Weed Management
in Rice, 2018


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

The weed management 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 2018 continued its focus on the efficacy of new and existing herbicides, alternative crop establishment methods, and herbicide resistance strategies to deal with the 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.

Predominant weeds in the 2018 research plots included late watergrass, ducksalad, ricefield bulrush, and smallflower umbrella sedge, followed by barnyardgrass, monochoria, and water hyssop. Sprangletop and redstem were present in low densities. 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®. 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

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 and offers an exceptional level of crop safety. However, there is a need to examine its use with other herbicides to improve grass and broadleaf weed control.

Butte® was tested in a continuous flood system both alone and in combination with other herbicides. Applied at 1-leaf stage of rice, Butte® provided excellent control of sedges, ducksalad, monochoria, and sprangletop. However, it was less effective in late watergrass and barnyardgrass control.

Butte® followed by an into-the-water application of Granite® GR at 3-leaf stage of rice, a follow-up foliar application of Granite® SC at 5-leaf stage of rice or Loyant® at mid-tiller stage of rice provided exceptional control of all weeds. A follow-up application of Regiment® at midtiller or full tiller stage of rice greatly improved barnyardgrass and watergrass control.

Low-cost herbicide programs

Weed management in California rice incurs significant costs. Most herbicides are relatively expensive. In some instances, the high cost of resistance management may challenge the economic viability of weed control programs. The weed control efficacy of several low-cost herbicide programs was compared with the efficacy of more expensive programs.

A low-cost program using Cerano® 5 MEG provided excellent control of grass weeds and very good to excellent control of all other primary weeds. A program with Granite® SC and a tank mix of Abolish® and Regiment® were similarly effective.

These low-cost programs may offer satisfactory weed control, especially in fields with no resistance problems. However, they may require more vigilance of factors influencing herbicide effectiveness, such as weed species present, water management, and timing of herbicide applications. In addition, frequent herbicide rotation is suggested to mitigate the potential for development of herbicide resistance.

Evaluating algaecides

Algaecides applied early in the season could help decrease the presence of algae, which can interfere with rice establishment. In a field experiment, complex copper algaecides, hydrogen peroxide, zinc sulfate, and herbicides with activity on algae were compared against the conventional copper sulfate used for algae control.

All compounds were applied three days after flooding except for Goal® 2XL, which was applied two days before flooding. Algae response was very dynamic, with rapid changes over time. Maximum control was achieved with the Goal® applications (96%). Zinc sulfate gave the lowest control (21%). Cutrine®-Plus, Cutrine®-Ultra, liquid peroxide, and copper sulfate gave 70%, 60%, 60%, and 47% control, respectively.

In general, algae started to recover within a few days of algaecide application. The fastest algae recovery was observed in the Cutrine®-Plus and Cutrine®-Ultra treatments, while algae coverage remained low in the other treatments eight days after treatment. All the tested algaecides have the potential to reduce early algae bloom. However, further investigation is needed to determine optimal application timing and potential effects on rice.

Fertilizers and algae

Growth of algae in the beginning of the rice growing season is associated with the availability of fertilizers in the soil. Once the rice is flooded, algae grow rapidly and develop a mat on the surface of the water that interferes with the emergence and establishment of rice seedlings. Researchers examined how the rate of nitrogen and phosphorous fertilizers, individually or in a mixture, can influence an algae bloom.

Two rates of nitrogen in the form of urea and two rates of phosphorous in the form of triple super phosphate were spread and lightly mixed with the soil at the beginning of the season. Thirty-six PVC pipes were placed in the field as experimental units. Water temperature and light intensity were recorded during the experiment. Algae coverage was observed eight times over 30 days.

The algae coverage within the pipes showed high fluctuations over time because of changes in temperature and standing water levels. As might be expected, algae coverage and dry biomass was higher in the treatments with higher fertilizer rates. Furthermore, in treatments where both nitrogen and phosphorous were applied, the algae coverage and biomass were higher than when either fertilizer was applied alone.

Results from this experiment suggest that application of fertilizers at lower rates in the beginning of the season could reduce the risk of algae bloom. However, the effect of reduced fertilizer rate on the growth and development of rice needs further investigation.

Herbicide research

Twelve separate studies were conducted to evaluate different formulations and new active ingredients in continuously flooded rice systems.

Evaluation of ROXY™

An oxyfluorfen-resistant rice trait (ROXY™) was discovered in M-206 by plant breeders at the Rice Experiment Station. Oxyfluorfen has good activity on a broad range of rice weeds. The ROXY™ experimental line (17Y3000) was evaluated at normal and delayed planting dates for crop tolerance and weed control with four rates of the herbicide (Goal® 2XL).

Under both planting times, all rates of oxyfluorfen provided excellent control of broadleaf weeds, grasses, and sedges. Preliminary results also indicate some improvement in initial crop vigor may be achieved by adjusting planting time. Oxyfluorfen may provide only a fair level of ricefield bulrush control and has little activity on sprangletop.

With further developmental work, ROXY™ technology can be a valuable addition to address weed management in California rice.

Research on pyraclonil

Pyraclonil is a new PPO-inhibiting active ingredient, which is being evaluated by Nichino America. This new chemistry has activity on a broad spectrum of rice weeds. NAI-1883, a granular formulation of this compound, was evaluated alone and in combination with other herbicides.

The stand-alone application was exceptionally effective in controlling all weeds in a research field. However, pyraclonil was slightly weaker on ricefield bulrush and ineffective against bearded sprangletop at the evaluated rates. A later application timing of NAI-1883 following Cerano 5 MEG was equally effective in controlling all weeds except barnyardgrass.

In summary, pyraclonil is a very promising chemistry for weed control in California rice, most importantly for controlling and managing a variety of herbicide-resistant weeds.

New herbicide Loyant®

Loyant® is a new product from Dow. Its active ingredient, florpyrauxifen-benzyl (Rinskor™), is being evaluated for California rice. This new chemistry can be a valuable tool for herbicide resistance management.

Efficacy of Loyant® was evaluated in several weed control programs. Loyant® was safe on rice and provided nearly complete control of all weeds present in the field. All the other programs that included Loyant® provided perfect control of nearly all weed species with excellent crop safety. Loyant® alone or in combination with Granite® is not expected to provide satisfactory control of sprangletop.

In summary, Loyant® is a promising tool for weed control in California rice. It will be an excellent addition to control herbicide-resistant watergrass and smallflower umbrella sedge.

New grass herbicide

A new, low-rate herbicide with a novel mode of action from FMC was tested mainly for watergrass control. It has a wider window of application than other herbicides.

A field experiment evaluated weed control and crop safety when this new grass herbicide was applied alone and in combinations at different rates and timing. It showed excellent safety on rice and outstanding watergrass control.

Work will continue with a focus on different formulations of this herbicide.


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®. The purpose of this study is to have more than one company to market clomazone for California rice to increase competition and reduce costs to growers.

Developing management alternatives

Decades of herbicide use have resulted in widespread herbicide resistance in rice fields. Management of resistance while maintaining economically viable yields is becoming increasingly difficult for many growers. Research on drill-seeded rice systems has been underway for a number of years. This technique delays crop emergence and should allow for most grasses to be controlled by glyphosate or other broad-spectrum herbicides, resulting in better weed control.

Field trials and greenhouse studies were conducted in 2018 to learn more about this issue with M-105, M-205, M-206, and M-209. In the greenhouse study, M-205 and M-209 had higher seedling vigor, emergence rates, and total emergence than did M-105 and M-206. M-105 performed poorly at all depths below a half inch. When planted to a depth of 2 inches, M-105, M-205, M-206, and M-209 had emergence rates of 78%, 81%, 81%, and 93%, respectively. In addition, M-209 reached 50% emergence about two days earlier than M-206 and M-105. These results suggest that M-205 and M-209 could emerge at sufficient rates and amounts to provide a window for good pre-emergence weed control with glyphosate while provide economically competitive yields.

The field study consisted of plots drilled to one inch or two inches and planted with M-206 or M-209. Both cultivars and both seeding depths reached emergence five days after planting. Glyphosate was then applied, with minimal rice injury and no stunting. Glyphosate alone reduced barnyardgrass by more than 50% in all treated plots, with the remainder of the herbicide program ensuring a season free of this weed. Bearded sprangletop germinated later in the season and was not effectively controlled with glyphosate, necessitating an application of Clincher®. Yields for untreated plots were zero for both cultivars and both seeding depths. Yields for treated plots of either cultivar were no different across seeding depths or across three herbicide treatments.

Bearded sprangletop is a problematic weed in California rice production. Flooding was thought to suppress its growth. After many years of continuous rice production, however, anecdotal evidence suggests that certain populations of this weed can tolerate flood pressure.

A study at the Rice Experiment Station tested flooding tolerance of two bearded sprangletop populations against three irrigation depths. One population was resistant to clomazone, the other susceptible to this herbicide. There was no emergence of bearded sprangletop in the 8-inch flood depth of either population. With a continuous 4-inch flood, only the resistant population survived flooding pressure and produced significantly more tillers and seed. This suggests that there may be a fitness advantage related to clomazone resistance. Further testing is needed.

Growing degree days were calculated for bearded sprangletop, smallflower umbrella sedge, and barnyardgrass at three sites. Studies were conducted in late April, mid-May, and early June to ensure a wide range of temperatures. Smallflower umbrella sedge germinated first, followed by barnyardgrass and bearded sprangletop. In addition, bearded sprangletop germinated during a wider window than the other species. This suggests that herbicide application at day of seeding is not optimum timing for bearded sprangletop control.

Rice field survey

Surveys of California rice fields help to shed light on herbicide resistance in major weeds such as smallflower umbrella sedge and bearded sprangletop. Information gained from these efforts will help in the development of programs to manage resistant biotypes.

In 2018, research expanded into ALS-inhibitor cross-resistance in smallflower umbrella sedge to include a study of resistance in other types of herbicide resistance. A study revealed no resistance to Shark® or Abolish®, but most populations tested showed some tolerance to Stam® up to three times the label rate. Research to elucidate the mechanisms of herbicide resistance for this weed will focus next on genetic and molecular studies.

Seed from 31 populations of bearded sprangletop were submitted by growers from California rice fields to screen for clomazone resistance. In greenhouse experiments at the Rice Experiment Station, four populations were confirmed as resistant. In an additional study, three populations showed resistance to clomazone at above the labeled rates.

Diagnostics and detection continue

Greenhouse testing of suspected herbicide resistant weeds was conducted on more than 230 samples from 2017. 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 showed 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. The protocol is similar to 2017. However, the new herbicide Loyant® has been added to test for resistance in smallflower umbrella sedge and barnyardgrass.