Weed Control in Rice - 98


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Project Leader and Principal UC Investigators

Albert J. Fischer, Assistant Professor, Weed Science Group, University of California, Davis

James E. Hill, Extension Agronomist and Chair, Department of Agronomy and Range Science, UC Davis

W. Mick Canevari, UC Farm Advisor, San Joaquin County

David Cheetham, Postgraduate Researcher, Department of Agronomy, University of California, Davis

Randall G. "Cass" Mutters, UC Farm Advisor, Butte County

Stacey R. Roberts, Postgraduate Researcher, Department of Agronomy and Range Science, UC Davis

Steven C. Scardaci, UC Farm Advisor, Colusa, Glenn and Yolo counties

John F. "Jack" Williams, UC Farm Advisor, Sutter and Yuba counties

Evaluation of new and existing herbicides, development of transgenic rice and management for herbicide resistance are the three areas topping 1998 weed control research. Progress in these areas is reported below.

Promising new herbicides

Evaluation of carfentrazone (Shark) is continuing with researchers comparing surfactants, combinations with other herbicides and into-the-water applications. As in previous studies, carfentrazone controlled broadleaf and sedge weeds with excellent control of ricefield bulrush and good to very good control of smallflower umbrella sedge. Surfactants generally increased rice injury without improving weed control. Foliar and into-the-water applications were both effective, but into-the-water applications required approximately twice the rate for similar levels of control. Combinations of carfentrazone with trichlopyr (Grandstand) were very effective at half the normal use rate, thus offering growers an opportunity to cut costs.

WeedCtrl1.jpg (64768 bytes)Several candidate herbicides were evaluated during 1998 at the Rice Experiment Station. One of these, a Japanese herbicide called cyhalofop (Clincher), was tested for the first time last year in granular and foliar formulations. It looked particularly promising for watergrass and sprangletop control and has the capability for both early and later-timed treatments. Work is needed on efficacious rates and timing of applications, even though it is similar chemistry to fenoxaprop (Whip). It also appears to be less damaging to rice. However, neither of these herbicides proved very effective against resistant strains of watergrass.

Bispyrabac-sodium (Regiment) has been tested at the Rice Experiment Station for more than 10 years and is expected to be registered as a watergrass and barnyardgrass herbicide next year. It is a postemergence herbicide that also partially controls ricefield bulrush. It is not as effective against sprangletop.

Other herbicides examined included:

  • Clomazone -- good watergrass control, 100 percent sprangletop control at highest rate and earliest timing; some injury to rice observed.
  • IR-5878 - tested on broad spectrum of weeds; control best at earliest timing (1-2 leaf stage) and higher rates.
  • IR-5790 --offered better control of broadleaf weeds at later timing (5-6 leaf stage); activity increased in combination with propanil.

These herbicides are in early stages of testing to develop a base of information on their suitability for use in California rice.

Propanil revisited

Because of problems with herbicide resistance, propanil use is expanding in California. Two potential low-drift formulations of propanil were compared for efficacy and safety to rice. Both (Super Wham SC and Stam EDF) were found to be similar in activity, providing good watergrass and sedge control. However, sprangletop control was weak to fair when not in combination with another grass herbicide.

Transgenic Rice

The use of transgenic rice cultivars with ˘built in÷ resistance to broad spectrum herbicides such as glufosinate (Liberty) is progressing. Last year for the first time, research was conducted on a California variety (M-202). Single applications were good, but split applications of glufosinate were better. Depending on weed pressure and initial timing, growers will be able to determine if a second application is needed when bioengineered rice comes into commercial production.

Watergrass resistance

Continuous rice culture and the restricted availability of grass herbicides have led to strains of watergrass showing resistance to different herbicides. However, recent testing showed that resistance to propanil is currently infrequent and low, suggesting that for the short-term this active ingredient will be a useful management tool. Researchers caution that overuse of propanil will accelerate resistance, as has happened in other rice growing regions. It is important that watergrass control be diversified by preventive, mechanical and cultural practices aimed at eliminating the survival, seed production and dispersal of plants that escape herbicide treatment. Avoidance of herbicides with the same mode of action should help delay the buildup of resistance.

Rice/Weed Competition

The detection of resistance to herbicides throughout the rice growing areas of California involving most of the herbicides currently available for control in rice poses a serious problem for rice growers. Low-cost practices capable of delaying selection pressure are needed. Work in 1998 evaluated the potential for enhancing the competitive ability of rice.

ducksalad.gif (24404 bytes)Weed scientists conducted experiments to determine the requirements for weed suppression and the specific rice traits responsible for exerting competitive pressure. One experiment evaluated the response of four weeds (early watergrass, late watergrass, smallflower umbrella sedge and redstem) to shade to determine how much these weeds might be suppressed by rice plants with faster growth or a different type of canopy. All four species were affected, although by varying degrees. Results suggest the effect of reduced light is most important at early stages of weed growth.

Limiting nitrogen availability to watergrass was also shown to significantly reduce weed growth. In order to suppress weeds emerging simultaneously with rice, the crop must be capable of competing strongly for light and nutrients. Further research on below-ground competition is needed.

The second experiment found substantial variability in the ability of 11 rice cultivars (including some taller obsolete varieties) to compete with watergrass. Researchers believe it may be possible to improve competitive ability without returning to tall lodging-prone cultivars wjile maintaining high yield potential. These experiments suggest that competitive ability can be improved as a low-cost and environmentally safe component of integrated weed management in concert with herbicide use.

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