations of this compound were evaluated. Two can be used effectively in California rice. In summary, NA-1777 is a promising tool for controlling a variety of herbicide resistant weeds.


Rinskor™ is a new Dow Chemical Co. active ingredient related to triclopyr (Grandstand®) to control ricefield bulrush and redstem. However, Rinskor™ has broad-spectrum control that includes grasses, sedges, and broadleaf weeds.

In three field studies, Rinksor™ demonstrated excellent safety on rice with good control of watergrass, sedges, and broadleaf weeds. It is weak on sprangletop. Work on Rinksor™ will continue to improve efficacy and to examine granular formulations that will meet California’s needs. Dow aims to have this product available for California rice producers in the next several years.


TVE29 is a new, low rate herbicide with a novel mode of action that could be extremely helpful in managing resistant grasses. A crop safety field experiment examined TVE29 applied alone and in combination with other herbicides at different rates and timing. It demonstrated excellent crop safety and outstanding grass control. Evaluation of this active ingredient in different formulations will continue.


Several new formulations of clomazone were tested for weed control and crop safety. In general, research showed that crop safety and weed control of new formulations are similar to Cerano®. One formulation was slightly better than Cerano®.

Developing management alternatives

Drill-seeded rice systems offer flexibility for herbicide use when proximity to nontarget sensitive crops restricts aerial applications. However, this system may affect the relative emergence of rice and weeds. Use of a nonselective herbicide such as glyphosate may be more economically feasible in these systems.

A field study using glyphosate alone and in combination with other herbicides was conducted with rice planted at three depths. As expected, weeds adapted to dryland seedbeds such as barnyardgrass, sprangletop, watergrass, and smallflower umbrella sedge were better established, whereas aquatic weeds such as ricefield bulrush, ducksalad, and redstem were almost eliminated.

Soil type and rice variety may affect seedling emergence. Therefore, greenhouse research is underway to help identify rice varieties that are a good fit for deep planting in order to take advantage of differential weed and rice emergence.

Sprangletop is a major weed problem in some California rice fields. In the past, sprangletop was controlled by elevating field water depth. Recently, however, there have been reports of sprangletop emerging from deep water.

In 2017, a field study was conducted to study sprangletop emergence from water depths of 2, 4 and 8 inches. Clomazone resistant and susceptible sprangletop populations were planted at each depth. Neither of the two populations emerged from water depth of 8 inches. Clomazone susceptible populations did not emerge from a 4-inch water depth. However, clomazone resistant populations were able to emerge from a 4-inch water depth. At 2 inches both populations were able to emerge, but growth and seed production were more robust in the clomazone resistant population.

Smallflower umbrella sedge resistance

Smallflower umbrella sedge is a major weed in California rice. Historically, it was controlled with ALS inhibitors. However, extensive use of these herbicides led to development of resistance.

A study in 2017 surveyed more than 60 rice fields for ALS resistance. All populations were treated with four ALS inhibitors that belong to different chemistries. The study showed widespread ALS resistance in California rice fields, although the pattern of resistance varied. Research is underway to study the mechanisms of resistance.

Rice field survey

Bearded sprangletop seeds were collected from 32 rice fields and germinated in a greenhouse. Seedlings were tested for clomazone resistance. Four populations showed resistance to clomazone. However, the resistant plants were suppressed with clomazone but not killed. This response suggests that metabolic resistance may be at work. Research is underway to define the level of resistance, competitiveness of resistant populations, and mode of resistance in order to prevent further development of resistance and to manage clomazone resistance.

Resistance diagnostics and detection

Greenhouse testing of suspected herbicide resistant weeds was conducted on more than 120 samples from 2016. Growers and PCAs submitted weed seed samples, including barnyardgrass, early and late watergrass, smallflower umbrella sedge, sprangletop, ricefield bulrush, and redstem. Responses of these weeds to several herbicides confirmed resistance.

Most of the samples tested show resistance to at least one herbicide. Several samples showed multiple resistance. Each grower has been provided with an extensive report that includes photos of plant response to different herbicides and recommendations for herbicide alternatives.

Work will continue to test suspected resistant weed populations provided by growers and PCAs. This approach has been well received. For 2017, more than 230 samples were received, a significantly higher amount than the previous year and six times the number of samples received in 2014.