Weed Management
in Rice, 2019


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 2019 focused on the efficacy of new and existing herbicides in mixtures or sequential combinations, alternative crop establishment methods to reduce weed pressure, 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.

Predominant weeds in the 2019 research plots included late watergrass, ducksalad, ricefield bulrush, and smallflower umbrella sedge, followed by barnyardgrass, monochoria, water hyssop, redstem, and sprangletop. All of these weeds are susceptible to herbicides registered for use in California rice.

Several into-the-water herbicides are available for controlling weeds in continuously flooded rice, including Bolero®, Butte®, Cerano®, Granite® GR and SC, League® MVP, Shark® H20, and propanil. 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

This is the second year that Butte®, a granular mixture of benzobicyclon and halosulfuron, has been available to California rice growers. The benzobicyclon component adds a new mode of action for control of broadleaf weeds, grasses, and sedges. Halosulfuron is an ALS-inhibiting herbicide. 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 such as Clincher®, Cerano®, Granite®, Regiment®, and propanil to improve grass weed control. The combination of Butte®, followed by other herbicides provided good weed control in 2019.

Butte® followed by into-the-water applications of Granite® GR at the 3.5-leaf stage of rice, and a foliar application of Regiment®, Granite® SC or Grandstand® provided exceptional control of all weeds. The lone exception was smallflower umbrella sedge control was below 90% when Butte® was followed by Grandstand® or Regiment®. The choice of the appropriate follow-up application or inclusion of a granular herbicide largely depends on the weed population pressure and resistance status of the weeds in the field.

Cerano® rate and timing

Leather’s method is used in water-seeded systems by draining the field within the first few days after seeding and reflooding once the crop has established shallow roots (about a week after seeding). This allows for good crop establishment but also gives weeds the opportunity to become highly competitive. Many growers use Cerano® (clomazone) to control sprangletop and barnyardgrass. However, if growers are using the Leather’s method, Cerano® is not an option because of labeled water-holding requirements.

A study examined Cerano® applied at day of seeding, as well as after a Leather’s method to determine whether weed control was adequate and whether there was any crop damage. Cerano® 5MEG was applied at the rate of 8, 10, and 12 pounds/acre at the two timings. To control broadleaf weeds, an application of Granite® GR and SuperWham! ® was included in all treatments. In general, weed control was better when Cerano® was applied at day of seeding. Cerano® applied after leathering injured rice. A rate of 8 to 10 pounds/acre provided adequate grass control.

Evaluating algaecides

California rice production is challenged by nuisance algae in the beginning of the growing season. Rapid, early formation of algal mats at the time of flooding can prevent the establishment of newly emerged rice seedlings. Aerial application of copper sulfate does not provide consistent control. In 2019, two bucket experiments and one field study were conducted with 10 chemicals containing possible algaecide activity.

The results from the bucket experiments showed that chelated copper (Algimicyn®) had the highest level of control at 86%, followed by Protox herbicides (Goal® 2XL and Ronstar®) at 70%. Blue stone copper, Cutrine®-Plus and Cutrine®-Ultra controlled algae similarly in the range of 50% to 70%. Hydrogen peroxide controlled algae by only 50%, and its efficacy dropped dramatically after five days.

In the field experiment, maximum algae control (96%) was achieved with a pre-emergence application of Goal® 2XL. Cutrine®-Plus, Cutrine®-Ultra and liquid peroxide provided similar levels of algae control in the range of 60% to 70%. Rice was able to establish in all the algaecide treatments except the Goal® 2XL treatment, which severely damaged the rice crop.

All the tested algaecides have the potential to reduce the early bloom of algae. However, further research is needed to determine the optimal time of application and their effects on rice.

Herbicide research

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

Research on pyraclonil

Pyraclonil is a new PPO-inhibiting active ingredient under development by Nichino America. Its mode of action is a new chemistry with activity on a broad spectrum of rice weeds and no known resistance issues. Previous studies demonstrated that pyraclonil would be best used as part of a comprehensive weed control program. It is less effective on sprangletop, smallflower umbrella sedge, and ricefield bulrush.

A great deal of weed management research focuses on new and existing herbicides in mixtures or sequential combinations. This is the Hamilton Road site where much of this work takes place.
In 2019, pyraclonil was tested as part of a program that included Butte®, Cerano®, Bolero®, Regiment®, Granite® GR, SuperWham!®, and Loyant®. Results showed outstanding weed control, including smallflower umbrella sedge and broadleaf weeds when pyraclonil was applied in combination with Bolero® UltraMax, Butte®, and Loyant®. Additionally, rice did not show any signs of injury.

A separate study with pyraclonil applied day of seeding on M-105, M-205, M-206, M-209, S-102, and Koshihikari showed all varieties exhibited good tolerance to this compound.

Best and most effective application timing is day of seeding. Testing thus far suggests pyraclonil holds promise as a new tool for managing a variety of herbicide-resistant weeds in California rice fields.

New herbicide Loyant®

Loyant® is a new product under development by Corteva Agriscience. Its active ingredient, florpyrauxfen-benzyl (Rinskor™), is being evaluated for California rice. It is a post-emergent herbicide in liquid formulation meant to control broadleaf weeds, grasses, and sedges. Loyant® is an auxin-type herbicide with a mode of action that has no known resistance issues in California rice.

Two separate studies were conducted in 2019 to evaluate Loyant® in programs with Clincher®, Granite®, RebelEX®, Cerano®, Bolero®, and Butte®. Loyant® with Butte® and Loyant® with Bolero® gave nearly complete weed control. It was also found to be safe on rice.

Loyant® is another promising tool for weed control, in general, and for managing herbicide-resistant watergrass and smallflower umbrella sedge in particular.

New grass herbicide

Work continued on a new, low-rate herbicide with a novel mode of action from FMC mainly for watergrass control. It has a wider window of application than other herbicides and could be a valuable tool in managing herbicide-resistant grasses. Three separate studies were conducted on this herbicide in 2019. Researchers evaluated time of application, formulation, and potential herbicide partners. It showed excellent safety on rice and outstanding watergrass control.

Work will continue with a focus on optimal application timing.

New clomazone formulations

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.

Weedy rice research

Weedy red rice, or simply weedy rice, is an emerging issue for California growers. In 2019, field experiments were conducted to observe the timing and conditions for emergence of several weedy rice types. All weedy rice types emerged within 14 days of flooding. Type 3, the weed suspected of existing in California for the longest period, had the highest rate of emergence.

In addition to field studies, research continued on temperature and water conditions favorable to weedy rice under controlled conditions. Results thus far indicate there isn’t a large difference in germination patterns for all weedy types at 85 degrees Fahrenheit. However, some differences were noted at 60 degrees Fahrenheit.

Germination results will be combined with data from an emergence experiment to create predictive models for the timing of weedy rice emergence. This research is continuing in 2020.

Drill seeded rice

Drill seeding rice is practiced widely in the southern U.S., yet it is not commonly practiced in California. With this strategy, the crop and weeds emerge at the same time. Weed management options are thus limited to rice-safe herbicides.

Research in 2019 focused on water management and an herbicide program centering on a post-plant, pre-emergent application of glyphosate. Cultivars included M-206 and M-209. Weather was a major factor in this research, as multiple spring storms delayed planting by a month. In addition, the rain caused germination of most watergrass before field work.

As watergrass was largely not present during the season, later germinating bearded sprangletop was able to compete more effectively with rice and sedges. Glyphosate alone reduced bearded sprangletop by more than 50% in all treated plots, with the remainder of the herbicide treatment program ensuring a weed-free season.

Sprangletop control

Bearded sprangletop is a problematic weed in California rice production. Flooding was thought to suppress its germination, emergence, and growth. After many years of continuous rice production, however, some biotypes appear to survive.

A study at the Rice Experiment Station tested the flooding tolerance of two biotypes, one clomazone resistant and the other not, at 2, 4, and 8 inches of continuous flooding depth. At 2 inches, neither biotype was controlled. With a 4-inch flood, only the resistant biotype survived flooding. There was no emergence of bearded sprangletop in the 8-inch flood depth of either biotype. Thus, continuous flooding can still be used as a management tool to control bearded sprangletop.

The results of this study indicate that clomazone-resistant bearded sprangletop is more likely to spread throughout the Sacramento Valley because it can tolerate standard flooding practices and survive applications of commonly used herbicides. Control of bearded sprangletop will require the use of integrated weed management. Growers should consider rotating crops where possible, rotating herbicides with different modes of action, using weed-free rice seed, and increasing flood levels when possible.

Rice field survey

Location of bearded sprangletop populations tested for clomozone resistance. Closed circles indicate a field cloosecion site. Open circles indicate a known sysceptible field site. Stars indicate clomazone resistant population site.
Decades of herbicide use has resulted in widespread resistance in rice fields. Surveys of California rice fields help shed light on herbicide resistance in weeds such as smallflower umbrella sedge and bearded sprangletop. Research into the mechanisms of resistance also will help in the development of management programs.

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 the development of resistance in this sedge.

In 2019, research expanded into ALS-inhibitor cross-resistance in smallflower umbrella sedge. Herbicides used included Londax®, Halomax®, Regiment®, and Granite® at rates up to 12 times the label rate. Results confirm that most of the resistance is dose-dependent. Resistance mechanisms are likely to be nontarget site based. However, one population showed strong survival to all rates of applied herbicides and may have target-site resistance. If confirmed, this would be the first known case of smallflower umbrella sedge target-site ALS resistance in California.

A genetic study is underway to determine whether alteration to the ALS gene may be causing resistance. Also, a greenhouse study is looking into the mechanism of resistance.

Bearded sprangletop has been controlled with clomazone since 2004. The continuous use of clomazone and lack of crop rotation in rice fields has resulted in resistance to clomazone in several bearded sprangletop populations.

In a survey of clomazone resistance in bearded sprangletop, 17 out of 21 populations were found to be resistant. The other four were not. All resistant populations were slightly injured by clomazone seven days after treatment. However, three weeks after treatment, the plants recovered and resumed normal growth.

The resistant populations came from different fields in Colusa and Sutter counties. Locations of these fields suggest that resistant populations may have evolved independently multiple times.

Resistant populations were in fields with diverse herbicide use histories. Clomazone was used in most fields in the study. Growers suspected clomazone resistance, but only 5% of the populations tested were resistant. Improper application timing may have been at the root of failure to control bearded sprangletop.

In a dose-response study, susceptible plants became symptomatic at clomazone rates as low as one-quarter of label rates. Rates required to cause mortality of resistant bearded sprangletop was more than four times the label rate.

To determine the mechanism of resistance, researchers investigated clomazone absorption, translocation, and metabolism under controlled environmental conditions with two resistant populations and one susceptible population. This research confirmed that the resistant and susceptible populations of bearded sprangletop metabolized clomazone differently.

Diagnostics and detection

Greenhouse testing of suspected herbicide resistant weeds was conducted on more than 157 seed samples from 2018. 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.