Weed Control - 95


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Project Leader and Principal UC Investigators

David E. Bayer, Professor, Weed Science Program, Department of Vegetable Crops, UC Davis

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

Ernest Roncoroni, Staff Research Associate, Department of Vegetable Crops, UC Davis

Michael Hair, Postgraduate Researcher, Dept. of Agronomy & Range Science, UC Davis

David Brandon, Research Assistant, Weed Science Program, UC Davis

Damon Roush, Research Assistant, Weed Science Program, UC Davis

Steven C. Seardaci - UC Farm Advisor, Colusa county

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

Michael Carriere - Research assistant, Dept. of Agronomy & Range Science, UC Davis

Weed control is one o the biggest ongoing challenges facing the California rice industry. With the advent of Londax-resistant weeds four years ago, the development of an effective strategy to battle broadleaf weeds has become critical. Objectives guiding weed control research are:
  • To develop chemical methods of weed control in rice and to improve herbicide efficacy and safety.
  • To study the biology and physiology of rice weeds in the field, greenhouse and laboratory.
  • To study Londax-resistant weeds and to develop strategies for their control.

Weed1.jpg (63772 bytes)New Chemicals Analyzed

Last year researchers continued to evaluate the new herbicides V-10029 and F-8426, as well as an older herbicide, Grandstand (trichlopyr). In early postemergence field tests V- 1002 9, primarily a barnyardgrass/watergrass herbicide, was not as effective alone as it was in combination with Abolish. A combination of V-10029 with Prowl caused permanent injury to rice. V-10029 also showed reasonably good activity on ricefield bulrush.

Both F-8426 and Grandstand are broadleaf herbicides. Grandstand has a current registration in the southern United States. Research in California will lay the necessary groundwork for registration here. In tests last year both these herbicides effectively controlled ricefield bulrush, the predominant sedge on the Rice Experiment Station and one of the two major sedge weeds afflicting the rice industry.

Herbicide Timing

Timing is critical for effective herbicide control - to ensure that weeds are knocked out at their most vulnerable stage of development and to prevent damage to emerging rice plants. In studies with Grandstand and MCPA, severe injury occurred at early 2-3 leaf stage treatments, indicating that rice should have at least one tiller (5 leaf stage) before treatment with these herbicides. At 20 to 25 days after seeding both chemicals controlled ricefield bulrush and ducksalad with minimal injury to rice.

Researchers found several different formulations of 2,4-D (2,4-D Amine, Hl- DEP, and Solution) to be equally effective. All applications were made at the 1-2 tiller stage (30 days after seeding) to avoid injury to rice. 2,4-D Amine in combination with Grand- stand gave excellent control at all rates.

Extensive testing of Whip showed that early applications (4-5 leaf to 1 tiller) controlled sprangletop and watergrass effectively, although late-season watergrass reinfested the plots. Applications of Whip in the 2-4 tiller stage gave season-long control of these weeds.

Herbicide Injury

In recent years a few California rice fields have shown mid-season injury from Bolero or Abolish (thiobencarb) treatments. Researchers believe the cause of the injury is not the herbicide itself but a breakdown product or metabolite called deschlorothiobencarb (DCF). DCT is created through thiobencarb's interaction with anaerobic microorganisms.

In greenhouse studies DCT was isolated from soils collected in fields that had previously shown damage. Treatment with the metabolite resulted in severe injury to rice.

Londax Strategies Explored

Control strategies for Londax-resistant weeds were tested in a resistant California arrowhead field at the Rice Experiment Station. Neither Londax granules or later foliar applications of Londax controlled the resistant weeds. Applications of MCPA, 2,4-D or F-8426 alone or in combination with Londax did, how- ever, provide control of resistant weeds.

Londax Resistance Surveyed

Weed2.jpg (126458 bytes)University of California scientists, DuPont Co. representatives and pest control advisers continued their survey of Londax resistance. Since first being discovered in 1992, weeds resistant to Londax's mode of action have increased substantially. Four important weed species are involved - California arrowhead, smaliflower umbrella- plant, ricefield bulrush and redstem.

California arrowhead appears to be the most resistant weed, with resistance turning up in approximately 60 percent of more than 4,100 fields surveyed. Resistance by redstem and smallflower umbrellaplant (a sedge) was found in approximately 35 percent of the fields and ricefield bulrush in 19 percent.

Resistance has been documented in every rice-producing county. About 20 percent of the fields in the areas where resistance was originally found are now beyond economic treatment with Londax. But Londax resistance affected less than 10 percent of the total rice acreage.

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