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September 6, 2006

Manipulating weed management practices can reduce herbicide dependency in rice

A University of California, Davis research team has found that different rice establishment methods can keep weeds from developing, reduce herbicide dependency, lower fuel use, and help reduce herbicide-resistant weeds in rice.

   Barnyardgrass flower head.

Barnyardgrass flower head.
Photo by Jack Kelly Clark

California’s rice industry produces nearly 2 million tons of rice annually, making it the second largest rice growing state in the nation and contributing nearly $500 million dollars to the state's economy. For the rural Sacramento Valley communities of Colusa, Butte, Sutter, and Yuba counties, rice is the predominant crop. California's rice is exported to Asia, the Middle East and Mediterranean markets, and is distributed throughout the United States.

Weed scientist Al Fischer and Jim Hill, UC Cooperative Extension specialist, who coordinated the project, say, “As California rice growers find their herbicide options dwindling because of widespread herbicide resistance in the major weeds of rice, the need for non-chemical means of manipulating weed management practices is imperative. Both molinate and thiobencarb had been for years the most relevant herbicides for grass control in rice, but due to resistance they no longer control watergrass. Molinate registration will expire by 2008. In addition, rice straw burning restrictions and subsequent increased straw incorporation has increased soil weed seed banks by protecting the seeds from scavengers like birds and rodents.”

With funding from the UC Statewide Integrated Pest Management Program, CalFed, and USDA, Fischer and his research team conducted field studies to find diverse weed management options in rice cropping systems to reduce dependency on herbicides.

Fischer experimented with rice establishment treatments including water- or drill-seeded rice, fall or spring tillage, no-till, and stale-seedbed techniques to assess their effects on weed recruitment and control.

During the three-year study, the lowest weed infestation occurred where rice was water-seeded after a stale seedbed without spring tillage. “In the stale seedbed technique, growers use irrigation flushes to promote weed emergence, followed by pre-plant burn-down application of glyphosate (Roundup) for which resistance has not evolved in weeds of rice.” says Fischer.  “This method depletes weed populations from the upper soil layer and markedly diminishes the amount of weeds emerging with the crop; glyphosate is considered an environmentally benign herbicide.

“Success with the stale-seedbed technique depends on keeping seedbeds moist and allowing sufficient time for most weeds to emerge prior to glyphosate application. If the stale-seedbed technique is followed by limited or no soil disturbance prior to seeding rice, very little weed control is needed thereafter.”

In addition, experiments show eliminating spring tillage (preparing land for crop planting by plowing or disking) reduced weeds by 75 percent in the water-seeded treatments, suggesting that this may be an effective cultural technique to further reduce weed populations in rice. Less tillage also means lowering the costs for fuel—a very real concern in the face of ever increasing energy prices.

“We designed the systems with grower participation to address specific key concepts,” says Cass Mutters, University of California farm advisor and part of the research team. “Dry-seeded and water-seeded rice can drastically alter the species composition of weed populations that emerge with the crop. Aquatic weeds are strongly disfavored in drill-seeded rice, while grasses like barnyardgrass or sprangletop are suppressed by water seeding. By alternating stand establishment systems we can break weed life cycles to reduce weed infestations and weed seed input to the soil seed-bank. Also, drill-seeded systems and the stale seedbed technique allow using herbicides with new modes of action to control weeds that have evolved resistance to herbicides currently used with water-seeded rice.  A crop established without weed interference rapidly develops a competitive canopy that suppresses further weed establishment.”

Weed ecologist Mike Moechnig says,Modeling of weed recruitment and growth is being evaluated for this experiment to identify stand establishment sequences that may reduce seed-banks of problematic weed species. Results from this research will be used to develop integrated weed management programs by breaking weed life cycles through rotation of stand establishment methods, alternating herbicide modes of action, as well as effective crop interference.

“This research gives rice growers the essential tools to control expanding populations of herbicide resistant weed biotypes,” says Fischer. “In many rice-growing areas, the crop is grown in heavy-textured rice-only soils where no other crops are economically viable, and it is these cases that we are trying to address in this work.  A viable and practical solution to manage herbicide resistant weeds in these rice-only rotations is to alternate conventional water seeding with new rice establishment methods.”

The potential benefits of no tillage, or conservation tillage, include water conservation, dust suppression, reduced pesticide runoff into surface water, lowered labor needs and costs, and fuel savings. In addition, limiting tillage helps to keep carbon in the ground and prevent the buildup of greenhouse gases in the atmosphere.

Besides expanding options for weed control, implementing alternative rice establishment systems implies diversifying tillage operations and water management. Hill coordinates efforts to implement appropriate fertilizer regimes for the new systems and to assess their potential benefits on water quality. Soil scientist Chris van Kessel, associate researcher Bruce Lindquist and PhD student Kaden Koffler conduct research on plant nutrients, fertilization and water quality. Cass Mutters is involved with agronomic research and oversees field operations, and farm advisor Chris Greer coordinates outreach activities.

Members of Fischer and Hill’s research team are:

From Plant Sciences at UC Davis:

  • Chris van Kessel, professor and chair
  • Michael Moechnig, postdoctoral scholar; currently South Dakota Cooperative Extension weed specialist
  • Bruce Linquist, associated researcher
  • Kaden Koffler, PhD student
  • Ray Wennig, senior research associate
  • James Eckert, senior research associate
  • Steve Bickley, post graduate researcher

From UC Cooperative Extension: farm advisors Randall “Cass” Mutters, Butte County, and Christopher Greer, Colusa County.


High-resolution image (85KB) "Barnyardgrass flower head." Photo credit: Courtesy of UC Statewide IPM Program, Jack Kelly Clark. Photos are for use with this release only. All other uses see Legal Notices.


Stephanie Klunk, Communications Specialist
UC Statewide IPM Program
(530) 754-6724

Albert Fischer, Weed Scientist
Plant Sciences, UC Davis
(530) 752-7386

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