Research into weed control continues on a number of fronts. Studies of the effectiveness of new and existing compounds, combinations and sequential applications are a major emphasis of this work. 

Concern over herbicide resistance is generating a great deal of new research into other areas.  For instance, scientists are documenting how alternative crop establishment methods alter weed dynamics and thus diversify herbicide options. 

Lab work is also helping to tease apart the biological basis for herbicide resistance, giving weed scientists yet another tool in understanding the most challenging production problem in California rice.

Herbicide studies

Herbicide efficiency of currently registered and potential new herbicides has become a more crucial issue in recent years with the development of herbicide-resistant weeds.  Following are highlights of work on studies of new compounds, combinations and sequential applications.

Cerano (clomazone), a pigment synthesis inhibitor, is a grass herbicide that has been tested for seven seasons at the Rice Experiment Station and for two seasons at a known, resistant late-watergrass site in Glenn County.  This compound works well on watergrass and sprangletop but has been less effective against herbicide-resistant late watergrass.  In combination with either Shark (carfentrazone) or Super Wham (propanil), Cerano provided very good broad-spectrum weed control.  Damage to rice (bleaching and stunting) associated with use of this compound can be reduced and yield loss prevented when floodwater depth is not lower than four inches at the time of application.

Regiment (byspyribac-sodium) is a post-emergent herbicide applied with a silicone surfactant.  It is effective on watergrass, ricefield bulrush (that is not resistant to ALS inhibitors) and demonstrates good activity on California arrowhead.  Regiment was tested in both continuous flood and pinpoint systems alone, in tank mixes and in sequences.  Regiment performed well in pinpoint systems in sequence with Clincher (cyhalofop-butyl), and as a tank mix with Abolish; and also as a follow-up treatment to Cerano, Bolero and Sofit in continuously flooded systems.

Clincher is a registered, post-emergent ACCase inhibitor that controls watergrass and sprangletop and is very safe on rice.  Clincher performed well in sequential programs for pinpoint systems.  Good broad-spectrum control was obtained when Clincher was followed by Super Wham. This sequence is important in sites where watergrass can be resistant to Clincher.

Shark was tested at different timings following Cerano in a continuously flooded system.  The best broad-spectrum control, yield and safety to rice occurred when Shark followed Cerano at the two-to-four leaf stage of rice.  The dry application into water reduces the potential for non-target drift.  This herbicide is important to California rice, filling a niche created by widespread Londax (bensulfuron) resistance.

Prowl (pendimethalin) is a meristematic inhibitor that interferes with plant cellular division and early growth.  The EC formulation is labeled for use in dry/drill-seeded rice in California, while a new water-based formulation is being tested in water-seeded rice.  At identical application rates, the EC formulation was somewhat more effective.  Tank mixes of Prowl with Super Wham are being tested to improve residual control against watergrass in pinpoint systems.

IR-5878, an experimental ALS inhibitor, has been tested at the Rice Experiment Station for seven seasons.  It has worked well in pinpoint and continuously flooded systems but provided poor control of ALS resistant sedge biotypes.

Sofit (pretilachlor + safener)) differs in mode of action with currently available rice herbicides in California.  The pre-flood surface application provided excellent broad-spectrum control but caused severe stand reduction.  Given Sofit’s different mode of action and effectiveness on resistant late watergrass, however, further research with this chemical is warranted.

Granite GR (penoxsulam) is an ALS inhibiting post-flood herbicide for selective broad-spectrum control.  It’s not active on sprangletop and weak on resistant late watergrass.  This formulation was expected to be available for use in 2005.  It performed well on susceptible weeds as a sequential following Cerano and Bolero, or followed by propanil or Clincher.  As with all ALS-inhibiting compounds, Granite can fail to control certain biotypes that are resistant to the mode of action of this compound.

Alternative rice systems

Alternative rice establishment systems are at the center of a long-term experiment to incorporate the benefits of crop rotation in a crop that can seldom be rotated. These systems include conventional water-seeded rice; conventional drill-seeded rice; water-seeded rice after spring tillage and a stale seedbed; water-seeded rice after a stale seedbed without spring tillage; and drill-seeded rice after a stale seedbed without spring tillage.  The stale seedbed technique involves flushing the field with irrigation to promote weed emergence; weeds are then killed prior to seeding with a total non-selective herbicide like glyphosate (Roundup).  They were evaluated for their potential to shift and reduce weed species recruitment and to facilitate the use of herbicides such as pendimethalin and glyphosate that are capable of controlling weed biotypes resistant to herbicides used in conventional water-seeded rice.

Species compositions of weed communities were distinctly different among establishment systems. Sedge and broadleaf weeds dominated water-seeded systems. Sprangletop and barnyardgrass dominated drill-seeded systems. 

This study demonstrated the potential for manipulating weed species recruitment and to enable the use of herbicides to control weed biotypes resistant to herbicides used in conventional water-seeded systems.  For instance, glyphosate used in the stale seedbed systems reduced weed populations by 85 to 100 percent in drill-seeded rice and by nearly 100 percent in the water-seeded systems.  A pre-plant application of glyphosate onto emerged weeds was the only herbicide application required to achieve nearly 100 percent weed control over the entire season in the water-seeded/stale seedbed/ no till system.  Prowl and Clincher gave more than 89 percent grass control in drill-seeded systems.

Growers may be able to minimize recruitment of entire groups of weed species by alternating among years.  This strategy may effectively reduce selection pressure favoring resistance.

Resistance studies

A study is under way to better understand the current distribution of herbicide-resistant early and late watergrass and barnyardgrass.  Researchers collected 240 samples in the fall of 2003 throughout the rice-growing regions of the Sacramento Valley, including some samples submitted by growers.  All seed samples are being tested for resistance to Bolero, Ordram, Regiment and Clincher.  Substantial progress has been reported in the identification of the biochemical basis of herbicide resistance in watergrass.

Molecular analysis of a large collection of smallflower umbrella sedge gathered from throughout the state has also been undertaken to determine patterns of resistance in ALS-inhibiting herbicides.  This molecular “fingerprinting”  should prove useful in stewardship of compounds such as penoxsulam and bispyribac-sodium.

Scientists also conducted a second season of experiments aimed at delivering criteria for breeding competitive rice cultivars that will delay the onset of herbicide resistance.