Weed Control
in Rice - 2008



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Project Leader and Principal  Investigators

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


Weed control studies in this ongoing project are conducted at the Rice Experiment Station, in the fields of cooperating growers, and in laboratories and greenhouses. This work is focused on preventing and managing herbicide-resistant weeds, development of efficacious broad-spectrum weed control, and compliance with environmental and personal safety requirements. Specific research areas cover new and existing herbicides, as well as combinations and sequential applications of different compounds; alternative crop-establishment methods; and strategies to eliminate or reduce herbicide resistance.

Herbicide studies

Herbicide test plots in 2008 were located on two sites at the Rice Experiment Station and one site in Glenn County. These tests examined registered and potential new herbicides for effectiveness, safety and compatibility. Rice varieties used in this work were M-205 and M-206, which contributed 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 recently released 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.

Prowl H2O applied alone as a delayed pre-emergent provided 46 percent watergrass/barnyardgrass control and 33 percent sprangletop control at 40 days after seeding. Prowl H2O did not perform as well as in previous seasons. Control of barnyardgrass and watergrass improved with the addition of Super Wham.

This material does not have post-emergence activity. Best results can be expected when applied prior to weed emergence. Tank mixes with other materials such as Clincher, Regiment, Whip or Super Wham improved late-season watergrass control and rice yields. The combination of Granite SC, Prowl H2O and Clincher provided excellent watergrass control.

Harbinger is an EC (emulsifiable concentrate) formulation of this compound designed for use with a safening agent. It is to be applied to a dry soil surface after seed has been lightly incorporated. Yields were poor for all treatment combinations and high levels of weeds made harvesting difficult in highly variable fields.

 Strada WG (orthosulfamuron) is an ALS inhibitor for broad-spectrum control of watergrass and smallflower umbrellasedge that does not harm rice. Tests were conducted on two formulations a water-dispersible granular form of this compound for pinpoint applications and a granular formulation for into-the-water treatments in continuously flooded rice.

In the pinpoint system, promising results were reported with an early application of Cerano that was followed by an application of Strada. All applications of Strada in combination with propanil following Cerano provided excellent grass and ricefield bulrush control.

The granular Strada GR applied into-the-water following an early application of Cerano, or followed by propanil, was also promising.

Granite (penoxsulam) is an ALS-inhibiting herbicide applied after flooding for selective control of susceptible watergrass and barnyardgrass, broadleaf weeds and sedges. The granular formulation Granite GR was commercially available for the first time in 2005. It was tested in combination with Cerano, propanil, Clincher and Shark  (carfentrazone). Most treatments provided good to excellent weed control. Good treatments included Cerano followed by Granite, Granite followed by Clincher, Granite followed by Stam and Shark applied same day as Granite.

Rice culture studies continue

Testing on the three major systems of rice culture
continuous flood, pinpoint flood, and dry or drill-seeded continued in 2008. Research is seeking the best herbicide and management combinations for each of these systems.
A comprehensive strategy relying on different herbicide combinations will protect against resistance.

Continuous flood system

The first month after seeding is the critical period of competition between weeds and rice. Best yields are obtained in continuously flooded fields when herbicides provide at least 95 percent broad-spectrum weed control during this period. Good weed control in this system can be achieved with early treatments, which generally provided excellent yields. Treatments consisting of an early application followed by a late-season treatment generally were no better than early treatments. However, they can be useful in preventing growth and seed production by late-emerging weeds. Late herbicide treatments produced generally lower and less consistent yields.

Granular formulations applied early into-the-water are excellent non-drift tools for this system. Sequences of Cerano, followed by either Granite GR or propanil provided excellent broad-spectrum control. Some stunting and dark green color of rice could be noticed after a Granite GR treatment. Other treatments that worked well following Cerano were Strada GR followed by propanil, Strada WG plus propanil, Shark plus Granite GR, and Shark plus Londax.

Another promising combination is Cerano followed by a tank mix of propanil and a new Valent Corporation herbicide, V-10142 (imazosulfuron), which may be available in the near future.

Good sequences can also start with Granite GR applied into the water, followed by either propanil or Clincher. Shark followed by Granite GR improved ricefield bulrush control and provided good broadleaf and sedge control. Shark plus Londax, followed by Clincher was also efficacious and provided good yield. Valent is also developing another into-the-water granular herbicide, V-10219, which is a combination of Bolero and imazosulfuron. Rice yields for these treatments were in the range of 5,200 pounds/acre to 7,250 pounds/acre at 14 percent moisture.

Pinpoint flood system

Weed infestations in the pinpoint system can have a stronger impact on yield because of the temporary elimination of flooding. Smallflower umbrellasedge and sprangletop are common weeds in this system. Short drainage periods (48 hours or less) will, however, help prevent these species from getting established.

Good control (95 percent or greater) during the first month after planting in this system is also essential for the best yields. This is best achieved with early applications. Split early and late applications gave similar results. Late applications were not as successful.

The best broad-spectrum treatments were Clincher tank-mixed with Granite SC and propanil, Granite SC followed by propanil, and Clincher followed by Regiment. Most treatments had similar yields.

In another experiment in the pinpoint system, a single application of propanil at the one-tiller stage of rice gave total control of resistant late watergrass, smallflower umbrellasedge and ducksalad, the main weeds present.

Clincher followed by propanil, Regiment followed by propanil, or a tank mix of Regiment plus Abolish followed by propanil also gave excellent control.

Drill-seeded system

Drill-seeded M-206 was flushed three times for establishment with a permanent flood applied when rice was at the five-leaf stage. The main weeds in this system were watergrass and sprangletop. Residual herbicides Prowl H2O and Abolish applied in delayed pre-emergence were ineffective in 2008 trials in contrast to prior trials.

Early season control of watergrass and sprangletop that lasted through the season generally led to the highest yields in this trial. For the second year, the best yields were achieved with Regiment plus Abolish, followed by Clincher. Other treatments that worked well in this system were Granite SC followed by Clincher; or tank mixes of Granite SC, Prowl H2O and Clincher or Prowl H2O, Regiment, and Whip.

Granite SC is a fluid formulation for foliar application. It was labeled for California in 2006 and was in good supply in 2007. It was tested in a pinpoint flood system with floodwater dropped at the three-to-four leaf stage of rice. High-yielding treatments included Clincher tank mixed with Granite SC followed by Stam, a tank mix of Granite SC and Stam followed by Clincher, and Granite alone. Granite SC will not control sprangletop; therefore Clincher is needed for control of this weed.

V-10142 (imazosulfuron) is a Valent Corporation dispersible granule. It is intended as a tank-mix partner for follow-up spray treatments after an into-the-water herbicide. Treatments with Cerano followed by a tank mix of this herbicide with propanil or Bolero followed by a tank mix with Regiment provided good weed control and yield. This compound is an ALS inhibitor, so it not recommended for use with similar herbicides such as Granite or Londax.

V-10219 (thiobencarb plus imazosulfuron) is a Valent Corporation combination granule being tested for into-the-water application. This compound was tested at three rates with interesting results.

Halomax 75 (halosulfuron) is another ALS herbicide that needs the same precautions for use as other compounds with a similar mode of action. It was in the final stages of EPA approval for California in April 2009. It was tested as a stand-alone treatment, but would work better in a program addressing weeds such as sprangletop and watergrass.

Stam 4SC is a liquid suspension of propanil that was tested as a stand-alone treatment in a pinpoint system. It provided good weed control.

 Mimic control on a Glenn County farm

Mimic is resistant to all available herbicides for watergrass control except propanil. In a continuously flooded system two basins were treated with either Cerano or Granite GR. In the Cerano basin the best treatment was this herbicide followed by either propanil or Granite SC. Best results in the Granite GR basin were obtained when this herbicide was followed by propanil either alone or mixed with Clincher. Paddy yields with these treatments ranged from 9,000 pounds/acre to 10,000 pounds/acre at this site.

Alternative stand establishment

Research also continued on alternative rice establishment systems for their potential to reduce weeds.  The systems include drill-seeded rice, water seeding, no-till options, and the use of a stale seedbed technique (promoting early weed emergence through irrigation flushing followed by glyphosate).

After four years of maintaining the same establishment technique on a large-plot experiment at the Rice Experiment Station, treatments were rotated to determine the usefulness of this strategy.

All drill seeding was replaced by water seeding and vice versa to offset weed adaptations and all treatments, except a conventional water-seeded control. Weeds were allowed to germinate prior to seeding, followed by a glyphosate (stale seedbed) treatment. This dramatically reduces the weed pressure on the crop as long as the soil surface is not disturbed after glyphosate application.

Only three of the original treatments were implemented in 2008: water-seeded conventional, water-seeded stale seedbed, and drill-seeded stale seedbed. In addition, the two stale-seedbed treatments were not spring-tilled.

Evaluations of weed cover and yield indicate that the drill-seeded treatments needed follow-up herbicide applications. The water-seeded, no-till, stale seedbeds did not need the follow-up herbicide treatments. This suggests strongly that switching establishment systems in conjunction with the use of a stale seedbed technique is effective in controlling weeds in these systems.

Mimic (late watergrass)

Late watergrass (mimic) is one of the most important weeds in California rice fields. It has evolved resistance to most available herbicides, severely limiting control options. Accurately predicting rice yield loss based on simple early-season measurements can reduce guesswork involved in deciding among weed control options. A series of competition experiments were conducted between rice and four late watergrass biotypes. Resistant biotypes had a slightly smaller effect on yields than susceptible ones.

Early watergrass

Early watergrass is an aggressive weed in California that has evolved resistance to several herbicides. To provide insight into the origins and spread of resistance, 434 individual weeds from 23 populations in rice fields across California were analyzed. Findings suggest that resistant biotypes of early watergrass are spreading by seed dispersal and by genetic adaptation.


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