Weed Control in Rice - 2010




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Project Leader and Principal  Investigators

Albert Fischer, associate professor, 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 safety requirements. Field testing in 2010 took place at the Rice Experiment Station and in a cooperating grower’s field, where the weed spectrum includes heavy infestation of late watergrass that is strongly resistant to multiple herbicides. This project also conducted studies of variety tolerance to several herbicides and collaborated in an alternative stand management study.

Herbicide studies

Herbicide test plots examine registered and potential new herbicides for effectiveness, safety, and compatibility. Rice varieties used in this work were M-205 and M-206, which contribute to reduced lodging and improved harvest yield.

Prowl® (pendimethalin) is a selective herbicide for controlling annual grasses such as barnyardgrass and sprangletop and certain broadleaf weeds as they germinate and emerge. Prowl® H2O is a water-based capsule formulation that has been developed for use in dry and drill-seeded rice. It needs to be applied to moist soil without any standing water. Flooding causes this chemical to degrade faster and lose efficacy.

Prowl® H2O applied alone as a delayed pre-emergent (DPRE) provided 49% watergrass/barnyardgrass control and 33% sprangletop control at 40 days after seeding. However, that figure dropped to 25% at 60 days after seeding. Sprangletop control was 75% at the later date. Prowl® H2O is not effective against already emerged weeds. Best performance with this compound can be expected when applied prior to weed emergence. Therefore, grass control was greatly improved when Prowl® H2O DPRE was followed by Super Wham!®.

When weeds were already emerging at time of application, a tank mixture of Prowl® H2O, Clincher®, and Super Wham!® greatly improved control and yield.

The combination of Granite® SC, Prowl® H2O, and Clincher® applied at the two- to three-leaf stage of rice provided outstanding grass control, although yields were no better than other, less elaborate treatments.

Strada® WG (orthosulfamuron) is an ALS inhibitor for broad-spectrum control of watergrass and smallflower umbrella sedge that exhibits little impact on rice. Tests were conducted on two formulations—a water-dispersible granular form for pinpoint applications and a granular formulation for into-the-water treatments in continuously flooded rice.

An experiment was conducted to determine whether the wettable granule could replace the other formulation for spreading to reduce costs. Overall, weed control and yield were best with the granular formulation, although yield was not statistically superior. The wettable formulation is unlikely to be labeled. An application at a later timing in a tank mix with Ultra Stam® afforded less control of weeds and lower yields than the other treatments.

In a continuously flooded experiment, excellent weed control and good yields were reported when Cerano® followed an early application of Strada®, which could then be followed by Stam®.

Granite® (penoxsulam) is an ALS-inhibiting herbicide applied after flooding for selective control of susceptible watergrass and barnyardgrass, broadleaf weeds, and sedges. It is not active against sprangletop.

The granular formulation—Granite® GR—has been available since 2005. It was tested in combination with a follow-up application of Ultra Stam® 4SC and as a follow-up to Cerano®. Both treatments provided excellent control of broad-spectrum weeds, ricefield bulrush, and ducksalad/monochoria.

Granite® SC is a fluid formulation for foliar application that was labeled for California in 2006. It was tested in a pinpoint flood system with floodwater dropped at the three- to four-leaf stage of rice. Alone or in combination, Granite® SC provided excellent control and good yields.

Ultra Stam® 4SC is a liquid suspension of propanil tested in both continuously flooded experiments and in the pinpoint system. Applied alone following Cerano®, it provided better weed control than Super Wham!®. In the pinpoint trial, all formulations of propanil performed similarly.

Bombard™ is a new granulated formulation of clomazone. Weed suppressive activity and yield appear similar to Cerano.® Best results were at one-leaf stage of watergrass rather than day of rice seeding.

Rice culture studies

Testing on the three major systems of rice culture—continuous flood, pinpoint flood, and dry or drill-seeded—continued in 2010. This research is seeking the best herbicide and management combinations for each of these systems.

Continuous flood

In the continuous flood trial, good weed control was achieved with early treatments. Best results were obtained when herbicide programs provided at least 95% broad-spectrum weed control in the first month after seeding.

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

Cerano® applied early is a very good grass herbicide and provides good broad-spectrum control when followed by propanil, Strada® GR, Sandea®, Granite® GR or Strada® GR followed by Stam® 80EDF (propanil). Granite® GR, followed by Ultra Stam® 4SC provided excellent broad-spectrum control of rice weeds. Other treatments that worked well in this system were Abolish® followed by Super Wham!® or Bolero® Ultramax followed by Super Wham!® (or Regiment®).

The new clomazone formulation Bombard™ was also tested in this system. Bombard™ is a prilled rather than extruded granule. Yields of field-rate treatments were not statistically different for the two formulations.

Pinpoint system

The pinpoint system used in California requires early draining to expose emerging weeds to foliar herbicides. However, exposure of soil surface to air also favors the establishment of other weeds—smallflower umbrella sedge, barnyardgrass, and sprangletop. Thus, it is important that fields be rapidly reflooded within 48 hours of herbicide application.

Follow-up applications can be made at the one-to-two tiller stage of rice with water lowered to expose 70% of weed foliage to the spray. Some of the best broad-spectrum treatments were Regiment® followed by Super Wham!®, Clincher® tank-mixed with Granite® SC followed by propanil, Granite® SC alone or tank mixed with Ultra Stam® 80EDF followed by Clincher®, Regiment® tank mixed with Abolish®, Regiment® alone, or Clincher® followed by Ultra Stam® 80EDF.

Drill-seeded system

The drill-seeded plot was flushed with water three times for establishment. A permanent flood was applied when rice was at five-leaf stage. Significant yield losses were associated with infestations by the main weeds in this system—watergrass, barnyardgrass, and sprangletop.

As in the previous year, there were no stand-alone treatments that provided sufficient weed control and good yields in the drill-seeded trial. Several combinations that provided excellent weed control and good yield were Clincher® followed by Super Wham!®, a tank mix of Prowl® H2O with Granite® SC and Clincher®, Granite® SC followed by Clincher®, a tank mix of Prowl® H20, Super Wham!® and Clincher®, Granite® SC with Clincher® followed by Super Wham!®, or Regiment® with Abolish® followed by Super Wham!®.

Variety tolerance

Several recently available herbicides appear to impact rice during the critical establishment period, when a well-developed canopy is crucial for suppressing early weed growth.

In some instances, Cerano® has been noted to reduce stand and cause bleaching of rice plants. Granite® GR and Granite® SC can stunt root development and cause shorter stature and darker green plant growth. Regiment® has been noted to cause some stunting of rice and occasional yellowing of foliage. Researchers conducted tests to observe variety response to some of these herbicides in 2009 and again in 2010. Specifically, they examined Cerano®,

Granite® GR, Granite® SC, and Regiment® on six predominant California rice varieties—Calmochi 101, L-206, M-202, M-205, M-206, and S-102—to determine whether these herbicides may impact rice growth and yield in the absence of weeds. Prior to seeding, the field was subjected to a stale seedbed treatment with glyphosate. A mid-season propanil treatment was needed to keep plots weed-free throughout the season. Tests were conducted with Granite® GR and Cerano® in a continuous flood trial and Granite® SC and Regiment® in a pinpoint system.

All varieties responded similarly to the herbicides. No herbicide injury, stand loss, delay in heading, nor yield reductions were observed for either herbicide in the pinpoint system. Effects in the continuous flood trial were variable. However, results showed that injury to rice can occur with Cerano® and Granite® applied into-the-water when rates are doubled. This might occur in areas of grower fields where applications overlap. Given the variation in observations from both years, the experiments will be conducted again in 2011.

Alternative stand establishment

Alternative stand establishment techniques developed at the Rice Experiment Station continue to show promise.

A cooperating grower in Glenn County used a spring-tilled stale seedbed technique on a 10-acre field plagued with resistant late watergrass or “mimic.” Glyphosate eliminated weeds germinated with an early irrigation prior to flooding and seeding rice.

This technique has proven very successful in reducing or eliminating watergrass and sprangletop competition during the growing season. The hope is that several years of using this technique will give growers a reliable tool to reduce the weed seedbank in soil to allow a transition back into more conventional high yield production practices. A neighboring grower tried this technique on several fields infested with resistant late watergrass and produced excellent yields. A field where the stale seedbed technique had been used the previous two years has successfully been reverted to conventional rice production.

Since late watergrass has evolved resistance to almost all available grass herbicides in California, there is a pressing need to refine alternative control methods. Accurate predictions of weed seed germination would help this alternative stand establishment technique. Several controlled laboratory experiments were conducted to better understand germination rates, thresholds, and patterns under varying simulated environmental conditions.

Diagnostic services

Scientists continue to screen potentially resistant watergrass and barnyardgrass samples against susceptible and resistant lines. During the past four seasons, results of testing against commonly used herbicides at standard field rates have been well received by growers and pest control advisers.


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