Weed Control in Rice-2000 



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Project Leader and Principal UC Investigators

Albert Fischer, professor, Weed Science Group, Vegetable Crops Dept., UC Davis


Weed control studies in 2000 focused on promising new herbicides and combinations of existing compounds; safe and effective herbicide use; weed biology and rice-weed competition; herbicide resistance and management; and transgenic herbicide-resistant rice. Progress in these areas is discussed below.

Promising new herbicides

18WeedCntrl.jpg (8810 bytes)Work continued with Regiment (bispyribac-sodium), which can be used from the five-leaf stage until full tillering of rice to control watergrass. It also controls ricefield bulrush and has some activity on California arrowhead at higher rates. It is characterized as a strong suppressant of herbicide-resistant watergrass. Early applications can cause some stunting on rice. Combinations with Abolish (thiobencarb) appear to be a good option for early use since low rates can be used.

Shark (carfentrazone) continues to work well in foliar applications made from five-leaf stage until mid tillering. Efficacy is lost at later stages. Shark provides good control of ricefield bulrush and California arrowhead. It is also active on smallflower umbrellasedge and redstem. The herbicide will be available for ground-only applications in 2001. Very early applications can result in rice foliage burning. Into-the-water applications for earlier timing can eliminate the need to drain fields and also improves activity on smallflower umbrellasedge but rates need to be higher than for foliar applications.

Command (clomazone) was studied for a third year in 2000. Applied in granular form, it provided excellent control of watergrass and sprangletop. It should be applied very early in rice growth - between half to a maximum of 1.5 leaf stage. Effectiveness decreases substantially and injury to rice increases beyond this point. Command is effective against resistant watergrass. Bleaching occurs for a week to 10 days and may cause yield reductions where injury is high. Different varieties show different levels of tolerance.

Clincher (cyhalofop) was also studied for a third year in 2000. Clincher is a safe and excellent watergrass and sprangletop herbicide that can be applied from two-leaf stage up to panicle initiation. Higher rates can be used as application timing is delayed, such as for "rescue treatments" beyond the mid-tillered stage of rice. Watergrass resistant to whip is also resistant Clincher. Section 18 has been requested for 2001.

Herbicide combinations

A great deal of emphasis was devoted last year to testing herbicide combinations aimed at broad spectrum control and at strategies for delaying resistance to grass herbicides.

Regiment in combination with Abolish has been tested for five years at different rates and timings. Early use (four- to six-leaf stage of rice) at low rates delivered good control of watergrass, sprangletop, ricefield bulrush, smallflower umbrellasedge and ducksalad. This combination also provided excellent control of resistant early and late watergrass, smallflower umbrellasedge, California arrowhead and redstem at the two- to three-tiller stage.

Super Wham (propanil) in combination with Abolish has been tested for four years. It can be used without crop oil at the three- to four-leaf stage of rice to control watergrass, sprangletop, ricefield bulrush, smallflower umbrellasedge, ducksalad, monochoria, California arrowhead and redstem.

Clincher in combination with Super Wham has been tested for two years. It can be used at the three- to six-leaf stage and at the one- to three-tiller stage of rice to control sprangletop and watergrass, as well as ricefield bulrush and smallflower umbrellasedge This mixture is also a good tool to protect these grass herbicides from the development of resistance in watergrass.

Duet - propanil in combination with bensulfuron - is a formulated mixture that has been tested for two years at the four-leaf and two-tiller stages of rice. It provided good control of watergrass, ricefield bulrush, duck salad and monochoria. Control of watergrass was better than Super Wham at the same rates. There are, however, concerns about bensulfuron (Londax®) resistance.

19WeedCntrl2.jpg (9216 bytes)Other interesting mixtures tested this season for the first time include Clincher in combination with Shark; Clincher in combination with Grandstand (trichlopyr); and Regiment in combination with Super Wham. Weed scientists also were encouraged by sequential applications of Clincher at four- to five-leaf stage of rice, followed by Regiment at full tillering; Clincher at four-five leaf, followed by Super Wham at full tillering; Command at half leaf, followed by Shark at four-five leaf; and Command at half leaf, followed by Super Wham at four-five leaf.

New compounds

Weed scientists also tested some new introductions and experimental compounds in 2000. Sempra (halosulfuron) is an active ALS inhibitor for broadleaf and sedge control, which also afforded strong suppression of watergrass. Combinations of this compound with other herbicides were examined. Super Duet (propanil, bensulfuron and carbaryl) demonstrated high levels of efficacy and safety at four-leaf stage on a broad spectrum of weeds. This product will be tested again in 2001 at both an early and later timing. Proper rates and timing of applications are still being examined for the grass herbicide BAS 625H (Aura).

Resistance studies

Thirty-five early and late watergrass samples from growers' fields were tested for resistance in 2000. As in the previous year, a high percentage of samples tested showed resistance, and most of them were resistant to more than one herbicide. Information from this study was incorporated into a GPS map that has been useful in the submission of registration packages for new herbicides. This information is also helping UC farm advisors and PCAs develop herbicide programs for sites with resistance.

Work is in progress in the study of resistance management on a cooperator's farm in Glenn County. Mixtures and sequential applications to prevent reseeding of herbicide-resistant watergrass is showing promise.

Bioassay and laboratory studies with bispyribac (Regiment) have raised concerns about possible cross-resistance (i.e. common mechanism) with bensulfuron (Londax®) in watergrass and smallflower umbrellasedge. This underscores the importance of using this herbicide in tank mixtures, sequential applications or seasonal rotation with herbicides that have different modes of action and mechanism of selectivity to rice. The frequent use of this compound on the same field is strongly discouraged. Deep water and competition from well-established rice stands are essential to prevent the survival and reproduction of resistant weeds.

Yield loss studies

Studies on yield loss from weed infestations continued to develop information useful to support registration of new herbicides. In the 2000 season yield loss from sprangletop and smallflower umbrellaplant were evaluated in mature rice using field plots and GPS (global positioning) technology. Visual ratings proved more accurate than actual measurements - not unusual for this type of work, scientists report. Further analysis of the data is still taking place, but it is clear that both weeds are capable of greatly reducing rice yields.

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