Weed Control in Rice-03



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Project Leader and Principal UC Investigators

Albert Fischer, weed science professor, Dept. of Vegetable Crops, UC Davis


This ongoing project is researching the use of new and existing herbicides, including combinations of herbicides and sequential applications. Concern about herbicide resistance is giving rise to new efforts to develop rotation strategies that will prolong the life of weed control chemicals important to rice culture.

A major effort was begun last year to begin a long-term experiment on alternative systems for establishing rice. The objective is to incorporate the benefits of crop rotation in a crop that can seldom be rotated. The aim is to provide strategic alternatives for managing herbicide-resistant weeds that, besides alleviating the specific weed problem, would also help prolong the useful life of the chemicals available by delaying the onset and spread of resistance. These systems are based on four basic principles:

  • Depletion of weed seed in the soil through minimum soil disturbance
  • Flushing fields and controlling emerged weeds prior to sowing rice and drill-seeding to permit use of a broader range of herbicides
  • Alternation of aerobic and anaerobic seedbed conditions to shift the composition of weed communities
  • Incorporation of the benefits of crop rotation in a farm feasible manner

Researchers are very encouraged by initial work on this approach and will expand to large-plot experiments in 2004. This study is accompanied by on-farm experiments in collaboration with UC Cooperative Extension personnel. Models for predicting how best to deal with different weed infestation scenarios are in development.

Herbicide Studies

Weed scientists continued efficacy studies on new compounds, combinations and sequential applications.

Cerano (clomazone), a new watergrass and sprangletop herbicide, continues to demonstrate its effectiveness applied into water at various timings ranging from pre-seeding to day of seeding and up to the one-leaf stage of rice post flood. A reduction in crop injury at RES's Hamilton Road research site underscores the value of land leveling to keep water at a uniform depth.

Some tolerance to clomazone may be developing in watergrass that has multiple resistance to other herbicides. For broad-spectrum control, Cerano can be followed by later applications of Shark (carfentrazone), propanil, Londax (bensulfuronmethyl) or Regiment (bispyribac-sodium).

Regiment applied at the one-to-three tiller stage of rice afforded good control of watergrass, ricefield bulrush and California arrowhead. Water must be lowered to expose at least 70 percent of the weed foliage in most cases and higher rates may be required in fields with resistant late watergrass. To avoid injury to rice, Regiment should not be applied much before appearance of the first tiller. This herbicide can be used in sequential applications - either preceded by or followed by Clincher (cyhalofop-butyl) - for additional control of sprangletop. Regiment in combination with Abolish (thiobencarb) continues to perform synergistically with applications made from the four-to-six stage of rice.

Clincher is a post-emergent herbicide that is highly active on susceptible watergrass and sprangletop. It is very safe on rice and can be applied at increasing rates from the two-leaf stage of rice up to just before panicle initiation. It can also be applied in sequential applications for broad-spectrum control. Clincher will not control watergrass resistant to Whip (fenoxaprop). Followed by propanil at the one-to-three tiller stage, Clincher applied at the three-to-six leaf stage of rice was one of the four best broad-spectrum treatments. Followed by Regiment instead of propanil at the same timing, additional control of ricefield bulrush and California arrowhead can be expected. Tank mixes of Clincher and propanil may result in antagonistic effects that reduce sprangletop control, which is why sequential applications of these herbicides is preferred.

Shark is a useful sedge and broadleaf herbicide that can be applied at the two-to-three leaf stage of rice as a direct-dry application, in combination with other into-water herbicides and in sequential weed control operations. Excellent broad-spectrum control was obtained with Shark applied at the two-to-three leaf stage of rice following Cerano. It also performed excellently when formulated as an extruded granule for drift control and handler safety. Excellent broad-spectrum control resulted when Clincher followed Shark at the one-to-three leaf stage.

At a site heavily infested with resistant late watergrass, Clincher was applied at the two-to-six leaf stage of rice mainly for sprangletop and followed by propanil at the one-to-three tiller stage, resulting in 88 percent watergrass control and broad-spectrum activity. An excellent treatment for resistant late watergrass control was the synergistic tank mixture of Abolish and Regiment applied at the five-to-six leaf stage.

Several new herbicides are also being tested for broad-spectrum control-IR 5878 (anthrasulfuron), Sempra (halosulfuron) and DE-638 (penoxsulam). These ALS-inhibitors performed well in foliar and into-the-water applications. Combinations or sequential applications with other herbicides are required at sites with resistant watergrass or Londax-resistant weeds.

Another "interesting" compound is Sofit (pretilachlor+safener). Used pre-flood in continuously flooded rice, it controlled herbicide-resistant watergrass, smallflower umbrellasedge, ricefield bulrush and ducksalad. It is weak on California arrowhead and rates need to be adjusted to maximize safety to rice. In dry-seeded rice Sofit provided good broad-spectrum control and excellent sprangletop control.

Herbicide resistance

Scientists also provided a service to growers testing watergrass and barnyardgrass samples for signs of resistance. Barnyardgrass, early and late watergrass were tested against Ordram (molinate), Bolero (thiobencarb), Regiment (bispyribac-sodium) and Clincher (cyhalofop-butyl). Herbicide resistance, including multiple resistance in some samples, was present in a higher percentage of samples tested. This underscores the need to develop alternative weed management strategies to reduce the rate at which these compounds lose their effectiveness.

Genetic Purity

A study to determine the risk of gene flow from transgenic rice was completed. Prompted by the needs of genetic purity preservation as established in the California Rice Certification Act, a large experiment was continued this year to establish maximum distances of pollen flow from transgenic to non-transgenic rice. Natural outcrossing from transgenic rice to non-transgenic rice was detected no further than 1.8 meters from the transgenic sources, with rates for outcrossing from 0.010 to 0.415 percent.

Work will continue to identify traits for enhancing rice competitiveness with weeds and the mechanisms of resistance and dispersal of herbicide-resistant weeds.


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