Weed Control in Rice, 2013


Project Leader

Albert Fischer, professor, Weed Science Program, Dept. of Plant Sciences, UC Davis

The weed control project seeks to assist California rice growers in the prevention and management of herbicide-resistant weeds, to achieve economic and timely broad-spectrum weed control, and to comply with personal and environmental safety requirements. Field testing in 2013 took place at the Rice Experiment Station, and in a cooperating grower’s field where the weed spectrum includes the late watergrass type that is strongly resistant to multiple herbicides.

Herbicide research

Herbicide test plots examine registered and new herbicides for effectiveness, safety, and compatibility.

League® MVP is a granular mixture of thiobencarb and imazosulfuron developed by Valent U.S.A, Corp., for use in California rice production. It was tested in a continuous flood system. Applied into the water at the 2-leaf stage of rice, League® MVP fully controlled ricefield bulrush. It also had good control of susceptible watergrass and ducksalad. A follow-up application of Regiment® at the 1-2 tiller stage of rice will improve control of multiple herbicide-resistant watergrass.

GWN 10252 is a Gowan Co. granular mixture of benzobicyclon and halosulfuron. It was tested under a continuous flood, both alone and in combination with other herbicides. Phytotoxicity to rice was very low. Applied at the 1-leaf stage of rice, it had good early control of watergrass, as well as excellent ricefield bulrush and ducksalad control. A follow-up watergrass herbicide is necessary for 100% control. Cerano® followed by GWN 10252 at the 1-leaf stage of rice provided excellent broad-spectrum control throughout the season.

Clincher® (cyhalofop-butyl) is an herbicide from Dow AgroSciences that is used exclusively against grasses. Clincher® G is an experimental granular formulation that was tested under a continuous flood in several programs that included other herbicides. It had very low phytotoxicity on rice, except at very high rates. It effectively controlled susceptible watergrass (84%) when applied alone. When applied 24 hours before Shark® H20, antagonism was prevented and control of watergrass increased to more than 90%. Control of ricefield bulrush increased to 100%. Studies of possible antagonisms between these compounds are necessary.

Rice culture systems

Herbicide testing continued in 2013 on the three major systems of rice culture—continuous flood, pinpoint flood, and dry or drill-seeded. This ongoing research seeks the best herbicide and management combinations for each of these systems.

Continuous Flood

In the continuous flood trial, early herbicide treatment is essential for good weed control. Best results are obtained when herbicide programs give at least 95% broad-spectrum weed control in the first critical month after seeding.

Provided a uniform, four-inch water depth can be maintained, the continuous flood system can eliminate sprangletop and suppress watergrass. Granular herbicide formulations applied early into-the-water are excellent nondrift tools for this system.

Four separate trials were conducted in 2013 to examine combinations and experimental herbicides. Testing of the clomazone formulation Bombard™ continued. It is a prilled formulation, instead of an extruded granule (Cerano®). Both formulations provided strong watergrass suppression.

The granular formulation of Granite® GR alone gave excellent (96%) control of watergrass. (However, it will not control herbicide-resistant late watergrass or “mimic.”) Granite® GR also afforded good control of bulrush and excellent control of ducksalad.

Cerano® applied day of seeding, followed by Granite® GR at the 2.5-leaf stage of rice gave excellent broad-spectrum control. A tank mix of SuperWham!® and Grandstand® applied at the 1-2 tiller stage of rice worked well as a clean up. Grandstand® is used to control ricefield bulrush and late-emerging redstem plants.

Shark® H20 is a good herbicide for controlling ALS inhibitor- or propanil-resistant sedges. Applied at the 1-2 leaf stage of rice, it suppressed ricefield bulrush and ducksalad. Cerano® followed by Shark® H20 provided strong broad-spectrum suppression by 40 days after seeding. A follow-up application of SuperWham!® at the 1-2 tiller stage can be used to clean up watergrass and sedge. Regiment® can also be used to control watergrass “escapes.”

Granite® GR in combination with Shark® H20 applied into the water at the 2.5-leaf stage of rice provided excellent control of watergrass, ricefield bulrush, and ducksalad, although some injury to rice was noticeable at 7 and 14 days after treatment.

Bolero® Ultramax, a granular into-the-water herbicide, applied at 1-2 leaf stage of rice provided good watergrass control and activity on ricefield bulrush. Bolero® Ultramax followed by Granite® GR at the 2-3 leaf stage of rice gave 100% control of watergrass, ricefield bulrush, and ducksalad. Bolero® Ultramax followed by Regiment® provided excellent ricefield bulrush control but failed to control ducksalad.

Pinpoint System

The pinpoint system is used in California when rice requires early draining for establishment or when early weed exposure to foliar herbicides is needed.

Prevailing weeds in this experiment were early and late watergrass, ricefield bulrush, ducksalad, and waterhyssop. Excellent (100%) broad-spectrum control in this system was achieved with a tank mix of Regiment® and Abolish®. Granite® SC and Clincher®, followed by SuperWham!® provided excellent broad-spectrum control (97%), as did Granite® SC alone at the 3-4 leaf stage of rice (over 98%). Clincher® controlled watergrass, but a tank mix of Clincher® and SuperWham! ® lowered the efficacy of both herbicides. Control of both watergrass and ricefield bulrush was still greater than 80%.

Leathers’ method

The Leathers’ method is a water-seeded system in which the flood water is dropped after seeding to allow improved stand establishment. In 2013, irrigation water was drained four days after seeding, when the rice was pegging. It was reflooded eight days after seeding, when rice was about two inches tall. All initial herbicide applications were made after reflooding while weeds were still very small. Follow-up applications were made at the 1-tiller stage of rice.

This approach shortens the period during which the soil surface is not flooded, thus helping suppress sprangletop and barnyardgrass, and enables the use of granular herbicides to minimize drift issues. To utilize this method, growers must have the ability to drain and reflood fields quickly. Prevailing weeds in this system were early and late watergrass, ricefield bulrush, ducksalad, and water hyssop.

Excellent overall control was achieved in a program of Cerano® applied after reflooding, followed by Granite® GR two days later, or followed by GWN 10252 (benzobicyclon+halosulfuron), also two days later. A follow-up application of SuperWham!® with Grandstand® at the 1-tiller stage of rice can control watergrass escapes and late-emerging ricefield bulrush and redstem. Granite® GR applied after reflooding also gave excellent broad-spectrum control. Propanil with Grandstand® at the 1-tiller stage of rice can be used for clean up.

Drill-seeded system

The drill-seeded system offers flexibility for herbicide use when proximity to sensitive crops limits aerial herbicide applications. Drill seeding favors weeds adapted to dryland seedbeds, such as sprangletop and barnyardgrass. But it helps limit recruitment of aquatic species such as bulrush, ducksalad, and redstem. Drill seeding is useful for alternating with water-seeded systems that have problematic aquatic weed pressure.

Weed competition can cause significant yield loss under drill seeding. Thus, early treatments affording 95% or greater weed control are necessary for optimum yields. Prowl® H20 is a pre-emergence herbicide that limits weed emergence from soils and thus protects rice from weed emergence after seeding and prior to permanent flood.  It is best used in combination with other herbicides that control already emerged weed seedlings. The tank mixture of Prowl® H20, SuperWham! ®, and Clincher® applied at the 2-3 leaf stage of rice is effective when some grasses have already emerged, while Prowl® H20 in the soil prevents emergence of new weed seedlings. Also, overall excellent control was achieved with a tank mix of Granite®SC, Prowl® H20, and Clincher® applied at 2-3 leaf stage. This is the third year that this three-way tank mix has had excellent broad-spectrum weed control. Abolish® can be useful as a delayed pre-emergent to control sprangletop, watergrass, and propanil-resistant smallflower umbrellasedge. Abolish® followed by SuperWham! ® gave good broad-spectrum control.

Other excellent treatments were Granite®SC tank-mixed with Clincher® at the 2-3 leaf stage of rice, followed by SuperWham!® after permanent flood, and Clincher® followed by SuperWham! ®. Granite® SC followed by Clincher® provided excellent grass control but less sprangletop control.

Weed modeling

Research continued on a model to predict germination and emergence for late watergrass and smallflower umbrellasedge, two species that have evolved herbicide resistance. Field and greenhouse experiments conducted in 2013 sought to better identify possible differences between resistant and susceptible biotypes. In large field plots, soil moisture and temperature information was collected under different irrigation regimes.

Smallflower umbrellasedge initiated emergence earlier under the flooded irrigation treatment than under the drill-seeded treatment. The rate of emergence was faster under drill seeding, but the total number of emerged plants was much lower than in the continuously flooded treatments.

Late watergrass emergence was not influenced by irrigation treatment. This confirms what is already known—that late watergrass can be either an aquatic or dryland weed. The total number of emerged plants was slightly lower in the drill-seeded treatments than in the continuously flooded treatments.

Field data from this study will be compared to data from laboratory and pot studies. This will help assess the validity of the models for predicting emergence and early growth.

To be practical for field use, the model’s predictive capabilities must extend beyond germination to include early growth—shortly after emergence from soil. This will facilitate optimal timing for late watergrass control, particularly in stale-seedbed systems.

Experiments were conducted on populations of herbicide-resistant and susceptible late watergrass under different watering regimes in the field. Daily measurements of plant height and leaf number were recorded until plants reached approximately the 5-leaf stage to tillering. Early growth varied across all treatments with soil moisture, temperature, and population. Plants in the flooded treatments were taller and had less biomass than plants in the daily flush treatments. Evaluation of the data is in progress and will help elucidate differences between resistant and susceptible plants.

Herbicide resistance

Concern has grown among rice growers and pest control advisers about some smallflower umbrellasedge and ricefield bulrush populations that have become resistant to propanil.

In 2013 smallflower umbrellasedge and ricefield bulrush plants were hand-harvested in rice fields throughout the Sacramento Valley where resistance to propanil is suspected. A greenhouse study confirmed resistance and examined the mechanism of resistance in both species.

Research into alternative management methods, as well as new herbicide options, are continuing. Currently, Shark® H20 is a good alternative for controlling both sedges.

Herbicide resistance may not be just the result of repeated use, but may also be influenced by broader environmental adaptation. It is important to understand the relationship between resistance to herbicides and the gene expression involved in tolerance to specific environmental stresses. Mechanisms conferring herbicide resistance may be part of a plant’s stress adaptation in its original environment, such as deep-flood tolerance in Asian rice paddies. An experiment revealed that resistant and susceptible late watergrass differ in expression of certain genes when treated with an herbicide or are subjected to submergence stress. Future work will explore the genetic link between resistance and tolerance as a way to elucidate novel opportunities for crop and weed management in the field.

Invasive weeds

Winged primrose-willow (Ludwigia decurrens) is a quarantined invasive weed that was discovered in Butte County in 2011. Detected in irrigation canals and some rice fields, it can grow to six feet or higher. Weed scientists have evaluated its characteristics and options for its control.

Applying a total herbicide like glyphosate can improve control of this weed. Vigilant surveillance is necessary to ensure no escapes or new germination of the weed. The best method of control is a tank mix of Grandstand® with SuperWham!®.

Sound recommendations to prevent seed spread of this weed include hand removal of seed-producing plants with destruction of the plant material, cleaning tillage equipment and combines when leaving known infested fields, and tilling or harvesting known infested fields last. Spraying known infestations should be done early in the season prior to the weed’s yellow flowers being visible. With diligence this noxious weed can be contained and possibly eradicated over time.

Indian marshweed (Limnophila indica) is another quarantined weed that has become a concern in rice production. Greenhouse experiments found good control with early application of Londax® or Sandea® at normal field rates. On well-established plants, field rates of Londax® and League® MVP hindered flowering. All of these herbicides have yet to be field-tested for control of this weed.