...results suggest that straw chopping and flooding appear to be more effective than straw incorporation for limiting resistant watergrass seed.

Promising New Herbicides

Research continued on Command (clomazone) for a fourth season at the Rice Experiment Station (RES) and at two on-farm locations. This compound is a pigment synthesis inhibitor, a new mode of action, is designed to control herbicide-resistant watergrass and sprangletop.

Command is applied in a granular form and has repeatedly proven effective against watergrass very early in rice growth. Beyond early stages, however, effectiveness decreases and rice injury occurs. Last year, a warmer than usual spring may have contributed to the injury problem. Loss of rice foliage resulting from this damage reduced rice competitiveness and allowed new flushes of watergrass to emerge.

Some varieties—Akitakomachi, Calmochi-101, CH-201, M-104, M-202, M-205 and S-102—were more sensitive to Command when applied at high rates. Symptoms include foliar chlorosis and bleaching, stunting and stand reduction. Noticeably tolerant varieties were A-201, CT-201, L-204 and L-205.

Command could become a very useful tool for early watergrass and sprangletop control. However, this herbicide may not be appropriate for use with certain varieties unless new techniques are developed to reduce rice injury. An expanded experiment will be conducted again in 2002 in a flooded levee plot system.

Regiment (bispyribac-sodium) is a new post-emergent herbicide that will be available in 2002 to control resistant watergrass. Applied with a silicone surfactant, it can be effective on watergrass, ricefield bulrush and California arrowhead at the one- to three-tiller stage of rice. Water must be lowered to expose at least 70 percent of the weed foliage. Rates must be increased with resistant watergrass to be effective. Early application before first rice tiller can cause crop injury. Combinations of Regiment with Abolish (thiobencarb) showed an enhanced or synergistic watergrass control. Regiment in combination with propanil, however, was less effective on watergrass. Similarly, combinations of Regiment with either Clincher (cyhalofop) or Grandstand (trichlopyr) antagonized watergrass control.

Clincher (cyhalofop) is a post-emergent herbicide highly active on susceptible watergrass and sprangletop. It has been tested at the RES four years and at off-station field locations for two seasons. Clincher is very safe on rice and can be applied from two-leaf stage up to before panicle initiation. It can also be applied in sequential applications for broad-spectrum control. Draining fields for post-emergent herbicide applications usually encourages new weed emergence, so reflooding in a timely manner is important. This herbicide will not control watergrass resistant to Whip (fenoxaprop). Clincher combined with propanil did not provide effective sprangletop or watergrass control. Clincher mixed with Grandstand antagonized watergrass control.

However, Clincher applied at the five-leaf stage of rice, followed by a sequential application of Regiment at the one- to three- tiller stage provided excellent control of watergrass, sprangletop and California arrowhead. Similarly, Clincher followed by propanil or Sempra (halosulfuron) resulted in excellent broad-spectrum control.

Shark (carfentrazone) is a useful sedge and broadleaf herbicide that can be applied as post-emergent in a direct spray or dry application, in combination with other into-water herbicides and in sequential weed control operations. Foliar tank-mix applications of Shark and propanil injured rice and reduced weed control. Shark mixed with Clincher at the five- to six-leaf stage of rice antagonized watergrass control.

Other Formulations

Duet (a combination of propanil and bensulfuron) was tested for a third season at the RES and a second year off-station at another location with watergrass resistance. Duet provided fair-to-good watergrass control and excellent control of sedges and broadleaf weeds at both locations.

BAS 625H, a cyclohexadione ACCase inhibitor, was tested for a third season at RES for its effectiveness as a grass herbicide. Compared to similar compounds, it was safer to rice than Ricestar (feoxaprop and safener) but did not provide the same level of control or safety as Clincher. It performed very well against sprangletop and, according to the manufacturer, is more effective on early watergrass than late watergrass.

IR-5878 is an ALS inhibitor applied as an early post emergent. It was tested for a fourth season at the RES and provided good watergrass and sedge control. In combination with propanil it provided enhanced weed control, although stunting of rice was observed for about 10 days. The rice fully recovered.

Sempra (halosulfuron) is an ALS inhibitor used with a similar range of control as Londax (bensulfuron). It was tested as a foliar spray and an into-the-water treatment at RES for a second season. Sempra appears to be somewhat more active than Londax. Combinations with propanil resulted in good watergrass and excellent sedge control. Because of its similar mode of action, Sempra will be restricted to sites where Londax resistance does not occur.

Clearfield Rice

The ClearfieldŽ rice system was used for the first time at the RES in 2001. This system is being developed by BASF and is currently targeted at the Southern rice states for the control of red rice. California weed scientists have evaluated it as an option for control of watergrass and sprangletop. BASF has not yet developed it in California.

Clearfield rice is not transgenic rice but is based on mutant breeding technology. It is designed to incorporate resistance to broad-spectrum imidazlinone herbicides such as Newpath (imazethapyr) and Raptor (imazethapox) that would otherwise be lethal to rice.

Newpath and Raptor provided excellent control of watergrass, sprangletop and ricefield bulrush. Regiment controlled watergrass and ricefield bulrush. All herbicides in this trial, including Londax, were weak against smallflower umbrellasedge and redstem, which may have become resistant to this and other ALS inhibitors in this field. For this reason, the Clearfield rice system may have a tough time being completely effective in those areas where broadleaf and sedges have developed resistance to ALS inhibitors.

Genetic Purity

Prompted by the need to ensure genetic purity as established by the California Rice Certification Act (CRCA), a large experiment was initiated in 2001 to determine maximum distances necessary to prevent pollen migration from transgenic to non-transgenic rice.

Herbicide-resistant rice developed with genetic engineering could provide a tool for overcoming the crippling effects of herbicide resistance. However, concerns have been raised about the potential for “outcrossing” genetic traits through cross pollination from one plant to another. The CRCA calls for the development of procedures to ensure identity preservation, including isolation distances. Wind speed and turbulence, temperature, relative humidity and other climatic conditions come into play. The isolation distances given by seed-certification agencies are, for the most part, educated guesses, so a two-year field study has been established to monitor potential outcrossing of transgenic rice to conventional rice.

Two experiments were initiated with the transgenic rices Liberty-Link M-202 and Round-Up Ready M-202, which are resistant to glufosinate and glyphosate, respectively. The trials were planted at the RES and will monitor outcrossing of these traits to conventional M-202. Results for this experiment are currently being evaluated at UC Davis and at Mid-West Seed, Inc. in Brookings, South Dakota.

Herbicide Toxicity

Another study was conducted in 2001 to determine the effect of herbicides on rice yields in the absence of weeds. The herbicides to be tested were applied at their recommended rates and at twice the recommended rate. The study demonstrated that visual symptoms of injury on rice (e.g. bleaching, stunting, leaf bronzing, spotting) recorded shortly after herbicide application are related to yield reduction, particularly when scoring exceeds 10 percent. Command applied at double the standard rate resulted in significantly lower yields Clincher was the safest herbicide.

Rice/Weed Competition

A two-year greenhouse study to identify rice traits that enhance competitiveness against weeds was completed. Previous experiments showed that about 30 days after seeding, rice is capable of suppressing watergrass, thus greatly reducing the need for late herbicide applications. Results of this study are still being processed. However, promising correlations between rice traits and watergrass suppression suggests that enhancing rice competitiveness while maintaining good yields is feasible.

Resistant Watergrass

Herbicide resistance in watergrass is nearing epidemic proportions in California rice fields. Watergrasses are the major weeds of rice and the development of resistance deprives farmers of essential tools for weed control and necessitates increased herbicide rates and frequency of application.

An experiment in a Glenn County field with a chronic lack of effective watergrass control continued for a third year to evaluate a systems approach to reducing resistance. The three-pronged study compared annual rotation of herbicides with different modes of action or tank mixes and sequential applications; use of transgenic rice resistant to environmentally friendly broad-spectrum herbicides; and the use of straw management to reduce weed seed survival.

Results clearly indicate that tank mixes or sequential applications of herbicides with different modes of action were effective in suppressing resistant watergrass. A late-season application to control “escapes” and prevent their seed production was deemed essential. The major emphasis in a program to manage herbicide-resistant weeds should be that of preventing reseeding into the soil seedbank reservoir. Also, results suggest that straw chopping and flooding appear to be more effective than straw incorporation for limiting resistant watergrass seed. Information from these experiments will improve knowledge of watergrass dynamics and suggests alternative cultural practices that could potentially increase control, reduce expense and decrease herbicide use.