Weed Control in Rice-99


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Project Leader and Principal UC Investigators

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

Michael D. Carriere, Interim specialist, Department of Agronomy and Range Science, UC Davis

James E. Hill, Specialist, Department of Agronomy and Range Science, UC Davis

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

David Cheetham,  Department of Agronomy and Range Science/Dept of Vegetable Crops, UC 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

M. W. Hair, Department of Agronomy and Range Science, UC Davis

Weed scientists evaluate promising new herbicides, combinations of existing compounds and other research topics affecting the most challenging area of rice management. Reports on transgenic rice, herbicide resistance and weed biology are also summarized below.

Promising New Herbicides

Weed scientists continue examining a variety of compounds at the Rice Experiment Station.   Clomazone(Command®) is a grass herbicide providing good watergrass and excellent sprangletop control.  The mode of action for this herbicide — a pigment synthesis inhibitor — is different than that of all other grass herbicides available, or currently being tested.  Because of its chemistry, clomazone has the potential for becoming a useful tool in the management of herbicide resistance in watergrass.  A granular formulation of this compound proved promising for drift control and into-the-water applications.  Further work will focus on refining rates and timing.

IR-5878, a new ALS inhibiting compound, was tested for the second year.  It showed good broad-spectrum control, although it was somewhat weak on sprangletop.  In general, it tended to perform better at earlier timings.

BAS 625H (sefoxydim) was tested at RES for the first time in 1999.  An experimental herbicide for watergrass control, it needs further work for improved activity and safety to the crop.

Other grass herbicides on which work continued from last year included:

• Cyhalofop (Clincher®) — effective on watergrass and sprangletop with a better margin of safety on rice than Whip®, a similar compound.  A granular formulation of this herbicide allows into-the-water applications, although the availability of this formulation in the future is uncertain.  This herbicide may face resistance problems in areas with resistance to Whip®.

• Bispyribac-sodium (Regiment®) — good watergrass herbicide that can be useful to suppress herbicide-resistant forms of these weeds.  It is a postemergence herbicide that is also effective on barnyardgrass and moderately active on ricefield bulrush.  However, it is an ALS inhibitor (like Londax®) and resistant watergrass biotypes have been detected.  Repeated use on the same area must be avoided, as well as areas with strong bensulfuron (Londax®) resistance.

• Carfentrazone (Shark®) — Although this herbicide’s registration was revoked last year after reports of drift damage, the California Department of Pesticide Registration allowed experimental work to be carried out.  Special attention was paid to into-the-water applications, which proved safer for rice than foliar applications even though higher rates were needed to afford similar control.  Future work will test the dry flowable formulation applied directly into water.   Carfentrazone provided good control of ricefield bulrush and California arrowhead but was slightly less active on smallflower umbrellaplant and redstem.   Into-the-water applications provided better control of smallflower umbrellaplant.

Herbicide Combinations

Combinations of herbicides are a powerful tool for growers to expand the spectrum of weed control and as a tool to delay the development of herbicide resistance.  Several combinations and sequential applications studied during 1999 include:

• The standard treatment with thiobencarb (Abolish®) followed by propanil (SuperWham®) resulted in enhanced watergrass control and good sprangletop control.   This mixture should be useful to delay herbicide resistance in watergrass.

• Tank mixtures of cyhalofop and propanil were tested as a tool to delay resistance in watergrass while adding sprangletop control.  This mixture controlled watergrass, sedges and sprangletop.

• Bispyribac-sodium tank mixed with thiobencarb provided excellent control of watergrass and sprangletop, and the surfactant normally required for bispyribac was not needed.

• Duet®, the formulated mixture of propanil combined with bensulfuron, proved better than propanil alone on watergrass and ricefield bulrush.

• Cyhalofop, applied at the three-leaf stage of rice, followed by trichlopyr (Grandstand®) at mid-tillering provided broad spectrum control, including watergrass and ricefield bulrush.

• Good results and no antagonisms were observed when cyhalofop was followed by either bensulfuron or carfentrazone.

• Good compatibility and sedge control in addition to watergrass control was observed in tank mixtures of bispyribac-sodium and carfentrazone applied at the 5-6 leaf stage.

Trichlopyr was effective on redstem and therefore appears to be a good complement for carfentrazone.

Transgenic Rice

Work on transgenic glufosinate-resistant transgenic rice (Liberty-Link rice) expanded in 1999 to include Roundup-Ready rice efficacy tests.  Results are promising and sequential applications of glufosinate were excellent on watergrass and sprangletop.   Mixtures of glufosinate with either carfentrazone or bensulfuron improved control of smallflower umbrella sedge; however, some injury to rice was observed when glufosinate was tank mixed with carfentrazone.

Herbicide Resistance Studies

Screening for herbicide resistance on watergrass seed samples submitted by growers continued.   Similar patterns of resistance were observed but a larger number of herbicide-resistant early watergrass biotypes were detected than in previous years.   A clear understanding of the mechanism of resistance to bispyribac in watergrass was developed, leading to specific guidelines for use of this herbicide.

Collaborative studies with the University of Kyoto in Japan suggest that seed dispersal among rice fields plays a key role in the expansion of herbicide resistance in California watergrass.   With supplemental funding from the California Department of Pesticide Regulation, a long-term, large-plot experiment was initiated on a grower’s field to test herbicide use strategies for managing herbicide resistance in early watergrass.  A portion of these plots is also being used to test herbicides for short-term, in-season control of resistant early watergrass. 

Integrated weed management will be key to delaying the development of resistance in both early and late forms of watergrass.   Basic principles of herbicide use include the avoidance of repeated applications of herbicides with the same mechanism of action by alternating herbicides in successive years and by using herbicide combinations of watergrass herbicides with different modes of action.   Knowing the patterns of resistance within a given field through the testing of samples will help choose the most effective herbicide sequences or mixtures.  Great emphasis is required to prevent seed dispersal of herbicide-resistant weeds among fields. Judicious herbicide use is paramount to avoid losing more chemicals to herbicide resistance.

Weed Biology

Given the scarcity of registered herbicides for use in rice and the risk of herbicide resistance in rice weeds, new opportunities for weed suppression are crucial.     With that in mind, researchers are attempting to define critical periods in the growth cycle of the rice crop that can be used to compete with and suppress weeds. One experiment demonstrated that rice competition suppresses watergrass emerging 30 days after seeding (DAS) or later when a continuous flood of 4-inch water is maintained.   Additional weed control beyond this point was unnecessary.  Watergrass that emerged before 30 days, however, did impact rice yields.  This study also showed that initial weed control could be delayed up to 15 DAS without compromising yields.

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Researchers also established yield loss projections caused by sprangletop infestations.   Significant yield reductions occurred at infestation levels above 10 sprangletop panicles per square foot or 25 percent canopy cover.  If yield loss-weed infestation relationships are to be useful in making decisions about herbicide use, weed infestation levels need to be determined early in the growing season.  Low weed infestations may not warrant the cost of controlling them.  If not controlled, however, these infestation will become larger and economically significant in the future.

 Watergrass has proven to be somewhat shade tolerant, adjusting its growth and photosynthesis to compete with rice — but at a price: compromised root growth.   Experimental evidence suggests that nitrogen fertility can be manipulated to take advantage of this weakness in combination with  shading by the rice crop to afford a nonchemical means of watergrass control.  Similar experiments with redstem indicate that it may be more difficult to control with shade.

Another experiment to evaluate the ability of 11 different rice cultivars to compete against watergrass showed considerable variability in competitiveness among cultivars — an asset for determining the rice traits associated with the ability to compete and suppress watergrass seed production.

The above research demonstrates that rice canopy shading and nutrient availability can lead to management alternatives to control some of the most troublesome weeds of rice. Rice with more vigorous root systems, for instance, could compete effectively with watergrass for nitrogen.  Similarly, early canopy development in competitive rice cultivars would also prevent late emerging weeds.   Manipulating crop competition and nutrient availability could significantly shorten the period during which watergrass needs to be controlled with herbicides, leading to more economical and environmentally safe weed control, and assisting in the prevention of herbicide resistance.

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